A method for evaluating different formation dip shale gas reservoirs for rate of transmissibility

Abstract

This invention relates to a method for evaluating the escape rate of shale gas reservoirs with different dip angles, employing the following technical solution: including the following steps: Step (1) Calculate the drilling angle θ of the plunger sample based on the shale bedding type, calculate the length L of the plunger sample based on the plunger diameter d, and drill the plunger sample based on the drilling angle θ and length L; Step (2) Test the minimum starting pressure P0 of the plunger sample using methane gas; Step (3) Calculate the fluid pressure P of the formation where the plunger sample is located. 流 hydrostatic pressure P 水 Minimum starting pressure P0 and static rock pressure P 岩 Calculate the test pressure Δ for permeability testing p and confining pressure P 围 Step (4) is performed according to the test pressure P determined in step (3). 测 and confining pressure P 围 The permeability of the plunger sample is tested to obtain the permeability k; step (5) calculate the gas escape rate R; step (6) evaluate the shale gas escape capacity of the layer where the sample is located based on the gas escape rate R. The larger the escape rate R is, the stronger the shale gas escape capacity is. Then calculate the shallowest burial depth H where the layer where the sample is located can be effectively preserved.

Description

Technical Field

[0001] This invention relates to the field of shale gas development and evaluation in petroleum geology engineering, and more specifically, to a method for evaluating the escape rate of shale gas reservoirs with different formation dip angles. Background Technology

[0002] Shale gas is a very important unconventional natural gas resource with enormous economic and social value. However, shale gas development also faces many technical challenges, one of which is how to evaluate the escape rate of shale gas, i.e., the rate at which shale gas escapes from the reservoir. The escape rate of shale gas is related to factors such as the reservoir dip angle, permeability, formation pressure, and shale bedding. Changes in these factors affect the preservation and development of shale gas. Therefore, methods for evaluating the escape rate of shale gas reservoirs at different formation dip angles are a significant technical issue. The formation dip angle determines the migration direction, storage space, and sealing conditions of shale gas, thus affecting its enrichment and distribution. Shallow shale gas (shale gas buried at depths below 2500 meters) is particularly affected by the formation dip angle, mainly in the following aspects:

[0003] (1) Influences on the migration direction and distance of shale gas. Generally speaking, the greater the dip angle of the formation, the more horizontal the migration direction of shale gas from the source rock to the reservoir, the longer the migration distance, and the greater the migration loss, thus the lower the shale gas preservation capacity. Conversely, the smaller the dip angle of the formation, the more vertical the migration direction of shale gas from the source rock to the reservoir, the shorter the migration distance, and the smaller the migration loss, thus the higher the shale gas preservation capacity.

[0004] (2) Impact on shale gas storage space and sealing conditions. Generally, the greater the formation dip angle, the smaller the shale gas storage space and the worse the sealing conditions, because an increased formation dip angle leads to unfavorable factors such as fracture development, pore compression, and water activity. These factors reduce the adsorption capacity and free gas content of shale gas, increasing the risk of leakage and loss. Conversely, the smaller the formation dip angle, the larger the shale gas storage space and the better the sealing conditions, because a smaller formation dip angle is conducive to fracture closure, pore preservation, and water stability. These factors improve the adsorption capacity and free gas content of shale gas, reducing the risk of leakage and loss.

[0005] (3) Influence on the distribution and enrichment patterns of shale gas. Generally speaking, the greater the dip angle of the strata, the more complex the distribution pattern of shale gas and the more diverse the enrichment patterns. This is because an increase in the dip angle of the strata leads to a stronger control of geological structures and fluid dynamics over shale gas accumulation. These factors result in differentiated enrichment of shale gas in different geological units, different structural locations, and different fluid systems. Conversely, the smaller the dip angle of the strata, the simpler the distribution pattern of shale gas and the more homogeneous the enrichment patterns. This is because a decrease in the dip angle of the strata leads to a weaker control of geological structures and fluid dynamics over shale gas accumulation. These factors result in a homogeneous distribution of shale gas throughout the basin or region.

[0006] In summary, formation dip angle has a significant and complex impact on shallow shale gas preservation. Current research in this area lacks a dedicated methodology; most studies rely on macroscopic descriptions such as tectonic movements to qualitatively analyze the influence of formation dip angle on shale gas preservation, or use single experimental methods to test samples at specific formation dip angles. Further research is needed to develop an accurate qualitative and quantitative method to evaluate the effect of formation dip angle on shale gas escape rates. Summary of the Invention

[0007] To overcome the problems in the prior art, this invention provides a method for evaluating the gas escape rate of shale gas reservoirs with different formation dip angles. This method can drill plunger samples based on the shale bedding type and formation dip angle, perform minimum start-up pressure and permeability tests on the plunger samples, calculate the gas escape rate, evaluate the shale gas escape capacity, and calculate the shallowest effective burial depth and the minimum and optimal well placement distances, thereby providing a scientific basis and guidance for shale gas development.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for evaluating the escape rate of shale gas reservoirs with different formation dip angles includes the following steps:

[0010] Step (1) Calculate the drilling angle θ of the plunger sample according to the bedding type of shale, calculate the length L of the plunger sample according to the plunger diameter d, and drill the plunger sample according to the drilling angle θ and the length L.

[0011] Step (2) Use methane gas to test the minimum starting pressure P0 of the plunger sample;

[0012] Step (3) Based on the fluid pressure P of the formation where the plunger sample is located 流 hydrostatic pressure P 水 Minimum starting pressure P0 and static rock pressure P 岩 Calculate the test pressure Δp and confining pressure P for permeability testing. 围 Use the following formula:

[0013] Δp=P 流 -P0-P 水

[0014] P 围 =P 岩

[0015] Step (4) The test pressure P determined in step (3) is used. 测 and confining pressure P 围 The permeability of the plunger sample was measured to obtain the permeability k.

[0016] Step (5) Calculate the gas escape rate R using the following formula:

[0017]

[0018] Where R is the gas escape rate, Q is the gas flow rate, A is the cross-sectional area of ​​the plunger sample, k is the permeability, ΔP is the test pressure, μ is the gas viscosity, L is the length of the plunger sample, and P... 流 For fluid pressure, P 水 P0 is the hydrostatic pressure and P1 is the minimum starting pressure.

[0019] Step (6) Evaluate the shale gas dispersion capacity of the layer where the sample is located based on the gas dispersion rate R. The larger the dispersion rate R, the stronger the shale gas dispersion capacity.

[0020] Step (7) Calculate the shallowest burial depth H that the sample can be effectively preserved in the stratigraphic layer based on the gas escape rate R obtained in step (6), using the following formula:

[0021]

[0022] Where ρ is the fluid density and g is the gravitational acceleration.

[0023] Step (8) Calculate the minimum distance D between the shale gas well and the formation outcrop point at this stratum based on the shallowest burial depth H in step (7). 小 Use the following formula:

[0024]

[0025] Where β is the dip angle of the strata.

[0026] Furthermore, in order to improve the efficiency of well site deployment, a reasonable well site spacing can be quickly determined based on the dissipation rate and development cycle in step (5) using the following formula, so that the well sites are neither too far apart nor too close together, thereby maximizing development benefits:

[0027] D 优 =R×t

[0028] Among them, D 优 The optimal well spacing is given by t, which represents the development cycle.

[0029] Preferably, the bedding types in step (1) include horizontal bedding, wavy bedding, and unidirectional bedding. When the bedding type is horizontal bedding or wavy bedding, the plunger sample drilling angle θ = β; when the bedding type is unidirectional bedding, the plunger sample drilling angle θ = α + β, where α is the angle between the unidirectional bedding fracture and the formation bedding plane.

[0030] Preferably, the length L of the plunger sample in step (1) should be greater than d×tan60° to avoid the influence of end effect.

[0031] Preferably, when the test pressure Δp calculated in step (3) is less than 0, it is evaluated that the layer where the sample is located has self-sealing properties and poor dissipation ability.

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] This invention, through the design of a series of testing procedures and steps, forms a user-friendly and highly practical evaluation method for the gas escape rate of shale gas reservoirs at different formation dip angles. This method can accurately test the seepage rate of different shale gas reservoirs under different formation dip angles and conditions, and can also calculate the shallowest burial depth and minimum distance from the outcrop, providing a certain reference for shale gas well location deployment. This invention also improves the sensitivity and accuracy of testing by introducing methane as a gas for seepage capacity experiments and permeability tests, while also considering the adsorption and desorption of methane in shale gas reservoirs, thus more closely reflecting the actual physical characteristics of shale gas reservoirs. Furthermore, this invention provides a quantitative indicator of the escape capacity of shale gas reservoirs at different formation dip angles by introducing a formula for calculating the gas escape rate, facilitating comparison and analysis. Finally, this invention provides a simple method for shale gas well site selection by introducing formulas for calculating the shallowest burial depth and minimum distance, allowing for rapid estimation of suitable well locations based on formation dip angle and fluid pressure, avoiding resource waste or development difficulties caused by wells that are too shallow or too far away. This invention also provides an effective tool for optimizing shale gas well locations by introducing a calculation formula for the optimal well location distance. It can quickly determine an optimal well location distance based on the dispersion rate and development cycle, so that the well location can make full use of the formation's dispersion capacity, save development costs, and achieve maximum development benefits. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the layering type of the present invention; Detailed Implementation

[0035] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0036] The purpose of this invention is to provide a method for evaluating the escape rate of shale gas reservoirs with different formation dip angles. To achieve the above objective, the technical solution of this invention includes the following steps: determining the drilling angle and length of the plunger sample, drilling the plunger according to the angle and length, minimum starting pressure test, determining the permeability test pressure and confining pressure, permeability test, escape rate, shallowest burial depth, minimum exposure distance, and calculation of optimal well spacing.

[0037] Step (1) Calculate the drilling angle θ of the plunger sample based on the bedding type of the shale. When the bedding type is horizontal bedding and wavy bedding, the drilling angle θ = β; when the bedding type is unidirectional bedding, the drilling angle θ = α + β, where α is the angle between the unidirectional bedding fracture and the formation bedding plane. Calculate the length L of the plunger sample based on the plunger diameter d. To avoid end effects, according to the requirements of conditional experiments and permeability testing, the plunger sample L ≥ d × tan60°. The experimental core sample was horizontally bedding. Samples with five different formation dip angles β (0°, 30°, 45°, 60°, and 90°) were simulated for drilling. The core plunger diameter d was 2.5 cm, and the length L was 5 cm.

[0038] Stratification types such as Figure 1 As shown, numerous experimental results indicate that the horizontal permeability of shale is much greater than its vertical permeability. The primary reason for this is the presence of numerous bedding fractures in shale. These fractures not only provide ample reservoir space but are also the largest contributor to shale permeability, serving as one of the main channels for shale oil and gas flow. Bedding can be classified according to its occurrence and origin into horizontal bedding, parallel bedding, grain-scale bedding, cross-bedding, wavy bedding, graded bedding, and massive bedding. Massive bedding, such as in mudstone, is an indistinct form of bedding. Shale primarily exhibits horizontal bedding, cross-bedding, and wavy bedding. Cross-bedding can be further divided into uniaxial cross-bedding and cross-bedding. Conceptually, horizontal bedding is parallel to the bedding plane, and wavy bedding is also generally parallel to the bedding plane. In these two types of bedding, the dip angle of the strata is equal to the dip angle of the bedding fracture. Cross-bedding is oblique to the separating plane between strata. In this case, the angle is recalculated. Since cross-bedding involves multiple cross-beddings in different directions, it is difficult to calculate uniformly and is relatively rare in actual strata. Therefore, this patent does not consider the occurrence of cross-bedding and only applies to horizontal bedding, wavy bedding, and unidirectional cross-bedding.

[0039] Step (2) Use methane gas to test the minimum starting pressure P0 of the plunger sample and record the test results of the minimum starting pressure P0 of the above 5 samples;

[0040] Drilling angle Minimum starting pressure gradient (MPa / m) <![CDATA[Minimum starting pressure P0 (Mpa)]]> 90° 0.0012 2.64 60° 0.0017 3.74 45° 0.0019 4.18 30° 0.0021 4.62 0° 0.0028 6.16

[0041] Note: Methane was used as the experimental gas to fully consider the adsorption and desorption of methane in shale gas reservoirs, which is closer to the characteristics of shale gas reservoirs under actual formation conditions.

[0042] Step (3) Calculate the fluid pressure P based on the formation information of the plunger sample. 流 hydrostatic pressure P 水 Harmonious rock pressure P 岩 The sample depth was 2200m, the formation pressure coefficient was 1.3, and the density of water was 1g / cm³. 3 Calculate hydrostatic pressure P 流 The fluid pressure is 22 MPa, P 流 The pressure is 28.6 MPa, based on the average rock density of 2.6 g / cm³. 3 Calculate static rock pressure P 岩 The pressure is 57.2 MPa. Then, calculate the test pressure Δp and confining pressure P for the permeability test. 围 Use the following formula:

[0043] Δp=P 流 -P0-P 水

[0044] P 围 =P 岩

[0045]

[0046] Note: To fully simulate the pressure conditions under formation conditions, fluid pressure P is used. 流 Subtract hydrostatic pressure P 水 The pressure difference Δp between the minimum starting pressure P0 and the minimum starting pressure P0 is the pressure at which methane gas flows underground. 岩 The confining pressure is for testing.

[0047] Step (4) The test pressure P determined in step (3) is used. 测 and confining pressure P 围 The permeability of the plunger sample was measured to obtain the permeability k.

[0048] Drilling angle Permeability k(md) 90° 0.0003111 60° 0.000127 45° 0.0000859 30° 0.0000367 0° 0.0000133

[0049] Step (5) Calculate the gas escape rate R using the following formula:

[0050]

[0051] Where R is the gas escape rate, Q is the gas flow rate, A is the cross-sectional area of ​​the plunger sample, k is the permeability, ΔP is the test pressure, μ is the gas viscosity, L is the length of the plunger sample, and P... 流 For fluid pressure, P 水 P0 is the hydrostatic pressure and P1 is the minimum starting pressure.

[0052] Note: Based on a sample depth of 2200m, a surface temperature of 20℃, and a geothermal gradient of 3℃ / 100m, the formation temperature is calculated to be 86℃, and the fluid pressure P... 流 The pressure is 28.6 MPa, and the viscosity of methane under these temperature and pressure conditions is 12.49 μPa·s.

[0053] Step (6) Evaluate the shale gas dissipation capacity of the layer where the sample is located based on the gas dissipation rate R. The larger the dissipation rate R, the stronger the shale gas dissipation capacity.

[0054] Step (7) Calculate the shallowest burial depth H that the sample can be effectively preserved in the stratigraphic layer based on the gas escape rate R obtained in step (6), using the following formula:

[0055]

[0056] Where ρ is the fluid density and g is the gravitational acceleration.

[0057] Step (8) Calculate the minimum distance D between the shale gas well and the formation outcrop point at this stratum based on the shallowest burial depth H in step (7). 小 Use the following formula:

[0058]

[0059] Where β is the dip angle of the strata.

[0060]

[0061]

[0062] Note: When the formation dip angle β is 90°, under the formation conditions in this case, shale gas cannot be effectively preserved, and shale gas wells will not be deployed under these conditions; therefore, the minimum distance D is not calculated. When the formation dip angle β is 0°, under the formation conditions in this case, the formation will not emerge at the surface; therefore, there is no minimum distance D between the shale gas well and the formation outcrop. 小 .

[0063] To improve the efficiency of well site deployment, a reasonable well site spacing can be quickly determined based on the escaping rate calculated in step (5) and the development cycle using the following formula, ensuring that the well sites are neither too far apart nor too close together, thereby maximizing development benefits:

[0064] D 优 =R×t

[0065] Among them, D 优 The optimal well spacing is given by t, which represents the development cycle.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any brief modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention are permitted.

Claims

1. A method for evaluating the escape rate of shale gas reservoirs with different formation dip angles, characterized in that, Includes the following steps: Step (1): Calculate the drilling angle θ of the plunger sample according to the bedding type of shale, calculate the length L of the plunger sample according to the plunger diameter d, and drill the plunger sample according to the drilling angle θ and the length L. Step (2): Use methane gas to test the minimum starting pressure P0 of the plunger sample; Step (3), based on the fluid pressure P of the formation where the plunger sample is located... 流 hydrostatic pressure P 水 Minimum starting pressure P0 and static rock pressure P 岩 Calculate the test pressure Δp and confining pressure P for permeability testing. 围 Use the following formula: Δp=P 流 -P0-P 水 P 围 =P 岩 Step (4), according to the test pressure P determined in step (3) 测 and confining pressure P 围 The permeability of the plunger sample was measured to obtain the permeability k. Step (5), calculate the gas escape rate R using the following formula: Where R is the gas escape rate, Q is the gas flow rate, A is the cross-sectional area of ​​the plunger sample, K is the permeability, ΔP is the test pressure, μ is the gas viscosity, L is the length of the plunger sample, and P... 流 For fluid pressure, P 水 P0 is the hydrostatic pressure, and P1 is the minimum starting pressure. Step (6): Evaluate the shale gas dissipation capacity of the layer where the sample is located based on the gas dissipation rate R. The larger the dissipation rate R, the stronger the shale gas dissipation capacity. Step (7): Calculate the shallowest burial depth H that the sample can be effectively preserved in based on the gas escape rate R obtained in step (6), using the following formula: ； Where ρ is the fluid density and g is the gravitational acceleration; Step (8) Calculate the minimum distance D between the shale gas well and the formation outcrop point at this stratum, based on the shallowest burial depth H obtained in step (7). 小 Use the following formula: Where β is the dip angle of the strata.

2. The method according to claim 1, characterized in that, To improve the efficiency of well site deployment, a reasonable well site spacing can be quickly determined based on the dissipation rate R and development cycle t in step (5) using the following formula, thereby maximizing development benefits: D 优 =R×t Among them, D 优 The optimal well spacing is given by t, which represents the development cycle.

3. The method according to claim 1, characterized in that, The bedding types in step (1) include horizontal bedding, wavy bedding and unidirectional bedding; when the bedding type is horizontal bedding and wavy bedding, the plunger sample drilling angle θ = β; when the bedding type is unidirectional bedding, the plunger sample drilling angle θ = α + β, where α is the angle between the unidirectional bedding fracture and the formation bedding plane.

4. The method according to claim 1, characterized in that: The length L of the plunger sample in step (1) should be greater than d×tan60° to avoid the influence of end effect.

5. The method according to claim 1, characterized in that: When the test pressure Δp calculated in step (3) is less than 0, it is evaluated that the layer where the sample is located has self-sealing properties and poor dissipation ability.

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