WO2011071412A2 - Electrical cased-hole logging method - Google Patents

Abstract

The invention relates to the field of geophysical well surveying and is intended for determining the electrical resistivity of rocks in cased holes. The essence of the invention is the use of a multi-electrode probe in the form of two or more measuring units arranged consecutively and at equal distances along the axis of the borehole and each consisting of three measuring electrodes. Beyond the measuring electrodes, two current electrodes are mounted symmetrically relative to the middle of the probe. Bipolar square d.c. pulses are supplied alternately to the current electrodes of the probe. The electric potential and its first-order differences are measured and digitized. All of the digitized signal quanta are processed and filtered appropriately and on the basis thereof the electrical resistivity of the rock formations surrounding the casing string is determined in each measuring unit at several points along the borehole axis. The technical result is an increase in logging speed, a reduction in interference, and an increase in the dynamic range.

Description

 Method for cased hole electric logging

The invention relates to the field of geophysical studies of wells and is intended to determine simultaneously at several discrete points of electrical resistivity of rocks located equidistant along the axis of the borehole surrounding a cased metal column well.

 A known method of electric logging of cased wells based on a bipolar symmetric five-electrode probe, where the potential of the electric field, its singing and second difference (Kashik A.S., Rykhlinsky N.I., etc., is measured wells. Patent JYQ 2176802 from 02.20.2001. Bull. 34. 2001.) [1]. In the study by this method, the distortions associated with the instability of the linear electrical resistance of the casing string due to maintaining the current in the probe current electrodes of such a magnitude that causes the appearance of an extremum of the electric potential at its measurement point are eliminated. The disadvantage of this method is that when a current is supplied to the current electrodes of the probe, the power cable passes by the electric lines of the receiving electrodes. Because of this, induction induction on the receiving circuits arises, which significantly reduces the dynamic range of measuring the specific electrical resistances of rock formations surrounding the column to 25 Ohm.m with an error of more than 10%. In addition, it has a low write speed.

There is a known method for electric cased hole logging based on a bipolar symmetric five-electrode probe (Kashik AS, Rykhlinsky NI, etc. A method for electric cased hole logging. Patent No. 2229735 of 04/22/2003. Bull. JSTs 15. 2004.) [2], where induction interference are eliminated. But in this way, the extremum of the potential is maintained using the automatic analog auto-compensator located in the downhole tool, which is controlled in the downhole tool with useful signals in the nanovolt range, which are many times lower than the interference signals associated with thermal noise, induction noise, telluric currents, contact electrode potentials, etc., which leads to unstable operation of this auto-compensator and makes it impossible to control its operation. Therefore, this method has not found application in cased hole logging practice. It also has a low write speed.

 Note that any known method of electric logging of wells cased by a solid metal casing is set in conditions of working with useful signals in the nanovolt range, which are many times lower than interference signals if they are not suppressed.

We also note that the above methods, like other known methods, are based on differential measurement between two pairs of measuring electrodes of a probe of the second electric potential difference or using a bridge consisting of two identical electrical resistances (the first option), or by separately measuring both differences of electric potentials with two separate meters, followed by subtracting their readings at the output (second option). Moreover, the second option also has the disadvantage that it is technically difficult to create two amplifiers with the same and stable gain, suitable for differential measurement of the second potential difference. Such methods of measuring the second difference in electric potentials are in principle permissible in cased hole logging, provided that the linear base between the two pairs of measuring electrodes is constant. But since the measuring electrodes of cased hole electric logging devices are constructed in the form of clamping structures, and the diameter of the casing string is unstable, for example, due to its swelling after perforation or due to the peculiarities of its rolling technology, the distance changes when changing the electrode clamp diameter between their pairs can vary by up to one centimeter. Since the resistivity of the column and the resistivity of the rocks surrounding it differ by 10 ⁷ times or more, such a change in the distance between the measuring electrodes in the differential measurement of the second difference in electrical potentials can lead to an error that is many times higher than the permissible one.

 The proposed method solves the problem of increasing the recording speed and eliminating the above disturbances against the background of useful measured signals and, as a result of this, solves the problem of increasing the dynamic range of determining the true specific electrical resistance of rock formations surrounding the well over 100 Ohm.m with a measurement error of up to 5%.

This technical result is achieved by the fact that in the method of electric logging of cased wells with a multi-electrode probe, made in the form of measuring electrodes sequentially and equally spaced along the axis of the well and installed outside them symmetrically with respect to the middle of the probe of two current electrodes into which bipolar rectangular pulses of constant electric current are alternately supplied, and at each of the current supplies after a specified time after the attenuation of the induction noise associated with the reverse polarity of the current, measure the electric field potential of one of the measuring electrodes and the first potential difference between the extreme measuring electrodes; on the basis of the indicated measured electrical signals, when the resultant difference in the electric field potentials between the extreme measuring electrodes resulting from the total action of the currents of both current electrodes is determined, the electrical resistance of the rock formations surrounding the well is determined;

according to the invention, the electrodes of a multi-electrode probe are distributed into groups of measuring nodes, each of which consists of three nearby measuring electrodes;

the number of measuring nodes is two or more;

measuring the first potential difference is carried out between the extreme measuring electrodes of each measuring node and additionally measuring the first electric potential difference between one of the extreme measuring electrodes and the central of each measuring node at each of the electric current supplies to both current electrodes;

digitize the measured potential signals, the first potential differences and currents supplied to the current electrodes, in which each group of quantization potential quanta and its first differences and groups of current quanta are summed, averaged and, using the obtained values, determined simultaneously at several measuring points nodes along the axis of the borehole specific electrical resistance Stress i of rock formations surrounding the well according to the formula

where zi is the electrical resistance of the well section, determined for each i-th measuring unit according to the formula

K ί is the focusing coefficient determined from the condition that the resulting normalized potential difference between the extreme measuring electrodes of each measuring node is equal to zero from the equation

where i varies from 1 to n, are the electric field potentials of one of the

measuring electrodes of the probe when applying current, respectively, to the first A \ and second A2 current electrodes of the probe; - measurable by one - measurable

by the body, the first potential differences of the electric field between the extreme measuring electrode Mi and the central Mi + i of each measuring unit when applying currents, respectively, to the first A \ and second A current probe electrodes; - measured by one / - meter

the first potential differences of the electric field between the extreme measuring electrodes Mi and Mi + 2 of each measuring unit when applying currents, respectively, to the first A \ and second Ar current probe electrodes;

I A \ _> ^ A2 - currents of the current electrodes of the probe A 1 and Ar; - dimensionless current modules I _A , I _A2 ;

 B is the geometric coefficient of the probe for each measuring unit.

токовые электроды зонда, расположенные за пределами измерительных электродов симмет- рично относительно центра зонда. The invention is illustrated by drawings, where in FIG. 1 shows a block diagram of a downhole tool that implements the proposed method. Here 1 is the body of the downhole tool; 2- first measuring electrode Mi; 3- second measuring electrode Mr, 4- third measuring electrode Ms; 5th - measuring electrode Mi; 6- i + 1- measuring electrode Mi +1; 7- i + 2- measuring electrode Mi +2; 8-p - measuring electrode Mn; 9- p + \ - measuring electrode Mn + \; 10 - p + 2 - measuring electrode

probe current electrodes located outside the measuring electrodes symmetrically with respect to the probe center.

 Mi, Mi, Mi - electrodes making up the first measuring unit; Mi, Ms, M4 - electrodes making up the second measuring unit; Mi, Mi + l, Mi + 2- electrodes making up the i-th measuring unit; Mn, Mn + 1, Mn + 2 - electrodes making up the fifth measuring unit;

меж- ду крайними измерительными электродами Mi и Mi +2 каждого измери- тельного узла. Figure 2-a is a graph of the distribution of electric potential along the casing from the action of the current I _A from the current electrode A \. Figure 2-b shows a graph of the distribution of electric potential along the casing from the current I _I FROM the current electrode Ar, taking into account the scaling focusing coefficient K коэффициента obtained from the condition that the potential difference is equal to zero

between the extreme measuring electrodes Mi and Mi +2 of each measuring node.

 As noted above, any version of the method of electric logging cased by a solid metal casing wells is set in the conditions for working with useful signals in the nanovoltaic range, which are many times lower than interference signals, among which are: interference associated with a change the distance between the measuring electrodes of the probe due to a change in the inner diameter of the casing string and, as a consequence of this, a change in the angle of inclination of the levers of the pressure contacts of these electrodes; interference due to inconstancy of the running electrical resistance of the casing; interference caused by the inconstancy of the supply current of the probe current electrodes caused by both the insufficient stability of the power source for operation in the nanovolt range and the inconstancy of the electrical resistance of the current circuit; interference due to induction interference of the line probe current electrodes on the line of the probe measuring electrodes; interference due to contact electrode potentials; thermal noise; telluric interference; random impulse noise.

To combat the above-mentioned interference, the measured electric potential, its first differences and the currents of both current electrodes are digitized. Also, for successful filtering of useful signals from thermal, telluric, and other random interference, a high frequency of supply of the probe current electrodes is required, but it cannot be higher than 0.25 Hertz due to the influence of induction pickups. To eliminate the distorting effect of induction pickups that occur when combining current and measuring lines, it is preferable to supply the current electrode of the probe with alternating rectangular pulses of direct current, where the interference associated with induction pickups, some time after the polarity reversal of rectangular - DC pulses disappear (see [2]). In this case, the measurement and digitization of signals in the measuring circuits must be started after the attenuation of the emissions associated with the process of switching current in the probe current electrodes.

 The time interval between switching the current and the beginning of the measurement of the signals of the first potential differences, as shown by experimental studies, depends on the length of the current and measuring lines located together with one another. When measuring the first potential differences, this interval is not less than 0.4 seconds, since the current and measuring lines of the first potential differences are combined only in the interval of several meters, equal to the length of the measuring lines of the first differences. In order to avoid the distorting effect of induction interference on the results of measuring the first potential differences, information is in the time interval from the beginning of current switching to 0.4 sec. do not use.

When measuring potential, this interval is already at least one second, since the current and measuring lines in this case are combined in the interval of several thousand meters, that is, over the entire length of kA a mouth cable connecting the downhole tool to surface equipment. Based on the foregoing, the most optimal is the frequency of bipolar rectangular current pulses of 0.25 Hertz or less.

 Consider the principle of eliminating the distorting effect of the inconstancy of the electrical resistance of the casing string.

In logging, when solving gas and oil exploration problems, the medium under study is approximated as two-dimensionally inhomogeneous in the coordinates Z and V. At the same time, the well is not an ideal linear electrode, i.e., its linear electric resistance Ω along the coordinate Ζ between the extreme measuring electrodes is unstable and can vary from one section to another in several

time.

We place in the well, at point A, the source from which electric current I is supplied to the medium under study, and determine the distribution of the electric potential along its axis. It is known [1] that

and only provided that Ω _Γ / Ω _ζ »1,

Where

U ( ^ζ ) is the electric potential in the well at the observation point with coordinate Ζ;

Ι _ζ ( ^ζ ) -electric current through the cross section of the well with the same coordinate; J _r (z) is the current flowing from the borehole wall into the surrounding rock per unit of the depth interval (linear current density with dimension [A / m]);

 Ωγ is the electrical resistance exerted by the medium to the current

Ω _ζ - (as already noted above) the electrical resistance of the well cut between the extreme measuring electrodes to the axial direction current, functionally dependent on the coordinate Ζ due to the inconstancy of the geometric and other parameters of the well.

и с центром в точке наблюдения (средний измерительный электрод Mi+ l ). К замкнутой поверхности этого цилиндрического отрезка применим уравнение непрерывности вектора плотности тока

, взятое в интегральной форме, т. е.

Select a segment of the well column at point Ζ with height

and centered on the observation point (average measuring electrode Mi + l). To the closed surface of this cylindrical segment we apply the equation of continuity of the current density vector

taken in integral form, i.e.

The surface S consists of the bases of the cylinder $ _p and _q and its side surface S _b . Therefore, the left side of equation (3) represents the sum of three flows

таким образом, согласно (3), имеем

thus, according to (3), we have

_и в пределе при

_:

where from

_and in the limit at

_:

We differentiate expression (1) with respect to Ζ, taking into account that Ω _ζ is a function of the electric resistance of the wellbore, which changes in a real well with a change in the coordinate Ζ, i.e.

 :

Substituting equations (2) and (5) into equation (6), we obtain the equation of the distribution of the source potential along the axis of the well with the electrical resistance Ω _ζ that is not constant along its axis [1]

 An analysis of equation (7) shows that measuring the electric potential and its second derivative does not determine the desired ratio

₅ сильно за- висящего от изменчивости электрического сопротивления ствола скважи- ны. Ω _Ζ / Ω _Γ due to the presence in this equation of the term

_{5 is} highly dependent on the variability of the electrical resistance of the wellbore.

т. е.

. Следовательно, из уравнения (7) исключается член, содержащий неопределенную величину

₅ и это уравнение в точке z ~ ^zMi+\ принимает следующий вид:

The method of electric logging [1], the measurement results of which are practically unaffected by the inconstancy of the linear electrical resistance of the column, differs in that, thanks to the use of appropriate technical methods and means, the distribution curve for potential along the axis of the well becomes extremum in the field of measuring electrodes (in the coordinate

i.e.

. Therefore, a term containing an indefinite quantity is excluded from equation (7)

₅ and this equation at the point z ~ ^z Mi + \ takes the following form:

where from

Based on equation (9), by measuring the electrical resistance of the column segment between the extreme measuring electrodes of the measuring unit, the potential and its second derivative at a point with coordinate ^z Mi + \ in the presence of an extremum, it is possible to determine the desired electrical resistance of the formations surrounding the well rocks

Q _r at this point.

 The extremum of the potential at the location of the measuring electrodes is achieved by selection in the sources

Достижение экстремума в точке измерения

означает исклю- чение осевой составляющей тока

) , которая в скважине, при воз- буждении исследуемой среды однополюсным источником, многократно больше радиальной составляющей

. На практике для измерения сопротивления Ω_Γ вместо второй производной потенциала из (9) исполь- зуют пропорциональную ей вторую конечную разность потенциалов

Α _λ and Л ₂ , located on both sides at the same distance from the middle electrode Mi +1 (measuring point), of currents of values such that the potential difference between the two electrodes Mi and Mi +2 symmetric with respect to Mi +1 is equal to zero, i.e. .

Reaching an extremum at a measurement point

means the exclusion of the axial component of the current

), which in the well, upon excitation of the medium under study by a single-pole source, is much larger than the radial component

. In practice, to measure the resistance Ω _Γ, instead of the second derivative of the potential from (9), a second finite potential difference proportional to it is used

. Согласно закону Ома в этой точке осевая составляющая плотности тока вдоль оси скважины равна нулю

Thus, by the method of electric logging of cased wells, it is possible to determine the resistance Ω _Γ provided that the current is focused at the signal receiving site, i.e. if in the center of each measuring unit Mi + l the extremum of the electric field potential is maintained

. According to Ohm's law, at this point, the axial component of the current density along the axis of the well is zero

 Implementation of the proposed electric logging method is carried out on the basis of determining the electrical resistivity

R _p the rock formations surrounding the cased hole according to formulas (9) and (10), i.e.

между крайними измерительными электродами Mi и Mi+2 каждого измерительного узла, где

- соответственно, электрический по- тенциал поля одного из измерительных электродов и вторая разность элек- трических потенциалов на участке электропроводящего цилиндра между внешними измерительными электродами Mi и Mi+2 каждого измеритель- ного узла при равенстве нулю первой результирующей разности потен- циалов между этими электродами, вольты; when the condition of equality to zero of the first resultant from the action of both current electrodes of the difference of electric potentials

between the extreme measuring electrodes Mi and Mi + 2 of each measuring node, where

- accordingly, the electric potential of the field of one of the measuring electrodes and the second difference of electric potentials on the portion of the electrically conductive cylinder between the external measuring electrodes Mi and Mi + 2 of each measuring node, when the first resulting potential difference between these electrodes is equal to zero, volts;

 h is the geometric coefficient of each measuring node of the probe, meters.

Note that due to the fact that the electrical resistance of the casing string is more than 10 ⁷ times lower than the resistance of the rocks surrounding it, the main share of the sources A \ and Ar flows through the casing and, as a result, the potential of the casing in the section A \ Ar is almost - nak, i.e. it is much more orders of magnitude than the second difference of electric potentials. As a result, the electric potential of the field can be measured with respect to any measuring electrode. Then the electrical resistivity of the layers in each measuring node is determined by the formula:

Where K i is the focusing coefficient, determined from the condition that the resulting normalized potential difference between the extreme measuring electrodes of the probe from the equation be equal to zero

- соответственно, зависящие от токов первого А\ и второго Аг токовых электродов зонда потенциалы электрического поля любого из измерительных электродов зонда;

- accordingly, the potentials of the electric field of any of the measuring electrodes of the probe, depending on the currents of the first A \ and second Ar of the probe current electrodes;

- respectively

the first and second potential differences of the electric field, depending on the currents of the first A \ and second Ar of the probe current electrodes;

I A _{\ 5} I A2 - currents of the current electrodes of the probe A 1 and Ar; - dimensionless modules of currents I _{A \} , 1 _A2 , obtained after

summation and averaging of their quanta filtered;

 ki is the geometric coefficient of the probe for each measuring unit.

 Ω ζ ί is the electrical resistance of the well section, measured between the extreme measuring electrodes of each measuring node of the probe.

Удельное электрическое сопротивление Р„ в данном примере кон- кретного выполнения получено из формулы (12). Как уже отмечалось вы- ше, эта формула выведена из предпосылки, что результирующая осевая составляющая тока, текущего вдоль высокопроводящей металлической колонны между измерительными электродами Mi и Mi+2 каждого изме- рительного узла, равна нулю. Благодаря этому, в частности, искажающее влияние непостоянства электрического сопротивления колонны на резуль- таты измерения отсутствует, и процессор после обработки сигналов опре- деляет по формуле (12) истинное сопротивление пластов в каждом изме- рительном узле, что подтверждено моделированием на математических моделях. The electrical resistance zi of the column section between the extreme measuring electrodes of the probe is usually determined by the formula

The electrical resistivity P „in this particular example is obtained from formula (12). As noted above, this formula is deduced from the assumption that the resulting axial component of the current flowing along the highly conductive metal column between the measuring electrodes Mi and Mi + 2 of each measuring unit is zero. Owing to this, in particular, there is no distorting effect of the inconstancy of the electrical resistance of the column on the measurement results, and the processor after processing the signals determines the true formation resistance in each measuring unit using formula (12), which is confirmed by modeling on mathematical models.

But, as noted above, methods based on the differential measurement between two pairs of measuring electrodes of the probe of the second electric potential difference, both by using a bridge consisting of two identical electrical resistances, and by separately measuring both electric potential differences by two separate meters, followed by subtraction at the output of their readings, they do not have the necessary accuracy of measuring this difference under conditions when the ratio of electrical resistivity is rock formations surrounding the column to its resistance is 10 ⁷ times or more (in practice, this ratio always exists). Therefore, in the proposed method, from the formula (12), it is necessary to exclude terms containing the second potential difference. For

this, we use Fig. 2 and formula (13), whence it follows that when using the coefficient κΐ

From the formula (12), we single out the denominator containing the differentially measured second potential differences ( _D

simplification of the analysis, we assume that the currents are equal to unity).

Then, taking into account formulas (10) and (15-17):

Now, taking into account (18), formula (12) for determining the electrical resistivity _r takes the form:

Formula (19) quantitatively for determining the electrical resistivity D does not differ from formula (12), but qualitatively it differs in that it replaces the differentially measured second potential differences by integrally measured

между одним из внешних измерительных электродов Mi и центральным Mi +1. Благодаря этому значительно повы - шается точность определения истинного удельного электрического сопро- тивления Pi . the first potential differences measured by the same meter

between one of the external measuring electrodes Mi and the central Mi +1. Due to this, significantly increased - The accuracy of determining the true electrical resistivity Pi is improved.

In the proposed method, the geometric coefficient of the probe h and the linearity range between the true electrical resistivity P _t and the readings of the device created by this method are determined using a grid mathematical model (V. Druskin, L. Knizhnerman. Method for solving direct problems of electric logging and DC electrical intelligence. Izv. AN SSSR, ser. "Physics of the Earth", 1987,

N2 4, p.63-71) [4], finding for given ₂ values of elec-

ternary potentials and their differences

R which

substitute in the formula (19). The device based on the proposed method has been tested in wells. The error in determining the electrical resistivity pi for cased hole logging is not more than 5%.

Claims

Claim  A method for electric logging of cased wells with a multielectrode probe made in the form of measuring electrodes sequentially and equally spaced along the axis of the well and installed outside them symmetrically with respect to the middle of the probe of two current electrodes, into which bipolar rectangular pulses of direct electric current are alternately supplied, and at each of the current supplies, after a specified time after the attenuation of the inductive noise associated with current reversal, the electric potential is measured o fields of one of the measuring electrodes and the first potential difference between the extreme measuring electrodes; on the basis of the indicated measured electrical signals, when the resultant from the total action of the currents of both current electrodes is equal to the potential difference of the electric field between the extreme measuring electrodes, the electrical resistance of the rock formations surrounding the well is determined; characterized in that electrodes of a multielectrode probe are distributed into groups of measuring nodes, each of which consists of three nearby measuring electrodes; the number of measuring nodes is two or more; measuring the first potential difference is carried out between the extreme measuring electrodes of each measuring node and additionally measuring the first electric potential difference between one of the extreme measuring electrodes and the central of each measuring node at each of the electric current supplies to both current electrodes; digitize the measured potential signals, the first potential differences and currents supplied to the current electrodes, in which each group of quantization potential quanta and its first differences and groups of current quanta are summed, averaged and, using the obtained values, determined simultaneously at several measuring points nodes along the axis of the well, electrical resistivity pi of rock formations surrounding the well according to the formula

Where Ω, ζ ί - electric resistance of the well section, determined for each of that measuring unit according to the formula

K i is the focusing coefficient, determined from the condition that the resulting normalized potential difference between the extreme measuring electrodes of each measuring node is equal to zero from the equation

where i varies from 1 to n, are the electric field potentials of one of the

probe measuring electrodes when applying current, respectively, to the first A i and second Ai current probe electrodes;

- the first potential differences of the electric field measured by one measurer between the extreme measuring electrode Mi and the central Mi + i of each measuring unit when currents are applied, respectively, to the first

and second Ar current probe electrodes; - measurable by one - measurable

by the body, the first potential differences of the electric field between the extreme measuring electrodes Mi and Mi + 2 of each measuring node when applying currents, respectively, to the first A \ and second Ar current electrodes of the probe; 1 A \, 1 Ar - currents of the current electrodes of the probe A \ and Ar, - dimensionless current modules 1 _A \, I _A2 ;  ki- is the geometric coefficient of the probe for each measuring unit.