CA1216025A - Method and apparatus for reducing groeningen effect errors in resistivity measurements of an earth formation - Google Patents
Method and apparatus for reducing groeningen effect errors in resistivity measurements of an earth formationInfo
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- CA1216025A CA1216025A CA000454930A CA454930A CA1216025A CA 1216025 A CA1216025 A CA 1216025A CA 000454930 A CA000454930 A CA 000454930A CA 454930 A CA454930 A CA 454930A CA 1216025 A CA1216025 A CA 1216025A
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- sonde
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
- G01V3/20—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with propagation of electric current
- G01V3/24—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with propagation of electric current using ac
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- Environmental & Geological Engineering (AREA)
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- General Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
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- Measurement Of Resistance Or Impedance (AREA)
- Geophysics And Detection Of Objects (AREA)
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- Apparatuses And Processes For Manufacturing Resistors (AREA)
Abstract
ABSTRACT
A method and apparatus are described for reducing "Groeningen" effect and other anomaly induced errors in resistivity measurements of an earth formation. Such errors are typically noted during resistivity measurements of an earth layer and are generally referred to as "Groeningen" effect or "casing" effect. A sonde having a plurality of voltage measuring electrodes and current emitting electrodes is provided and suspended in a borehole by means of a conductive cable. The sonde is attached to the conductive cable by means of a length of insulated cable and two reference electrodes are disposed above the sonde. A survey current and at least two focusing currents are emitted from the sonde at alternate times or on alternate frequencies and various voltages induced by each current are measured and utilized to calculate a corresponding set of transfer impedances for that current. By utilizing the calculated transfer impedances, the relationships of focusing currents to survey current which is necessary to properly focus the sonde may be calculated.
With the relationship of the survey current and focusing currents thus defined, the apparent resistivity of the formation may be expressed as a function of the transfer impedances and the currents without the necessity of actually altering the amounts of focusing current. One of the focusing currents is operated at a very low frequency to minimize the effect of a resistive anomaly and, by measuring the voltages induced by that current between widely spaced electrodes, the sonde may be operated at a faster rate than would otherwise be possible.
A method and apparatus are described for reducing "Groeningen" effect and other anomaly induced errors in resistivity measurements of an earth formation. Such errors are typically noted during resistivity measurements of an earth layer and are generally referred to as "Groeningen" effect or "casing" effect. A sonde having a plurality of voltage measuring electrodes and current emitting electrodes is provided and suspended in a borehole by means of a conductive cable. The sonde is attached to the conductive cable by means of a length of insulated cable and two reference electrodes are disposed above the sonde. A survey current and at least two focusing currents are emitted from the sonde at alternate times or on alternate frequencies and various voltages induced by each current are measured and utilized to calculate a corresponding set of transfer impedances for that current. By utilizing the calculated transfer impedances, the relationships of focusing currents to survey current which is necessary to properly focus the sonde may be calculated.
With the relationship of the survey current and focusing currents thus defined, the apparent resistivity of the formation may be expressed as a function of the transfer impedances and the currents without the necessity of actually altering the amounts of focusing current. One of the focusing currents is operated at a very low frequency to minimize the effect of a resistive anomaly and, by measuring the voltages induced by that current between widely spaced electrodes, the sonde may be operated at a faster rate than would otherwise be possible.
Description
~216Q~i Method and appara s for reducin~ groeningen effect errors in resistivity measurements of an earth formation Cross-reference to related applications This application is related to copending Canadian Application Serial Number 454,929, filed on May 23, 1984.
~ackground of the Invention This invention relates to geological formation exploration in general and in particular to the utilization of resistivity measurements in the exploration of geologi-cal formations. More particularly, this invention relates to a method and apparatus for reducing anomaly induced errors in such resistivity measurements of geological formations.
It is well known in the prior art that the sedimentary portion of the earth's surface is generally comprised of successive layers or beds which generally do not have a constant thickness. Each of these beds will typically exhibit a certain resistivity characteristic which can be highly useful in the evaluation of a part-icular borehole with regard to the presence of hydrocarbon deposits. The resistivity characteristics of a particular formation are generally investigated by introducing a resistivity measurement sonde into the borehole. Such sondes are generally lowered into a borehole on a cable utili~ing a section of insulated cable generally known as the "bridle" which is generally disposed between the cable and the sonde.
. .
~5~325 While disposed in the borehole, the resistivity measuring sonde is utilized to generate a survey current and generally one or more focusing or "bucking" currents which are utilized to obtain deeper lateral penetration of the formation by the survey current. In 5 previously known resistivity measurement sondes, voltage measurements taken between electrodes disposed on the sonde are utilized to constantly adjust the amount of focusing current necessary $o optimize the penetration of the survey current into the formation.
A return electrode is utilized at the surface to provide a return for 10 the various currents and permit the current measurements necessary to determine formation resistivity. A voltage reference electrode is also generally utilized and is generally located at a point between the lower part of the conductive cable and the sonde.
~Vhile this system has worked well for many years, certain 15 field conditions have been encountered which cause errors in the calculated or apparent resistivity of the formation. One such error is induced due to an anomaly in the resistance of a formation above the sonde which causes variations in the return path of the survey and focusing currents to the return electrode. ~1hen attempting 20 resistivity measurements in a low resistance earth formation helow a highly resistive bed, the survey and focusing currents tend to return along the well casing or cable and induce a greater than normal voltage at the reference electrode disposed above the sonde. This results in an error in calculation due to the fact that the voltage 25 differential between the sonde and the reference electrode will not be equal to the voltage differential between the sonde and the theoretical point at in~lnity utilized in these calculations.
This particular error, sometimes referred to as the "~roeningen" effect or "casing" effect is particularly distressing in 30 that the resultant resistivity measurements are similar in nature to those associated with petrochemical deposits when such deposits are not present. The expense associated with drilling and testing these wellbores makes it desirable that a method and apparatus be defined to correct for or recluce the errors induced by such resistive 35 anomalies. One method of correcting for this error involves the utilization of very low frequency currents; however, since several cycles of current are nccessary to complete a single resistivity ~Z16~Z~
measurement, the speed at which the sonde must be operated in this method is quite slow. The expense associated with these wells and equipment dictates that these measurements be taken as rapidly as possible, thereby eliminating this approach as a viable alternative.
Summary of the Invention . .
It is therefore one o~ject of the present invention to provide an improved apparatus for reducing anomaly induced errors in resistivity measurements of lo earth formations.
It is another object of the present invention to provide an improved method for reducing anomaly induced errors in resistivity measurements of earth formations.
It is yet another object of the present invention to provide an improved method for reducing anomaly induced errors in resistivity measurements of earth formations which permits the resistivity measurement device to be operated at normal speeds.
It is another object of the present invention to provide an improved method for reducing anomaly induced errors in resistivity measurements of earth formations which can be utilized in conjunction with known resistivity measurement sondes.
In accordance with an aspect of the invention there is provided a method for reducing anomaly induced errors in resistivity measurements of an earth formation traversed by a borehole in which the resistivity measure-ment is accomplished utilizing a sonde suspended in said borehole from a conductive cable in the borehole, said sonde having a plurality of voltage measuring electrodes and current emitting electrodes disposed thereon and a first reference electrode disposed above said sonde, com-prising emitting a survey current from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said survey current at at least a selected one of said voltage measuring electrodes; dis-~2~602~
- 3a -posing a second reference electrode between said first reference voltage electrode and said sonde; emitting a first focusing current at a first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said first focusing current between said sonde and said second refer-ence electrode; emitting a second focusing current at a fre~uency substantially below said first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said second focusing current between said second reference electrode and said first reference electrode; and combining the voltage induced by said first focusing current and the voltage induced by said second focusing current to obtain a resultant total voltage measurement between said sonde and said first reference electrode wherein said resultant total voltage measurement has a reduced anomaly induced error.
In accordance with another aspect of the invention ~0 there is prov~ded a method for reducing anomaly induced errors in resistivity measurements of an earth formation traversed by a borehole, in which the resistivity measure-ment is accomplished utilizing a sonde suspended in said borehole from a conductive cable in said borehole, said sonde having a plurality of voltage measuring electrodes and current emitting electrodes disposed thereon, com-prising suspending said sonde from said conductive cable utilizing a length of insulated cable; disposing a fîrst reference electrode above said sonde; disposing a second reference electrode on said insulated cable at a point between said first reference electrode and said sonde;
emitting a survey current from a selected one of said current emitting electrodes; measuring the voltages induced by said survey current at at least one of said voltage measuring electrodes; computing at least a first set of transfer impedances utilizing said voltages induced at at least one of said voltage measuring electrodes and said ' ' ~21~0Z5 - 3b -survey current; emitting a first focusing current from a selected one of said current emitting electrodes; measuring the voltages induced by said first focusing current between said sonde and said second reference electrode; computing at least a second set of transfer impedances utilizing said voltages induced between said sonde and said second refer-ence electrode and said first focusing current; emitting a second focusing current from a selected one of said current emitting electrodes at a frequency substantially below the frequency of said first focusing current; measuring the voltages induced by said second focusing current between said second reference electrode and said first reference electrode; computing at least a third set of transfer impedances utilizing said voltages induced between said second reference electrode and said first reference elec-trode and said second focusing current; and determining the resistivity of the earth formation as a function of said first, second and third sets of transfer impedances.
The foregoing objects are achieved as is now described. A sonde having a plurality of voltage measuring electrodes and current emitting electrodes is provided and suspended in a borehole by means of a conductive cable.
The sonde is attached to the conductive cable by means of a length of insulated ca~le and two reference electrodes are attached to the cable above the sonde. A survey cur-rent and at least two focusing currents are emitted from the sonde at alternate times or on alternate frequencies and various voltages induced by each current are measured and utilized to calculate a corresponding set of transrer impedances for that current. By utilizing the calculated transfer impedances, the relationships of focusin~ currents to survey current which is necessary to properly focus the sonde may be calculated. With the relationship of the survey current and focusing currents thus defined, the apparent resistivity of the formation may be expressed as a function of the transfer impedances and the currents without the necessity of actually altering the amounts of focusing current. One of the focusing currents is ~z~
operated at a ~rery low frequency to minimize the effect of a resistive anomaly and, by measuring the voltages induced by that current between ~ridely spaced electrodes, the sonde may be operated at a faster rate than would otherwise be possible.
s BRIEF DESCRIPTION OF THE DRAWINGS
_ Tlle novel features believed characteristic of the invention are set forth in the appended claims. The invention itself; however, as well as a preferred mode of use, further objects and advantages 10 thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
Figure 1 is a partially schematic, partially diagrammatic view of the resistivity measuring apparatus of the preser,t invention;
15 and Figure 2 is a block diagram of the circuitry of the resistivity measuring apparatus of the present invention.
I)ETAILED DESCRIPTION OF THE INVENTION
With reference now to the figures, and in particular with reference to Figure 1, there is depicted a partially schematic, partially diagrammatic view of the resistivity measuring apparatus of the present invention. The techniques disclosed herein are similar in nature to the techniques disclosed in the above-referenced related 25 applicatioll and that applîcation is hereby incorporated herein by reference thereto.
~s in the copending application, the method and apparatus disclosed herein will find application with many different types of resistivity measuring devices; however, for purposes of explanation, 30 the embodiment disclosed herein is described with respect to a so-called "deep latcrolog. " The "deep laterolog" is designed to measure formation resistivities at greater distances from the borehole und, in particular, beyond the "invaded zone" vhere the presence of drilling fluids may alter the formation resist,vity.
As in most typical installations of this type, the system includes a sonde 12 suspended in a borehole 14 by means of a ~vireline cable lG. Electrical conductors (not shown) disposed within ~Z16()~5 wireline cable lfi are coupled to various electronic processing devices contained in van 18. Sonde 12 includes a plurality of voltage measuring electrodes ~ilL and Ml U ~ 2L 2 U ' ~L 1 U
which are disposed on the surface of sonde 12 on either side of a 5 survey current emitting electrode Ao~ The subscripts U and L
signify the upper and lower of each pair of a pair of symmetrical electrodes. Sonde 12 also includes a plurality of focus current emitting electrOdes A1L~ A1U' ~2L a 2U
Also depis~ted in Figure 1 is a voltage reference electrode or 10 torpedo N which is separated from sonde 12 by a length of insulated cable 32, typically referred to as a "bridle. " Located between sonde 1 ' and torpedo N is an additional voltage measu~ing electrode N1~
Electrode N1 is particularly useful in the method disclosed herein for reducing errors in the formation resistivity measurements which may 15 be caused by resistive anomalies such as zone 36.
As is depicted in Figure 1, a typical path of return for survey current 38 is altered by the highly resistive nature of zone 36. Rather than proceeding radially outward to return electrode B, the path of survey current is altered by the presence of wireline 20 cable 16 and well casing 40. Those skilled in the art ~vill appreciate that the electrcmagnetic phenomena known as "skin effect" will enhance this alteration of return path and induce a greater than normal voltage at electrode N. This error may be minimized by utilizing very low frequency currents (typically less than one hertz);
25 however, since it is generally acknowledged that three cycles of current are necessary for each resistivity reading, a resistivity log of several thousand feet of borehole could become very time consuming.
Fortunately, the " Groeningen" effect varies very slo~-ly with depth and may be compensated for by utilizing this novel 30 technique of independent voltage and current measurements which permits sonde 12 to be operated at normal logging speeds while simultaneously measuring and COmpUtillg a "focused" condition which is corrected for the errors induced by the effect.
In orcler to understand the novel method and apparatus of 35 the present invention, it is necessary to understand the operation of a conventional deep laterolog system.
~51~2~
Generally, a survey current I0 is emitted by the central electrode Ao and a plurality of focusing currents are emitted from s A1L, A1U, A2L and A2U. The ratios between focusing currents and survey current are constantly adjusted to maintain a 5 zero or null voltage gradient between elec~rode pairs Ml and M2 and between A1 and A2.
In order to simplify the notation utilized herein, the average voltage of symmetrical electrode pairs, without U and subscripts, is written utilizing the rule expressed in equation (1):
M; Mj U jL
~ackground of the Invention This invention relates to geological formation exploration in general and in particular to the utilization of resistivity measurements in the exploration of geologi-cal formations. More particularly, this invention relates to a method and apparatus for reducing anomaly induced errors in such resistivity measurements of geological formations.
It is well known in the prior art that the sedimentary portion of the earth's surface is generally comprised of successive layers or beds which generally do not have a constant thickness. Each of these beds will typically exhibit a certain resistivity characteristic which can be highly useful in the evaluation of a part-icular borehole with regard to the presence of hydrocarbon deposits. The resistivity characteristics of a particular formation are generally investigated by introducing a resistivity measurement sonde into the borehole. Such sondes are generally lowered into a borehole on a cable utili~ing a section of insulated cable generally known as the "bridle" which is generally disposed between the cable and the sonde.
. .
~5~325 While disposed in the borehole, the resistivity measuring sonde is utilized to generate a survey current and generally one or more focusing or "bucking" currents which are utilized to obtain deeper lateral penetration of the formation by the survey current. In 5 previously known resistivity measurement sondes, voltage measurements taken between electrodes disposed on the sonde are utilized to constantly adjust the amount of focusing current necessary $o optimize the penetration of the survey current into the formation.
A return electrode is utilized at the surface to provide a return for 10 the various currents and permit the current measurements necessary to determine formation resistivity. A voltage reference electrode is also generally utilized and is generally located at a point between the lower part of the conductive cable and the sonde.
~Vhile this system has worked well for many years, certain 15 field conditions have been encountered which cause errors in the calculated or apparent resistivity of the formation. One such error is induced due to an anomaly in the resistance of a formation above the sonde which causes variations in the return path of the survey and focusing currents to the return electrode. ~1hen attempting 20 resistivity measurements in a low resistance earth formation helow a highly resistive bed, the survey and focusing currents tend to return along the well casing or cable and induce a greater than normal voltage at the reference electrode disposed above the sonde. This results in an error in calculation due to the fact that the voltage 25 differential between the sonde and the reference electrode will not be equal to the voltage differential between the sonde and the theoretical point at in~lnity utilized in these calculations.
This particular error, sometimes referred to as the "~roeningen" effect or "casing" effect is particularly distressing in 30 that the resultant resistivity measurements are similar in nature to those associated with petrochemical deposits when such deposits are not present. The expense associated with drilling and testing these wellbores makes it desirable that a method and apparatus be defined to correct for or recluce the errors induced by such resistive 35 anomalies. One method of correcting for this error involves the utilization of very low frequency currents; however, since several cycles of current are nccessary to complete a single resistivity ~Z16~Z~
measurement, the speed at which the sonde must be operated in this method is quite slow. The expense associated with these wells and equipment dictates that these measurements be taken as rapidly as possible, thereby eliminating this approach as a viable alternative.
Summary of the Invention . .
It is therefore one o~ject of the present invention to provide an improved apparatus for reducing anomaly induced errors in resistivity measurements of lo earth formations.
It is another object of the present invention to provide an improved method for reducing anomaly induced errors in resistivity measurements of earth formations.
It is yet another object of the present invention to provide an improved method for reducing anomaly induced errors in resistivity measurements of earth formations which permits the resistivity measurement device to be operated at normal speeds.
It is another object of the present invention to provide an improved method for reducing anomaly induced errors in resistivity measurements of earth formations which can be utilized in conjunction with known resistivity measurement sondes.
In accordance with an aspect of the invention there is provided a method for reducing anomaly induced errors in resistivity measurements of an earth formation traversed by a borehole in which the resistivity measure-ment is accomplished utilizing a sonde suspended in said borehole from a conductive cable in the borehole, said sonde having a plurality of voltage measuring electrodes and current emitting electrodes disposed thereon and a first reference electrode disposed above said sonde, com-prising emitting a survey current from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said survey current at at least a selected one of said voltage measuring electrodes; dis-~2~602~
- 3a -posing a second reference electrode between said first reference voltage electrode and said sonde; emitting a first focusing current at a first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said first focusing current between said sonde and said second refer-ence electrode; emitting a second focusing current at a fre~uency substantially below said first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said second focusing current between said second reference electrode and said first reference electrode; and combining the voltage induced by said first focusing current and the voltage induced by said second focusing current to obtain a resultant total voltage measurement between said sonde and said first reference electrode wherein said resultant total voltage measurement has a reduced anomaly induced error.
In accordance with another aspect of the invention ~0 there is prov~ded a method for reducing anomaly induced errors in resistivity measurements of an earth formation traversed by a borehole, in which the resistivity measure-ment is accomplished utilizing a sonde suspended in said borehole from a conductive cable in said borehole, said sonde having a plurality of voltage measuring electrodes and current emitting electrodes disposed thereon, com-prising suspending said sonde from said conductive cable utilizing a length of insulated cable; disposing a fîrst reference electrode above said sonde; disposing a second reference electrode on said insulated cable at a point between said first reference electrode and said sonde;
emitting a survey current from a selected one of said current emitting electrodes; measuring the voltages induced by said survey current at at least one of said voltage measuring electrodes; computing at least a first set of transfer impedances utilizing said voltages induced at at least one of said voltage measuring electrodes and said ' ' ~21~0Z5 - 3b -survey current; emitting a first focusing current from a selected one of said current emitting electrodes; measuring the voltages induced by said first focusing current between said sonde and said second reference electrode; computing at least a second set of transfer impedances utilizing said voltages induced between said sonde and said second refer-ence electrode and said first focusing current; emitting a second focusing current from a selected one of said current emitting electrodes at a frequency substantially below the frequency of said first focusing current; measuring the voltages induced by said second focusing current between said second reference electrode and said first reference electrode; computing at least a third set of transfer impedances utilizing said voltages induced between said second reference electrode and said first reference elec-trode and said second focusing current; and determining the resistivity of the earth formation as a function of said first, second and third sets of transfer impedances.
The foregoing objects are achieved as is now described. A sonde having a plurality of voltage measuring electrodes and current emitting electrodes is provided and suspended in a borehole by means of a conductive cable.
The sonde is attached to the conductive cable by means of a length of insulated ca~le and two reference electrodes are attached to the cable above the sonde. A survey cur-rent and at least two focusing currents are emitted from the sonde at alternate times or on alternate frequencies and various voltages induced by each current are measured and utilized to calculate a corresponding set of transrer impedances for that current. By utilizing the calculated transfer impedances, the relationships of focusin~ currents to survey current which is necessary to properly focus the sonde may be calculated. With the relationship of the survey current and focusing currents thus defined, the apparent resistivity of the formation may be expressed as a function of the transfer impedances and the currents without the necessity of actually altering the amounts of focusing current. One of the focusing currents is ~z~
operated at a ~rery low frequency to minimize the effect of a resistive anomaly and, by measuring the voltages induced by that current between ~ridely spaced electrodes, the sonde may be operated at a faster rate than would otherwise be possible.
s BRIEF DESCRIPTION OF THE DRAWINGS
_ Tlle novel features believed characteristic of the invention are set forth in the appended claims. The invention itself; however, as well as a preferred mode of use, further objects and advantages 10 thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
Figure 1 is a partially schematic, partially diagrammatic view of the resistivity measuring apparatus of the preser,t invention;
15 and Figure 2 is a block diagram of the circuitry of the resistivity measuring apparatus of the present invention.
I)ETAILED DESCRIPTION OF THE INVENTION
With reference now to the figures, and in particular with reference to Figure 1, there is depicted a partially schematic, partially diagrammatic view of the resistivity measuring apparatus of the present invention. The techniques disclosed herein are similar in nature to the techniques disclosed in the above-referenced related 25 applicatioll and that applîcation is hereby incorporated herein by reference thereto.
~s in the copending application, the method and apparatus disclosed herein will find application with many different types of resistivity measuring devices; however, for purposes of explanation, 30 the embodiment disclosed herein is described with respect to a so-called "deep latcrolog. " The "deep laterolog" is designed to measure formation resistivities at greater distances from the borehole und, in particular, beyond the "invaded zone" vhere the presence of drilling fluids may alter the formation resist,vity.
As in most typical installations of this type, the system includes a sonde 12 suspended in a borehole 14 by means of a ~vireline cable lG. Electrical conductors (not shown) disposed within ~Z16()~5 wireline cable lfi are coupled to various electronic processing devices contained in van 18. Sonde 12 includes a plurality of voltage measuring electrodes ~ilL and Ml U ~ 2L 2 U ' ~L 1 U
which are disposed on the surface of sonde 12 on either side of a 5 survey current emitting electrode Ao~ The subscripts U and L
signify the upper and lower of each pair of a pair of symmetrical electrodes. Sonde 12 also includes a plurality of focus current emitting electrOdes A1L~ A1U' ~2L a 2U
Also depis~ted in Figure 1 is a voltage reference electrode or 10 torpedo N which is separated from sonde 12 by a length of insulated cable 32, typically referred to as a "bridle. " Located between sonde 1 ' and torpedo N is an additional voltage measu~ing electrode N1~
Electrode N1 is particularly useful in the method disclosed herein for reducing errors in the formation resistivity measurements which may 15 be caused by resistive anomalies such as zone 36.
As is depicted in Figure 1, a typical path of return for survey current 38 is altered by the highly resistive nature of zone 36. Rather than proceeding radially outward to return electrode B, the path of survey current is altered by the presence of wireline 20 cable 16 and well casing 40. Those skilled in the art ~vill appreciate that the electrcmagnetic phenomena known as "skin effect" will enhance this alteration of return path and induce a greater than normal voltage at electrode N. This error may be minimized by utilizing very low frequency currents (typically less than one hertz);
25 however, since it is generally acknowledged that three cycles of current are necessary for each resistivity reading, a resistivity log of several thousand feet of borehole could become very time consuming.
Fortunately, the " Groeningen" effect varies very slo~-ly with depth and may be compensated for by utilizing this novel 30 technique of independent voltage and current measurements which permits sonde 12 to be operated at normal logging speeds while simultaneously measuring and COmpUtillg a "focused" condition which is corrected for the errors induced by the effect.
In orcler to understand the novel method and apparatus of 35 the present invention, it is necessary to understand the operation of a conventional deep laterolog system.
~51~2~
Generally, a survey current I0 is emitted by the central electrode Ao and a plurality of focusing currents are emitted from s A1L, A1U, A2L and A2U. The ratios between focusing currents and survey current are constantly adjusted to maintain a 5 zero or null voltage gradient between elec~rode pairs Ml and M2 and between A1 and A2.
In order to simplify the notation utilized herein, the average voltage of symmetrical electrode pairs, without U and subscripts, is written utilizing the rule expressed in equation (1):
M; Mj U jL
2 ~1) 15 Similarly, the sum of currents flowing from (or to) symmetrical electrodes will be written without the U and L subscript as follows:
1u + I1L) (2) When the aforementioned zero voltage gradient is achieved and the sonde is ~Ifocused,~ the apparent resistivity of a formation can be expressed by:
Ra = K [V~1i VNo]
0 ( 3 ) 25 Wrhere No is a r emote electrode which is ideally at electrical infinity.
In actual practice, the voltage measurement is taken with respect to torpedo N. In ger.eral, torpedo N is far enough away from sonde 12 that no error occurs. However, as depicted in Figure 1, nmeasurements taken beneath a zone of high resistivity can be 30 perturbed by the presence of the current flowing down wireline cable 16 to sonde 12.
The method of the preser.t invention utilizes four nominal value currents which are emitted from sonde 12. A survey current Io is emitted from electrode Ao~ Thrce separate focusing currents are also utilized. Currcnt 11 is cmitted from electrodes AlU and A"~
current I2 is emitted from electrodes A2 U and A2L; and, current I2 is also emitted from electrodes A2U and A2L.
~LZ1~2~
Each of these currents is emitted independently which, as those skilled in the art will appreciate, can be accomplished by alternately emitting each current or by continuously emitting all currents at various different frequencies. No attempt is made during this process to alter the amount of focusing current being emitted to actually balance sonde 12. In the preferred mode of the present invention, currents Io and Il are alternately emitted at several hundreds of hertz, current I2 is emitted at less than 100 hertz and current I*2 at a very low frequency (less than one hertz).
Preferably, during the period of time that each current is emitted, a series of five voltage potentials are measured:
Voltage Measuring Point Vl between Ml and N
V2 between Ml and M2 v3 between Al and A2 v4 between Ml and N
v5 between Nl and N
Each of these voltages is utilized in conjunction with each current to generate five transfer impedances for sonde 12 for each current source. In this manner, as in the above-referenced copending Canadian Application Serial No~ 454,929, the principle of superposition in linear cir-cuitry permits the amounts of various currents necessary to achieve a desired voltage level to be simply and easily computed utilizing the transfer impedances. The matrix of thus calculated transfer impedances is as follows:
1u + I1L) (2) When the aforementioned zero voltage gradient is achieved and the sonde is ~Ifocused,~ the apparent resistivity of a formation can be expressed by:
Ra = K [V~1i VNo]
0 ( 3 ) 25 Wrhere No is a r emote electrode which is ideally at electrical infinity.
In actual practice, the voltage measurement is taken with respect to torpedo N. In ger.eral, torpedo N is far enough away from sonde 12 that no error occurs. However, as depicted in Figure 1, nmeasurements taken beneath a zone of high resistivity can be 30 perturbed by the presence of the current flowing down wireline cable 16 to sonde 12.
The method of the preser.t invention utilizes four nominal value currents which are emitted from sonde 12. A survey current Io is emitted from electrode Ao~ Thrce separate focusing currents are also utilized. Currcnt 11 is cmitted from electrodes AlU and A"~
current I2 is emitted from electrodes A2 U and A2L; and, current I2 is also emitted from electrodes A2U and A2L.
~LZ1~2~
Each of these currents is emitted independently which, as those skilled in the art will appreciate, can be accomplished by alternately emitting each current or by continuously emitting all currents at various different frequencies. No attempt is made during this process to alter the amount of focusing current being emitted to actually balance sonde 12. In the preferred mode of the present invention, currents Io and Il are alternately emitted at several hundreds of hertz, current I2 is emitted at less than 100 hertz and current I*2 at a very low frequency (less than one hertz).
Preferably, during the period of time that each current is emitted, a series of five voltage potentials are measured:
Voltage Measuring Point Vl between Ml and N
V2 between Ml and M2 v3 between Al and A2 v4 between Ml and N
v5 between Nl and N
Each of these voltages is utilized in conjunction with each current to generate five transfer impedances for sonde 12 for each current source. In this manner, as in the above-referenced copending Canadian Application Serial No~ 454,929, the principle of superposition in linear cir-cuitry permits the amounts of various currents necessary to achieve a desired voltage level to be simply and easily computed utilizing the transfer impedances. The matrix of thus calculated transfer impedances is as follows:
3~ V1 V2 V3 V4 V5 lo aOl a02 ao3 ao4 ao5 I1 a11 al2 al3 a14 a15 a21 a22 a23 a24 a25 I2 a31 a32 a33 a34 ~35 (4) By now setting Io equal to unity and writing the conditions necessary to balance sonde 12, the theoret-ical amount of focusing currents necessary to balance the tool may be derived. The balance conditions are:
~L2~Q2~
Io = 1 (5~
V2 = ~7M~ - VM2 aO2IO + al2Il + a22I2 = (6) V3 = ~1 A2 ~ aO3IO + al311 + a23I2 By solving equations (5), (6) and (7) we can determine the exact amount of focusing current necessary to balance sonde 12 without the limitations generally imposed by physical constraints in existing systems.
Io = 1 (8) aO2a23 ~ aO3a22 Il =
a222l3 ~ al2a23 aO2al3 ~ ao3al2 8l2a23 - a22al3 (10) Given the precise amount of focusing current needed to balance sonde 12, the apparent resistivity of sonde 12 can be expressed as follows:
R = K (aO1I0 + a~ + a21 2) 0 (11) As those skilled in the art will appreciate, by substituting equations (9) and (10) into equation (11) the apparent resisti~vity of the formation may be expressed as a function of transfer impedances with Io being constrained to unity.
This expression of the apparent resistivity of the formation is not corrected for the resistive anomaly of zone 36. This error can be seen to be an error in transfer impedance a21 due to the spurious volta~e induced between electrodes M1 and N by the altered current path. In order to reduce this error it is only necessary to replace 35 transfer impedance a21 by a combination of two different transfer impedances, only one of which can be measured at a very low frequency. Indeed, since the voltage difference between electrodes M1 and N is equal to the voltage difl`erence between electrodes M1 and . , .
~Z1~02~
g N1 plus the voltage difference between electrodes N1 and M, transfer impedance a21 can be replaced by a combination of transfer impedances a24 and a35, where a24 is measured at a less than one hundred hertz and a35 is measured at a very low frequency. By 5 having measured the voltage induced between electrodes N1 and N at a very low frequency, this measurement can be combined with the voltage induced between electrodes M1 and N1 at normal survey frequencies to generate a corrected voltage measurement as expressed in equation (12~: 0 a [aO110 + a11I1 + (a24 + a35)I2] /Io (12) Those ordinarily skilled in the art will appreciate that the physical distances between electrodes Ml, Nl and N are much greater than the 15 distances between electrocles disposed on sonde 12, and this physical separation will permit the measurement of the voltage between electrode N1 and N to be accomplished at a very low frequency.
A ~lock diagram of the circuitry necessary to perform this method is depicted in Figure 2. As can be seen, the voltages at each 20 eiectrode on sonde 12 are coupled to appropriate band pass filters 50, 52, 54, 56 and 58, in those embodiments in which differing fre~uencies are utilized. The outputs of each band pass filter are then amplified by ampli~lers 60, 62, 64, 66 and 68 and coupled to analog-to-digital converters 70, 72 j 74, 76 and 78. Similarly, the 2 5 amount of current emitted from each electrode is coupled to an appropriate analog to-digital converter 80 ~ 82, 84 and 86 . The outputs of each analog-to-digital converter are then coupled to an appropriately programmed digital processing device 88 which is utilized, in a preferred embodiment of the present invention, to 30 calculate the necessary transfer impedances and to control the selective application of various survey and focusing currents in a manner well known in the art.
Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed 35 in a limiting sense. Various modifications of the disclose~l embodiment as urell as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the 1.2~ Z~;~
description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments that fall within the true scope of the invention.
~L2~Q2~
Io = 1 (5~
V2 = ~7M~ - VM2 aO2IO + al2Il + a22I2 = (6) V3 = ~1 A2 ~ aO3IO + al311 + a23I2 By solving equations (5), (6) and (7) we can determine the exact amount of focusing current necessary to balance sonde 12 without the limitations generally imposed by physical constraints in existing systems.
Io = 1 (8) aO2a23 ~ aO3a22 Il =
a222l3 ~ al2a23 aO2al3 ~ ao3al2 8l2a23 - a22al3 (10) Given the precise amount of focusing current needed to balance sonde 12, the apparent resistivity of sonde 12 can be expressed as follows:
R = K (aO1I0 + a~ + a21 2) 0 (11) As those skilled in the art will appreciate, by substituting equations (9) and (10) into equation (11) the apparent resisti~vity of the formation may be expressed as a function of transfer impedances with Io being constrained to unity.
This expression of the apparent resistivity of the formation is not corrected for the resistive anomaly of zone 36. This error can be seen to be an error in transfer impedance a21 due to the spurious volta~e induced between electrodes M1 and N by the altered current path. In order to reduce this error it is only necessary to replace 35 transfer impedance a21 by a combination of two different transfer impedances, only one of which can be measured at a very low frequency. Indeed, since the voltage difference between electrodes M1 and N is equal to the voltage difl`erence between electrodes M1 and . , .
~Z1~02~
g N1 plus the voltage difference between electrodes N1 and M, transfer impedance a21 can be replaced by a combination of transfer impedances a24 and a35, where a24 is measured at a less than one hundred hertz and a35 is measured at a very low frequency. By 5 having measured the voltage induced between electrodes N1 and N at a very low frequency, this measurement can be combined with the voltage induced between electrodes M1 and N1 at normal survey frequencies to generate a corrected voltage measurement as expressed in equation (12~: 0 a [aO110 + a11I1 + (a24 + a35)I2] /Io (12) Those ordinarily skilled in the art will appreciate that the physical distances between electrodes Ml, Nl and N are much greater than the 15 distances between electrocles disposed on sonde 12, and this physical separation will permit the measurement of the voltage between electrode N1 and N to be accomplished at a very low frequency.
A ~lock diagram of the circuitry necessary to perform this method is depicted in Figure 2. As can be seen, the voltages at each 20 eiectrode on sonde 12 are coupled to appropriate band pass filters 50, 52, 54, 56 and 58, in those embodiments in which differing fre~uencies are utilized. The outputs of each band pass filter are then amplified by ampli~lers 60, 62, 64, 66 and 68 and coupled to analog-to-digital converters 70, 72 j 74, 76 and 78. Similarly, the 2 5 amount of current emitted from each electrode is coupled to an appropriate analog to-digital converter 80 ~ 82, 84 and 86 . The outputs of each analog-to-digital converter are then coupled to an appropriately programmed digital processing device 88 which is utilized, in a preferred embodiment of the present invention, to 30 calculate the necessary transfer impedances and to control the selective application of various survey and focusing currents in a manner well known in the art.
Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed 35 in a limiting sense. Various modifications of the disclose~l embodiment as urell as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the 1.2~ Z~;~
description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments that fall within the true scope of the invention.
Claims (11)
1. A method for reducing anomaly induced errors in resistivity measurements of an earth formation traversed by a borehole in which the resistivity measurement is accomplished utilizing a sonde suspended in said borehole from a conductive cable in the borehole, said sonde having a plurality of voltage measuring electrodes and current emitting electrodes disposed thereon and a first reference electrode disposed above said sonde, comprising:
emitting a survey current from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said survey current at at least a selected one of said voltage measuring electrodes;
disposing a second reference electrode between said first reference voltage electrode and said sonde;
emitting a first focusing current at a first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said first focusing current between said sonde and said second reference electrode;
emitting a second focusing current at a frequency substantially below said first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said second focusing current between said second reference electrode and said first reference electrode; and combining the voltage induced by said first focusing current and the voltage induced by said second focusing current to obtain a resultant total voltage measurement between said sonde and said first reference electrode wherein said resultant total voltage measurement has a reduced anomaly induced error.
emitting a survey current from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said survey current at at least a selected one of said voltage measuring electrodes;
disposing a second reference electrode between said first reference voltage electrode and said sonde;
emitting a first focusing current at a first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said first focusing current between said sonde and said second reference electrode;
emitting a second focusing current at a frequency substantially below said first selected frequency from a selected one of said plurality of current emitting electrodes and measuring the voltage induced by said second focusing current between said second reference electrode and said first reference electrode; and combining the voltage induced by said first focusing current and the voltage induced by said second focusing current to obtain a resultant total voltage measurement between said sonde and said first reference electrode wherein said resultant total voltage measurement has a reduced anomaly induced error.
2. The method for reducing anomaly induced errors in resistivity measurements of an earth formation according to Claim 1 further including the step of calculating a first set of transfer impedances for said sonde by means of measuring said survey current and said voltage induced by said survey current.
3. The method for reducing anomaly induced errors in resistivity measurements of an earth formation according to Claim 2 further including the step of calculating a second set of transfer impedances for said sonde by means of measuring said first focusing current and said voltage induced by said first focusing current.
4. The method for reducing anomaly induced errors in resistivity measurements of an earth formation according to Claim 3 further including the step of calculating a third set of transfer impedances for said sonde by means of measuring said second focusing current and the voltage induced by said second focusing current.
5. The method for reducing anomaly induced errors in resistivity measurements of an earth formation according to Claim 4 further including the step of calculating the amount of focusing current needed to properly focus said sonde by utilizing said first, second and third sets of transfer impedances.
6. The method for reducing anomaly induced errors in resistivity measurements of an earth formation according to Claim 5 further including the step of calculating the resistivity of an earth formation transversed by said sonde as a function of said survey current, said first focusing current, said second focusing current and said first, second and third sets of transfer impedances.
7. Apparatus for correcting anomaly induced errors in resistivity measurements of an earth formation traversed by a borehole in which the resistivity measurement is accomplished utilizing voltage measurements between a sonde suspended in said borehole from a conductive cable and a first reference electrode disposed above said sonde, comprising:
means for emitting a first focusing current from said sonde at a selected frequency;
a second reference electrode disposed between said first reference electrode and said sonde;
means for measuring a first voltage induced by said first focusing current between said sonde and said second reference electrode;
means for emitting a second focusing current from said sonde at a frequency substantially below said selected frequency;
means for measuring a second voltage induced by said second focusing current between said second reference electrode and said first reference electrode; and means for combining said first voltage and said second voltage wherein said resultant voltage is a corrected measurement of the voltage measurement between said sonde and said first reference electrode.
means for emitting a first focusing current from said sonde at a selected frequency;
a second reference electrode disposed between said first reference electrode and said sonde;
means for measuring a first voltage induced by said first focusing current between said sonde and said second reference electrode;
means for emitting a second focusing current from said sonde at a frequency substantially below said selected frequency;
means for measuring a second voltage induced by said second focusing current between said second reference electrode and said first reference electrode; and means for combining said first voltage and said second voltage wherein said resultant voltage is a corrected measurement of the voltage measurement between said sonde and said first reference electrode.
8. The apparatus for correcting anomaly induced errors in resistivity measurements of an earth formation according to Claim 7 further including an elongated insulated cable disposed between said sonde and said first reference electrode.
9. The apparatus for correcting anomaly induced errors in resistivity measurements of an earth formation according to Claim 8 wherein said second reference electrode is disposed on said elongated insulated cable.
10. The apparatus for correcting anomaly induced errors in resistivity measurements of an earth formation according to Claim 7 wherein said sonde includes a plurality of current emitting electrodes and wherein said survey current is emitted from a first selected current emitting electrodes and wherein said first and second focusing currents are emitted from a second selected current emitting electrode.
11. A method for reducing anomaly induced errors in resistivity measurements of an earth formation traversed by a borehole, in which the resistivity measurement is accomplished utilizing a sonde suspended in said borehole from a conductive cable in said borehole, said sonde having a plurality of voltage measuring electrodes and current emitting electrodes disposed thereon, comprising:
suspending said sonde from said conductive cable utilizing a length of insulated cable;
disposing a first reference electrode above said sonde;
disposing a second reference electrode on said insulated cable at a point between said first reference electrode and said sonde;
emitting a survey current from a selected one of said current emitting electrodes;
measuring the voltages induced by said survey current at at least one of said voltage measuring electrodes;
computing at least a first set of transfer impedances utilizing said voltages induced at at least one of said voltage measuring electrodes and said survey current;
emitting a first focusing current from a selected one of said current emitting electrodes;
measuring the voltages induced by said first focusing current between said sonde and said second reference electrode;
computing at least a second set of transfer impedances utilizing said voltages induced between said sonde and said second reference electrode and said first focusing current;
emitting a second focusing current from a selected one of said current emitting electrodes at a frequency substantially below the frequency of said first focusing current;
measuring the voltages induced by said second focusing current between said second reference electrode and said first reference electrode;
computing at least a third set of transfer impedances utilizing said voltages induced between said second reference electrode and said first reference electrode and said second focusing current; and determining the resistivity of the earth formation as a function of said first, second and third sets of transfer impedances.
suspending said sonde from said conductive cable utilizing a length of insulated cable;
disposing a first reference electrode above said sonde;
disposing a second reference electrode on said insulated cable at a point between said first reference electrode and said sonde;
emitting a survey current from a selected one of said current emitting electrodes;
measuring the voltages induced by said survey current at at least one of said voltage measuring electrodes;
computing at least a first set of transfer impedances utilizing said voltages induced at at least one of said voltage measuring electrodes and said survey current;
emitting a first focusing current from a selected one of said current emitting electrodes;
measuring the voltages induced by said first focusing current between said sonde and said second reference electrode;
computing at least a second set of transfer impedances utilizing said voltages induced between said sonde and said second reference electrode and said first focusing current;
emitting a second focusing current from a selected one of said current emitting electrodes at a frequency substantially below the frequency of said first focusing current;
measuring the voltages induced by said second focusing current between said second reference electrode and said first reference electrode;
computing at least a third set of transfer impedances utilizing said voltages induced between said second reference electrode and said first reference electrode and said second focusing current; and determining the resistivity of the earth formation as a function of said first, second and third sets of transfer impedances.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US49927383A | 1983-05-31 | 1983-05-31 | |
| US499,273 | 1983-05-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1216025A true CA1216025A (en) | 1986-12-30 |
Family
ID=23984591
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000454930A Expired CA1216025A (en) | 1983-05-31 | 1984-05-23 | Method and apparatus for reducing groeningen effect errors in resistivity measurements of an earth formation |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1216025A (en) |
| FR (1) | FR2547063B1 (en) |
| GB (1) | GB2141237B (en) |
| NO (1) | NO159322C (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB8924934D0 (en) * | 1989-11-04 | 1989-12-28 | Natural Environment Res | Method for use in assessing the physical state of ground materials and apparatus for use therein |
| CA2878919A1 (en) * | 2012-07-13 | 2014-01-16 | Halliburton Energy Services, Inc. | System and method of focusing an array laterolog |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2653294A (en) * | 1949-04-21 | 1953-09-22 | Phillips Petroleum Co | Apparatus for electrical well logging |
| US4335353A (en) * | 1979-06-18 | 1982-06-15 | Schlumberger Technology Corporation | Method and apparatus for detecting an anomaly in a resistivity measurement of an earth formation |
-
1984
- 1984-05-23 CA CA000454930A patent/CA1216025A/en not_active Expired
- 1984-05-29 GB GB08413679A patent/GB2141237B/en not_active Expired
- 1984-05-29 FR FR8408401A patent/FR2547063B1/en not_active Expired - Fee Related
- 1984-05-30 NO NO842169A patent/NO159322C/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| FR2547063A1 (en) | 1984-12-07 |
| GB8413679D0 (en) | 1984-07-04 |
| NO159322B (en) | 1988-09-05 |
| NO159322C (en) | 1988-12-14 |
| FR2547063B1 (en) | 1993-04-30 |
| NO842169L (en) | 1984-12-03 |
| GB2141237A (en) | 1984-12-12 |
| GB2141237B (en) | 1986-10-22 |
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