MXPA01010725A - A method and apparatus for determining the resistivity of a formation through which a cased well passes - Google Patents

Abstract

The invention relates to a method of surveying the resistivity of a geological formation through which a borehole provided with metal casing passes, in which method a leakage current is caused to leak into said formation outside the casing, and said leakage current is determined on a casing section at a certain level, the leakage current being indicative of the resistivity of the formation. According to a characteristic of the invention, the resistivity is determined on the basis of the leakage current by applying a factor that depends on the distance z between said level and the surface.

Description

METHOD AND APPARATUS TO DETERMINE THE RESISTIVITY OF ON DEPOSIT THROUGH WHICH A COATED WELL PASSES The invention relates to determining the resistivity of geological deposits through which a well provided with metallic coating passes. The importance of the resistivity graph for oil drilling is well known. It is known that the resistivity of a reservoir depends essentially on the fluid it contains. A reservoir containing salt water, which is conductive, has resistivity that is much greater than a reservoir filled with hydrocarbons, and therefore resistivity measurements are of irreplaceable value for locating hydrocarbon deposits. Resistivity logging has been performed widely and for many years, in particular by means of apparatuses having electrodes, but existing techniques have a field of application that is limited to wells that are not coated, or "uncoated wells" as they are referred to. in the terminology of the oil industry. The presence in the metallic coating well, which has resistivity that is the same compared to the typical values for the geological deposits (approximately 2 x 10 ~ 7 ohm.m for the steel coating compared to the values in the range of 1 ohm .ma 1000 ohm.m for a reservoir), represents a considerable barrier to sending electric currents in the formations surrounding the coating. As a result, in particular, it is not possible to obtain well resistivity measurements that are in production, since the wells are provided with coating. Therefore, it can be very advantageous to make it possible to measure the resistivity in sections of coated wells. These measurements, taken in a production well at the deposit level, can make it possible to locate the hydrocarbon-water interfaces and in this way how to monitor the positions of these interfaces vary with time, as to monitor the behavior of the hydrocarbon deposit and how to optimize the extraction of it. It may also be possible to obtain a resistivity measurement in a well (or in a well section) for which no measurement was taken before the coating was placed in place, in particular to supplement the knowledge of the deposit and optionally detect the productive layers that were not initially located. The proposals have been made in this object in the literature. The basic measurement principle presented in US Pat. No. 2 459 196 is to cause a current to flow through the coating under conditions such that the current is filtered or lost in the reservoir. This loss is a function of reservoir resistivity: the more the reservoir is conductive, the greater the current loss. By measuring the current loss, it is possible to determine reservoir resistivity. The current loss can be evaluated by measuring the voltage drop between the electrodes placed at different depths in the well. Patent Document US 2 729 784 describes a measurement method using two pairs of measuring electrodes a, b and b, c spaced apart along the coating, electrodes a and c are in principle equidistant from electrode b. The current electrodes are placed on either side of the measuring electrodes to inject the currents in the coating in opposite directions. A servo feedback circuit controls the injected current to place the external measurement electrodes at the same potential, so as to cancel out the effect of the coating resistance that varies in sections (a, b) and (b, c) as defined by the measuring electrodes. A value for the filtering current at the level of the central electrode b is obtained by measuring the voltage drop across each of the electrode pairs a, b and b, c, and by taking the difference between the voltage drops, whose difference is established to be proportional to the filtering current. French Patent Document 2 207 278 provides for the use of three uniformly spaced measurement electrodes as in US Patent Document 2 729 784 for measuring current filtration, and describes a two-stage method: a first step for measuring resistance of the cladding section defined by the external measuring electrodes, during which the current stage is caused to flow along the cladding so that there is no filtration in the reservoir; and a second stage during which the current can seep into the reservoir. For that purpose, a current injection system is provided which comprises an emission electrode and two return electrodes, one near the measurement electrodes which is effective during the first stage, and the other measurement electrode is located at the surface and is active during the second stage. US Pat. No. 4,796,186 describes a two-step method of the same type as the method described in the aforementioned French Patent Document 2 207 278, and using the same electrode configuration. This provides a circuit to cancel the resistance effect that varies between the two sections of the coating. That circuit comprises amplifiers connected in each pair of measuring electrodes to supply respective voltage drops in their output powers. One of the amplifiers is a variable gain amplifier, its gain is adjusted during the first stage to cancel the difference between the outputs of the amplifiers. US Pat. No. 4,820,989 describes an identical compensation technique. Using Ohm's law, in order to determine reservoir resistivity, in addition to knowing the filtering current when it is measured using one of the indicated methods, it is also necessary to know the potential difference with reference to the infinity of the coating at the measurement level . In the aforementioned documents, that potential difference is measured by means of a reference electrode located on the surface, and at a sufficient distance from the aforementioned surface return electrode. The use of a reference electrode suffers from operational disadvantages. The corresponding measurement must be taken separately from the aforementioned measurements, and in this way represents an additional step that increases the total duration of the operations. It also represents a source of error, which is possible for the potential of the reference electrode that is affected by various phenomena. Proposals have been made to omit said reference electrode. Patent Document US 5 510 712 proposes to apply currents to the coating in two places that separate in the longitudinal direction. Similarly, US Pat. No. 5,543,715 proposes an additional current electrode. These proposals suffer from the disadvantage of complicating the measuring apparatus and in particular increasing its length. In one aspect, the invention provides a method for studying the resistivity of a geological reservoir through which passes a probing well provided with metallic coating, in which method a filtration stream is caused to filter into the reservoir out of the coating, and the filtration current is determined in a coating section at a certain level, the filtration current is indicative of reservoir resistivity, the method is characterized by the fact that the resistivity is determined at the base of the filtration stream by applying a factor that depends on the distance z between the level and the surface. In a preferred implementation, the factor takes into account the length of the coating. The invention will be understood upon reading the following description given with reference to the accompanying drawings, in which: Figure 1 summarizes the principle of measurement resistivity in a coated well; Figure 2 shows diagrammatically the apparatus located at the bottom of the perforation designed to complement the principle; Figures 3A, 3B and 3C show different operating states of the apparatus shown in Figure 2; and Figure 4 shows, by way of example, a result obtained by the resistivity of a reservoir by applying the method described below to determine the potential of the coating. The principle of measuring resistivity in a coated well is to cause the current to flow along the coating with a return that is remote, to allow the current to seep into the geological deposits through which the well passes, and to evaluate the filtration current: at any given level under the well, the higher the reservoir conductivity that surrounds the well at the level, the higher the filtration current. This can be expressed in mathematical terms by an exponentially decreasing relationship for the current flowing through the coating, with a rate of decrease, at any given level that is a function of the reservoir resistivity ratio Rt to the resistivity Rc of the coating . The diagram in Figure 1 shows a section of a well 10 having an axis X-X 'and is provided with metallic coating 11. • The desired level (or depth) at which the measurement will be taken is referred to as b. Consideration is given to a section of the liner (a, c) that extends on either side of level b. If the current flows through the coating with a return that is remote (ie at the surface level) the current loss can be expressed in terms of electrical circuit diagram by a shunt that is placed between the coating level b and the infinity . The resistance of the shunt is representative of the resistivity Rt of the reservoir at the level of the electrode b. Using Ohm's law in this way it is possible to write: Rt = k (Vb, oo / Ifor) [1] where k is a geometric constant that can be determined by the calibration measurements, Vb, w is the coating potential in the level b with a reference in infinity, and Ifor is the filtering current in level b. When approaching a discrete variation, it is possible to describe a current loss at level b as a difference between the input current at level b and the output current. The filtering current Ifor is thus expressed as the difference between the lab and Ibc currents (which are increased by being constant) that flow respectively in the coating sections (A, b) and (b, c): Ifor = Iab -Ibc [2] or Ifor = Vab / Rab - Vbc / Rbc [2 '] where Vab and Rbc are the potential drops respectively along the section (a, b) and along the section (b, c) of the coating, and Rab and Rbc are the values of the resistance respectively of the section ab and of the section be of the coating. It is assumed initially that the current applied to the coating is CD. In view of the relationship between the resistivity of the coating and the usual resistivity values of the deposits, whose ratio is in the range of 107 to 1010, the loss of current over a length corresponding to a resolution that is acceptable for a reservoir resistance measurement, for example in the range of 30 cm to 1 m, is very small. The difference between the potential Vab and Vbc drops that can be attributed to the current loss is therefore usually a very small amount. As a result, doubts, even small uncertainties, have to do with the terms of the difference that has the most influence. For various reasons (localized corrosion, non-uniformity of the coating material or variation in thickness), the resistance values per unit length of sections (a, b) and (b, c) of coating - may differ from the value corresponding to the nominal characteristics of the coating, and above all may be different from each other.

An uncertainty also affects the lengths of the sections (a, b) and (b, c) of the coating since the lengths depend on the positions of the contact points in which the electrodes are in contact with the coating, whose positions are known only with relatively poor accuracy. Figure 2 shows diagrammatically the apparatus for implementing the principle described above. The apparatus comprises a probe 12 suitable for moving in a petroleum borehole 10 provided with the liner 11, and which is suspended from the end of an electrical cable 13 connecting the surface equipment 14 comprising the data acquisition and the medium of processing and a supply 16 of electrical energy. The probe 12 is provided with three measuring electrodes a, b and c that can be placed in contact with the coating, thereby defining the sections (a, b) and (b, c) of length coating that are appropriately in the range from 40 cm to 80 cm. In the embodiment shown, the electrodes a, b and c are mounted on arms 17 articulated to the probe 12. By means of mechanisms of known type that are necessary to describe in detail herein, these arms can be thrown out of the probe to place the electrodes in contact with the coating, and then return them in the retracted position once the measurements have been completed. The electrodes are designed so that, once they are in contact with the coating, their positions remain as stationary as possible, and so that the electrical contact with the coating is optimal. A probe of this type can be made on the basis of the instrument commercially used by Schlu berger for the service "CPET", as indicated in US Pat. No. 5,563,514. That instrument, which is designed to evaluate the cathodic protection of the coating and the state of corrosion thereof, is provided with twelve measuring electrodes distributed on four levels separated in the longitudinal direction, the distance between the levels is approximately 60 cm, and the three electrodes in each level are arranged symmetrically around the axis of the instrument , that is, with an angular space of 120 ° between the adjacent electrodes.To measure reservoir resistivity, three electrodes a, b, c are sufficient, but it is possible to use a large number of levels, for example, as in the instrument mentioned above, four levels that can form two groups of three consecutive levels, to acquire more information and to take measurements correspond teeth at two different depths simultaneously. In such cases, each set of three consecutive electrodes is associated with the processing circuits described below. With respect to the number of electrodes per level, a single electrode is sufficient. The probe is further provided with current electrodes arranged on each side of the ao electrodes, mainly a higher Inl electrode and a lower In2 electrode, remote from the electrodes a and c which may be in the same order or a little larger than the distance between the electrodes. the electrodes of an oro, for example several meters. The isolation accessories 18, such as accessories of the type AH169 commonly used by Schlumberger, are placed on either side of the central portion of the probe, whose central portion carries the measuring electrodes a, b and c to isolate the central portion of the electrodes Inl and In2 current. The current Inl and In2 electrodes can be made in the form of conventional centralizers for coated wells. The wells normally provided in such centralisers as elements that come into contact with the coating are then replaced by elements that serve as current electrodes, and electrical conductors are provided to connect to the electrode forming elements. The apparatus is also provided with a remote return In3 electrode preferably placed at surface level, in the wellhead (if the well is deep enough) or at some distance from the well head, and with means for feeding the current electrodes to establish the various circuits described below with reference to Figures 3A to 3C . The medium comprises the aforementioned surface current source 16, and depending on the case, an additional source located on the probe, as well as suitable switching circuits. The diagrams given in Figures 3A to 3C show measurement steps corresponding to the various current pass circuits that can be established by means of the apparatus described above. As explained below, two (or three) of such steps are sufficient to obtain the desired results. These diagrams show a processing circuit that includes amplifiers Dab and Dbc whose inputs are respectively connected to the electrodes a and b and the electrodes b and c, and which supply at their outputs the voltage drops Vab and Vbc in the cladding sections defined by the electrodes, and a Dabc amplifier connected to the Dab and Dbc amplifiers and which supplies the Vabc difference between the voltage Vab and Vcb in its output power. This circuit is preferably located in the probe 12 located at the bottom of the borehole. It is supplemented by the calculation means preferably belonging to the data acquisition and to the processing means 15 of the surface equipment, whose calculation means receives the voltages of the processing circuit and the other pertinent data and the supply of resistivity values Rt. . The data is conveyed conventionally by the cable 13 in digital form, an analog-to-digital converter (not shown) that is provided in the probe 12 and connected to the processing circuit. The stage shown in Figure 3A calibrates the measurement system formed by the measuring electrodes a, b and c and the coating sections 11 they define. In this stage, a current is applied to the coating by means of a circuit formed by Inl as an injection electrode and by In2 as a near-return electrode, by placing the switching circuits in the appropriate position. In this way, the stream substantially does not penetrate the reservoir surrounding the well. The current preferably is low frequency CA, for example, having a frequency in the range of 1 Hz to 5 Hz, but the following reasoning assumes that the current is CD. With the applied current referred to as It, the output voltages of the amplifiers are as follows: VbC (7 = Rbc.It [3 '] Vabcc = (Rab-Rbc) It [3"] The stage shown in Figure 3B It uses a current application circuit made of a superior Inl electrode and the remote In3 electrode, the applied current is of the same type as in the first stage, mainly AC of the same frequency. Under these conditions, current leakage occurs as it is described in the foregoing with reference to Figure 1, whose filtering is a function of reservoir resistivity at the electrode level B. With the current flowing downwards towards the sections (a, b) and (b, c) of coating referred to as Id and the filtering current referred to as Ifor as in the above, the output voltages of the amplifiers are as follows: Vaby = Rab.dt [4] Vbcr = Rbc (Id-Ifor) [ 4 '] Vabcy = (Rab-Rbc). Id + Rbc.Ifor [4"] By combining these expressions, it is possible to deduce cir stream Ifor filtration: Ifor = It. [Vabcr- (Vabcc.Vab | j '/ Vabc)] / (Vabc-Vabcc [5] The stage shown in Figure 3C differs from the stage shown in Figure 3B only in that the lower In3 electrode is used in place of the Upper Inl electrode for applying current, the return is provided by the surface electrode In3 As in the stage in Figure 3B, the current therefore leaks into the reservoir, but the current flows upwards through the sections ( to, b) and (b, c) of coating. This current is referred to as Ih and the voltages obtained are referred to as VabB, V cB / and VabcB. It should be noted that, by virtue of the principle of superposition, the current circuit shown in Figure 3A and made of the electrodes Inl and In2, is equivalent with respect to the electrical magnitudes (current and voltage) in the difference between the circuit shown in Figure 3B and the circuit shown in Figure 3C, if the current applied respectively by the electrodes Inl and In2 is the same. Therefore symbolically. CIRCUIT 3A = CIRCUIT 3B - CIRCUIT 3C The current and voltage values in the expression [5] above and corresponding to the stage shown in Figure 3A can thus be replaced, according to the invention with the differences between the corresponding values obtained respectively in the steps shown in Figures 3A and 3C: in this way, Vabc = Vacy-acß, etc. This makes it possible to replace the stage shown in Figure 3A with the stage shown in Figure 3C. The advantage of this solution is that the current application circuit is simplified. In this regard, it should be noted that the stage shown in Figure 3A requires either a current source in the probe located at the bottom of the borehole, or a current source on the surface and connected to the two additional lines in the borehole. cable 13. In order to determine the resistivity Rt, of the reservoir, once the filtering current Ifor has been calculated in this way, the task of determining the potential of the coating with respect to an infinity reference Vb, 8, remains explains in the above. This is done as described in the aforementioned literature, by means of a reference electrode that can be placed on the surface, remote from the In3 electrode, surface return, or preferably located in the well, for example in the cable portion. isolated or the "flange" that connects the device located at the bottom of the bore to the cable. In this way, it is possible to measure the potential difference Vbs between the coating at the level of the measuring electrode b and the reference electrode. Using the aforementioned equation [1], the ratio K.Vbs / Ifor is formed, where K is the aforementioned constant, so as to deduce reservoir resistivity Rt. This voltage measurement Vbs can not be performed simultaneously with the other measurements mentioned above due to the coupling phenomenon in the cable. A method of the invention offers the advantage of avoiding the use of a reference electrode and the additional operation represented by measuring the voltage Vbs, and consists of determining the coating potential by calculation.

Using Ohm's law, the potential can be obtained as the product of the total applied to the current multiplied by the resistance of the coating. In the invention, it is appropriate to apply the following relationship: Rt = A (z) .Gblty / Ifor [6] in which A (z) is a term that corrects the effect of the proximity of the lining pads, and depends on the depth z ^ of the measurement electrode b, Gb is a term that characterizes the geometric conformation and the properties of the coating at the level of the electrode b, and Ity is the total current applied by the upper electrode Inl during the step described in the Figure 3B. The measurements to be taken have to do with the hydrocarbon production zones, which are generally located at a limited distance above the lining shoe. The proximity of the lining shoe amplifies the filtration of current in the reservoir since it constitutes a considerable discontinuity for the current flowing below the Inl electrode. Compared to the length characteristic of the aforementioned exponential decay, only a short distance is available for this current in which it filters from the coating. The purpose of the term A (z) is to correct this effect.

Specifically, it has been found that a satisfactory correction is obtained by using the following expression for the term A (z): A (z) = arg sh. { 2z / (lc-z) > [6 '] where lc is the total length of the lining, the length of which is known from the borehole in question. The depth z is more accurate than the distance from the measurement level b to the surface. It is measured in a well-known way in the field of petroleum logging. Naturally, other mathematical expressions can be considered with the condition that they give a result comparable to that of the expression [6 '] - The term Gb is appropriately defined by the following relation: Gb = (pDc.h / e) .Rab [6] "] where Dc is the outer diameter of the coating at measurement level b, h is the length of the coating section defined by the electrodes a and b, Rab is as indicated above, the strength of the coating on the section ( a, b) and e is the coating thickness at level B. For the outer diameter Dc, a nominal value is known, and this value is satisfactory For the thickness e, it is possible, by approximation, to use a nominal value deduced from the available nominal values (outer diameter and weight per unit length) and from the density of the steel constituting the coating, however, it is observed that the thickness e may differ from this nominal value, and in addition ede vary with time, due to corrosion phenomena. Thus, it is preferable to determine its value at level b as a function of the measurements taken at the level in view of the fact that the resistance of the coating also depends on its sectional area s ^ (R = pl / s), and therefore in the thickness e. For this purpose, the measurement step described in the foregoing with reference to Figure 3A is used. As it appears from the relation [3], the resistance Rab of the coating on the section (a, b) can be determined on the basis of the voltage Vab ^ - and the applied current It. The resistivity of the steel at the depth z ^ is also determined, as a function of temperature at the depth, and is measured as is conventional in the field of petroleum logging. A value e (z) of specific thickness at level b is derived from it. As explained in the above, it is possible to advantageously replace the measurement of Figure 3A with the difference between the measurements of Figures 3B and 3C. This applies to determine Rab. The quantities Vab ^ and It are then obtained by taking the difference between the correspondence quantities in steps 3B and 3C, being remembered that the current applied to the coating is the same in both stages. The corresponding calculation is performed in the aforementioned calculation means located in the surface equipment. For this purpose, the calculation means receives all the relevant data from the probe 12 located at the bottom of the bore or from the surface device for the depth data z. In addition, the values of the parameters involved in the calculation are pre-stored in the calculation medium. Figure 4 provides an example of the results obtained by applying the method described above to calculate reservoir resistivity Rt to the data obtained in a test well. The studied section extended from a depth (represented along a horizontal axis) of less than 500 m to a depth of more than more than 1100 m. The thick line curve corresponds to previously known resistivity values, the shaded line curve corresponds to applying the above method to the data acquired by means of the apparatus as described in the present specification. It should be noted that the shaded line curve coincides almost exactly with the thick line curve which shows the effectiveness of the previous method. For reasons of simplification, in the above description it is assumed that the applied current is CD. In fact, CA is used, preferably at a lower frequency, which is appropriately in the range of 1 Hz to 5 Hz. In view of the reactive effects due to the coating metal (bark effect), the sections (a, b ) and (b, c) of the coating are characterized by their complex impedance Zab, Zbc values, in which the resistors Rab and Rbc are the real portions, and the currents flowing through the sections (a, b) and (b, c) coating and the voltages through its terminals are complex magnitudes, each of which includes a component in quadrature relative to the applied current. The filtering current is then determined by applying the relationship [5] above with the Vab ^, Vabco Vaby, and Vabcy complex voltages. A Jfor of complex filtering stream is obtained in this way so that the real Re (Jfor) portion must be determined in order to calculate the resistivity Rt of the reservoir. According to the invention, it is observed that the filter stream Jfor has a constant phase relationship with the current Id (or Ih) flowing through the coating at the level of the electrode b during step 3B (or, respectively, 3C) over the entire coating section, while the current Id (or Ih) is the phase changed relative to the current applied in steps 3B, 3C, by a value that varies over the section of the coating, which depends on numerous parameters , and that it is therefore difficult to predict, and therefore the Jfor component that is in phase with the current Id is used to determine the actual portion of Jfor. The complex current Id is obtained on the basis of the measurements of steps 3A and 3B (advantageously possible for the measurements of step 3A to be obtained by means of the difference between the measurements of step 3B and the measurements of the stage 3C, as described in the above). With the references used in the above, the relations [3] and [4] become: Vab = Zab.lt Vaby = Zab. id therefore: Id = (Vabr / Vab). It If fd is the phase of the current Id relative to the current Injected, and fJ is the phase of the stream Jfor of filtration relative to the current It, the actual portion of Jfor is determined, in view of the above, by the following relation: Re (Jfor) = | Jfor | eos (fJ -fd) The corresponding processing means are preferably distributed between the probe 12 located at the bottom of the borehole and the aforesaid calculation means located in the middle 15 of the surface equipment. Suitably, the probe located at the bottom of the bore is provided with circuits with which the real and imaginary portions of the measured magnitudes are administered, and the calculations described above are performed by the calculation means located in the surface equipment.

Claims (6)

1. CLAIMS 1. A method to study the resistivity of a geological deposit through which passes a well of sounding provided with metallic coating, in whose method the resistivity (Rt) is determined in the base of a current (Ifor) of filtration that is causes filtering in the reservoir outside the coating, and the filtration current is determined in a coating section at a certain level (b), the method is characterized by the fact that a factor takes into account the discontinuities in the filtration stream which is applied to the filtering stream, the factor depends on the distance z between the level (b) and the surface. The method according to claim 1, wherein the factor comprises a correction term A (z), depends on the distance z, to correct the discontinuities in the filtration stream due to the proximity of the lining shoe multiplied by a Gb term that characterizes the geometric conformation and the properties of the coating section. 3. The method according to claim 2, wherein the correction term A (z) is the form arg sh. { 2z / (lc-z) > , where lc is the length of the coating. The method according to claim 2 or 3, wherein the term Gb is the form Gb = (pDc.h / e). Rab, where Dc is the outer diameter of the coating, e is the coating thickness at this level, h is its length, and Rab is the strength of the coating section. The method according to any of claims 1 to 4, wherein the coating section is defined by three electrodes (a, b, c) applied against the coating at separate positions in the longitudinal direction of the coating, and the Filtering current (Ifor) is determined at the level of the electrode (a, b) medium on the basis of the difference Vabc between the voltages Vab and Vbc measured through the terminals respectively of the electrodes (a, b) and of the electrodes (b, c). The method according to any of claims 1 to 5, in which the current applied to the coating is CA, and the component of the filtering current (Ifor) which is in a constant phase relationship with the flowing current Through the coating on the level it is determined to be able to determine the resistivity (Rt).