A method for determining an effective stress coefficient of a formation rock

Abstract

This invention discloses a method for determining the effective stress coefficient of formation rocks, belonging to the field of oil drilling technology, including collecting data from H points at different depths of the target formation. i Measured formation pressure P Pi ; Obtain the corresponding depth point H i The pressure σ of the overlying strata at the location Vi and effective stress σ Vi Based on Biot's effective stress formula, calculate the sum at different formation depths and plot the X-ray. i -Y i In a cross-plot, the slope of the linear fitted line passing through the origin is the effective stress coefficient of the formation rock. This invention does not rely on well logging sonic velocity; it can conveniently and efficiently obtain the effective stress coefficient of the formation rock by calculating the overlying rock pressure using well logging volumetric density and combining it with measured formation pressure.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil drilling and completion, and particularly relates to a method for determining an effective stress coefficient of formation rock. BACKGROUND

[0002] Biot effective stress theory formula The contribution of formation pressure to effective stress is described, wherein the rock effective stress coefficient a determines the degree of contribution of formation pressure, and plays a key determining role in accurately calculating effective stress. At the same time, the effective stress coefficient is also an important parameter for drilling and completion wellbore stability analysis, sand production prediction, hydraulic fracturing design, reservoir permeability analysis, and stress sensitivity research, and has wide application and important role in the field of petroleum engineering. Therefore, it is of great significance and practical effect to obtain accurate and reliable effective stress coefficient of formation rock.

[0003] Generally, the rock effective stress coefficient is obtained through indoor variable confining pressure- variable internal pressure experiment, indirect calculation of acoustic wave velocity test, and rock physical model method. For the first method, although the formation effective stress coefficient can be directly obtained through downhole core, the downhole core is difficult to obtain, the rock sample is difficult to prepare and easy to break, the test period is long, the parameter acquisition efficiency is low, and the application is not convenient; the second method can indirectly calculate the effective stress coefficient by using acoustic logging longitudinal wave, transverse wave velocity or indoor rock sample acoustic wave velocity test, and the continuous effective stress coefficient profile of the formation can be obtained by using logging curve, but this method obtains dynamic effective stress coefficient, which needs to be corrected to static effective stress coefficient for effective stress calculation. If the static effective stress coefficient test and correction are lacking due to the lack of downhole core, it will cause error in effective stress calculation, for example, the existing patent CN115144904A discloses a kind of formation pressure prediction method, device, computer equipment and storage medium. The rock physical model method is to determine the rock effective stress coefficient by determining the rock bulk modulus and rock matrix particle modulus through a suitable rock solid phase component physical model, but some rock physical parameters of this method must be obtained through indoor test, many factors are considered, the model is complex, and the solution is difficult. Therefore, a method is needed which can quickly and accurately determine the formation effective stress coefficient, and can fully utilize the existing logging data and be convenient for popularization and application. SUMMARY

[0004] The present application aims to solve the above-mentioned problems in the prior art, and provides a method for determining the effective stress coefficient of formation rock, which can conveniently obtain the effective stress coefficient of formation rock by calculating the overburden pressure of overlying rock through logging bulk density combined with the measured formation pressure.

[0005] In order to achieve the above-mentioned application purposes, the technical scheme of the present application is as follows:

[0006] A method for determining the effective stress coefficient of formation rock, comprising the following steps:

[0007] collecting the measured formation pressure P i of different depth points H Pi of a target formation;

[0008] obtaining the overburden pressure and the effective stress at the corresponding depth points;

[0009] calculating and at different depth points H i of the formation according to the Biot effective stress formula, and drawing a crossplot of - The slope of the linear fitting line passing through the origin in the crossplot is the effective stress coefficient of the target formation rock.

[0010] Further, the overburden pressure at the corresponding depth points is calculated according to the following formula:

[0011] ; (1)

[0012] In the formula: is the overburden pressure at a certain depth point H i of the formation, MPa; is the logging rock volume density, kg / m 3 ; is the gravitational acceleration, generally taken as 9.801, m / s 2 ; H i is the depth of a certain point of the formation, m.

[0013] Further, the effective stress at the corresponding depth points H i is calculated according to the following formula:

[0014] ; (2)

[0015] ; (3)

[0016] In the formula: is the effective stress at a certain depth point H i of the formation, MPa; is the initial pore ratio of the formation rock, dimensionless; is the pore ratio at a certain depth point H i of the formation, dimensionless; is the compression index of the formation, dimensionless; For a point in the stratum at a depth H i Porosity at that location, %.

[0017] Furthermore, according to Biot's effective stress formula The effective stress coefficient of the formation rock is determined by the following formula. :

[0018] (4)

[0019] (5)

[0020] (6)

[0021] Furthermore, the measured formation pressure at different depths of the target formation can be obtained through drill pipe testing, bottom hole pressure measurement during drilling, drilling fluid density inversion, or oil and gas reservoir testing methods. The required measured formation pressure of the target formation should be at least five different depth points.

[0022] Furthermore, if the well logging volume density of a certain formation... Missing information; Gardner's formula is used to calculate based on logging P-wave travel time or P-wave velocity:

[0023] (7)

[0024] (8)

[0025] In the formula: P-wave transit time in formation rock logging, µs / ft; Let be the rock P-wave velocity, in m / s. The logging P-wave transit time is the reciprocal of the P-wave velocity.

[0026] Furthermore, the initial porosity of the strata rocks Compression index Determined by using standard plunger rock samples for uniaxial or triaxial compression tests.

[0027] Furthermore, at least three standard plunger rock samples should be used for the compression test; for shale formations, empirical values ​​can be used. and .

[0028] Furthermore, at a certain depth H in the strata i porosity at Determined by calculations based on well logging P-wave velocity, compensated neutrons, or resistivity.

[0029] In summary, the present invention has the following advantages:

[0030] The present application can conveniently obtain the effective stress coefficient of the stratum rock by calculating the overburden pressure of the stratum volume density combined with the measured stratum pressure, and does not depend on the logging acoustic wave velocity, has high efficiency, simple required steps, and can truly reflect the real effective stress coefficient of the stratum in situ state. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort.

[0032] Figure 1 The flow chart of the method for determining the effective stress coefficient of the stratum rock in the present application;

[0033] Figure 2 The crossplot in the present application - DETAILED DESCRIPTION

[0034] In order to more clearly illustrate the present application, the present application will be further described in combination with the preferred embodiments and the drawings. It should be understood by those skilled in the art that the following specific description is illustrative rather than limiting, and the protection scope of the present application should not be limited by this. The terms "first", "second" and the like in the specification and claims of the present application and the above drawings are used to distinguish different objects, and are not used to describe a specific order. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device including a series of steps or units is not limited to the listed steps or units, but can optionally include steps or units not listed, or can optionally include steps or units inherent to the process, method or device.

[0035] The present application provides a method for determining the effective stress coefficient of the stratum rock, as shown in the figure, comprising the following steps: Figure 1

[0036] Step a), collecting the measured stratum pressure P i of the target stratum at different depths H Pi , and arranging according to the following table:

[0037]

[0038] In the table, H i is the depth of a certain point i of the stratum, in meters; P P1 is the stratum pressure at the depth H1 of the stratum, and so on.​​

[0039] Step b) Calculate the corresponding depth point H in step a) based on the well logging formation bulk density. i Pressure of the overlying strata at the location:

[0040] (1)

[0041] In the formula: For a point at a depth H in the strata i Pressure of the overlying strata at the location, MPa; The volumetric density of the rock in the well logging test is expressed in kg / m³. 3 ; The acceleration due to gravity is typically taken as 9.801 m / s². 2 H i Let m be the depth of a certain point in the stratum.

[0042] For the depth point H corresponding to step a) i Calculated pressure of overlying strata Organize according to the following table:

[0043]

[0044] In the table: This represents the pressure of the overlying strata at a depth of H1, and so on.

[0045] c) Calculate the corresponding depth point H in step a). i Effective stress at:

[0046] (2)

[0047] (3)

[0048] In the formula: For a point at a depth H in the strata i Effective stress at the point, MPa; The initial void ratio of the strata rocks is dimensionless. For a point at a depth H in the strata i The porosity at that location is dimensionless. is the compressibility index of the formation, dimensionless; For a point at a depth H in the strata i Porosity at that location, %.

[0049] For the depth point H corresponding to step a) i Calculated effective stress of the formation Organize according to the following table:

[0050]

[0051] In the table: the formation effective stress at the formation depth H1, and so on.

[0052] d) the formation rock effective stress coefficient is determined according to the Biot effective stress formula

[0053] (4)

[0054] (5)

[0055] (6)

[0056] In the formula, H is the formation depth, P is the formation pressure, P0 is the overburden pressure, and σ is the effective stress. The formation rock effective stress coefficient is dimensionless.

[0057] The measured formation pressures at different depth points, the overburden pressures, and the effective stresses calculated in steps a), b), and c) are used to calculate and and draw a - crossplot as shown in FIG. 2, in which the slope of the linear fitting line passing through the origin is the formation rock effective stress coefficient. Figure 2 Example 2 This example provides a method for determining the formation rock effective stress coefficient. On the basis of Example 1, in step a), the measured formation pressures at different depths of the target formation can be obtained by drill pipe testing (midway testing), while-drilling bottom hole pressure measurement, drilling fluid density inversion, oil and gas layer testing, and the like. The measured formation pressures of the target formation required are not less than 5 different depth points.

[0058] In step b), if the bulk density of a certain formation is missing, the Gardner formula can be used to calculate

[0059] (7)

[0060]

[0061] (8)

[0062] In the formula, t is the formation rock logging compressional wave slowness, us / ft; and v is the rock compressional wave velocity, m / s. The logging compressional wave slowness is the reciprocal of the compressional wave velocity.

[0063]

[0064] ​​​​​​​In step c), the initial porosity ratio of the formation rock and the compression index Determined by using standard plunger rock samples (25mm x 50mm) in uniaxial compression or triaxial compression tests, the standard plunger rock samples used in the compression tests are not less than 3; for shale formations, empirical values can be used and .

[0065] In step c), the porosity of the formation at a certain point depth H i can be determined by calculating the logging longitudinal wave velocity, compensated neutron, resistivity, etc.

[0066] The beneficial effects of the present application are that the effective stress coefficient of the formation rock can be conveniently obtained by calculating the overburden pressure of the formation using the logging bulk density combined with the measured formation pressure, and does not depend on the logging acoustic wave velocity, has high efficiency, requires simple steps, and can truly reflect the real effective stress coefficient of the formation in situ.

[0067] Although the specific embodiments of the present application are described in detail with reference to the accompanying drawings, it should not be understood as limiting the scope of protection of the present patent. Various modifications and variations made by those skilled in the art within the scope described in the claims are still within the scope of protection of the present patent.

[0068] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Any simple modification or equivalent change made according to the technical essence of the present application to the above embodiment falls within the scope of protection of the present application.​

Claims

1. A method for determining the effective stress coefficient of formation rocks, characterized in that, Includes the following steps: Collect H points at different depths of the target strata i Measured formation pressure P Pi ; Obtain the pressure of the overlying strata at the corresponding depth point. and effective stress ; Based on Biot's effective stress formula, calculate H at different formation depths. i place and and draw - Intersection diagram; The slope of the linear fitting line passing through the origin in the intersection diagram is the effective stress coefficient of the target stratum rock.

2. The method for determining the effective stress coefficient of formation rocks according to claim 1, characterized in that, Pressure of the overlying strata at the corresponding depth point Calculate using the following formula: （1）； In the formula: For a certain depth point H in the strata i Pressure of the overlying strata at the location, MPa; The volumetric density of the rock in the well logging test is expressed in kg / m³. 3 ; Let gravitational acceleration be 9.801 m / s². 2 H i Let m be the depth of a certain point in the stratum.

3. The method for determining the effective stress coefficient of formation rocks according to claim 1, characterized in that, Corresponding depth point H i Effective stress at Calculate using the following formula: （2）； （3）； In the formula: For a point in the stratum at a depth H i Effective stress at the point, MPa; The initial void ratio of the strata rocks is dimensionless. For a point in the stratum at a depth H i The porosity at that location is dimensionless. is the compressibility index of the formation, dimensionless; For a point in the stratum at a depth H i Porosity at that location, %.

4. The method for determining the effective stress coefficient of formation rocks according to claim 1, characterized in that, According to Biot's effective stress formula The effective stress coefficient of the formation rock is determined by the following formula. : （4）； （5）； （6）。 5. The method for determining the effective stress coefficient of formation rocks according to claim 1, characterized in that, The measured formation pressure at different depths of the target formation is obtained through drill pipe testing, bottom hole pressure measurement during drilling, drilling fluid density inversion, or oil and gas reservoir testing. The required measured formation pressure of the target formation is no less than 5 different depth points.

6. The method for determining the effective stress coefficient of formation rock according to claim 2, characterized in that, If the volumetric density of rock in a certain formation is measured in well logging... Missing information; Gardner's formula is used to calculate based on logging P-wave travel time or P-wave velocity: （7） （8） In the formula: P-wave transit time in formation rock logging, µs / ft; Let be the rock P-wave velocity, m / s; where the logging P-wave transit time is the reciprocal of the P-wave velocity.

7. The method for determining the effective stress coefficient of formation rock according to claim 3, characterized in that, Initial porosity of strata rocks Compression index Determined by using standard plunger rock samples for uniaxial or triaxial compression tests.

8. The method for determining the effective stress coefficient of formation rocks according to claim 7, characterized in that, No fewer than three standard plunger rock samples should be used for compression tests; empirical values ​​should be used for shale formations. and .

9. The method for determining the effective stress coefficient of formation rocks according to claim 3, characterized in that, At a depth H of a certain point in the strata i porosity at Determined by calculations based on well logging P-wave velocity, compensated neutrons, or resistivity.

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