A method for resistivity correction in highly deviated and horizontal wells in bedded formations

Abstract

The application discloses a bedding anisotropy stratum large-inclination well and horizontal well resistivity correction method, which comprises the following steps: (1) selecting several discrete well inclination data and corresponding resistivity logging response values in the same set of bedding anisotropy stratum according to stratum layering data, well inclination data and resistivity logging data; (2) fitting the selected data set according to a given correction model to determine r0, r 90 and the value of B; (3) combining the well inclination data to calculate the correction coefficient alpha (theta) of the target layer at different measuring depths; (4) according to the definition of the correction coefficient, the correction model of the resistivity curve is derived; (5) according to the correction model, the resistivity of the bedding anisotropy stratum large-inclination well and horizontal well is corrected. The method can correct the resistivity logging data, can get rid of the dependence on rock physical experiments, has low error rate, is simple and convenient to operate, and saves the cost of downhole sample coring and rock physical experiment.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of geophysical well logging interpretation, and in particular to a correction method for resistivity logging data of bedded anisotropic formations in horizontal wells and high-deviation wells. BACKGROUND

[0002] With shale and other unconventional oil and gas becoming the focus of oil and gas exploration and development in China, more and more high-deviation wells and horizontal wells are put into use. Geophysical well logging is an oil and gas exploration engineering technology that involves placing instruments in a well to continuously measure and record the rock physical response characteristics of formations at different depths. Accurate and reliable geophysical well logging data are an important basis for quantitative evaluation of reservoir parameters. Among various geophysical well logging data, resistivity logging data are almost indispensable for oil and gas saturation evaluation of various reservoirs and are a must-measured item for each oil and gas well. The development of bedded structure in shale and other unconventional oil and gas reservoirs results in differences in formation resistivity measured in different directions, i.e., anisotropy. The resistivity anisotropy of bedded rock makes the resistivity logging response values of the same formation in different-deviation wells different, especially the resistivity logging values in high-deviation wells and horizontal wells and the resistivity logging values in vertical wells differ greatly, which causes difficulties in accurate evaluation of reservoir saturation. Therefore, it is necessary to correct the resistivity logging values in high-deviation wells and horizontal well sections for anisotropy so that they have consistent value ranges with the same formation under the condition of a vertical well.

[0003] A Chinese invention patent with the application publication number CN112115592A discloses a high-deviation / horizontal well resistivity correction method based on rock physical experiments. The method constructs a resistivity anisotropy inversion model, calculates resistivity anisotropy coefficients in combination with rock physical experiment calibration, and finally determines a resistivity correction formula. An important step of the method is to calibrate the parameters in the correction model based on rock physical experiments on core samples taken downhole. The inventors found through numerical simulation that when the bedded angle is constant, the length of the rock sample will affect the measured resistivity value of the bedded rock sample in the laboratory. Therefore, the calibration of the parameters of the correction model using rock physical experiments has certain errors. SUMMARY

[0004] Based on the above technical problems, the present application proposes a bedded formation high-deviation well and horizontal well resistivity correction method, which can eliminate the dependence on rock physical experiments, reduce the error rate, and save the implementation cost of the scheme.

[0005] The technical solution adopted by the present application is as follows:

[0006] A bedded formation high-deviation well and horizontal well resistivity correction method, comprising the following steps:

[0007] Step 1: selecting several discrete inclination data θ in the same set of bedded formation according to the bedded formation data, inclination data and resistivity logging data i , i = 1, 2, 3… and corresponding resistivity logging response value r i , i = 1, 2, 3…;

[0008] Step 2: fitting the selected data set according to the given model to determine the values of r0, r 90 and B according to formula (1);

[0009] r θ = (r0-r 90 )cos B θ+r 90 (1)

[0010] In formula (1), r θ is the resistivity of the formation when the inclination angle is θ; r0 is the resistivity of the formation when the inclination angle is 0°, corresponding to the case of a vertical well; r 90 is the resistivity of the sample when the inclination angle is 90°, corresponding to the case of a horizontal well; and B is a constant coefficient of the model.

[0011] Then the value of A is calculated from A is a constant coefficient of the model.

[0012] For the same set of formation or adjacent bedded formation, the values of A and B are considered fixed.

[0013] Step 3: calculating the correction coefficient α(θ) of the target layer at different measurement depths according to formula (2) combined with the inclination data;

[0014]

[0015] Step 4: according to the definition of the correction coefficient, the correction model of the resistivity curve is derived as:

[0016]

[0017] In formula (3), r 校正后 is the corrected resistivity curve, and r 校正前 is the uncorrected resistivity curve.

[0018] Step 5: correcting the resistivity of the high-deviation well and horizontal well in the bedded anisotropic formation according to the correction model of the resistivity curve obtained in step 4.

[0019] Preferably, the method is applicable to ordinary resistivity logging and lateral resistivity logging.

[0020] For inductive resistivity logging, formula (1) needs to be converted to formula (4):

[0021] r θ = (r 90 -r0)sin B θ+r0 (4)

[0022] Formula (2) needs to be converted into formula (5):

[0023]

[0024] The beneficial technical effects of the present application are:

[0025] The present application is based on rock physical experiment and numerical simulation to obtain an empirical formula model suitable for resistivity correction of bedded formation, and further obtain a resistivity correction method for high-deviation well and horizontal well in bedded anisotropic formation based on the empirical formula model. The method for resistivity logging data correction can get rid of the dependence on rock physical experiment, has low error rate, is simple to operate, and saves the cost of downhole sample coring and rock physical experiment. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a schematic diagram for selecting basic data in step (1) of the method of the present application;

[0027] Figure 2 It is a well inclination angle and resistivity curve measured when a pilot well (vertical well) drills Longyi formation;

[0028] Figure 3 It is a well inclination angle and resistivity curve measured when a horizontal well drills Longyi formation;

[0029] Figure 4 It is a schematic diagram of the position relationship of selected data points on a logging profile;

[0030] Figure 5 It is a fitting result of the selected data by using formula (4);

[0031] Figure 6 It is a resistivity correction effect display based on the present application;

[0032] Figure 7 It is a relationship between the correction coefficient and the cosine value of bedding angle obtained by numerical simulation;

[0033] Figure 8 It is a circuit density distribution after applying voltage to samples of different lengths;

[0034] Figure 9 It is a graph of the relationship between the anisotropy coefficient and the cosine value of bedding angle under different rock sample lengths;

[0035] Figure 10The relationship between the collected 6 block bedded shale samples and the resistivity and the bedding angle. DETAILED DESCRIPTION

[0036] The application will be further described below in conjunction with the drawings and specific embodiments.

[0037] A bedded formation high angle deviated well and horizontal well resistivity correction method, comprising the following steps:

[0038] Step 1: According to the formation layering data, well deviation data and resistivity logging data, a plurality of discrete well deviation data θ i , i = 1, 2, 3... and corresponding resistivity logging response value r i , i = 1, 2, 3... in the same set of bedded formation are selected, as shown by the dots in Figure 1 .

[0039] Step 2: According to formula (1), the selected data set (well deviation data, resistivity logging response value) is fitted according to the given model, and the values of r0, r 90 and B are determined.

[0040] r θ = (r0-r 90 ) cos B θ + r 90 (1)

[0041] In formula (1), r θ is the resistivity of the formation when the well deviation angle is θ; r0 is the resistivity of the formation when the well deviation angle is 0°, corresponding to the vertical well case; r 90 is the resistivity of the formation when the well deviation angle is 90°, corresponding to the horizontal well case; and B is a model constant.

[0042] Then the value of A is calculated from ; A is a model constant.

[0043] For the same set of formations or adjacent bedded formations, the values of A and B are considered fixed.

[0044] Step 3: According to formula (2), the correction coefficient α(θ) of the target layer at different measurement depths is calculated in combination with the well deviation data.

[0045]

[0046] Step 4: According to the definition of the correction coefficient: the correction model of the resistivity curve is derived:

[0047]

[0048] In formula (3), r 校正后The corrected resistivity curve, r 校正前 The resistivity curve before correction.

[0049] Step 5: Correct the resistivity of wells with high deviation and horizontal wells in anisotropic formations based on the correction model of the resistivity curve obtained in Step 4.

[0050] The basic applicable condition for the above implementation scheme is that the resistivity obtained from logging is maximum when the well inclination angle is 0°, i.e., the current path generated by the logging instrument is perpendicular to the bedding plane. In other words, the above scheme can be directly applied to conventional resistivity logging and lateral resistivity logging. For inductive resistivity logging, when the well inclination angle is 0°, the eddies generated by the instrument in the formation flow parallel to the bedding plane, resulting in the minimum measured resistivity. However, when the well inclination angle is 90°, the eddies generated by the instrument in the formation flow perpendicular to the bedding plane, resulting in the maximum measured resistivity.

[0051] Therefore, for inductive resistivity logging, formula (1) needs to be converted to formula (4):

[0052] r θ =(r 90 -r0)sin B θ+r0 (4)

[0053] Formula (2) needs to be converted to Formula (5):

[0054]

[0055] The preferred conditions for implementing the above-mentioned resistivity correction methods for highly deviated wells and horizontal wells in stratigraphic formations are as follows:

[0056] (1) The target well and target layer that need to be corrected have continuous well deviation data, geological stratification data and resistivity logging curves of the same type.

[0057] (2) Assume that the bedding strata are horizontal in space, that is, the dip angle of the strata and the dip angle of the bedding plane are close to 0°. When the dip angle of the bedding plane is much greater than 0°, all the "well inclination angles" θ in the scheme need to be converted into the angle between the well axis and the normal to the bedding plane according to the well trajectory.

[0058] The following example, using a shale gas well in the Sichuan Basin, illustrates the implementation steps and effects of this invention.

[0059] Figure 2 The figure shows the well inclination angle and resistivity logging (deep lateral) data of the pilot well (vertical well) in the Longmaxi Formation. It can be seen that the pilot well vertically penetrates the Longmaxi Formation's first section shale, with the well inclination angle always remaining at around 0°. The measured resistivity of the Longmaxi Formation's first section shale is concentrated between 10 and 20 Ω·m (the resistivity channels in the figure are logarithmic scales).

[0060] Figure 3 The figure shows the partial inclination and resistivity logging (deep induction) data of the horizontal well drilled into the Longmaxi Formation Long 1 section stratum of the same well mouth. It can be seen that, in the part above the depth of 3770m, the inclination angle remains about 10°. When the depth reaches 3770m, the inclination angle gradually increases, and when the depth reaches 4015m into the Long 1 section stratum, the resistivity also gradually increases. When the depth reaches about 4360m, the inclination angle remains at about 90°, entering the horizontal well section. The resistivity measured in the Long 1 section stratum of the horizontal well section remains at about 30Ω·m, which is obviously higher than that of the horizontal well section.

[0061] The following begins to demonstrate the resistivity correction method based on the present application:

[0062] First step: read several discrete inclination data and resistivity data in the build-up section. Table 1 is several discrete inclination data and corresponding resistivity logging response values read in the horizontal well. Due to the polarization phenomenon of the layering near the high angle well, the resistivity curve appears to have a sharp spike, so the data points corresponding to the abnormal spikes should be avoided when reading. Figure 4 The middle black dot shows the relative position relationship of the read data points on the logging curve.

[0063] Second step: according to the selected data, use formula (4) to determine the model parameters. It should be noted that the resistivity logging data for this correction is induction logging data, so formula (4) is used to fit the data. The fitting result is shown in Figure 5 According to the fitting result, r 90 = 29.42Ω·m, r0= 15.47Ω·m, B= 50.03, the fitting correlation coefficient R 2 = 0.92.

[0064] Third step: calculate the correction coefficient corresponding to different inclination angles.

[0065]

[0066] Fourth step: correct the logging data according to formula (3), and the correction result is shown in Figure 6 From the figure, it can be seen that the corrected curve retains the fluctuation characteristics of the curve before correction. With the continuous increase of the inclination angle, the correction amount of the corrected curve also increases. In the horizontal well section, the corrected resistivity curve returns to the resistivity range interval (10-20Ω·m) of the pilot well (vertical well), and the correction effect is good.

[0067] Table 1

[0068] Depth (m) Deviation angle (°) Resistivity (Ω-m) 4129.375 56.072 14.401 4176.5 68.43 15.487 4188.75 71.548 17.795 4203.75 74.957 20.042 4213.625 77.2 18.058 4245.375 79.745 20.359 4267 81.591 23.746 4318.75 85.905 28.246

[0069] The principle involved in the bedding-resistivity correction method for highly deviated wells and horizontal wells of the present application is described below:

[0070] (1) Numerical simulation

[0071] The resistivity petrophysical experiment process of the bedding-resistivity core sample was simulated by using a commercial electromagnetic field simulation software. The simulation conditions were that a voltage difference of 2 V was applied to the top and bottom surfaces of the core sample to simulate the diode method resistivity measurement, and the measurement direction was the long axis direction of the sample. The resistances of different samples were obtained by software analysis and calculation, and then the resistivity of the sample was calculated:

[0072]

[0073] In formula (7), r is the resistivity of the sample, R is the resistance of the sample, S is the cross-sectional area of the sample, and L is the length of the sample.

[0074] Let r θ be the resistivity of the sample when the bedding angle is θ. In particular, r0 is the resistivity of the sample when the bedding angle is 0°, which corresponds to the case of a straight well. r 90 is the resistivity of the sample when the bedding angle is 90°, which corresponds to the case of a horizontal well. In order to correct the resistivity measured in a highly deviated well or a horizontal well to the resistivity in a straight well, the correction coefficient of the resistivity when the bedding angle (or well deviation angle) is θ is defined as

[0075] Figure 7 The relationship between the correction coefficient and cosθ obtained by numerical simulation is shown. It is found that they can be well fitted by formula (8):

[0076] α θ = (1-A)cos B θ+A (8)

[0077] In the formula, A and B are model constants.

[0078] According to formula (8), when the bedding angle is 0°, α θ = 1; when the bedding angle is 90°,

[0079] Using the same method, the relationship between the correction coefficient and the bedding angle of the core sample at different lengths was simulated. Figure 8 The current density distribution of three samples of different lengths after applying a voltage excitation signal is shown when the bedding angle is the same (50°). When the bedding angle is fixed, the smaller the length of the core sample, the more high-conductivity strips there are to connect the two end surfaces of the sample, and the higher the current density in these high-conductivity strips, resulting in a lower resistivity of the sample. For example, Figure 9As shown, when the bedding plane is oblique to the long axis of the core sample, the sample length affects the resistivity measurement results. The shorter the sample length, the lower the measured resistivity. When the bedding plane is horizontal or perpendicular to the long axis of the sample, the sample length does not affect the resistivity measurement results.

[0080] Although the sample length affects the resistivity measurement results, formula (8) still applies to samples of different lengths. Figure 9 The solid black line in the middle is the fitting curve of the simulation results of three sample lengths. Therefore, the sample length will affect the accuracy of resistivity measurement in rock physics experiments, and thus affect the accuracy of the existing correction scheme (patent publication number CN112115592A), but will not affect the applicability of formula (8), only that samples of different lengths have different coefficients.

[0081] (2) Rock Physics Experiment

[0082] Six bedding shale samples from the same region at similar depths were collected. Each sample had different bedding angles and the angle between its major axis and bedding plane. Figure 10 As shown in (a) in the figure. Figure 10 Figure (b) shows the relationship between the sample resistivity and the stratification angle obtained by the diode method. In formula (8), Since there were no samples in the experiment with stratification angles exactly 0° and 90°, in order to fit the experimental data, formula (8) was transformed into:

[0083] r θ =(r0-r 90 cos B θ+r 90 (1)

[0084] The measured resistivity of the sample is directly fitted according to formula (1), and the fitting result is as follows: Figure 10 As shown by the black solid line in (b), the fitted correlation coefficient R0 2 The value is as high as 0.95. It can be seen that formula (1) is also highly adaptable to the actual rock physics experimental results of core samples.

[0085] For any parts not mentioned above, existing technologies can be adopted or referenced.

[0086] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method of resistivity correction for high angle deviated and horizontal wells in bedded formations, characterized by The method comprises the following steps: Step 1: selecting several discrete inclination data θ i i = 1, 2, 3... and corresponding resistivity logging response value r i i = 1, 2, 3... in the same set of bedded formation according to stratigraphic layering data, inclination data and resistivity logging data Step 2: Fit the selected data set according to equation (1) to determine the values of r0, r 90 and B; r θ = (r0- r 90 )cos B θ + r 90 (1) In formula (1), r θ is the resistivity of the formation when the well deviation angle is θ; r0 is the resistivity of the formation when the well deviation angle is 0°, corresponding to the case of a vertical well; r 90 is the resistivity of the formation when the well deviation angle is 90°, corresponding to the case of a horizontal well; B is a model constant. followed by A is calculated; A is a model constant; For the same set of strata or adjacent bedded strata, the A and B values are fixed; Step 3: According to formula (2), the correction coefficient a(θ) of the target layer at different measuring depths is calculated in combination with the inclination data; Step 4: Define the correction factor according to the correction coefficient: The correction model for deriving the resistivity curve is obtained: In formula (3), r 校正后 is the corrected resistivity curve, r 校正前 is the uncorrected resistivity curve; Step 5: The resistivity of the high-deviation well and the horizontal well in the bedded anisotropic strata is corrected according to the correction model of the resistivity curve obtained in step 4.

2. The method for resistivity correction of highly deviated and horizontal wells in bedded formations of claim 1, wherein: The method is applicable to common resistivity logging and lateral resistivity logging; For the inductive resistivity logging, formula (1) needs to be converted into formula (4): r θ = (r 90 -r0)sin B θ+r0 (4) Formula (2) needs to be converted into formula (5):

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