RU2205271C2 - Method of borehole electrical research - Google Patents

Abstract

FIELD: geophysical research of boreholes, particularly, determination of rock electrical resistance in boreholes. SUBSTANCE: method includes obtaining of symmetrically detailed electric resistance curve close to actual values of resistance of rocks enclosing borehole by readings of lateral sonde. For this purpose, determined on the basis of lateral sonde reading by relationship of longitudinal resistance of medium overlying the point of measurement to transverse resistance of medium underlying the said point is coefficient of current transmission from one medium to the other. In this case, actual value of rock resistance is obtained by dividing the value of apparent resistance (by measurement with lateral sonde) by current transmission coefficient. EFFECT: higher efficiency of method. 2 dwg

Description

 The present invention relates to the field of geophysical research of wells and can be used to determine the electrical resistance of rocks in wells.

 A known method of measuring electrical resistance with a gradient probe (Dakhnov V. N. Electrical and magnetic methods for researching wells. - M .: Nedra, 1981, p. 44-56), which consists of three electrodes: direct current is passed through one of them, the two other electrodes measure the potential difference between them, the value of which depends on the resistance of the rocks.

 However, only an apparent resistance curve is obtained from the gradient probe, which does not accurately reflect the nature of the change in rock resistance. The curve has an asymmetric smoothed shape, false understatement and overestimation of readings regarding rock resistance due to violation of the uniform distribution of current due to reflection from high-resistance strata or retraction by low-resistance layers.

 The present invention is based on the task of developing a method for electric well research, which provides, according to the gradient probe, a symmetric-detailed curve of electrical resistance close to the actual resistance values of the rocks surrounding the well.

 The problem is solved in that in the method of electrical research of the well, which consists in the fact that the resistance of the rocks in the well is determined according to the readings of the gradient probe, according to the invention based on the readings of the gradient probe in relation to the longitudinal resistance of the medium lying from the measuring point to the transverse resistance of the underlying of this point of the medium, the coefficient of the passage of current from one medium to another is determined, while the actual value of the rock resistance is obtained by dividing the value of the apparent rotation obtained by measuring the gradient probe by the current transmission coefficient.

 In the future, the invention is illustrated by a specific example of its implementation and the accompanying drawings, where figure 1 shows the resistance curves obtained by the proposed method and methods of induction and side logging; figure 2 is a curve corrected by the proposed method based on previously obtained measurements.

 The proposed method of electrical research of the well is as follows.

При установлении коэффициента К_пр прохождения тока учитывают, что проходящий ток будет пересекать породы по вертикали преимущественно поперек напластованию пород, а отраженный ток будет направляться из скважины в породы горизонтально вдоль напластования пород. По этой причине при слоисто-неоднородных средах, какими являются исследуемые отложения горных пород, проходящий и отраженный токи будут встречать разное сопротивление, так как поперечное сопротивление будет выше, чем продольное, если даже вышележащая и нижележащая относительно точки замера среды будут иметь одинаковое строение.By the nature of the change in the readings of the gradient probe, which are scaled in units of resistance - Ohm • m, the ratio of the longitudinal and transverse resistances R _in and R _n , provided to the current, respectively, by the media lying above and below the measurement point, is determined and the current transmission coefficient from one well environment to another:
K _ol = R _in / R _n (1)
In the obtained values K _pr divide values of apparent resistivity derived from the measurements of the gradient probe, thereby correcting possible over- or overestimation of indications associated with the measurement current change curve gradient probe and obtained a symmetric curve resistance from values close to the resistance of rocks surrounding a borehole :

When establishing the coefficient K _{pr of the} passage of current, it is taken into account that the transmitted current will cross the rocks vertically mainly across the bedding, and the reflected current will be directed horizontally from the well into the rocks along the bedding. For this reason, with layered inhomogeneous media, such as the studied rock deposits, the transmitted and reflected currents will meet different resistance, since the transverse resistance will be higher than the longitudinal resistance, even if the overlying and underlying relative to the measuring point of the medium will have the same structure.

где КС₁, КС₂, ......,КС_n - значения кажущегося сопротивления, взятые с кривой градиент-зонда через 0,1 м, в интервале (+L), равном длине зонда ниже точки исследования.Given this, when determining the coefficient K _{pr of the} passage of current for the resistance R _{in the} overlying stratum, the total longitudinal resistance of the rocks is taken, and the underlying thickness - R _n is the total transverse resistance of the series of rocks, enclosed within the intervals equal to the length of the probe, up and down from the measurement point :

where KS ₁ , KS ₂ , ......, KS _n are the apparent resistance values taken from the gradient probe curve through 0.1 m in the interval (+ L) equal to the probe length below the point of study.

With a long gradient probe, the apparent resistance curve can be asymmetrically smoothed due to the significant distance MN between its measuring electrodes (MN = 0.5 ÷ 1 m). To ensure the required detail, the calculated resistance curve R _sim in such cases, the long-length gradient probe curve is pre-processed in the manner described above, but with the calculation of R _in and R _n for the intervals limited by the distance ± MN.

 Using the curves recorded by gradient probes of different lengths, symmetric resistance curves corresponding to different depths of the study are obtained.

 According to the proposed method, from the distorted curve A (Fig. 1) of the resistance obtained by measuring with a simple gradient probe, a symmetric resistance curve B is obtained, in vertical detail comparable to the effective resistance lateral or induction logging curves C, D using focusing loggers. Moreover, the proposed method has several advantages: it is easier to obtain multi-depth measurements than for lateral and induction logging; you can get the corrected curves E (figure 2), corresponding to the current level, based on previously obtained measurements of F gradient probe, which are often the main and only. In addition, screen effects that distort lateral logging curves are eliminated on the calculated curves.

Claims (1)

 The method of electric research of the well, which consists in the fact that the resistance of the rocks in the well is determined by the readings of the gradient probe, characterized in that, based on the readings of the gradient probe, the coefficient of the longitudinal resistance of the medium lying above the measuring point and the transverse resistance of the medium lying below this point is determined the passage of current from one medium to another, while the actual value of the resistance of the rocks is obtained by dividing the value of the apparent resistance obtained by measuring the grad nt probe, for current transmission coefficient.

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