A method for dividing and evaluating shale preservation conditions in complex structures

Abstract

The application discloses a kind of complex structure with shale preservation condition division evaluation method, comprising: based on the explanation result of geoscience, according to fault distance, extension length, fault layer series is classified;According to fault classification result, combined with structural style, burial depth, stratigraphic dip characteristics, structural zoning is carried out;Analysis of the coupling relationship between fracture and ground stress direction, calculate the fracture sealing parameter, qualitative evaluation of fracture preservation condition;Wherein, fracture sealing parameter is Z, Z=sinθ, wherein θ is the angle between fracture and ground stress direction;Based on fracture sealing parameter, the change rule of formation pressure with fracture distance, obtain fracture preservation effective distance D;According to stratigraphic dip, burial depth, fracture sealing parameter, establish standard, evaluate shale gas preservation condition grade.By structural style summary, fault classification quantitative evaluation, clear different structural types, based on the coupling relationship between fracture and ground stress direction to establish preservation condition evaluation method, implement favorable area boundary.

Description

Technical Field

[0001] This invention relates to the field of oil and gas exploration technology, and in particular to a method for classifying and evaluating the preservation conditions of shale in complex structural zones. This invention uses structural analysis to assess fault sealing characteristics, and then uses this as a basis to evaluate the preservation conditions in complex structural zones. Background Technology

[0002] In recent years, shale gas exploration and development in the Sichuan Basin has progressed rapidly, with the discovery of several shale gas fields. As exploration and development advance, the exploration of the Wufeng Formation-Longmaxi Formation has gradually shifted to two areas: deep to ultra-deep layers within the basin, and complex tectonic zones along the basin margin. These complex tectonic zones exhibit diverse structural morphologies and complex patterns, making research on the differences in shale gas formation and enrichment relatively weak. Preservation conditions are a crucial factor that cannot be ignored in shale gas exploration and evaluation, especially in tectonically complex areas. The shale sedimentary thickness distribution of the Wufeng Formation-Longmaxi Formation is relatively stable, and its evolution is relatively moderate. Finding breakthroughs in the residual distribution within tectonically complex areas and establishing targeted preservation condition evaluation methods are indispensable.

[0003] Previous studies have systematically investigated the preservation conditions of marine shale gas in my country. They have systematically analyzed the preservation conditions of shale gas from aspects such as tectonic evolution history, formation water conditions, caprock, natural gas composition, top and bottom plate conditions, and pressure coefficients. They have proposed that favorable top and bottom plate conditions are the foundation, and tectonic modification is the main controlling factor.

[0004] The complex tectonic zones along the basin margin exhibit diverse structural morphologies, with varying tectonic stress backgrounds and deformation mechanisms at different locations, making it impossible to apply a unified standard to assess their preservation conditions. Particularly in southeastern Sichuan, the region is affected by multiple phases of tectonic stress, resulting in various superimposed deformation layers at different locations along the basin margin. The impact of different tectonic phases on preservation remains unclear; factors such as fault grade and development level can influence shale gas enrichment and preservation. These complex environmental factors further complicate the assessment of shale preservation conditions in complex tectonic zones.

[0005] Therefore, establishing a set of evaluation methods for preservation conditions in structurally complex areas is currently a key challenge in shale gas exploration. Summary of the Invention

[0006] The purpose of this invention is to overcome the limitations of existing technologies, such as the diverse structural morphologies of complex shale reservoirs, complex tectonic stress backgrounds and deformations, and the lack of a unified evaluation method. This invention provides a method for classifying and evaluating the preservation conditions of shale in complex structural zones.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] A method for evaluating the preservation conditions of shale in complex structural zones includes the following steps:

[0009] Based on geophysical interpretation results, faults are classified according to fault displacement, extension length, and fault-crossing strata.

[0010] Based on the fault classification results, and combined with the structural style, burial depth, and stratigraphic dip characteristics, structural zoning is carried out;

[0011] Analyze the coupling relationship between the fracture and the geostress direction, calculate the fracture closure parameters, and qualitatively evaluate the fracture preservation conditions;

[0012] The fault closure parameter is Z, where Z = sinθ, and θ is the angle between the fault and the direction of the geostress.

[0013] Based on the fracture sealing parameters and the variation of formation pressure with fracture distance, the effective fracture preservation distance D is determined.

[0014] Based on the dip angle, burial depth, and fault sealing parameters, standards are established to evaluate the level of shale gas preservation conditions.

[0015] This invention presents a method for evaluating shale preservation conditions in complex tectonic zones. Based on the geological characteristics of complex tectonic zones along basin margins, particularly the influence of multiple phases of tectonic stress, it divides tectonic zones using fault grading and structural zoning. It comprehensively considers key preservation factors such as stratigraphic dip angle, burial depth, and sealing parameters, scientifically analyzing and evaluating shale preservation conditions. This method is of great significance for developing shale gas storage in complex tectonic zones and solves the problem of existing technologies' difficulty in evaluating complex geological conditions. The angle θ between the fault and the direction of geostress is obtained through geophysical data analysis or directly from geophysical data collection.

[0016] The derivation / calculation order of the process steps in this invention can be changed under permissible conditions without affecting the final judgment of shale preservation conditions. For example, the process of calculating the effective fracture preservation distance D in step S4 can be performed after the fracture closure parameters are calculated, but before the qualitative evaluation of fracture preservation conditions.

[0017] Furthermore, in the fault classification process, based on the results of detailed geophysical interpretation, faults are classified according to fault displacement, extension length, and fault-crossing strata.

[0018] Preferably, the fault classification includes two levels, named Level III and Level IV respectively; Level IV is further subdivided into four grades.

[0019] Preferably, the fault classification criteria are as follows:

[0020] Table 1 Standards for Shale Gas Fault Classification Parameters

[0021]

[0022] Preferably, when the fault is classified as Level III, the preservation conditions are affected; when the fault is classified as Level IV, further subdivided into Grade A and Grade B, the preservation conditions are somewhat affected; when the fault is classified as Level IV, Grade C and Grade D, the preservation conditions are unaffected.

[0023] Furthermore, in the process of dividing the structural zones, the reference elements include: the dip angle and burial depth of the strata, and the faults of Class III, Class IV, Grade A, and Class B in the fault classification.

[0024] Further, in step S4, the effective distance for preserving the fracture is calculated as D, where D = 2000 - a × Z, and the value of a ranges from 1900 to 2030.

[0025] Based on the sealing parameter Z and the variation of formation pressure with fault distance, the effective preservation distance D of the fault is determined. The effective preservation distance D is mainly for faults of grade III and grade A and B in grade IV, which are affected by preservation conditions, and its value is related to the fault sealing parameter Z.

[0026] Based on exploration experience, when Z = 1, the fault is completely closed, and its effective preservation distance is D = 0 m; when Z = 0, the fault is completely open, and its effective preservation distance is D = 2000 m. Therefore, a functional relationship is established between the effective preservation distance D and the fault sealing parameter Z: D = 2000 - a × Z, where a ranges from 1900 to 2030. Following exploration experience, setting the parameter a value between 1900 and 2030 allows for effective analysis and determination of the effective preservation distance D. In this invention, a value of a = 1922 is used, which effectively achieves the analysis and research of the effective preservation distance D.

[0027] The most direct destructive effect of faults on shale gas is manifested in the fact that "through-the-sky" faults can rupture through the upper (direct) caprock, becoming channels for shale gas loss and destroying shale gas reservoirs. Conversely, open faults that rupture through shale gas layers and connect to high-permeability layers can also cause shale gas to migrate outwards, reducing gas content. The magnitude of the impact of faults on shale gas preservation conditions is related to the degree of fault damage, primarily manifested in the width of the fault fracture zone; the wider the fracture zone, the greater the degree of fault damage. Experience in the Pingqiao and Jiaoshiba areas shows that the fracture zone width is related to the fault grade and the angle between the fault and the maximum horizontal geostress. The lower the fault grade and the larger the angle with the geostress, the smaller the fracture zone width and the better the fault sealing. Therefore, in the evaluation of preservation conditions, it is necessary to deduct the corresponding fracture zone width, i.e., the effective preservation distance D.

[0028] Furthermore, based on the dip angle and burial depth of the strata, different levels of shale gas preservation conditions are classified.

[0029] Preferably, the following shale gas preservation condition level evaluation criteria are used as a reference;

[0030] Table 2 Evaluation Criteria for Shale Gas Preservation Conditions

[0031]

[0032] Then, subtracting step S4 to obtain the effective storage distance, a storage condition evaluation chart is drawn.

[0033] The dip angle of a formation is primarily used to evaluate the foliation fractures of shale. During tectonic activity, the formation uplifts, and shale foliation can become unstable or slip due to differences in mechanical properties and reduced pressure from the overlying strata. Fissures become widespread, and analysis shows that the transverse permeability of shale is 2 to 8 times that of the vertical permeability. In the absence of faults and high-angle fractures, shale mainly migrates laterally along the foliation plane. The degree of migration is related to the normal stress on the foliation plane. The foliation plane is primarily sealed by the gravity of the overlying strata or the regional principal compressive stress. The normal stress on the foliation plane is mainly related to the burial of the strata and the dip angle of the formation. Figure 2 As shown, the formation pressure P can be derived from this.

[0034] P=F÷S'=G×cosα÷S′=ρ×S×Hg×cosα÷(S / cosα)=ρ·Hg·cos 2 α

[0035] In the formula, P is the overlying stratum pressure, F is the overlying stratum pressure, S is the area, S' is the slope area, α is the slope dip angle, ρ is the stratum density, H is the burial depth, and g is the gravity constant.

[0036] Therefore, the greater the dip angle of the strata and the shallower the burial depth, the smaller the normal stress on the foliation surface, and the worse the preservation conditions. Generally, a dip angle of less than 10° is good, 10-20° is relatively good, and greater than 20° results in poor preservation.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] The technology of this invention can summarize structural patterns, quantitatively evaluate fault classification, clarify different structural types, establish a preservation condition evaluation method based on the coupling relationship between faults and geostress directions, and determine the boundaries of favorable areas.

[0039] This invention is mainly applicable to the evaluation of favorable areas with complex geological structures in different basins, and has certain promotional significance. Attached Figure Description

[0040] Figure 1 This is an evaluation diagram of shale gas preservation conditions in a complex structural zone, as shown in Example 1.

[0041] Figure 2 This is a diagram illustrating the principle of normal stress calculation on the foliation surface. Detailed Implementation

[0042] The present invention will be further described in detail below with reference to experimental examples and specific embodiments. However, this should not be construed as limiting the scope of the above-mentioned subject matter of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0043] Example 1

[0044] Geophysical exploration in a certain area revealed information about the underlying structures of shale gas. The exploration results indicate that the underlying structures in this area are complex. The following method is used to evaluate the shale preservation conditions in the complex structural zone.

[0045] Step 1: Based on the detailed geophysical interpretation results, faults are classified according to fault displacement, extension length, and fault-crossing strata. The classification is carried out in accordance with Table 1, "Standard for Fault Classification Parameters of Shale Gas". The classification results are shown in Table 3.

[0046] Table 3 Summary of Fault Elements and Preservation Parameters

[0047]

[0048]

[0049] Step Two: Based on the fault classification results, combined with structural style, burial depth, and stratigraphic dip characteristics, structural zoning is performed; using the f1 fault and its burial depth as boundaries, it is divided into three zones: A, B, and C. Zone A is dominated by NWW stratigraphic strike, Zone B by NEE stratigraphic strike, and Zone C has a greater burial depth. The results are as follows: Figure 1 As shown.

[0050] Shale Gas Fault Classification Parameter Standard Table

[0051]

[0052] Step 3: Analyze the coupling relationship between the fracture and the geostress direction, calculate the fracture closure parameter, and qualitatively evaluate the preservation conditions of the fracture. Let the angle between the fracture and the geostress direction be θ, and the fracture closure parameter be Z, then Z = sinθ.

[0053] Step 4: Based on the fracture closure parameter Z and the variation of formation pressure with fracture distance, calculate the effective preservation distance D of the fracture, and subtract the effective distance D from both the hanging wall and footwall of the fault. For example... Figure 1 As shown, the corresponding distance D is deducted before and after the fault.

[0054] Step 5: Establish standards based on stratum dip angle, burial depth, and fault sealing parameters, and conduct preservation condition evaluation.

[0055] Shale gas preservation condition evaluation parameter standards

[0056]

[0057]

[0058] Based on the above-mentioned shale gas preservation condition evaluation parameters, and after deducting the effective distance D from both the hanging wall and footwall of the fault, the following plot was obtained: Figure 1 The shale gas preservation condition evaluation results shown in the diagram are divided into Class I, II, and III areas, which can provide good guidance for subsequent shale gas exploration and development.

Claims

1. A method for evaluating shale preservation conditions in complex structural belts, characterized by, The method comprises the following steps: Based on the result of geophysical interpretation, the fault classification is carried out according to the fault throw, the extension length and the penetrated stratigraphic system according to the following shale gas fault classification parameter standard: Shale gas fault classification parameter standard According to the fault classification result, the structural partition is carried out in combination with the structural style, the burial depth and the stratigraphic dip characteristics; The coupling relationship between the fault and the ground stress direction is analyzed, the fault sealing parameter is calculated, and the fault preservation condition is qualitatively evaluated; The fault sealing parameter is Z, and Z=sinθ, wherein θ is the included angle between the fault and the ground stress direction; Based on the fault sealing parameter and the variation law of the stratigraphic pressure with the fault distance, the effective distance D of the fault preservation is obtained; The effective distance D of the fault preservation is obtained, and D=2000-a×Z, wherein the value range of a is 1900-2030, and Z is the fault sealing parameter; According to the stratigraphic dip, the burial depth and the fault sealing parameter, the standard is established, the shale gas preservation condition grade is evaluated, the effective distance D of the preservation is deducted, and the preservation condition evaluation diagram is drawn.

2. The method according to claim 1, wherein the complexly-structured shale reservoir condition evaluation method is characterized by, In the fault classification process, the fault classification is carried out according to the fault throw, the extension length and the penetrated stratigraphic system based on the result of geophysical fine interpretation.

3. The method according to claim 1, wherein the complexly structured shale reservoir condition evaluation method is characterized by, In the division process of the structural partition, the reference elements include the stratigraphic dip, the burial depth and the A-class and B-class faults of the Ⅲ-class and Ⅳ-class faults in the fault classification.

4. The method according to claim 1, wherein the complexly-structured shale reservoir condition evaluation method is characterized by, According to the effective distance D of the fault preservation, the value of the fault classification Ⅲ-class and the A-class and B-class faults in the fault classification Ⅳ-class which have influence on the preservation condition is mainly taken, and the value is related to the fault sealing parameter Z; when Z=1, the fault is completely sealed, and the effective distance of the preservation is D=0m; when Z=0, the fault is completely opened, and the effective distance of the preservation is D=2000m.

5. The method of claim 1, wherein the complexly structured shale preservation condition evaluation method is characterized by, According to the stratigraphic dip and the burial depth, different shale gas preservation condition grades are divided.

6. The method according to claim 5, wherein the complexly-structured shale reservoir condition evaluation method is characterized by, The shale gas preservation condition grade evaluation standard is as follows: The shale gas preservation condition grade evaluation standard 。

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