Quantitative evaluation method for damage degree of fault to shale gas storage conditions

Abstract

The invention discloses a quantitative evaluation method for the damage degree of a fault to a shale gas storage condition. The method comprises the following steps: S1, depicting fault development characteristics of a target layer series of a to-be-detected region; s2, fault levels are divided according to the fault displacement, the extension length on the fault plane and the vertical broken layer; s3, three-dimensional seismic attribute extraction is carried out on the target stratum, natural fracture sensitive attributes are selected based on geological laws and single well core fracture statistical results, and the widths W of fault fracture zones of different levels are quantitatively described; s4, on the basis of the current stress field, the fluid pressure and the fault occurrence, the effective normal stress sigman borne by the three-dimensional space interruption layer is calculated; and S5, establishing a damage index R of the fault to the shale gas storage condition according to the fault fracture zone width W and the size sigma n of the effective normal stress borne by the fault plane, and evaluating the influence of the fault on the shale gas storage condition. Through the method, different scales, different occurrence states, different burial depths and different parts of the same fault can be evaluated.

Description

Technical Field

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

[0002] Over the past decade, shale gas, driving the majority of global natural gas production growth, has profoundly altered the global energy supply landscape. Previous studies have shown that preservation conditions are key factors influencing shale gas enrichment and production, and faults, as pathways for shale gas dissipation, have a significant impact on shale gas preservation conditions. Quantitatively assessing the extent to which faults damage shale gas preservation conditions is crucial for guiding shale gas exploration and development.

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

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

[0005] The purpose of the present invention is to overcome the problems of the prior art and disclose a method for quantitatively evaluating the degree of damage to shale gas preservation conditions caused by faults. The method of the present invention can be used to evaluate different scales, different occurrences, different burial depths and different parts of the same fault.

[0006] The object of the present invention is achieved through the following technical solutions:

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

[0008] S1: Characterize the fault development characteristics of the target stratum in the survey area, including: interpreting the faults of the target stratum based on the 3D seismic data volume, characterizing the fault depth, fault throw, strike, dip, inclination, vertical disconnection layer, and horizontal extension length;

[0009] S2: Fault levels are divided according to the fault throw, the length of the fault plane, and the vertical fault level;

[0010] S3: 3D seismic attribute extraction is performed on the target formation. Based on geological laws and single-well core fracture statistics, natural fracture sensitive attributes are selected to quantitatively characterize the width W of fault fracture zones of different levels.

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

[0012] S5: According to the width of the fault fracture zone W and the effective normal stress σ on the fault surface n The fault damage index R on shale gas preservation conditions was established to evaluate the impact of faults on shale gas preservation conditions.

[0013] According to a preferred embodiment, step S2 includes: breaking the target stratum or deeper strata upward to the surface, with a fault throw greater than 300m and an extension length of 8km-20km as a first-level fault;

[0014] The faults that break the regional cover upward from the target strata or deeper strata with a fault throw of 100m to 300m and an extension length of 6km to 8km are regarded as secondary faults;

[0015] Faults that are disconnected from the target strata with a throw of 40m to 100m and an extension length of 4km to 6km are considered third-order faults;

[0016] Faults that are disconnected from the target strata with a fault throw of 20m to 40m and an extension length of less than 4km are considered as fourth-order faults.

[0017] According to a preferred embodiment, in step S3, based on the natural fracture development pattern established in the field outcrop of the target formation and the statistical results of fracture density of single well cores in the target area, the seismic attribute that can best reflect the natural fracture development characteristics of the target formation is selected.

[0018] According to a preferred embodiment, in step S3, quantitatively characterizing the widths 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 perpendicular to the fault strike, and the seismic attribute values ​​of 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 distance decreases to the same value as the surrounding rock, the distance between the sampling points on both sides of the fault is the width W of the fault fracture zone.

[0020] The normalization of natural fracture sensitivity attributes is calculated using the following formula:

[0021]

[0022] Where SA is the preferred seismic attribute value, SAmin is the minimum value of all sample points’ seismic attributes, SA 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 harbor numerous fracture systems and small faults, serving as important pathways for shale gas dissipation. The width of these zones determines the extent of the fault's influence on shale gas preservation. Existing methods primarily rely on field observations to establish a statistical relationship between fault throw and fracture zones for prediction. However, these methods are unable to accurately characterize the width of fault fracture zones within formations. However, methods based on geophysical properties can quantitatively characterize the width of fault fracture zones at any time and scale within a formation.

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

[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] Where σ n is the effective normal stress on the fault plane, S H is the maximum horizontal principal stress, S h is the horizontal minimum principal stress, S v is the vertical principal stress, α is the fault dip, and β is the fault strike and maximum horizontal principal stress S H The acute angle in the direction of the pore is 0.044°, and P is the pore fluid pressure.

[0027] According to a preferred embodiment, in step S4, the vertical principal stress S v By integrating the density log curve with depth, we can obtain:

[0028]

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

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

[0031]

[0032]

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

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

[0035] Among them, drilling-induced fractures appear as two black vertical stripes with a 180° symmetrical distribution on the imaging logging image, parallel to the well axis and extending longer. The trend of drilling-induced fractures is in the direction of the horizontal maximum principal stress;

[0036] Wellbore collapse appears as two 180° symmetrical vertical long dark strips or dark blocks on the image. The long axis direction of the elliptical well caused by wellbore collapse is the horizontal minimum principal stress direction.

[0037] Existing methods for determining the sealing properties of shale faults are primarily based on the angle between the fault strike and the current maximum principal stress, assuming that the smaller the angle, the stronger the fault sealing properties. This method primarily qualitatively determines the relative size of fault sealing properties in two-dimensional space and cannot quantitatively characterize changes in fault sealing properties in three-dimensional space. In reality, the degree of fault opening and sealing in three-dimensional space is controlled by the fault's strike, current stress field, and pore fluid pressure. This method calculates the effective normal stress on the fault plane based on the fault's strike (strike, dip), pore fluid pressure, and the current stress field size and direction, while also considering the fault's morphological characteristics and stress state in three-dimensional space. This method can more accurately and realistically reflect changes in fault sealing properties in three dimensions, enabling a quantitative evaluation of the fault sealing properties of shale reservoirs.

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

[0039] R=W′ / σ′ n

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

[0041] The larger the R, the stronger the destructive effect of the fault on shale gas, the worse the gas content of shale gas, and the lower the production capacity; the smaller the R, the weaker the destructive effect of the fault on shale gas preservation conditions, the better the gas content of shale, and the greater the production capacity.

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

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

[0044] Among them, σ max is the maximum value of the effective normal stress of the section, σ min is the minimum value of the effective normal stress of the cross section; the normalized width of the fault fracture zone W' is:

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

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

[0047] The aforementioned main solution of the present invention and its various further options can be freely combined to form multiple solutions, all of which can be adopted and protected by the present invention. After understanding the solutions of the present invention, those skilled in the art will understand 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 these are not exhaustive here.

[0048] Beneficial effects of the present invention:

[0049] The present invention simultaneously considers the width of the fault fracture zone and the closure of the fault in three-dimensional space. The established shale gas preservation condition destruction index R can quantitatively characterize the degree of damage to shale gas preservation conditions caused by faults in three-dimensional space. It can evaluate different scales, different occurrences, different burial depths, and different parts of the same fault, with higher accuracy and wider applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is a planar distribution map of faults of different levels in a shale gas field in the southern part of a basin;

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

[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 It is a schematic diagram of the spatial forces on the fault plane;

[0055] Figure 4 It is a schematic diagram of the relationship between the maximum and minimum horizontal principal stresses and depth in the embodiment of the present application. DETAILED DESCRIPTION

[0056] The following describes the embodiments of the present invention through specific examples. 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. The 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 the following embodiments and features in the embodiments can be combined with each other unless they conflict.

[0057] Example 1

[0058] This invention discloses a method for quantitatively evaluating the degree to which faults damage shale gas preservation conditions. The invention is further described below with reference to accompanying figures and examples. It should be understood that the specific examples described herein are merely illustrative and not limiting of the invention. Furthermore, for ease of description, only portions relevant to the invention are included, not all of them. The examples are illustrated using a fault in a shale gas field in the southern part of a basin.

[0059] Step 1: Detailed characterization of the fault development characteristics of the Wufeng-Longmaxi shale formation

[0060] Based on the 3D seismic data, a detailed interpretation of the Wufeng-Longmaxi fault was conducted to characterize the fault depth, fault throw, strike, dip, inclination, vertical fault position, and horizontal extension length.

[0061] Step 2: Classify the fault order of the Wufeng Formation-Longmaxi Formation according to the fault throw, the length of the fault plane, and the vertical fault layer.

[0062] like Figure 1As shown, major faults that break upward from the Silurian (or deeper strata) to the surface (or may break to the surface) have a controlling effect on the structure. Faults with a throw greater than 300 m and an extension length of 8 km to 20 km are classified as first-order faults. Faults that break upward from the Silurian (or deeper strata) to the Permian or Triassic (or break from the Triassic but show no signs of breaking to the surface) with a throw between 100 m and 300 m and an extension length of 6 km to 8 km are classified as second-order faults. Faults that break upward from the Wufeng Formation to the interior of the Silurian (and may break downward to the Aodi) are classified as third-order faults with a throw between 40 m and 100 m and an extension length of 4 km to 6 km. Faults that break between the Wufeng and Longmaxi Formations, with a throw between 20 m and 40 m and an extension length of less than 4 km are classified as fourth-order faults.

[0063] Step 3: Extract 3D seismic attributes of the target formation. Based on geological laws and single well core fracture statistics, select natural fracture sensitive attributes and quantitatively characterize the width W of different fault fracture zones.

[0064] Seismic attribute extraction was carried out on the Wufeng-Longmaxi shale based on 3D seismic data, including coherence, maximum likelihood volume, ant volume, variance, fault enhancement analysis technology, and Canny edge detection. Further, based on the natural fracture development pattern established in the field outcrop of the target formation and the statistical results of fracture density of single well cores in the target area, the variance attribute was selected to characterize the fault fracture zone, with reference to Figure 2(a) 、 2(b) 、2(c).

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

[0066]

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

[0068] Step 4: Calculate the effective normal stress on the fault plane in three dimensions based on the current stress field, fluid pressure, and fault attitude.

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

[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] Where σ n is the effective normal stress on the fault plane, S H is the maximum horizontal principal stress MPa, S h is the minimum horizontal principal stress MPa, α is the fault dip, β is the fault strike and the maximum horizontal principal stress S H The acute angle in the direction of the pore is 0.044°, and P is the pore fluid pressure.

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

[0073]

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

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

[0076]

[0077]

[0078] Shale bedding is well developed, and the calculation of shale in-situ stress using an anisotropic model can more accurately obtain the in-situ stress of shale reservoirs. h and S H are the minimum and maximum horizontal stresses, MPa; P p is the formation pore pressure, MPa; α is the Biot coefficient, usually set to 1; ε H and ε h are the maximum and minimum tectonic strains in the horizontal direction, respectively. E is Young's modulus; E v , E h are the vertical Young's modulus and the horizontal Young's modulus, MPa. v , v h is the Poisson's ratio in the vertical and horizontal directions.

[0079] Since the wellbore is usually located at a certain distance from the fault, it is impossible to reflect the magnitude of the ground stress around the fault. Therefore, samples of different buried depths of the target layer can be collected through core wells, and emission experiments can be carried out to measure the maximum and minimum principal stresses of the horizontal layer. The correlation between the maximum and minimum principal stresses and the buried depth can be established, and the maximum and minimum principal stresses around the fault can be predicted in combination with the buried depth of the fault. Figure 4 shown.

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

[0081] Step 5: Establish the fault damage index R on shale gas preservation conditions based on the width of the fault fracture zone and the effective normal stress on the fault surface to 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 is the normalized effective normal stress of the cross section, σ max is the maximum value of the effective normal stress of the section, σ min is the minimum value of the effective normal stress of the cross section. W' is the normalized width of the fault fracture zone, W max is the maximum value of the effective normal stress of the section, W min is the minimum value of the effective normal stress of the section.

[0086] The larger the R, the more destructive the fault is to shale gas, the worse the gas content of the shale is, and the lower the production capacity is. The smaller the R, the weaker the destructive effect of the fault on shale gas preservation conditions, the better the gas content of the shale is, and the greater the production capacity is.

[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 in the scope of protection of the present invention.

Claims

1. A quantitative evaluation method for the degree of damage to shale gas preservation conditions caused by faults, characterized in that: The quantitative evaluation method comprises the following steps: S1: Characterize the fault development characteristics of the target stratum in the survey area, including: interpreting the faults of the target stratum based on the 3D seismic data volume, characterizing the fault depth, fault throw, strike, dip, inclination, vertical disconnection layer, and horizontal extension length; S2: Fault levels are divided according to the fault throw, the length of the fault plane, and the vertical fault level; S3: 3D seismic attribute extraction is performed on the target formation. Based on geological laws and single-well core fracture statistics, natural fracture sensitive attributes are selected to quantitatively characterize the width W of fault fracture zones of different levels. S4: Based on the current stress field, fluid pressure and fault attitude, calculate the effective normal stress σ on the fault plane in three-dimensional space n ; S5: According to the width of the fault fracture zone W and the effective normal stress σ on the fault surface n The fault damage index R on shale gas preservation conditions was established to evaluate the impact of faults on shale gas preservation conditions.

2. The quantitative evaluation method according to claim 1, wherein: Step S2 includes: breaking the target stratum or deeper strata upward to the surface with a fault throw greater than 300 m and an extension length of 8 km to 20 km as a first-level fault; The faults that break the regional cover upward from the target strata or deeper strata with a fault throw of 100m to 300m and an extension length of 6km to 8km are regarded as secondary faults; Faults that are disconnected from the target strata with a throw of 40m to 100m and an extension length of 4km to 6km are considered third-order faults; Faults that are disconnected from the target strata with a fault throw of 20m to 40m and an extension length of less than 4km are considered as fourth-order faults.

3. The quantitative evaluation method according to claim 1, wherein: In step S3, based on the natural fracture development pattern established from the field outcrop of the target formation and the statistical results of fracture density of single well cores in the target area, the seismic attribute that best reflects the natural fracture development characteristics of the target formation is selected.

4. The quantitative evaluation method according to claim 3, wherein: In step S3, quantitative characterization of the width of the fault fracture zones of different levels in the target area includes: The seismic attribute values ​​SA on both sides of the fault are sampled at equal intervals perpendicular to the fault strike, and the seismic attribute values ​​of 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 distance decreases to the same value as the surrounding rock, the distance between the sampling points on both sides of the fault is the width W of the fault fracture zone.

5. The quantitative evaluation method according to claim 1, wherein: In step S4, the effective normal stress on the fault plane is calculated based on the horizontal maximum and minimum principal stresses, vertical principal stresses, and pore fluid pressure, combined with the fault strike and dip angle: s n =(S H -P)*sin 2 a*sin 2 β+(S h -P)*sin 2 a*cos 2 β+(S v -P)cos 2 a Where σ n is the effective normal stress on the fault plane, S H is the maximum horizontal principal stress, S h is the horizontal minimum principal stress, S S is the vertical principal stress, α is the fault dip, and β is the fault strike and maximum horizontal principal stress S H The acute angle in the direction of the pore is 0.044°, and P is the pore fluid pressure.

6. The quantitative evaluation method according to claim 5, wherein: In step S4, the vertical principal stress S v By integrating the density log curve with depth, we can obtain: Where: S v is the vertical principal stress; ρ is the formation density; H represents the vertical depth; and h is the formation thickness.

7. The quantitative evaluation method according to claim 5, wherein: In step S4, the calculation formulas for the horizontal minimum principal stress and the horizontal maximum principal stress are as follows: Where S h and S H are the horizontal minimum principal stress and the horizontal maximum ground stress, P p is the formation pore pressure; α is the Biot coefficient; ε H and ε h are the maximum and minimum tectonic strains in the horizontal direction, E is the Young's modulus; E v , E h are the Young's modulus in the vertical direction and the Young's modulus in the horizontal direction, v v , v h is the Poisson's ratio in the vertical and horizontal directions.

8. The quantitative evaluation method according to claim 5, wherein: In step S4, the directions of the horizontal minimum principal stress and the horizontal maximum principal stress are determined based on the imaging logging to identify drilling-induced fractures and wellbore collapse; Among them, drilling-induced fractures appear as two black vertical stripes with a 180° symmetrical distribution on the imaging logging image, parallel to the well axis and extending longer. The trend of drilling-induced fractures is in the direction of the horizontal maximum principal stress; Wellbore collapse appears as two 180° symmetrical vertical long dark strips or dark blocks on the image. The long axis direction of the elliptical well caused by wellbore collapse is the horizontal minimum principal stress direction.

9. The quantitative evaluation method according to claim 5, wherein: In step S5, the fault destructive index R is calculated by the following formula: R=W' / σ′ n Where W' is the normalized width of the fault fracture zone, σ′ n is the normalized effective normal stress of the cross section; The larger the R, the stronger the destructive effect of the fault on shale gas, the worse the gas content of shale gas, and the lower the production capacity; the smaller the R, the weaker the destructive effect of the fault on shale gas preservation conditions, the better the gas content of shale, and the greater the production capacity.

10. The quantitative evaluation method according to claim 9, wherein: Normalized effective normal stress σ′ of the cross section n for: in n =(s n -s min ) / (s max -s min ) Among them, σ max is the maximum value of the effective normal stress of the section, σ min is the minimum value of the effective normal stress of the cross section; the normalized width of the fault fracture zone W' is: W'=(WW min ) / (IN max -IN min ) Where W' is the normalized width of the fault fracture zone, W max is the maximum value of the effective normal stress of the section, W min is the minimum value of the effective normal stress of the section.

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