Density Log Curve Correction Method for Tight Sandstone Based on the Combined Calibration of LLD and GR

Abstract

The present invention belongs to the technical field of oil and gas exploration and development, and particularly relates to a method for correcting density logging curves of tight sandstones based on joint calibration of LLD and GR. The method first determines the depth of the sandstone section; then determines the non-diameter-expanded section and the diameter-expanded section of the sandstone section; then performs regression analysis on the measured data of the non-diameter-expanded section, where the measured data includes measured density data, measured deep lateral resistivity, and measured natural gamma value at the corresponding depth, to obtain a relationship model for the non-diameter-expanded section, and uses the relationship model for the non-diameter-expanded section as the relationship model for the diameter-expanded section; finally, based on the deep lateral resistivity logging curve and the natural gamma logging curve of the diameter-expanded section, and the relationship model for the diameter-expanded section, a corrected density logging curve for the diameter-expanded section is obtained. The formation density determined by the present invention is more accurate and closer to the true formation density logging value. The method has strong operability, small influencing factors, and high calibration accuracy, and can improve the success rate of identifying tight sandstone gas layers in oil and gas field exploration and development.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas exploration and development, and particularly relates to a method for correcting density logging curves of tight sandstone based on combined calibration of LLD and GR. Background Technique

[0002] During the drilling process of oil and gas field exploration and development, the phenomenon of wellbore enlargement (hole enlargement) often occurs. Wellbore enlargement is mainly caused by the softening and collapse of the formation after being soaked by mud in the shale section, or may also be caused by the caving of the wellbore wall during the friction between the brittle formation and the drill bit. Since the density of the mud is much smaller than that of the rock, and the detection range of density logging is very small, which is measured close to the wellbore wall. When the hole enlargement is serious or the wellbore wall is very irregular, the density value of the formation measured by the density (DEN) logging, which characterizes the formation porosity, is often significantly lower than the actual formation density. Therefore, after the wellbore is enlarged, the curve data of curves with very shallow detection depths such as density curves cannot be directly used and need to be corrected.

[0003] For the correction of density (DEN) logging curves in the hole enlargement section, there are three traditional methods. One is to correct using caliper logging respectively according to the working principles of acoustic time difference, compensated density and compensated neutron logging instruments and the hole diameter of the open hole well with initial calibration. Each logging instrument company has given corresponding calibration charts. However, this method is not applicable to the correction of density (DEN) logging curves of tight quartz sandstone, mainly because the calibration charts have high requirements for the accuracy of caliper logging, and are developed for the characteristics of logging instruments of each company and to simulate different environmental conditions, with a certain scope of application. And the hole diameter in the enlarged section is often irregular, making it difficult to meet the accuracy required by the calibration charts, so the error of the correction result is large and the process is relatively cumbersome; the second is to correct using the values of acoustic (AC) and neutron (CNL) curves that have a high correlation with the density (DEN) curve. However, since the detection depths of these two logging curves are also very small, they are also distorted in the enlarged section, and the corrected density value is still inaccurate; the third is to correct using the deep lateral resistivity curve LLD curve with a large detection distance and not easily affected by hole enlargement. However, since the correlation between LLD and density (DEN) curves is relatively low, the general correction accuracy is low.

[0004] The Chinese invention patent with the authorization announcement number CN100510779C discloses a method for determining the formation density of an enlarged diameter interval by using acoustic curve calibration. This method first determines the enlarged diameter interval and the non-enlarged diameter interval, then establishes the regression relationship between the density logging value and the acoustic logging value in the non-enlarged diameter interval, and then extends the established regression relationship to the enlarged diameter interval to determine the formation density of the enlarged diameter interval according to the acoustic logging value of the enlarged diameter interval. This method has relatively high requirements for the quality of the acoustic logging curve. When the correlation coefficient between the acoustic wave and the density curve is high and there is no measurement depth error between the acoustic wave and the density result curves obtained by mutual conversion, the result can meet the working requirements. However, for the acoustic curve with poor quality, the error transmission of the curve calculated from the acoustic wave will be further enlarged, and the density value cannot be accurately corrected. Summary of the Invention

[0005] The present invention provides a method for calibrating the density logging curve of tight sandstone based on the combined calibration of LLD and GR to solve the problem that the existing technology cannot accurately correct the density value.

[0006] To solve the above technical problems, the technical solution of the present invention includes:

[0007] The present invention provides a method for calibrating the density logging curve of tight sandstone based on the combined calibration of LLD and GR, including the following steps:

[0008] 1) Determine the depth of the sandstone interval according to the logging curves in the target area.

[0009] 2) Determine the non-enlarged diameter interval and the enlarged diameter interval of the sandstone interval according to the hole diameter curve and the bit diameter.

[0010] 3) Conduct regression analysis on the measured data of the non-enlarged diameter interval. The measured data includes measured density data, measured deep lateral resistivity, and measured natural gamma value at the corresponding depth, to obtain the non-enlarged diameter interval relationship model, and use the non-enlarged diameter interval relationship model as the enlarged diameter interval relationship model; the non-enlarged diameter interval relationship model / enlarged diameter interval relationship model is:

[0011] DEN = a × lg(LLD) + b × GR + c

[0012] In the formula, DEN is the density of the non-enlarged diameter interval / enlarged diameter interval, LLD is the deep lateral resistivity of the non-enlarged diameter interval / enlarged diameter interval, and GR is the natural gamma value of the non-enlarged diameter interval / enlarged diameter interval;

[0013] 4) Obtain the calibrated density logging curve of the enlarged diameter interval according to the deep lateral resistivity logging curve and the natural gamma logging curve of the enlarged diameter interval, and the enlarged diameter interval relationship model.

[0014] The beneficial effects of the above technical solution are as follows: The present invention starts from the commonly used LLD logging curve and GR logging curve, uses the measured data of the non-diameter-expanded section (including density data, deep lateral resistivity, and natural gamma value corresponding to the depth) for regression analysis to obtain the relationship model of the non-diameter-expanded section, and extends this relationship model to the diameter-expanded section, taking the relationship model of the non-diameter-expanded section as the relationship model of the diameter-expanded section. Then, by using the LLD logging curve and GR logging curve of the diameter-expanded section, the density logging curve corrected for the diameter-expanded section can be generated, and the curve to be corrected is inversely deduced from the stable curve to achieve the purpose of curve correction. This method calibrates the density curve by combining the LLD logging curve and GR logging curve, effectively removing the density measurement error caused by the irregular change of the well diameter, and solving the problem that the density logging tool cannot accurately measure the formation density. Moreover, both the LLD logging curve and GR logging curve are less affected by the well diameter and have a certain correlation with density. In particular, the GR logging curve can effectively characterize the shale content of tight sandstone, and there is a good correlation between the shale content and the sandstone density, making the formation density determined by the present invention more accurate and closer to the true formation density logging value. This method has strong operability, small influencing factors, and high calibration accuracy, and can improve the success rate of identifying tight sandstone gas layers in oil and gas field exploration and development. And the results prove that there is a good corresponding relationship between the corrected density curve and the measured core porosity.

[0015] Further, in step 1), according to the natural gamma logging curve of the target area, the depth of the sandstone section is determined.

[0016] Further, in step 3), if there are at least two groups of measured data, select the group of measured data with the smallest tooth amplitude for regression analysis; the tooth amplitude is:

[0017] E tooth =(GR tooth -GR min ) / GR tooth

[0018] In the formula, Etooth is the tooth amplitude, GRtooth is the gamma value of the tooth, and GRmin is the minimum gamma value of this section of the logging curve.

[0019] Further, in step 2), the means of determining the non-diameter-expanded section and diameter-expanded section of the sandstone section according to the well diameter curve and bit diameter are as follows: According to the well diameter curve and bit diameter, determine the non-diameter-expanded point and diameter-expanded point: If a certain depth point satisfies ∣Dr - Db∣≤K, then this depth point is a non-diameter-expanded point; otherwise, this depth point is a diameter-expanded point; where Dr is the wellbore diameter, Db is the bit diameter, and K is the well diameter expansion rate; according to the diameter-expanded point and non-diameter-expanded point, if the thickness of the continuously diameter-expanded formation is greater than the given cut-off value, then this section of the formation is the diameter-expanded section; if the thickness of the continuously non-diameter-expanded formation is greater than the given cut-off value, then this section of the formation is the non-diameter-expanded section. Description of the Drawings

[0020] Figure 1 is a flowchart of an embodiment of the method for correcting density logging curves of tight sandstones based on the combined calibration of LLD and GR according to the present invention;

[0021] Figure 2 is a single-well diagram of logging curves in a non-borehole-enlarged section;

[0022] Figure 3 is the effect diagram of correcting the borehole diameter of the compensated density curve by LLD and GR in the borehole-enlarged section of the logging curve. Detailed implementation manner

[0023] Considering that both the LLD logging curve and the GR logging curve are less affected by the borehole diameter and have a certain correlation with density. In particular, the GR curve can effectively characterize the shale content of tight sandstones, and there is a good correlation between the shale content and the density of sandstones, the present invention proposes a method for determining the density in the borehole-enlarged section by using the combined calibration method of the LLD logging curve and the GR logging curve. This method can effectively remove the density measurement error caused by the irregular change of the borehole diameter. The following will combine the accompanying drawings to detail a method for correcting density logging curves of tight sandstones based on the combined calibration of LLD and GR according to the present invention.

[0024] An embodiment of the method for correcting density logging curves of tight sandstones based on the combined calibration of LLD and GR according to the present invention is applied to the correction of density logging curves of tight quartz sandstones, and its process is as Figure 1 shown, and the specific steps are as follows:

[0025] Step 1: Use the natural gamma logging curve (GR) in the target area to determine the depth of the tight quartz sandstone section.

[0026] Establish the corresponding relationship between the core, cuttings and GR obtained from the drilling in the target area, and give the limited value range, that is, the GR value is less than 60 API, and the thickness of the interlayer section with a GR value greater than 60 API does not exceed 3 m, and divide the top and bottom boundaries of the sandstone in the same stratigraphic unit.

[0027] For example, as in the Figure 2 shown well, the quartz sandstone section of this well is from 3896.260 m to 3901.860 m and from 3903.680 m to 3906.920 m.

[0028] Step 2: In the same sand body, use the borehole diameter curve to judge the depth of the borehole-enlarged section and determine the borehole-enlarged section and the non-borehole-enlarged section.

[0029] The formula for the borehole diameter expansion rate is:

[0030] K = (Dr - Db) / Db × 100% (1)

[0031] Wherein, K is the borehole diameter enlargement rate; Dr is the borehole diameter, in cm; Db is the bit diameter, in cm.

[0032] Denote the borehole diameter as Dr and the bit diameter as Db. Given an error limit K (K>0), if at a certain depth point |Dr - Db| ≤ K, it is a non-enlarged diameter point; otherwise, if |Dr - Db| > K, it is an enlarged diameter point. If the thickness of the formation with continuous diameter enlargement is greater than the given cut-off value ζ (ζ≥0), then this section of the formation is considered an enlarged diameter section; if the thickness of the formation with continuous non-enlarged diameter is greater than the given cut-off value ζ, then this section of the formation is considered a non-enlarged diameter section.

[0033] For example, as Figure 2 shown in the well, the selected bit diameter is Db = 21.59 cm, K < 1, ζ = 3.0 m. Then, according to the principle that the thickness of the formation with continuous non-enlarged diameter is greater than ζ, the non-enlarged diameter sections are from 3896.260 m to 3901.860 m and from 3903.680 m to 3906.920 m.

[0034] Another example, as Figure 3 shown in the well, the selected bit diameter is Db = 24.13 cm, K > 1, ζ = 3.0 m. Then, according to the principle that the thickness of the formation with continuous non-enlarged diameter is greater than ζ, the non-enlarged diameter section is from 3792.208 m to 3802.860 m.

[0035] Step 3: In the sandstone section of the non-enlarged diameter section, construct the relationship between DEN and lg(LLD), GR as shown in Equation (2). Using the measured density data (DEN) of the non-enlarged diameter section and the measured deep lateral resistivity (LLD) and measured natural gamma value (GR) at the corresponding depth, obtain the model parameters a, b, c in the relationship to get the relationship model of the non-enlarged diameter section. This relationship is:

[0036] DEN = a × lg(LLD) + b × GR + c (2)

[0037] Wherein, DEN is the measured compensated density, in g / cm 3 ; LLD is the measured deep lateral resistivity, in Ω·m; GR is the measured natural gamma value, in API; a, b, c are model parameters, which are obtained by substituting the formation density data of the non-enlarged diameter section and the logging data at the corresponding depth into the regression equation of Equation (2). Specifically:

[0038] Obtain the sample values of lg(LLD), GR, and DEN at intervals of 0.125 m thickness in the same set of sandstone above the top depth and below the bottom depth of the enlarged diameter section respectively. Twenty-five consecutive samples (3 m) are taken as a group, and the E tooth value is calculated for each group. Select the group with the smallest Etooth value (Etooth = 0.25) for multiple regression analysis to obtain the model parameters in Equation (2). Among them, Etooth The calculation formula for the value is as follows:

[0039] E tooth =(GR tooth -GR min ) / GR tooth (3)

[0040] In the formula, E tooth is the amplitude of the tooth, dimensionless; GR tooth is the gamma value of the tooth, API; GR min is the minimum gamma value of the logging curve of this section, API.

[0041] For example, as Figure 2 shown in the well, a data table of DEN, lg(LLD), and GR for the sandstone section with 25 data points of this well is established as shown in Table 1. The regression relationship for the non-caliper section obtained through regression analysis is as follows:

[0042] DEN = 0.0622 * lg(LLD) + 0.0009 * GR + 2.4048 (4)

[0043] Table 1 Logging data table for the sandstone section of the non-caliper well

[0044]

[0045] Step 4: Extend the established regression relationship to the caliper section, and use the relationship model of the non-caliper section as the relationship model of the caliper section. Then, based on the LLD logging curve and GR logging curve of the caliper section, as well as the relationship model of the caliper section, the corrected density logging curve of the caliper section can be obtained, and the purpose of curve correction can be achieved by inversely deriving the curve to be corrected from the stable curve.

[0046] For example, as Figure 3 shown in the well, for a well with a logging curve of a certain caliper section in this gas field, as long as the logging curves LLD and GR of this well are substituted into the density logging curve calculation formula of the present invention, the density logging curve of this well can be obtained. The corresponding relationship between the corrected compensated density curve and the measured core porosity is relatively good.

[0047] The present invention has relatively low requirements for the required data. The corrected density value is close to the true formation density logging value. Combining the corrected density with other logging data can be better used for reservoir evaluation. Conducting reservoir research on the corrected logging curve can significantly improve the reliability, effectiveness, and rationality of logging lithology identification, reservoir division, and interpretation.

Claims

1. A method for correcting density logging curves of tight sandstone based on the combined calibration of LLD and GR, characterized in that, It includes the following steps: 1) Determine the depth of the sandstone section according to the natural gamma log curve of the target area; 2) Determine the non-expanded section and the expanded section of the sandstone section according to the well diameter curve and the bit diameter. The means for determining the non-expanded section and the expanded section of the sandstone section are as follows: Determine the non-expansion point and the expansion point according to the well diameter curve and the bit diameter: If ∣Dr - Db∣≤K at a certain depth point, it is a non-expansion point; otherwise, it is an expansion point. Where Dr is the wellbore diameter, Db is the bit diameter, and K is the well diameter expansion rate; According to the expansion points and non-expansion points, if the thickness of the continuously expanded formation is greater than the given cut-off value, then this section of the formation is the expanded section; if the thickness of the continuously non-expanded formation is greater than the given cut-off value, then this section of the formation is the non-expanded section; 3) Conduct a regression analysis on the measured data of the non-expanded section. The measured data includes the measured density data, the measured deep lateral resistivity, and the measured natural gamma value at the corresponding depth, to obtain the non-expanded section relationship model, and use the non-expanded section relationship model as the expanded section relationship model. The non-expanded section relationship model / expanded section relationship model is: DEN = a×lg(LLD)+b×GR + c (2) In the formula, DEN is the density of the non-expanded section / expanded section, LLD is the deep lateral resistivity of the non-expanded section / expanded section, GR is the natural gamma value of the non-expanded section / expanded section; a, b, c are model parameters, which are obtained by substituting the formation density data of the non-expanded section and the logging data at the corresponding depth into the regression equation of formula (2); If there are at least two groups of measured data, select the group of measured data with the smallest tooth amplitude for regression analysis. The tooth amplitude is: E tooth = (GR tooth - GR min ) / GR tooth Where, E tooth is the amplitude of the tooth, GR tooth is the gamma value of the tooth, GR min is the minimum gamma value of this section of the logging curve; 4) Obtain the corrected density log curve of the expanded section according to the deep lateral resistivity log curve and the natural gamma log curve of the expanded section, and the expanded section relationship model.

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