Apparatus and method for resistivity well logging Number:6,777,940 from the United States Patent and Trademark Office (PTO) owispatent Well logging apparatus and methods for determining formation resistivity at multiple (>3) depths of investigation. At least one transmitter antenna and at least two receiver antennas are mounted in a logging tool housing, on substantially a common axis. The antennas are untuned coils of wire. Electromagnetic energy is emitted at multiple frequencies from the transmitter into the formation. The receiver antennas, which are spaced apart from each other and from the transmitter, detect reflected electromagnetic energy. Formation resistivity at multiple depths of investigation is determined using only phase differences in the reflected energy at the different frequencies, minimizing false indications of invasion due to mismatch of vertical response with attenuation measurements and also permitting correcting for the effects of varying dielectric constant of the formation. The apparatus minimizes the number of antennas, electronics complexity, required power, and measurement time required to determine resistivity at multiple depths of investigation.<H resistivity, Apparatus, Apparatus, and, m, 6777940.html, Patent, pto, 6777940

Title: Apparatus and method for resistivity well logging

Abstract: Well logging apparatus and methods for determining formation resistivity at multiple (>3) depths of investigation. At least one transmitter antenna and at least two receiver antennas are mounted in a logging tool housing, on substantially a common axis. The antennas are untuned coils of wire. Electromagnetic energy is emitted at multiple frequencies from the transmitter into the formation. The receiver antennas, which are spaced apart from each other and from the transmitter, detect reflected electromagnetic energy. Formation resistivity at multiple depths of investigation is determined using only phase differences in the reflected energy at the different frequencies, minimizing false indications of invasion due to mismatch of vertical response with attenuation measurements and also permitting correcting for the effects of varying dielectric constant of the formation. The apparatus minimizes the number of antennas, electronics complexity, required power, and measurement time required to determine resistivity at multiple depths of investigation.

Patent Number: 6,777,940 Issued on 08/17/2004 to Macune

Inventors: Macune; Don T. (Houston, TX)
Assignee: Ultima Labs, Inc. (Houston, TX)

Appl. No.: 10/291,440

Filed: November 8, 2002

Current U.S. Class: 324/338 ; 324/335

Current International Class: G01V 3/30 (20060101); G01V 3/18 (20060101)

Field of Search: 324/334-343 702/7

References Cited [Referenced By]

U.S. Patent Documents


5278507	January 1994	Bartel et al.
5452761	September 1995	Beard et al.
5650726	July 1997	Gasnier et al.
5869968	February 1999	Brooks et al.
6218841	April 2001	Wu
6218842	April 2001	Bittar et al.
6476609	November 2002	Bittar
6646441	November 2003	Thompson et al.
6703837	March 2004	Wisler et al.

Primary Examiner: Le; N.
Assistant Examiner: Kinder; Darrell
Attorney, Agent or Firm: Dominque & Waddell, PLC

Claims

I claim:

1. A measurement while drilling apparatus for determining resistivity of formations surrounding a borehole at multiple radial depths, comprising: a) an elongated housing comprising a first transmitter antenna and first and second receiver antennas mounted thereon, said first and second receiver antennas spaced apart from one another and spaced apart from said first transmitter antenna, and wherein said transmitter and receiver antennas comprise untuned antennas adapted to transmit and receive electromagnetic energy simultaneously at multiple frequencies; b) means for generating a multiple frequency electromagnetic energy waveform coupled to said first transmitter antenna, said waveform comprising a fundamental frequency element and at least two higher frequency harmonics of said fundamental frequency element; c) means for processing reflections of said electromagnetic energy waveform detected by said first and second receiver antennas at said fundamental frequency and said harmonic frequencies, coupled to said first and second receiver antennas, and for generating a digitized output signal therefrom for each of said frequencies; and d) means for receiving said output signals and for determining a phase difference between said signals detected at said first and second receiver antennas at each of said frequencies, and for determining formation resistivity at each of said detected frequencies as a function of only said determined phase difference, thereby determining formation resistivity at multiple distances from said borehole.

2. The apparatus of claim 1, wherein said means for generating a multiple frequency electromagnetic energy waveform comprises a switchmode circuit.

3. The apparatus of claim 1, wherein said means for processing reflections of said electromagnetic energy waveform comprises a radio frequency preamplifier coupled to said receiver antenna, and at least one radio frequency band pass filter coupled to said preamplifier.

4. The apparatus of claim 3, wherein said at least one radio frequency band pass filter comprises three radio frequency bandpass filters, each of said three radio frequency bandpass filters tuned to one of the transmitted frequencies.

5. The apparatus of claim 4, wherein said means for processing reflections of said electromagnetic energy waveform further comprises a single frequency local oscillator coupled to a mixer, with an output of said mixer coupled to an intermediate frequency bandpass filter, and wherein each of said three radio frequency bandpass filters has a bandwidth less than twice the frequency of said intermediate frequency band pass filter.

6. The apparatus of claim 5, further comprising an analog to digital converter adapted to sample data signals at rates greater than twice the bandwidth of said intermediate frequency bandpass filter and to digitize signals at frequencies in the audio frequency range or higher.

7. The apparatus of claim 4, wherein said means for processing reflections of said electromagnetic energy waveform further comprises a multiple frequency local oscillator coupled to a mixer, with an output of said mixer coupled to three intermediate band pass filters.

8. The apparatus of claim 4, further comprising an analog multiplexer coupled between said radio frequency preamplifier and said three radio frequency band pass filters, such that said analog multiplexer selects an input signal from one of two radio frequency preamplifiers connected to said receiver antennas.

9. The apparatus of claim 3, further comprising a radio frequency low pass filter having a cutoff frequency of about 2 MHz or higher, and further comprising an analog to digital converter adapted to sample data signals at rates of twice the bandwidth of said radio frequency low pass filter or faster.

10. The apparatus of claim 1, wherein said means for receiving said output signals and for determining a phase difference between said signals detected at said first and second receiver antennas at each of said frequencies comprises a digital signal processor adapted to determine the variables

and adapted to adjust the period of accumulation of R and X to achieve a desired minimum value for the sum (R.sup.2 +X.sup.2).

11. The apparatus of claim 1, wherein said first transmitter antenna and first and second receiver antennas are on a substantially common axis.

12. The apparatus of claim 11, further comprising: a) a second transmitter antenna mounted in said housing on said substantially common axis such that said first and second receiver antennas are between said first and second transmitter antennas; b) means for generating a multiple frequency electromagnetic energy waveform coupled to said second transmitter antenna, said waveform comprising a fundamental frequency element and at least two higher frequency harmonics of said fundamental frequency element,

and wherein said first and second receiver antennas additionally detect reflections of electromagnetic energy emitted from said second transmitter antenna for processing.

13. The apparatus of claim 12, wherein said means for generating a multiple frequency electromagnetic energy waveform comprises a switchmode circuit.

14. The apparatus of claim 12, wherein said means for processing reflections of said electromagnetic energy waveform comprises a radio frequency preamplifier coupled to said receiver antenna, and at least one radio frequency band pass filter coupled to said preamplifier.

15. The apparatus of claim 14, wherein said at least one radio frequency band pass filter comprises three radio frequency bandpass filters, each of said three radio frequency bandpass filters tuned to one of the transmitted frequencies.

16. The apparatus of claim 15, wherein said means for processing reflections of said electromagnetic energy waveform further comprises a single frequency local oscillator coupled to a mixer, with an output of said mixer coupled to an intermediate frequency band pass filter, and wherein each of said three radio frequency bandpass filters has a bandwidth less than twice the frequency of said intermediate band pass filter.

17. The apparatus of claim 16, further comprising an analog to digital converter adapted to sample data signals at rates greater than twice the bandwidth of said intermediate frequency bandpass filter and to digitize signals at frequencies in the audio frequency range or higher.

18. The apparatus of claim 15, wherein said means for processing reflections of said electromagnetic energy waveform further comprises a multiple frequency local oscillator coupled to a mixer, with an output of said mixer coupled to three intermediate band pass filters.

19. The apparatus of claim 14, further comprising a radio frequency low pass filter having a cutoff frequency of about 2 MHz or higher, and further comprising an analog to digital converter adapted to sample data signals at rates of twice the bandwidth of said radio frequency low pass filter or faster.

20. The apparatus of claim 12, wherein said means for receiving said output signals and for determining a phase difference between said signals detected at said first and second receiver antennas at each of said frequencies comprises a digital signal processor adapted to determine the variables

and adapted to adjust the period of accumulation of R and X to achieve a desired minimum and adapted to adjust the period of accumulation of R and X to achieve a desired minimum value for the sum (R.sup.2 +X.sup.2).

21. The apparatus of claim 12, wherein said means for receiving said output signals further comprises a means for determining an attenuation between said reflections detected at said first and second receiver antennas at each of said frequencies, and determining a formation dielectric constant at each of said detected frequencies as a function of said determined phase difference and said attenuation.

22. The apparatus of claim 1, wherein said means for receiving said output signals further comprises a means for determining an attenuation between said reflections detected at said first and second receiver antennas at each of said frequencies, and determining a formation dielectric constant at each of said detected frequencies as a function of said determined phase difference and said attenuation.

23. The apparatus of claim 1, further comprising a gamma radiation sensor and means for processing a signal received from said sensor to determine a gamma radiation value of a formation.

24. The apparatus of claim 23, further comprising an inclination sensor and means for processing a signal received from said sensor to determine an inclination of said housing.

25. An apparatus for multiple depth of investigation formation resistivity measurement, comprising: a) an elongated housing comprising a first transmitter antenna and first and second receiver antennas mounted thereon, said first and second receiver antennas spaced apart from one another and spaced apart from said first transmitter antenna, and wherein said transmitter and receiver antennas comprise untuned antennas adapted to transmit and receive electromagnetic energy simultaneously at multiple frequencies; b) a multiple frequency electromagnetic energy waveform generator coupled to said first transmitter antenna, said waveform comprising a fundamental frequency element and at least two higher frequency harmonics of said fundamental frequency element; c) a receiver circuit coupled to said first and second receiver antennas and adapted to process reflections of said electromagnetic energy waveform detected by said first and second receiver antennas at said fundamental frequency and said harmonic frequencies, and to generate a digitized output signal therefrom for each of said frequencies; and d) a digital signal processor receiving said output signals and determining a phase difference between said signals detected at said first and second receiver antennas at each of said frequencies, said digital signal processor further determining formation resistivity at each of said detected frequencies as a function of only said determined phase difference, thereby determining formation resistivity at multiple distances from said borehole.

26. The apparatus of claim 25, wherein said multiple frequency electromagnetic energy waveform generator comprises a switchmode circuit.

27. The apparatus of claim 25, wherein said receiver circuit coupled to said first and second receiver antennas comprises a radio frequency preamplifier coupled to said receiver antenna, and at least one radio frequency band pass filter coupled to said preamplifier.

28. The apparatus of claim 27, wherein said at least one radio frequency band pass filter comprises three radio frequency bandpass filters, each of said three radio frequency bandpass filters tuned to one of the transmitted frequencies.

29. The apparatus of claim 28, wherein said receiver circuit coupled to said first and second receiver antennas further comprises a single frequency local oscillator coupled to a mixer, with an output of said mixer coupled to an intermediate frequency bandpass filter, and wherein each of said three radio frequency bandpass filters has a bandwidth less than twice the frequency of said intermediate frequency bandpass filter.

30. The apparatus of claim 29, further comprising an analog to digital converter adapted to sample data signals at rates greater than twice the bandwidth of said intermediate frequency bandpass filter and to digitize signals at frequencies in the audio frequency range or higher.

31. The apparatus of claim 28, wherein said receiver circuit coupled to said first and second receiver antennas further comprises a multiple frequency local oscillator coupled to a mixer, with an output of said mixer coupled to three intermediate band pass filters.

32. The apparatus of claim 28, further comprising an analog multiplexer coupled between said radio frequency preamplifier and said three radio frequency band pass filters, such that said analog multiplexer selects an input signal from one of two radio frequency preamplifiers connected to said receiver antennas.

33. The apparatus of claim 27, further comprising a radio frequency low pass filter having a cutoff frequency of about 2 MHz or higher, and further comprising an analog to digital converter adapted to sample data signals at rates of twice the bandwidth of said radio frequency lowpass filter or faster.

34. The apparatus of claim 25, wherein said digital signal processor is adapted to determine the variables

and adapted to adjust the period of accumulation of R and X to achieve a desired minimum value for the sum (R.sup.2 +X.sup.2).

35. The apparatus of claim 25, wherein said first transmitter antenna and first and second receiver antennas are on a substantially common axis.

36. The apparatus of claim 35, further comprising a gamma radiation sensor and means for processing a signal receiver from said sensor to determine a gamma radiation value of a attenuation.

37. The apparatus of claim 36, further comprising a inclination sensor and means for processing a signal received from said sensor to determine an inclination of said housing.

38. The apparatus of claim 36, further comprising: a) a second transmitter antenna mounted in said housing on said substantially common axis such that said first and second receiver antennas are between said first and second transmitter antennas; b) a multiple frequency electromagnetic energy waveform generator coupled to said second transmitter antenna, said waveform comprising a fundamental frequency element and at least two higher frequency harmonics of said fundamental frequency element,

and wherein said first and second receiver antennas additionally detect reflections of electromagnetic energy emitted from said second transmitter antenna for processing.

39. The apparatus of claim 38, wherein said multiple frequency electromagnetic energy waveform generator comprises a switchmode circuit.

40. The apparatus of claim 38, wherein said receiver circuit coupled to said first and second receiver antennas comprises a radio frequency preamplifier coupled to said receiver antenna, and at least one radio frequency band pass filter coupled to said preamplifier.

41. The apparatus of claim 40, wherein said at least one radio frequency band pass filter comprises three radio frequency bandpass filters, each of said three radio frequency bandpass filters tuned to one of the transmitted frequencies.

42. The apparatus of claim 41, wherein said receiver circuit coupled to said first and second receiver antennas further comprises a single frequency local oscillator coupled to a mixer, with an output of said mixer coupled to an intermediate frequency band pass filter, and wherein each of said three radio frequency bandpass filters has a bandwidth less than twice the frequency of said intermediate frequency band pass filter.

43. The apparatus of claim 42, further comprising an analog to digital converter adapted to sample data signals at rates of greater than twice the bandwidth of said intermediate frequency bandpass filter and to digitize signals at frequencies in the audio frequency range or higher.

44. The apparatus of claim 41, wherein said receiver circuit coupled to said first and second receiver antennas further comprises a multiple frequency local oscillator coupled to a mixer, with an output of said mixer coupled to three intermediate frequency band pass filters.

45. The apparatus of claim 40, further comprising a radio frequency low pass filter having a cutoff frequency of about 2 MHz or higher, and further comprising an analog to digital converter adapted to sample data signals at rates of twice the bandwidth of said radio frequency low pass filter or faster.

46. The apparatus of claim 38, wherein said digital signal processor receiving said output signals and determining a phase difference between said signals is adapted to determine the variables

and adapted to adjust the period of accumulation of R and X to achieve a desired minimum value for the sum (R.sup.2 +X.sup.2).

47. The apparatus of claim 36, wherein said digital signal processor is further adapted to determine an attenuation between said reflections detected at said first and second receiver antennas at each of said frequencies, and to determine a formation dielectric constant at each of said detected frequencies as a function of said determined phase difference and said attenuation.

48. The apparatus of claim 25, wherein said digital signal processor is adapted to determine an attenuation between said reflections detected at said first and second receiver antennas at each of said frequencies, and to determine a formation dielectric constant at each of said detected frequencies as a function of said determined phase difference and said attenuation.

49. A method for determining formation resistivity at three or more depths radially from a borehole, comprising the steps of: a) providing a measurement while drilling tool comprising: i) an elongated housing comprising a first transmitter antenna and first and second receiver antennas mounted thereon, said first and second receiver antennas spaced apart from one another and spaced apart from said first transmitter antenna, and wherein said transmitter and receiver antennas comprise untuned antennas adapted to transmit and receive electromagnetic energy simultaneously at multiple frequencies; ii) a multiple frequency electromagnetic energy waveform generator coupled to said first transmitter antenna, said waveform comprising a fundamental frequency element and at least two higher frequency harmonics of said fundamental frequency element; iii) receiver circuits adapted to receive signals from said first and second receiver antennas caused by reflections of said electromagnetic energy waveform detected thereby at said fundamental frequency and said harmonic frequencies, coupled to said first and second receiver antennas, and for generating a digitized output signal therefrom for each of said frequencies; and iv) a digital signal processor for receiving said output signals and for determining a phase difference between said signals detected at said first and second receiver antennas at each of said frequencies, and for determining formation resistivity at each of said detected frequencies as a function of only said determined phase difference, b) generating a multiple frequency electromagnetic energy waveform generator from said first transmitter antenna, said waveform comprising a fundamental frequency element and at least two higher frequency harmonics of said fundamental frequency element; c) detecting signals from said first and second receiver antennas caused by reflections of said electromagnetic energy waveform detected thereby at said fundamental frequency and said harmonic frequencies; d) generating a digitized output signal for each of said frequencies; e) determining a phase difference between said signals at each of said frequencies, and determining formation resistivity at each of said detected frequencies as a function of only said determined phase difference.

Description

BACKGROUND

1. Field of the Invention

This invention relates to the field of electric logging of "wells" or earthen boreholes. In particular, the invention relates to well logging apparatus and methods for determining formation properties, such as resistivity, at several different distances extending radially from the borehole into the surrounding formation. The invention has general applications in the well logging art, but is particularly useful in measurement while drilling ("MWD") applications.

2. Description of the Related Art

Resistivity logging is a commonly used technique for evaluating potential hydrocarbon-bearing formations surrounding a borehole drilled into the earth. Porous formations are more resistive to a flow of electric current when they are saturated with hydrocarbons, and less resistive when saturated with water (which contains some amount of salt, rendering it more or less conductive). The formation immediately surrounding the borehole can be altered by invasion of borehole fluids during the drilling of the well, and can therefore exhibit a different resistivity than the formation farther from the borehole--so-called "virgin" formation. In order to determine the true resistivity of the virgin zone, the well logging device must be capable of performing measurements at multiple depths of investigation. The multiple depths permit mathematical correction of the different measured values.

Historically, resistivity logging tools, conveyed by wireline after the borehole has been drilled, have measured resistivity at three depths of investigation (shallow, medium, and deep). Mathematically, the three measurements are used to solve for three unknowns (Rt, Rxo, and Di). The shallow and medium measurements are used to correct the deep measurements to obtain a more accurate measurement of true virgin resistivity (Rt). The medium and deep readings are used to correct the shallow reading to obtain a more accurate reading of flushed zone resistivity (Rxo) (the flushed zone being the formation nearest the borehole, in which the original formation fluids have been at least partially displaced by drilling fluids). The three readings are also used to determine the depth of invasion Di (that is, how far drilling fluids have intruded into the formation), when a simple step invasion profile is assumed.

Large values for depth of invasion indicate zones of high permeability, which suggest potential high fluid flow rates, desirable for producing commercially significant quantities of hydrocarbons. Computed values for Rt and Rxo may be used for estimating water saturation (Sw), under certain favorable conditions. Low values of Sw indicate the presence of hydrocarbons in the formation.

The present invention relates to a type of resistivity well logging known as electromagnetic propagation logging. Propagation logging is well suited for determining resistivity by apparatus designed for use while drilling, so-called MWD tools. The basic principle of such measurement is a transmitter propagating electromagnetic energy into the formation, at a known frequency and strength, and reflections of that transmitted energy are detected by receivers spaced apart from the transmitter. Earlier generation MWD propagation resistivity devices provided only two depths of investigation, from the phase difference and attenuation measurements. By "phase difference" is meant a difference in timing between the transmitted and received signal. By "attenuation" is meant a lessening or decrease in the amplitude of the transmitted signal.

Separation of the curves is used to identify invasion; however, it is mathematically impossible to solve for the three desired unknowns (Rt, Rxo, Di) from only two measurements. Another disadvantage of the earlier generation tools is that the vertical response of the attenuation measurement is not as sharp as the vertical response of the phase difference measurement. As a result, separation of the curves results at bed boundaries (that is, the boundaries between beds of dissimilar rock type within a zone), even when invasion is not present. Also, it is known in the art that dielectric uncertainty can cause the phase difference and attenuation curves to separate even when no fluid invasion is present. In fact, the separation of phase difference and attenuation curves can be used to estimate the dielectric constant in thick beds.

Another disadvantage of the attenuation measurement is reduced dynamic range when compared to the phase difference measurement. As the formation resistivity increases, the attenuation measured between the two receiver antennas approaches a constant value, and the measurement becomes insensitive to changes in resistivity. In contrast, the phase difference measurement retains sensitivity to higher resistivity values and thus has a broader useful range. The limited dynamic range of the attenuation measurement sets an upper resistivity limit on the utility of apparatus employing this method for detecting invasion.

More recent propagation MWD resistivity devices have added measurements at additional depths of investigation. However, these prior art apparatus and method still have various limitations. One group of apparatus achieves the multiple depth resistivity measurements via additional transmitter and receiver antennas, each tuned to transmit or receive at the same frequency but spaced differently, thereby resulting in different depths of investigation. The additional transmitters and receivers, it will be appreciated, added greatly to cost and complexity of the tools.

Yet another group of apparatus employed multiple different frequencies to yield multiple depths of investigation (it being known in the art that different frequencies yield different depths of investigation, the lower frequencies yielding a deeper investigation, while higher frequencies yield a shallower depth of investigation). However, this group of tools still required multiple additional transmitters and receivers, each tuned to transmit or receive only a single frequency. Again, increased cost and complexity of tools resulted. Many of these prior art apparatus exhibit other limitations, such as high electrical power consumption.

The apparatus and method of the present invention provide resistivity measurements at multiple (three or more) depths of investigation while avoiding the disadvantages of related art methods and apparatus. The apparatus and method herein provide multiple resistivity measurements that have nearly equivalent vertical resolution and maximum dynamic range, by using only phase difference measurements. Since attenuation measurements are not used for additional depths of investigation, the attenuation measurements can be combined with the phase difference measurements to solve for the formation dielectric constant at multiple frequencies. The current apparatus minimizes the number of antennas required for either a borehole compensated and electronically compensated measurement (four), or alternatively for an uncompensated measurement (three), as a result minimizing manufacturing and maintenance cost and maximizing reliability. Furthermore, untuned coils are used for the transmitter and receiver antennas, allowing each coil to be used for more than one frequency and eliminating error caused by mutual inductance between adjacent series tuned receiver antennas. The apparatus minimizes electronics required to transmit multiple frequencies by using a switch-mode transmitter circuit, which has the further advantage of generating the desired frequencies simultaneously. The transmitter electronics disclosed are also simpler and more efficient than methods used in the prior art. Transmitter energy is minimized by using low noise electronics and coherent detection in the receiver. Time required to complete a measurement can be minimized by simultaneously detecting multiple frequencies in the receiver.

OBJECTS AND ADVANTAGES

Accordingly one of the objects of this invention is to provide resistivity measurements of a formation surrounding a borehole at multiple (three or more) depths of investigation into the formation. The advantage of this invention is that the additional measurements can be used to compute true or virgin formation resistivity, flushed zone resistivity, depth of investigation, and additional parameters useful in evaluating the economic potential of an oil or gas well.

Another object is to provide measurements at multiple depths of investigation with nearly equivalent vertical response and maximum dynamic range. The advantage is that differences of the measurements caused by mismatched vertical resolution and limited dynamic range are minimized, further minimizing false indications of invasion and error in determination of true formation resistivity.

Another object is to provide an estimate of formation dielectric constant by using the attenuation measurement in combination with the phase difference measurement at each frequency to determine both formation resistivity and dielectric constant. The advantage is that more accurate corrections for variations in dielectric constant can be applied to the resistivity data.

Another object is to minimize the number of transmitting and receiving antennas required for measurements at multiple depths of investigation. The advantage is that the device will have lower manufacturing and operating cost and greater reliability than prior art devices.

Another object is to use untuned coils for both the receiver and transmitter antennas. The advantage is that untuned coils are less expensive to build and maintain and are more reliable. A further advantage is that untuned coils can be used to transmit or receive multiple frequencies, unlike prior art devices, which require separate coils for each measurement frequency. Another advantage is that untuned receiver antennas do not suffer from errors caused by circulating currents in series-tuned receiver antennas.

Another object is to minimize the complexity and maximize the efficiency of electronics used to drive the transmitter antennas at multiple frequencies. The advantage is that manufacturing and maintenance costs will be reduced, reliability increased, and power consumption minimized, increasing battery life or allowing a smaller power source to be used.

Another object is to transmit and detect multiple frequencies simultaneously. The advantage is that the total time required for measurements of multiple depths of investigation is minimized compared to prior art devices and methods.

Another object is to minimize the required transmitter energy and maximize the dynamic range of the measurements by using low noise electronics and coherent detection. The advantage is that the reduced transmitter energy results in longer battery life or allows smaller power sources to be used, without compromising the accuracy and resolution of the measurements.

Further objects and advantages will become apparent from consideration of the drawings and ensuing description thereof.

SUMMARY OF THE INVENTION

The present invention is directed to a well logging apparatus having features that are responsive to a number of needs of the prior art, as discussed above. Most of the features of the invention as set forth herein generally have application to both wireline logging and measurement while drilling. However, some of the features hereof are particularly advantageous for use in a measuring while drilling apparatus.

In accordance with a feature of the invention, there is provided an apparatus and method for investigating earth formations in which resistivity is determined at three or more depths of investigation radially into the formation while using signals transmitted from either a single transmitter antenna or a pair of transmitter antennas placed symmetrically around a pair of receiving antennas. In an embodiment of this form of the invention using a single transmitter antenna, electromagnetic energy is transmitted at a first location in the borehole (the active transmitter antenna) and received at a second and third location (the receiving antenna pair). In an embodiment with two transmitter antennas, electromagnetic energy is also transmitted from a fourth location in the borehole and received at the second and third locations, following transmission from the first location. The measurements of the received signals from both transmissions are optionally combined to cancel errors resulting from the borehole or imbalance in the electronics.

The electromagnetic energy is transmitted at a fundamental frequency and also at harmonics of the fundamental frequency. The receivers at the second and third location determine the phase difference between the two receiving locations, at each frequency of interest. The formation resistivity at the deepest depth of investigation is determined from the phase difference measurement of the fundamental (lowest) frequency. The resistivity of the formation closer to the borehole is determined from the phase difference of the higher frequency harmonics. As the frequency increases, the measurement distance away from the borehole decreases. By using multiple frequencies, with each antenna capable of transmitting and receiving multiple frequencies, the total number of antennas required to obtain measurements at multiple depths of investigation is minimized.

The attenuation of the electromagnetic energy between the second and third location at each frequency is also determined, and is combined with the phase difference measurements at each frequency to simultaneously determine the formation dielectric constant as well as a dielectric-corrected resistivity. The vertical response of the phase difference measurements are well matched, so that they may be easily combined to evaluate the invasion profile of borehole fluids into the formation and determine true resistivity, flushed zone resistivity, and depth of invasion. Also, the phase difference measurements have a wider usable dynamic range than the attenuation measurements.

In accordance with another feature of the invention, there is provided an apparatus and method wherein all of the antennas are simple, untuned coils of wire. The elimination of tuning allows each antenna to be used at multiple frequencies, eliminating the need for individually tuned antenna coils for each frequency. Minimizing the number of antenna coils lowers manufacturing and maintenance cost, and improves reliability. The elimination of tuning also eliminates mutual coupling of magnetic fields in the closely spaced receiver coils due to circulating currents in low impedance series tuned antenna coils.

In accordance with another feature of the invention, there is provided an apparatus and method wherein the electronics used to drive the transmitter antennas consist of a simple switchmode amplifier topology driven by a derivative of the system clock. The pulsating waveform used to drive the transmitter antenna contains energy at the fundamental frequency and also at higher harmonics of the fundamental frequency. The pulsating waveform provides a simple, convenient method for generating the additional frequencies of interest. Switchmode operation of the electronics delivers high efficiency with minimal power dissipation and self heating in the electronics components.

In accordance with another feature of the invention, there is provided an apparatus and method wherein the receiver is capable of detecting multiple frequencies simultaneously. The signals received by the receiver antennas and amplified with a low noise RF amplifier are digitized directly and processed to extract the phase and amplitude information at each frequency of interest. The RF amplifier includes filtering to limit the bandwidth of the signal to be digitized, in order to prevent aliasing of noise into the measurement. The anti-aliasing filters insure good performance in conditions of low received signal to noise ratio, and allow less energy to be used to drive the transmitter. Detecting multiple frequencies simultaneously results in the shortest possible measurement time.

In accordance with another feature of the invention, there is provided an apparatus and method wherein transmitter energy is minimized by using low noise receiver electronics and coherent detection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a drilling rig and drill string including a measurement while drilling system in which an embodiment of the invention can be employed.

FIG. 2 is a block diagram of the propagation resistivity tool, showing antennas, printed circuit assemblies, and interconnections.

FIG. 3 shows the relationship for attenuation.

FIG. 4 shows the relationship for phase difference measured between the two receiver antennas vs. resistivity.

FIG. 5 shows the response of phase and attenuation measurements at three different frequencies to varying depths of invasion.

FIG. 6 compares the vertical resolution of the phase difference measurement vs. the attenuation measurement for 2 MHz.

FIG. 7 shows the relationship of phase difference and attenuation under varying conditions of both resistivity and dielectric constant.

FIG. 8 is the block diagram of the electronics on the transmitter printed circuit assembly.

FIGS. 9a-9d show various possible topologies for the receiver electronics.

FIG. 10 shows an improved receiver electronics design for systems with an unbalanced antenna array.

FIG. 11 shows a block diagram of the processor printed circuit assembly.

DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Overview of MWD System

FIG. 1 illustrates an embodiment of the invention in the form of an MWD or logging-while-drilling apparatus and method. A drilling rig (1) is located over a borehole formed in the earth by rotary drilling. A drill string is suspended within the borehole and typically includes drill pipe from the surface (2), one or more drill collars (3), a mud motor (optionally), and a drill bit (4) at its lower end. During rotary drilling, the drill string and drill bit attached thereto are rotated by a rotating table (5), which engages a kelly at the upper end of the drill string. The drill string is suspended from a hook (6) attached to a traveling block (not shown). The kelly is connected to the hook through a rotary swivel (7) which permits rotation of the drill string relative to the hook. Sensors and associated instrumentation (8) monitor movement and load of the hook and/or kelly in order to generate a record of depth of the drill bit vs. time. This instrumentation is commonly referred to as the drill string depth system.

Drilling fluid or mud is contained in a pit (9) and is pumped by a mud pump 10 into the bore of the drill string via a port in the swivel to flow downward through the center of the drill string. Additional sensors and associated instrumentation are present which monitor the circulating mud pressure at the top of the drill string. The drilling fluid exits the bore of the drill string via ports in the drill bit and circulates upward in the annulus (11) between the outside of the drill string and the borehole wall. The resulting circulating pressure in the mud column at the top of the drill string is proportional to the resistance to flow encountered by the mud as it circulates. The drilling fluid provides lubrication for the bit while drilling and flushes formation cuttings to the surface, where the fluid returns to the pit for recirculation. The arrows in FIG. 1 illustrate the direction of flow of the drilling fluid.

When a mud motor is present in the bottom hole assembly, the flow of drilling mud through the bore of the drill string turns the mud motor, which in turn rotates the drill bit. In this mode of operation, the borehole may be lengthened by sliding the drill pipe into the borehole while the mud motor rotates the drill bit. Typically a specialized drill collar known as a bent sub is included above the drill bit. The bent sub causes the drill bit to turn slightly away from a straight path, allowing a curved borehole to be created. The direction of the borehole is controlled by orienting the bent sub via the drill string from the surface. If the entire bottom hole assembly is rotated from the surface, it is still possible to drill a straight hole. By a combination of rotary drilling and sliding, the trajectory of the borehole can be accurately controlled.

Mounted within the drill string, preferably near the drill bit, are the. components of the MWD system. These components include means for sensing various directional, geophysical, mechanical, or other parameters of interest, processing the outputs of the sensors, storing the data, and transmitting data of interest to the surface. MWD subsystem (12) includes a measuring apparatus (13) which comprises antennas T1, R1, and R2.

A transmitting portion (14) of the downhole subsystem includes a controllable valve in the bore of the drill string. Closing of the valve creates increased resistance to flow of the drilling mud, resulting in a measurable increase in pressure at the top of the bore of the drill string at the surface. Opening of the valve results in a measurable decrease in pressure. By opening and closing the valve, a serial bit stream of digital data can be transmitted to the surface and recovered by monitoring corresponding variations in pressure at the top of the drill string. Other techniques for transmitting digital data by modulating the mud pressure measured at the surface are also in use. Another technique for transmitting digital data to the surface relies on transmission of low frequency electromagnetic energy from the MWD transmitter through the formation to a receiving antenna at the surface. The computer at the surface (8) typically combines the mud pulse or electromagnetic telemetry data recovered from the downhole transmitter with the drill bit depth system output to create a real time log of sensor data versus depth. The telemetry system may also include the capability of receiving commands from the surface.

All presently available methods for transmitting digital data to the surface are limited in their capacity such that only a subset of the data of interest may be transmitted in real time. Additional data is typically stored downhole in a downhole memory subsystem. The downhole memory subsystem includes a clock which has been previously synchronized with the clock at the surface used to track drill bit depth vs. time. Each set of sensor data stored downhole is saved with the time it was acquired, creating a file of sensor data vs. time. When the MWD assembly returns to the surface, the memory file is downloaded from the MWD tool and combined with the surface file of drill bit depth vs. time to create a log of sensor data versus depth. Additional processing or corrections may be applied to the sensor data at the surface. There may be a single memory subsystem serving multiple sensors, or each sensor may have its own internal memory for storage of sensor data vs. time.

An additional subsystem of the measurement while drilling system provides electrical power to the sensors and telemetry data transmitter. The power subsystem may consist of a turbine in the bore of the drill collar with alternator wherein electrical power is created from rotation of the turbine by circulation of drilling fluid. The turbine--alternator system provides power only when mud is circulating. Instead of a turbine with alternator, a high temperature battery may be used as the primary source of electrical power. The battery can provide only a limited number of hours of operation, but operates independently of mud circulation. Often a turbine-alternator is combined with a battery to provide unlimited flow-on operation with the capability to acquire sensor data when flow is off. An additional subsystem of the measurement while drilling system is the system master. The system master coordinates operation of the sensors with the telemetry data transmitter and data memory subsystem. Each sensor contains an analog to digital conversion system which converts the sensor output to a digital representation. The system master is responsible for transferring the digital data from the sensor to either the telemetry data transmitter or data memory subsystem.

Block Diagram--Antennas and System Electronics

FIG. 2 is a block diagram of the electronics in the measuring apparatus 13 of FIG. 1. FIG. 2 shows lower and upper transmitter antennas T1 and T2, respectively; however, while the ensuing description includes upper transmitter T2, it is understood that upper transmitter T2 may be omitted in order to minimize the overall length of the device, and that the scope of the invention comprises apparatus with either one or two transmitter antennas. Also, as widely understood from the Reciprocity Theorem for antennas and electromagnetic propagation, the position of transmitter and receiver antennas can be interchanged without affecting the measurement. In an apparatus with two receivers located between two transmitters, the location of transmitters and receivers can be interchanged. In an apparatus with only one transmitter and two receivers, the two receivers can be replaced by two transmitters, and the original single transmitter can be replaced by a single receiver.

With reference to FIG. 2, the electronics required for implementing the resistivity electronics comprise four separate printed circuit assemblies. Interconnections for system power and local power supplies are not shown. Lower and upper transmitter antennas T1 and T2 are driven by a means for generating a multiple frequency electromagnetic energy waveform therefrom, comprising separate transmitter boards 20 and 30, respectively. Transmitter boards 20 and 30 are located physically near transmitter antennas T1 and T2 to minimize crosstalk from the transmitter wiring to the receiver antennas and circuitry. Signals from the processor board 40 provide on/off control and a common time base required for synchronous detection. The time base is typically generated by a crystal oscillator on the processor board. The time base frequency is much higher than the measurement frequencies to minimize crosstalk to the receiver electronics. On the transmitter board(s), the higher frequency time base is divided down to generate the measurement frequencies.

A means for processing reflections of the electromagnetic energy detected by R1 and R2 comprises receiver electronics resident on a single printed circuit board 50. Receiver board 50 is located physically near receiver antennas R1 and R2 to minimize wiring and interconnections required for the small receiver antenna signal. Receiver board 50 amplifies the received RF signal, shifts the frequency of the signal down to the audio range using a local oscillator and mixer, filters the audio frequency signal, and digitizes the signal using analog to digital converters. The digital data output signal is then transferred to processor board 40.

A means for receiving said output signal and determining a phase difference and attenuation therebetween, comprises processor board 40 which determines the relative phase and amplitude of the received signals from the sampled data transferred from receiver board 50. Processor board 40 uses synchronous detection to determine amplitude and phase. Synchronous detection allows the bandwidth of the measurement to be significantly reduced, which improves the overall signal to noise ratio. The higher signal to noise ratio extends the dynamic range and resolution of the measurement without an increase in transmitted power.

Synchronous detection requires that the transmitter, local oscillator, and analog to digital converter all operate from a common time base. The common time base resides on processor board 40. Processor board 40 also provides control of the transmitter boards 30 and 40, selecting either T1 or T2 to be operational in a sequential manner. Processor board 40 can also cause both transmitter boards 20 and 30 to be in a low power "off" state between measurements. Processor board 40 also controls the frequency of the local oscillator on receiver board 50.

Following determination of relative phase and amplitude of both individual received signals, processor board 40 further comprises a means for determining formation resistivity at multiple detected frequencies, by determining the phase difference and attenuation (amplitude ratio) of the two signals. Processor board 40 then converts phase difference and/or attenuation to an equivalent resistivity, or performs additional processing to estimate and correct for error due to dielectric effects of the formation. Processor board 40 also responds to data requests from the MWD system master.

Optional sensors for detection of natural gamma radiation 60 from the formation and measurement of inclination 70 of the drill collar are also shown in FIG. 2. Sensors and electronics required for these measurements are commonly located in probes in the bore of the drill collar. However, the packaging of the electronics and design of the antennas of the resistivity collar allow the sensors and electronics to be located in the resistivity collar. The gamma radiation measurement benefits from increased sensitivity, as there is less steel surrounding the detector to interfere with the measurement. The inclination measurement also benefits in that the inclination sensor can be rigidly mounted to the resistivity collar, and is not subject to the uncertainty in orientation created by mounting of a probe inside the bore of the collar. Total system electronics are minimized by allowing the gamma radiation measurement and inclination measurement to share processing, communications, and power supply electronics with the resistivity measurement. The processor board performs counting of the gamma ray detector pulse output, to determine gamma counts per second. It also calculates inclination and tool face from the measured accelerometer outputs.

Transforms--Phase Difference and Attenuation vs. Resistivity vs. Frequency

For the resistivity measurement, transmitter boards 20 and 30 drive transmitter antennas T1 and T2 with a waveform that contains a fundamental (lowest) frequency and odd harmonics of the fundamental frequency. By way of example only and not limitation, a typical value for the fundamental frequency is 400 KHz, with odd harmonics at 1.2 MHz (3.times.fundamental frequency) and 2.0 MHz (5.times.fundamental frequency). The higher frequencies are more sensitive to regions at shallower distances radially from the tool, while the lowest frequency penetrates deepest and is sensitive to regions farthest radially from the tool. The relationship of the measured attenuation of the two received signals (from receiver antennas R1 and R2) to the resistivity of a homogeneous formation (without invasion) for a typical antenna spacing of 24" from transmitter to near receiver (e.g., from T1 to R1) and 30" from transmitter to far receiver (e.g., from T1 to R2) is shown in FIG. 3. The same relationship for the measured phase difference is shown in FIG. 4.

Note that for the attenuation curves shown in FIG. 3, attenuation becomes almost constant for resistivities above 10 ohmm. For the phase difference curves shown in FIG. 4, the measured phase difference decreases with increasing resistance, but remains sensitive to formation resistivity even above 100 ohmm. For both phase difference and attenuation, sensitivity to formation resistance increases with frequency. These graphs (FIGS. 3 and 4) illustrate the advantage of the phase difference measurement over the attenuation measurement for measuring high values of formation resistance.

FIG. 5 illustrates the response of the phase difference and attenuation measurements to invasion. This figure assumes a 10 ohmm resistivity for the virgin formation far from the borehole, with a 1 ohmm invaded zone. For no invasion (invasion depth=0 in.), all measurements read 10 ohmm. As the invasion depth increases from zero, the phase difference measurements respond initially. As the invasion depth exceeds 10", the attenuation measurements are also affected. The curves verify that the phase difference measurements are more sensitive to shallow invasion than the attenuation measurements. Note that at an invasion depth of 10 ohmm, the phase difference curves at the three frequencies are clearly separated. The 2 MHz measurement reads is most affected by the invasion and reads approx. 5.3 ohmm. The 400 KHz measurement is least affected and reads approx. 6.8 ohmm. Using the three distinct readings at three different frequencies, values for virgin resistivity Rt, invaded (or flushed) zone resistivity Rxo, and depth of invasion Di can be determined.

Vertical Resolution--Phase vs. Attenuation

FIG. 6 illustrates the superior vertical resolution of the phase difference-derived resistivity versus the attenuation-derived resistivity, with two different bed arrangements. This data is from Clark, et al. for 2 MHz only. In the upper illustration, thin 10ohmm beds of from 1/2 to four feet thick are located between adjacent thick 1 ohmm beds. In the lower illustration, thin 0.3 ohmm beds are located between adjacent thick 1 ohmm beds. Note that the phase difference response is much sharper than the attenuation response, and also that the phase difference and attenuation curves separate in the regions immediately adjacent and also within the thin bed. Without a-priori knowledge of the bed boundaries or bed resistivities, the separation of the curves could be falsely interpreted as due to invasion. This illustrates a fundamental limitation of combining the phase difference and attenuation measurements to perform invasion profiling.

Although not shown, the bed boundary response for phase difference measurements which differ only in measurement frequency are very well matched. Nearly equivalent vertical response combined with differing depth of investigation allows the user to determine values for Rt, Rxo, and Di with significantly fewer artifacts created by bed boundaries.

Simultaneous Inversion of Resistivity and Dielectric Constant

Although the attenuation resistivity has several disadvantages when combined with phase difference resistivity for invasion profiling, it is still useful for determination of and correction for the formation dielectric constant. The curves shown in FIGS. 3 and 4 for phase difference and attenuation vs. resistivity assume a constant dielectric constant of 10. However, the dielectric constant of geologic formations is known to vary widely.

Error in estimating the dielectric constant causes the phase difference and attenuation resistivity curves to separate in a homogeneous formation. If the dielectric constant of a formation is higher than expected, the attenuation will be smaller, falsely indicating higher formation resistivity. However, the phase difference will be greater, falsely indicating lower than actual formation resistivity.

If no invasion is present, or if invasion can be detected in some other way (e.g., by using separation of phase difference resistivity curves obtained at different frequendes), the attenuation and phase difference data can be combined to simultaneously solve for both formation resistivity and dielectric constant. The method is limited to thick beds where bed boundary effects do not cause the phase difference and attenuation resistivity curves to separate due to differences in vertical resolution.

FIG. 7 shows a typical data set from Wu, et al for 2 MHz measurement frequency and 34" transmitter-receiver spacing. Phase difference and attenuation are used to locate an (x,y) coordinate on the chart. The curves nearest the (x,y) coordinate provide values for Rt and dielectric constant. The look-up process may be computer automated using a variety of algorithms to provide a continuous estimate of Rt and dielectric constant from log data.

Transmitter Circuit Details

FIG. 8 shows details of the transmitter electronics circuitry (earlier described as transmitter board 20) and operation thereof. The circuitry is located on an individual printed circuit board (PCB) located in close proximity to transmitter antenna T1 and electrically isolated from the remainder of the measurement electronics. If the tool has two transmitter antennas to realize the benefits of borehole an/or electronics compensation, then a second transmitter PCB (earlier described as transmitter board 30) is located near second transmitter antenna T2.

The inputs to the transmitter PCB are a single power supply voltage (Vbus), a master clock frequency (MCLK) and an Enable signal to turn the transmitter electronics on or off. MCLK and Enable are digital logic levels.

The Vbus power supply voltage (typically 18-48VDC ) is converted to other voltages required to power the circuitry on the transmitter PCB. These local power supply voltages are not shared with circuitry on other PC boards in the system, in order to minimize crosstalk through the power supply connections to the receiver circuitry. Vcc powers logic on the transmitter PCB and Vg provides the required voltage for controlling the switching elements

The frequency of MCLK is much higher than the measurement frequencies. The higher frequency is divided down to form logic signals DrvA and DrvB. Using a higher frequency for MCLK avoids electrical crosstalk in the measurement frequency bands due to the wiring of the clock signal. The frequency of DrvA and DrvB equals the fundamental, or lowest measurement frequency (which by way of example only may be 400 KHz). The switch drive logic insures that there is a small deadtime when both DrvA and DrvB are low (inactive). This deadtime prevents large current surges through switches A and B that would result if both switches were ever turned on at the same time.

Switches A and B are generally N-channel MOSFET (metal oxide semiconductor field effect transistor) conductor devices. MOSFETs are selected that balance losses due to "on" resistance with losses due to switch drive requirements. The MOSFET must also be rated for the voltage at the connection to T1 when the switch is "off". For N-channel devices, the source is connected to ground and the drive voltage is also referenced to ground, which simplifies circuitry required to create the drive voltages. Also, N-channel devices provide lower "on" resistance for a given input capacitance when compared to P-channel devices, enabling a higher level of efficiency for the overall circuit.

The center tap of the primary of T1 is connected to supply voltage Vpri. Filter component Cp provides AC current at the fundamental frequency. Components T1, Cp, and switches A & B are placed physically very near each other to minimize radiation of undesirable magnetic fields due to current flow between or through these elements. Filter c