Note: Descriptions are shown in the official language in which they were submitted.
~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~
-- 3
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
--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~
-- 7
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
I2
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~;~
--10-
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.
