Comprehensive evaluation method for complexity of hydraulic fracture of unconventional oil and gas reservoir

Abstract

The invention provides an unconventional oil and gas reservoir hydraulic fracture complexity comprehensive evaluation method, electronic equipment, a storage medium and a program product, and relates to the field of oil and gas engineering. The method comprises the following steps: acquiring a pressure drop and pressure drop derivative double logarithmic curve based on pumping stop pressure drop data of a fractured well section, and calculating a near-wellbore crack complex index; drawing a G function derivative curve, reading a G function derivative value when the crack is stably closed, and calculating the complex index of the connected crack through the areas of the main crack and the secondary crack of the connected crack system; determining the length and the width of the fracture reformation area seam net according to the spatial distribution of the microseismic event points, and calculating the complex index of the fracture reformation area seam net; and according to the weights of the near-wellbore fracture complex index, the connected fracture complex index and the fracture network complex index of the fracturing transformation area, the comprehensive complex index of the fracture is determined. According to the method, the complexity of the hydraulic fracture is quantitatively evaluated from three dimensions in a comprehensive grading manner, and the method is more comprehensive and reliable compared with a single evaluation method for evaluating the complexity of the fracture.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of oil and gas engineering, and particularly relates to a method for comprehensively evaluating the complexity of hydraulic fractures in unconventional oil and gas reservoirs. BACKGROUND

[0002] Coalbed methane, shale gas, tight oil and gas and other unconventional oil and gas resources in China are stored in low porosity and low permeability reservoirs. Hydraulic fractures need to be formed in the reservoirs as oil and gas seepage channels through hydraulic fracturing, so that these oil and gas resources can be economically and effectively exploited. The more complex the shape of the hydraulic fractures is, the greater the contact area between the fractures and the reservoirs is, and the more oil and gas is beneficial to enter the hydraulic fractures and flow into the wellbore along the fractures, so that high and stable production is achieved. The complexity of the fractures directly reflects the effect of hydraulic fracturing.

[0003] At present, the evaluation methods of the complexity of the fractures mainly include: (1) microseismic monitoring and wide-area electromagnetic method are used to evaluate the swept range of single-stage fractures; (2) the complexity of the fractures is judged according to the fluctuation of the construction pressure curve or the water hammer effect; (3) the fracture network shape is inversely analyzed through well testing; and (4) the shape of the fracture expansion is obtained by fitting the construction curve through the fracture expansion numerical model.

[0004] The above methods have certain defects: the microseismic monitoring explains the distribution of the fractures through the monitored microseismic event points, which do not completely represent the connected hydraulic fractures, and the explained fracture range is too large; the wide-area electromagnetic method can obtain the shape characteristics of the fracture distribution, but cannot quantitatively describe the complexity of the fractures; the fluctuation of the construction pressure curve can roughly qualitatively judge the complexity of the fractures, but lacks an index for quantitatively describing the complexity of the fractures; the complexity of the fractures can be described through the decay rate of the water hammer effect, but it is an indirect explanation and has certain uncertainty due to the influence of complex geological features and the corresponding pressure response; the well testing analysis uses the production stage data, and can only explain the average fracture parameters of the whole well, and cannot evaluate the complexity of the single-stage hydraulic fractures; the accuracy of the simulation of the fracture expansion by fitting the construction curve depends on the accuracy of the three-dimensional geomechanical model, and the existing geological model still has certain uncertainty in the description of the natural fractures, resulting in the difference between the described shape of the fractures and the real shape. At present, there is still a lack of a method for comprehensively and quantitatively evaluating the complexity of the hydraulic fractures from multiple angles. SUMMARY

[0005] In order to solve the defects of the evaluation methods of the complexity of the hydraulic fractures in the prior art, the present application provides a method for comprehensively evaluating the complexity of the hydraulic fractures in unconventional oil and gas reservoirs, which comprises the following steps: S1: obtaining the pressure drop and the pressure drop derivative double logarithmic curve based on the pump-off pressure drop data of the fracturing well section, and calculating the near-wellbore fracture complexity index; S2: Plot the derivative curve of the G function, read the derivative value of the G function when the crack is stably closed, and calculate the complex index of the connected crack by the area of ​​the main crack and secondary crack in the connected crack system. S3: Determine the length and width of the fracture network in the fracturing area based on the spatial distribution of microseismic event points, and calculate the fracture network complexity index in the fracturing area. S4: Determine the comprehensive fracture complexity index based on the weights of the near-wellbore fracture complexity index, the interconnected fracture complexity index, and the fracture network complexity index in the fracturing stimulation zone.

[0006] Furthermore, the method for calculating the near-wellbore fracture complexity index in step S1 is as follows: (2) In the formula: The near-wellbore fracture complexity index is dimensionless. The pressure drop caused by the complex and tortuous morphology of the fractures near the wellbore, in MPa; The total pressure drop when the pump is stopped is measured in MPa.

[0007] Furthermore, in step S2, when the slope of the pressure drop derivative on the double logarithmic curve is 1, it indicates that the crack has begun to close stably.

[0008] Further, in step S2, the areas of the primary and secondary fractures in the connected fracture system are determined using the following method: When the main crack and secondary crack begin to close simultaneously under the action of closure pressure after the pump stops, the internal pressure equation of the crack is constructed, and the G-function is differentiated to obtain the first equation. The second equation is obtained based on the instantaneous mass balance equation at the moment of pump shutdown; By combining the first and second equations, the areas of the primary and secondary cracks can be solved.

[0009] Furthermore, the method for calculating the complexity index of connected fractures is as follows: (9) In the formula: The complexity index of the connected cracks is dimensionless; The area of ​​the main crack in the connected crack system; The area of ​​secondary cracks in the connected crack system.

[0010] Furthermore, the calculation method for the fracture network complexity index in the fracturing zone in step S3 is as follows: (10) In the formula: The fracture network complexity index of the fracturing zone is dimensionless. The width of the fracture network in the fracturing zone is in meters (m). The length of the fracture network in the fracturing zone is in meters (m).

[0011] Furthermore, the method for calculating the comprehensive complexity index of cracks in step S4 is as follows: (11) In the formula: The comprehensive complexity index of cracks; , , These are the weighting coefficients for the near-wellbore fracture complexity index, the interconnected fracture complexity index, and the fracture network complexity index in the fracturing stimulation zone, respectively.

[0012] The fracture complexity evaluation method, electronic device, storage medium, and program product provided in this application comprehensively evaluate the complexity of hydraulic fractures from three dimensions: near-wellbore fracture complexity, interconnected fracture complexity, and fracture network complexity in the fracturing stimulation zone. First, a near-wellbore fracture complexity index is obtained by using the proportion of pressure drop caused by near-wellbore fracture complexity to the total pressure drop during pump shutdown and the double logarithmic curve of pressure drop derivative. This index is used to determine the complexity of hydraulic fractures in the near-wellbore zone. Second, the surface areas of primary and secondary fractures are obtained by inverting the pump shutdown pressure drop data and the G-function derivative curve. The sum of these two areas represents the total surface area of ​​the interconnected fracture system. The interconnected fracture complexity index is obtained by using the proportion of the secondary fracture surface area to the total surface area of ​​the interconnected fracture system. This index is used to determine the complexity of the interconnected fractures. Third, the fracture network complexity index is obtained by using the ratio of fracture network width to length within the stimulation zone as interpreted by microseismic data. This index is used to determine the complexity of the fracture network in the stimulation zone. Finally, the weighted sum of the three complexity indices is defined as the comprehensive fracture complexity index, which is used to quantitatively evaluate the fracture complexity in a graded manner.

[0013] This method comprehensively and quantitatively evaluates the complexity of hydraulic fractures after fracturing from three dimensions, providing a more comprehensive and reliable assessment compared to existing single-evaluation methods. Existing single-evaluation methods offer a general assessment of fracture complexity and cannot guide the optimization of fracturing operation parameters based on the complexity of different fracture locations. This method can separately evaluate the complexity of near-wellbore fractures, the complexity of the interconnected fracture network, and the complexity of the fracture network in the stimulated zone (including unconnected discrete fractures, which also contribute to production). If the near-wellbore complexity is low, more natural near-wellbore fractures can be opened by stress disturbance and fracturing fluid viscosity adjustment, thereby increasing near-wellbore complexity. If the complexity of the interconnected fracture network is low, in-fracture temporary plugging or volumetric fracture-enhancing techniques can be used to increase the complexity of the interconnected fracture network. These measures collectively improve the overall efficiency of hydraulic fracturing stimulation. Attached Figure Description

[0014] Figure 1 A flowchart illustrating the method of this invention; Figure 2 The double logarithmic curves of pump stop pressure drop and pressure drop derivative provided in the embodiments of the present invention; Figure 3 The G function and its derivative curve provided for embodiments of the present invention; Figure 4 This is a schematic diagram of the main crack and secondary cracks in the interconnected crack system in an embodiment of the present invention; Figure 5 The microseismic monitoring data and the geometric dimensions of the modified area defined therefrom in this embodiment of the invention are shown. Figure 6 This is a comparison chart of the overall crack complexity index and the amount of liquid used per segment in an embodiment of the present invention. Detailed Implementation

[0015] The present invention will be further described below with reference to the accompanying drawings and embodiments. As shown in Figure 1, the present invention provides a method for evaluating the complexity of hydraulic fractures formed after fracturing in unconventional oil and gas reservoirs, comprising the following steps: S1: Obtain double logarithmic curves of pressure drop and pressure drop derivative based on the pump shutdown pressure drop data of the fractured well section, and calculate the near-wellbore fracture complexity index; The initial time of pump shutdown is defined as the time when the pump discharge begins to decrease during the pump shutdown phase, and the wellhead pressure at this time is defined as the initial pressure. Record the pressure data every second after the pump stops. The pressure drop at each time point was obtained. (1) In the formula: The pressure drop when the pump is stopped is measured in MPa. The pressure at the initial moment of pump shutdown, in MPa; The pressure is given in MPa at a certain moment after the pump stops. Calculate. Regarding time derivative Plot a double logarithmic curve of pump shutdown pressure drop versus pressure drop derivative, as follows: Figure 2 As shown. Figure 2 In the middle, the value when the pump stop pressure drop is stable is the total pump stop pressure drop. MPa. In the initial stage, the pressure drop and pressure drop derivative almost coincide. The pressure drop that occurs when the two are clearly separated is caused by the wellbore reservoir effect and the pressure drop due to the perforation orifice. MPa. The subsequent pressure drop is caused by the complex and tortuous morphology of the fractures near the wellbore. MPa. When the pressure drop derivative shows a segment with a slope of 1 on the double logarithmic curve, it indicates that the crack has begun to close stably.

[0016] The near-wellbore fracture complexity index is determined by the ratio of the pressure drop caused by complex near-wellbore fractures to the total pressure drop during pump shutdown. : (2) In the formula: The near-wellbore fracture complexity index is dimensionless. The pressure drop, measured in MPa, is caused by the complex and tortuous morphology of the fractures near the wellbore.

[0017] S2: Plot the derivative curve of the G function, read the derivative value of the G function when the crack is stably closed, and calculate the complex index of the connected crack by the area of ​​the main crack and secondary crack in the connected crack system. Calculate pump stop time : (3) In the formula: Pump shutdown time (in minutes); The time from the start of pumping to the present, in minutes; This is the total pumping time before stopping the pump, in minutes.

[0018] Define dimensionless time as Calculating the G function based on dimensionless time: (4) (5) In the formula: for The value; through and the corresponding time point calculate right derivative Plot the derivative curve of the G function, such as... Figure 3 As shown.

[0019] Read the derivative value of the G function when the crack closes stably ; Record the time when the slope of the double logarithmic curve of the pump stop pressure drop derivative appears with a value of 1. Calculate the time corresponding to Value, in Reading from the curve corresponding value.

[0020] like Figure 4 As shown, the interconnected fracture system formed by fracturing consists of primary fractures and secondary fractures. After pump shutdown, the primary and secondary fractures begin to close simultaneously under the closure pressure. At this time, the internal pressure equation of the fracture is: (6) In the formula: The pressure inside the crack is expressed in MPa. The closing pressure of the secondary crack is MPa; The filtration coefficient at the crack surface is m / min. 0.5 ; The flexibility of the secondary crack is expressed in m / MPa. The compliance of the main crack, in m / MPa; The fracturing fluid loss surface area on one side of the main fracture cluster in a horizontal well section is given in m. 2 ; Let m be the fracturing fluid filtration surface area on one side of a cluster of secondary fractures. 2 ; This represents the dimensionless closure time of the secondary fracture. Secondary fractures are primarily used to filter out and prevent the accumulation of fracturing fluid, with a compliance value of 0.

[0021] Differentiating both sides of equation (6) with respect to the G function, we get (7) In the formula: For the voltage drop derivative With time The moment when the double logarithmic curve begins to show a slope of 1 corresponds to value.

[0022] According to the instantaneous mass balance equation upon pump shutdown, the total volume of injected fracturing fluid is equal to the sum of the volume of fracturing fluid stored in the main fracture and the total volume of fracturing fluid lost through filtration. (8) In the formula: The instantaneous pump stop pressure is in MPa. The closing pressure of the main fracture, in MPa; The total volume of fracturing fluid injected into a certain fracturing section, in m 3 ; This represents the number of perforation clusters in this well section.

[0023] For each segment of a fractured well, only equations (7) and (8) contain the following: and Since the number of unknowns is unknown, we can solve equations (7) and (8) simultaneously to obtain the average surface area of ​​the single-sided main crack in a single cluster. and the average surface area of ​​single-cluster unilateral secondary cracks Furthermore, the area of ​​the main crack and secondary cracks in multiple clusters of cracks can be obtained through the superposition method.

[0024] Based on the obtained surface areas of the primary and secondary cracks, the complexity index of the connected cracks is calculated: (9) In the formula: The complexity index of the connected cracks is dimensionless.

[0025] S3: Determine the length and width of the fracture network in the fracturing area based on the spatial distribution of microseismic event points, and calculate the fracture network complexity index in the fracturing area. like Figure 5 As shown, the fracturing stimulation zone is divided based on the distribution of microseismic events, the length and width of the fracture network are determined, and the fracture network complexity index of the fracturing stimulation zone is calculated. (10) In the formula: The fracture network complexity index of the fracturing zone is dimensionless. The width of the fracture network in the fracturing zone is in meters (m). The length of the fracture network in the fracturing zone is in meters (m).

[0026] S4: Determine the comprehensive fracture complexity index based on the weights of the near-wellbore fracture complexity index, the interconnected fracture complexity index, and the fracture network complexity index in the fracturing stimulation zone; (11) In the formula: The comprehensive complexity index of cracks; , , These are the weighting coefficients for the three complexity indices. Those skilled in the art can weight these indices according to the importance of fracture complexity at different locations to the production effectiveness of fracturing wells, systematically and comprehensively evaluating the fracture complexity index. Under normal circumstances, one can choose... , , .

[0027] The following uses relevant data from a horizontal well in deep shale gas as an example to evaluate the complexity of hydraulic fractures using the present invention: (1) Based on the pressure drop data of the pump stoppage in the fractured section, the pressure drop and pressure drop derivative double logarithmic curves are plotted, and the near-wellbore fracture complexity index of each section is calculated as shown in Table 1.

[0028] Table 1. Complexity index of near-wellbore fractures in different well sections

[0029] (2) The complexity index of the interconnected fractures in each well section is further calculated based on the pump stop pressure drop data of the fracturing section and the surface area of ​​the primary and secondary fractures obtained by inversion of the derivative curve of the G function, as shown in Table 2.

[0030] Table 2 Complexity index of interconnected fractures in different well sections

[0031] (3) The fracture network complexity index of each well section is calculated based on the distribution of microseismic event points in the fractured well section. Table 3 shows the fracture network complexity index of each well section.

[0032] Table 3. Complexity Index of Screw Network in Different Well Sections of the Stimulated Area

[0033] (4) By weighted summation of the above three complexity indices and based on the fracture complexity judgment criteria in Table 4, the comprehensive fracture complexity index and complexity of each well section are shown in Table 5.

[0034] Table 4 Criteria for Judging Crack Complexity

[0035] Table 5. Comprehensive Fracturing Complexity Index and Complexity Level in Different Well Sections

[0036] For deep shale, the evaluation criteria for fracture complexity are shown in Table 5. It can be seen that among the 26 hydraulic fractures in this well, 9 are simple fractures, 7 are moderately complex fractures, and 10 are moderately complex fractures.

[0037] In hydraulic fracturing, generally, the larger the volume of fracturing fluid used, the more connected natural fractures there are, and the more complex the fracture morphology becomes. Figure 6 The relationship between the fracture complexity index calculated using the method provided by this invention and the fluid consumption of a single fracturing section in the well can be seen. It can be seen that the trend of the comprehensive complexity index along the horizontal wellbore is consistent with the trend of the fluid consumption of each section, which also confirms the reliability and practicality of the method provided by this invention.

[0038] Therefore, this invention provides a comprehensive evaluation method for the complexity of hydraulic fractures in unconventional oil and gas reservoirs. Compared with existing methods, this method comprehensively and quantitatively evaluates the complexity of hydraulic fractures through three dimensions, which is more comprehensive and reliable than a single evaluation method.

[0039] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A comprehensive evaluation method for the complexity of hydraulic fractures in unconventional oil and gas reservoirs, characterized in that, Includes the following steps: S1: Obtain double logarithmic curves of pressure drop and pressure drop derivative based on the pump shutdown pressure drop data of the fractured well section, and calculate the near-wellbore fracture complexity index; S2: Plot the derivative curve of the G function, read the derivative value of the G function when the crack is stably closed, and calculate the complex index of the connected crack by the area of ​​the main crack and secondary crack in the connected crack system. S3: Determine the length and width of the fracture network in the fracturing area based on the spatial distribution of microseismic event points, and calculate the fracture network complexity index in the fracturing area. S4: Determine the comprehensive fracture complexity index based on the weights of the near-wellbore fracture complexity index, the interconnected fracture complexity index, and the fracture network complexity index in the fracturing stimulation zone.

2. The comprehensive evaluation method for crack complexity according to claim 1, characterized in that, The method for calculating the near-wellbore fracture complexity index in step S1 is as follows: （2） In the formula: The near-wellbore fracture complexity index is dimensionless. The pressure drop caused by the complex and tortuous morphology of the fractures near the wellbore, in MPa; The total pressure drop when the pump is stopped is measured in MPa.

3. The comprehensive evaluation method for crack complexity according to claim 1, characterized in that, In step S2, when the slope of the pressure drop derivative on the double logarithmic curve is 1, it indicates that the crack has begun to close stably.

4. The comprehensive evaluation method for crack complexity according to claim 1, characterized in that, In step S2, the areas of the primary and secondary fractures in the connected fracture system are determined using the following method: When the main crack and secondary crack begin to close simultaneously under the action of closure pressure after the pump stops, the internal pressure equation of the crack is constructed, and the G-function is differentiated to obtain the first equation. The second equation is obtained based on the instantaneous mass balance equation at the moment of pump shutdown; By combining the first and second equations, the areas of the primary and secondary cracks can be solved.

5. The comprehensive evaluation method for crack complexity according to claim 1, characterized in that, The method for calculating the complexity index of connected cracks is as follows: （9） In the formula: The complexity index of the connected cracks is dimensionless; The area of ​​the main crack in the connected crack system; The area of ​​the secondary cracks in the connected crack system.

6. The comprehensive evaluation method for crack complexity according to claim 1, characterized in that, The calculation method for the fracture network complexity index in the fracturing zone in step S3 is as follows: （10） In the formula: The fracture network complexity index of the fracturing zone is dimensionless. The width of the fracture network in the fracturing zone is in meters (m). The length of the fracture network in the fracturing zone is in meters (m).

7. The comprehensive evaluation method for crack complexity according to claim 1, characterized in that, The method for calculating the comprehensive complexity index of cracks in step S4 is as follows: （11） In the formula: The comprehensive complexity index of cracks; , , These are the weighting coefficients for the near-wellbore fracture complexity index, the interconnected fracture complexity index, and the fracture network complexity index in the fracturing stimulation zone, respectively.

8. An electronic device, characterized in that, include: Memory, processor; The memory stores computer-executed instructions; The processor executes computer execution instructions stored in the memory, causing the processor to perform the method as described in any one of claims 1-7.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer-executable instructions, which, when executed by a processor, are used to implement the method as described in any one of claims 1-7.

10. A computer program product, characterized in that, Includes a computer program that, when executed by a processor, implements the method described in any one of claims 1-7.

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