A method for quantitatively evaluating the damage degree of a fault to shale gas preservation conditions

Abstract

This invention discloses a quantitative evaluation method for the degree of damage of faults to shale gas preservation conditions, comprising: S1: characterizing the fault development characteristics of the target strata in the area to be tested; S2: classifying the fault order according to the fault displacement, the length of the fault plane, and the vertically discontinuous strata; S3: extracting three-dimensional seismic attributes of the target strata, selecting sensitive attributes of natural fractures based on geological laws and statistical results of fractures in single-well cores, and quantitatively characterizing the width W of the fracture zone of different fault orders; S4: calculating the effective normal stress σ on the three-dimensionally discontinuous strata based on the current stress field, fluid pressure, and fault attitude. n S5: Based on the width W of the fault fracture zone and the magnitude σ of the effective normal stress on the fault plane. n A fault-related disruption index R is established to evaluate the impact of faults on shale gas preservation conditions. This method can be used to evaluate different scales, occurrences, burial depths, and different parts of the same fault.

Description

Technical Field

[0001] This invention belongs to the field of shale reservoir evaluation, and in particular relates to a quantitative evaluation method for the degree of damage of faults to shale gas preservation conditions. Background Technology

[0002] Over the past decade, shale gas has become the main driver of global natural gas production growth, profoundly altering the global energy supply landscape. Previous research has identified preservation conditions as a key factor determining the enrichment and high production of shale gas, and faults, acting as channels for shale gas loss, significantly impact these preservation conditions. Quantitatively evaluating the degree of damage caused by faults to shale gas preservation conditions is crucial for guiding shale gas exploration and development.

[0003] Currently, the impact of faults on preservation conditions is mainly assessed by judging the angle between the fault strike and the current maximum principal stress. It is believed that the larger the angle, the stronger the fault sealing and the weaker the damage to shale gas preservation conditions. However, this method can only qualitatively evaluate the relative magnitude of fault sealing in two-dimensional space. In three-dimensional space, the burial depth, dip angle, and strike of the fault at different locations are constantly changing. Existing methods cannot quantitatively characterize the changes in fault sealing in three-dimensional space and their impact on preservation conditions.

[0004] Furthermore, existing methods focus on the relationship between the fault surface and the stress field, neglecting the impact of fault fracture zones on preservation conditions. The width of these fracture zones determines the extent of the fault's influence on shale gas preservation conditions. Therefore, existing methods are prone to bias and misleading interpretations during the evaluation process, reducing the efficiency of shale gas exploration and development. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of the prior art and to disclose a quantitative evaluation method for the degree of damage of faults to shale gas preservation conditions. The method of this invention can evaluate different scales, different occurrences, different burial depths, and different parts of the same fault.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A quantitative evaluation method for the degree of damage caused by faults to shale gas preservation conditions, the quantitative evaluation method comprising the following steps:

[0008] S1: Characterize the fault development characteristics of the target stratigraphic system in the area to be measured, including: interpreting the faults of the target stratigraphic system based on the three-dimensional seismic data volume, and characterizing the fault depth, displacement, strike, dip, dip angle, vertical break position, and horizontal extension length.

[0009] S2: Fault order is determined based on fault displacement, fault plane extension length, and vertically dissected strata.

[0010] S3: Extract three-dimensional seismic attributes from the target strata. Based on geological patterns and statistical results of fractures in single-well cores, select sensitive attributes of natural fractures and quantitatively characterize the width W of fault fracture zones of different orders.

[0011] S4: Based on the current stress field, fluid pressure, and fault orientation, calculate the magnitude of the effective normal stress σ on the fault plane in three-dimensional space. n ;

[0012] S5: Based on the width W of the fault fracture zone and the magnitude σ of the effective normal stress on the fault plane. n Establish the fault-induced damage index R to shale gas preservation conditions and evaluate the impact of faults on shale gas preservation conditions.

[0013] According to a preferred embodiment, step S2 includes: designating a first-order fault as a fault that breaks upward from the target stratum or deeper strata to the ground surface, with a fault displacement greater than 300m and an extension length of 8km-20km.

[0014] A secondary fault is defined as a fault that breaks the regional caprock upward from the target strata or deeper strata, with a fault displacement between 100m and 300m and an extension length of 6km to 8km.

[0015] Faults that break off from the target strata, with a displacement between 40m and 100m and an extension length of 4km to 6km, are designated as third-order faults.

[0016] Faults that break off from the target strata, with a displacement between 20m and 40m and an extension length of less than 4km, are classified as fourth-order faults.

[0017] According to a preferred embodiment, in step S3, the seismic attribute that best reflects the development characteristics of natural fractures in the target strata is selected based on the natural fracture development pattern established by the outcrops of the target strata in the field and the statistical results of fracture density in single-well cores of the target area.

[0018] According to a preferred embodiment, step S3, which involves quantitatively characterizing the width of fault fracture zones of different levels in the target area, includes:

[0019] The seismic attribute values ​​SA on both sides of the fault are sampled at equal intervals according to the vertical fault strike, and the seismic attribute values ​​of the sampled points are normalized. As the distance between the sample points on both sides of the fault increases, the seismic attribute values ​​decrease. When the values ​​decrease to the same level as the surrounding rock values, the distance between the sample points on both sides of the fault is the width W of the fault fracture zone.

[0020] The normalization of the natural crack sensitivity attribute is calculated using the following formula:

[0021]

[0022] In the formula, SA is the preferred seismic attribute value.min SA is the minimum value of the seismic properties of all sample points. max is the maximum value of the seismic attributes of all sample points, and n is the number of sample points.

[0023] Fault fracture zones are deformed surrounding rock masses distributed near fault planes. These zones develop numerous fracture systems and minor fractures, serving as important channels for shale gas loss. The width of the fault fracture zone determines the extent of the fault's influence on shale gas preservation conditions. Existing methods primarily rely on field observations and descriptions, establishing statistical relationships between fault displacement and fracture zones for prediction. However, they cannot accurately characterize the width of fault fracture zones within strata. Methods based on geophysical properties can quantitatively characterize the width of fault fracture zones at any time and scale within strata.

[0024] According to a preferred embodiment, in step S4, based on the maximum and minimum horizontal principal stresses, the vertical principal stresses, and the pore fluid pressure, and in conjunction with the fault strike and dip angle, the magnitude of the effective normal stress on the fault plane is calculated:

[0025] σ n =(S H -P)*sin 2 α*sin 2 β+(S h -P)*sin 2 α*cos 2 β+(S v -P)cos 2 α

[0026] In the formula, σ n S is the effective normal stress borne by the fault plane. H For the maximum horizontal principal stress, S h For the minimum principal stress in the horizontal direction, S v Let α be the vertical principal stress, α be the fault dip angle, and β be the fault strike and the maximum horizontal principal stress S. H The acute angle between the directions, where P is the pore fluid pressure.

[0027] According to a preferred embodiment, in step S4, the magnitude of the vertical principal stress S v The following is obtained by integrating the density logging curve with respect to depth:

[0028]

[0029] In the formula: S v ρ represents the vertical principal stress; H represents the formation density; and h represents the vertical depth.

[0030] According to a preferred embodiment, in step S4, the formulas for calculating the minimum horizontal principal stress and the maximum horizontal principal stress are as follows:

[0031]

[0032]

[0033] In the formula S h and S H These represent the minimum horizontal principal stress and the maximum horizontal ground stress, respectively, P p ε is the formation pore pressure; α is the Biot coefficient; ε H and ε h These represent the maximum and minimum tectonic strains in the horizontal direction, respectively, and E is Young's modulus; E v E h These are the vertical Young's modulus and the horizontal Young's modulus, respectively. v v h For the vertical and horizontal Poisson's ratios.

[0034] According to a preferred embodiment, in step S4, the directions of the minimum horizontal principal stress and the maximum horizontal principal stress are determined based on imaging logging to identify drilling-induced fractures and wellbore collapse.

[0035] Among them, the drilling-induced fractures are two black vertical stripes that are symmetrically distributed at 180° on the imaging logging image. They are parallel to the well axis and extend for a long time. The direction of the drilling-induced fractures is the direction of the maximum horizontal principal stress.

[0036] Wellbore collapse appears in images as two 180° symmetrical vertical dark bands or blocks. The major axis of the elliptical well caused by wellbore collapse is in the direction of the horizontal minimum principal stress.

[0037] Existing methods for assessing shale fault sealing primarily rely on the angle between the fault strike and the current maximum principal stress, assuming a smaller angle indicates stronger fault sealing. This method mainly qualitatively determines the relative magnitude of fault sealing in two-dimensional space, failing to quantitatively characterize changes in fault sealing in three-dimensional space. In reality, the opening and closing degree of a fault in three-dimensional space is jointly controlled by the fault attitude, the current stress field, and pore fluid pressure. This invention calculates the effective normal stress on the fault plane based on the fault attitude (strike and dip), pore fluid pressure, and the magnitude and direction of the current stress field. It also considers the fault's morphological characteristics and stress state in three-dimensional space, enabling a more accurate and realistic reflection of changes in fault sealing in three-dimensional space, thus achieving a quantitative evaluation of shale reservoir fault sealing.

[0038] According to a preferred embodiment, in step S5, the fault destructive index R is calculated using the following formula:

[0039] R = W′ / σ′ n

[0040] Where W' is the normalized width of the fault fracture zone, σ′n This represents the normalized effective normal stress of the cross section.

[0041] A larger R value indicates a stronger destructive effect of the fault on shale gas, resulting in poorer gas content and lower production capacity; a smaller R value indicates a weaker destructive effect of the fault on shale gas preservation conditions, resulting in better gas content and higher production capacity.

[0042] According to a preferred embodiment, the normalized effective cross-sectional normal stress σ′ n for:

[0043] σ′ n =(σ n -σ min ) / (σ max -σ min )

[0044] Where, σ max σ is the maximum value of the effective normal stress of the cross section. min The minimum effective normal stress of the cross section is given by: The normalized fault fracture zone width W' is:

[0045] W' = (WW min ) / (W max -W min )

[0046] Where W' is the normalized width of the fault fracture zone, W max W is the maximum value of the effective normal stress of the cross section. min This represents the minimum effective normal stress of the cross section.

[0047] The aforementioned main solution of the present invention and its various further alternative solutions can be freely combined to form multiple solutions, all of which are solutions that can be adopted and are claimed by the present invention. Those skilled in the art, after understanding the solution of the present invention, will realize that there are many combinations based on existing technology and common knowledge, all of which are technical solutions to be protected by the present invention, and will not be exhaustively listed here.

[0048] The beneficial effects of this invention are:

[0049] This invention takes into account both the width of the fault fracture zone and the closure of the fault in three-dimensional space, and establishes a shale gas preservation condition destruction index R. This index can quantitatively characterize the degree of destruction of shale gas preservation conditions by faults in three-dimensional space. It can evaluate different scales, different occurrences, different burial depths, and different parts of the same fault, and has higher accuracy and wider applicability. Attached Figure Description

[0050] Figure 1 This is a plan view of the fault distribution of different levels in a shale gas field in the southern part of a basin.

[0051] Figure 2(a) is a schematic diagram of the fault-related fracture development zone;

[0052] Figure 2(b) is a schematic diagram of earthquake crack properties;

[0053] Figure 2(c) is a schematic diagram of the quantitative analysis of the fault fracture zone;

[0054] Figure 3 This is a schematic diagram of the forces acting on the cross-section.

[0055] Figure 4 This is a schematic diagram showing the relationship between the maximum and minimum horizontal principal stresses and depth in the embodiments of this application. Detailed Implementation

[0056] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0057] Example 1

[0058] This invention discloses a quantitative evaluation method for the degree of damage caused by faults to shale gas preservation conditions. The invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are merely for illustrative purposes and not for limiting the invention. Furthermore, it should be noted that, for ease of description, only parts related to this invention, not all of them, are included. An example is a fault in a shale gas field in the southern part of a basin.

[0059] Step 1: Finely depict the fault development characteristics of the Wufeng Formation-Longmaxi Formation shale.

[0060] Based on the three-dimensional seismic data volume, the Wufeng Formation-Longmaxi Formation faults are interpreted in detail, and the fault depth, displacement, strike, dip, dip angle, vertical fault location, and horizontal extension length are characterized.

[0061] Step 2: Classify the Wufeng-Longmaxi Formation fault order based on the fault displacement, the length of the fault plane, and the vertically dissected strata.

[0062] like Figure 1As shown, a major fault extending upwards from the Silurian strata (or deeper strata) to the surface (or possibly to the surface) and controlling tectonic activity, with a displacement greater than 300m and an extension length of 8km-20km, is classified as a first-order fault. A fault extending upwards from the Silurian strata (or deeper strata) to the Permian or Triassic strata (or breaking through the Triassic but without any indication of breaking to the surface), with a displacement between 100m and 300m and an extension length of 6km-8km, is classified as a second-order fault. A fault extending upwards from the Wufeng Formation to the interior of the Silurian strata (potentially breaking downwards through the Aodi), with a displacement between 40m and 100m and an extension length of 4km-6km, is classified as a third-order fault. A fault that breaks through the Wufeng and Longmaxi Formations, with a displacement between 20m and 40m and an extension length of <4km, is classified as a fourth-order fault.

[0063] Step 3 involves extracting three-dimensional seismic attributes from the target strata. Based on geological patterns and statistical results of fractures in single-well core samples, sensitive attributes of natural fractures are selected to quantitatively characterize the width W of fault fracture zones of different orders.

[0064] Seismic attribute extraction was performed on the Wufeng-Longmaxi Formation shale based on 3D seismic data, including coherence, maximum likelihood volume, ant volume, variance, fault enhancement analysis, and Canny edge detection. Furthermore, based on the natural fracture development model established from outcrops in the target strata and the statistical results of fracture density from single-well cores in the target area, variance attributes were optimized for fault fracture zone characterization, referencing... Figure 2(a) , 2(b) 2(c).

[0065] Vertically along the fault strike, seismic attribute values ​​are sampled at equal intervals on both sides of the fault, and the seismic attribute values ​​at the sampling points are normalized. As the distance from the fault increases, the seismic attribute values ​​decrease. When the values ​​decrease to the same level as the surrounding rock values, the distance between the sampling points on both sides of the fault represents the width of the fault fracture zone. This method is used to quantitatively characterize the width of fault fracture zones of different levels in the target area. The variance attribute normalization is calculated using the following formula:

[0066]

[0067] In the formula, S is the variance attribute, S min S is the minimum value of the variance attribute of all sample points. max is the maximum value of the variance attribute for all sample points, and n is the number of sample points.

[0068] Step 4: Based on the current stress field, fluid pressure, and fault orientation, calculate the magnitude of the effective normal stress on the fault plane in three-dimensional space.

[0069] (1) Based on the maximum and minimum horizontal principal stresses, vertical principal stresses, and pore fluid pressure, combined with the fault strike and dip angle, calculate the magnitude of the effective normal stress on the fault plane:

[0070] σ n =(S H -P)*sin 2 α*sin 2 β+(S h -P)*sin 2 α*cos 2 β+(S v -P)cos 2 α

[0071] In the formula σ n S is the effective normal stress borne by the fault plane. H The maximum horizontal principal stress is MPa, S h Let α be the fault dip angle and β be the fault strike and the maximum horizontal principal stress S. H The acute angle between the directions, where P is the pore fluid pressure.

[0072] (2) The magnitude of the vertical principal stress S v The formula is obtained by integrating the density logging curve with respect to depth as follows:

[0073]

[0074] In the formula: S v ρ is the vertical principal stress, MPa; ρ is the formation density, g / cm³. 3 H represents vertical depth, in meters (m); h represents formation thickness, in meters (m).

[0075] (3) The formulas for calculating the maximum and minimum horizontal principal stresses are as follows:

[0076]

[0077]

[0078] Shale bedding is well-developed. Using anisotropic models to calculate the magnitude of shale in-situ stress can more accurately obtain the magnitude of shale reservoir in-situ stress, where S... h and S H These represent the minimum and maximum horizontal ground stresses, in MPa; P p ε is the formation pore pressure, MPa; α is the Biot coefficient, usually taken as 1; H and ε h These represent the maximum and minimum tectonic strains in the horizontal direction, respectively. E is Young's modulus; E v E h These are the vertical Young's modulus and the horizontal Young's modulus, respectively, in MPa. v v h For the vertical and horizontal Poisson's ratios.

[0079] Since wellbore locations are typically some distance from faults, they cannot reflect the magnitude of in-situ stress around the fault. Therefore, core samples from different burial depths of the target layer can be collected and subjected to bioemission experiments to measure the maximum and minimum horizontal principal stresses. The correlation between the maximum and minimum horizontal principal stresses and burial depth can be established. Furthermore, combined with the fault burial depth, the magnitudes of the maximum and minimum principal stresses around the fault can be predicted. Figure 4 As shown.

[0080] (4) The direction of the maximum principal stress is determined based on imaging logging to identify drilling-induced fractures and wellbore collapse. The direction of the maximum horizontal principal stress in the study area is between 95° and 110°. Figure 3 As shown.

[0081] Step 5: Establish the fault-induced shale gas preservation condition destruction index R based on the width of the fault fracture zone and the magnitude of the effective normal stress on the fault plane, and evaluate the impact of the fault on shale gas preservation conditions.

[0082] R = W / σ n

[0083] σ′ n =(σ n -σ min ) / (σ max -σ min

[0084] W' = (WW min ) / (W max -W min

[0085] R represents the fault destructive index, σ′ n The normalized effective normal stress of the cross section, σ max σ is the maximum value of the effective normal stress of the cross section. min W is the minimum effective normal stress of the cross section. W' is the normalized width of the fault fracture zone. max W is the maximum value of the effective normal stress of the cross section. min This represents the minimum effective normal stress of the cross section.

[0086] A larger R value indicates a stronger destructive effect of the fault on shale gas, resulting in poorer gas content and lower production capacity. A smaller R value indicates a weaker destructive effect of the fault on shale gas preservation conditions, resulting in better gas content and higher production capacity.

[0087] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for quantitatively evaluating the degree of damage caused by faults to shale gas preservation conditions, characterized in that, The quantitative evaluation method includes the following steps: S1: Characterize the fault development characteristics of the target stratigraphic system in the area to be measured, including: interpreting the faults of the target stratigraphic system based on the three-dimensional seismic data volume, and characterizing the fault depth, displacement, strike, dip, dip angle, vertical break position, and horizontal extension length. S2: Fault order is determined based on fault displacement, fault plane extension length, and vertically dissected strata. Step S2 includes: a fault that breaks upward from the target strata or deeper strata to the ground, with a fault displacement > 300m and an extension length of 8km ≤ 20km, is designated as a first-order fault; The regional caprock will be broken upward from the target strata or deeper strata. Faults with a displacement of 100m and a length of 6km and a length of 8km are designated as secondary faults. Faults that break off from the target strata with a displacement of 40m and a length of 4km and a length of 6km and a displacement of 100m and a length of 4km and a length of 6km or less are classified as third-order faults. Faults that break off from the target strata with a displacement of 20m to 40m and an extension length of less than 4km are classified as fourth-order faults. S3: Perform 3D seismic attribute extraction on the target strata. Based on geological patterns and statistical results of fractures in single-well core samples, select sensitive attributes of natural fractures to quantitatively characterize the width of fault fracture zones of different orders. ; Step S3 involves quantitatively characterizing the width of fault fracture zones of different levels in the target area, including: Seismic attribute values ​​(SA) on both sides of the fault are sampled at equal intervals along the vertical fault strike, and the seismic attribute values ​​at the sampling points are normalized. As the distance between the sampling points on both sides of the fault increases, the seismic attribute values ​​decrease. When the values ​​decrease to the same level as the surrounding rock values, the distance between the sampling points on both sides of the fault is the width of the fault fracture zone. ; S4: Based on the current stress field, fluid pressure, and fault orientation, calculate the magnitude of the effective normal stress on the fault plane in three-dimensional space. ; S5: Based on the width of the fault fracture zone and the magnitude of the effective normal stress on the fault plane Establish the fault-induced shale gas preservation condition destruction index R to evaluate the impact of faults on shale gas preservation conditions; In step S5, the fault-induced damage index R to shale gas preservation conditions is calculated using the following formula: R= W’ / Where W' is the normalized width of the fault fracture zone. This represents the normalized effective normal stress of the cross section. A larger R value indicates a stronger destructive effect of the fault on shale gas, resulting in poorer gas content and lower production capacity; a smaller R value indicates a weaker destructive effect of the fault on shale gas preservation conditions, resulting in better gas content and higher production capacity.

2. The quantitative evaluation method as described in claim 1, characterized in that, In step S3, based on the natural fracture development model established by the outcrops of the target strata and the statistical results of fracture density in single-well cores of the target area, the seismic attribute that best reflects the natural fracture development characteristics of the target strata is selected.

3. The quantitative evaluation method as described in claim 1, characterized in that, In step S4, based on the maximum and minimum horizontal principal stresses, vertical principal stresses, and pore fluid pressure, and combined with the fault strike and dip angle, the magnitude of the effective normal stress on the fault plane is calculated: = + + In the formula, The effective normal stress borne by the fault plane. The maximum principal stress is horizontal. For the minimum principal stress in the horizontal direction, For vertical principal stress, Let β be the fault dip angle, and β be the fault strike and the maximum horizontal principal stress. acute angle of direction, This refers to the pore fluid pressure.

4. The quantitative evaluation method as described in claim 3, characterized in that, In step S4, the magnitude of the vertical principal stress The following is obtained by integrating the density logging curve with respect to depth: In the formula: These are the vertical principal stresses; H represents the formation density; H represents the vertical depth; h represents the formation thickness. This is the acceleration due to gravity.

5. The quantitative evaluation method as described in claim 3, characterized in that, In step S4, the formulas for calculating the minimum and maximum horizontal principal stresses are as follows: In the formula and These are the minimum horizontal principal stress and the maximum horizontal ground stress, respectively. Formation pore pressure; For Biot coefficients; These are the maximum and minimum tectonic strains in the horizontal direction, respectively, and E is Young's modulus; and These are the vertical Young's modulus and the horizontal Young's modulus, respectively. and For the vertical and horizontal Poisson's ratios.

6. The quantitative evaluation method as described in claim 3, characterized in that, In step S4, the directions of the minimum and maximum horizontal principal stresses are determined based on imaging logging to identify drilling-induced fractures and wellbore collapse. Among them, the drilling-induced fractures are two black vertical stripes that are symmetrically distributed at 180° on the imaging logging image. They are parallel to the well axis and extend for a long time. The direction of the drilling-induced fractures is the direction of the maximum horizontal principal stress. Wellbore collapse appears in images as two 180° symmetrical vertical dark bands or blocks. The major axis of the elliptical well caused by wellbore collapse is in the direction of the horizontal minimum principal stress.

7. The quantitative evaluation method as described in claim 1, characterized in that, Normalized effective normal stress of cross section for: = in, This represents the maximum value of the effective normal stress in the cross-section. The minimum effective normal stress of the cross section is given by: The normalized fault fracture zone width W' is: W’= Where W' is the normalized width of the fault fracture zone. This represents the maximum width of the fault fracture zone. This represents the minimum width of the fault fracture zone.

Images