Calculation method of effective fracture half-length in multi-stage fracturing of horizontal wells in shale reservoirs

Abstract

The present invention belongs to the technical field of oil reservoir development, and relates to a method for calculating the half-length of effective cracks in multi-stage fracturing of horizontal wells in shale reservoirs. The method comprises: step 1, collecting daily data of the mine; step 2, calculating the bottom hole flowing pressure of the horizontal well; step 3, calculating the material balance time; step 4, drawing a relationship curve between normalized pressure and material balance time; step 5, calculating the comprehensive crack half-length of the multi-stage fracturing cracks in the horizontal well; step 6, calculating the production capacity contribution rate of different fracturing stages by using tracer monitoring; step 7, calculating the half-length of effective cracks in the multi-stage fracturing of the horizontal well. The method of the present invention has clear technical ideas, is closely combined with production practice, has clear and reasonable operating steps, and the obtained half-length of the cracks in the multi-stage fracturing of the horizontal well has high accuracy and precision, realizes the quantitative evaluation of the half-length of the cracks in the multi-stage fracturing of the horizontal well, and makes up for the shortcomings of the existing technology.

Description

Technical Field

[0001] The invention belongs to the technical field of oil reservoir development and relates to a method for calculating the effective fracture half-length of multi-stage fracturing in horizontal wells of shale reservoirs. Background Art

[0002] Over the past decade, basic research and field trials in unconventional oil and gas have effectively driven the development of China's unconventional oil and gas industry. By effectively drawing on techniques such as slickwater fracturing and hybrid fracturing used in North American unconventional oil and gas production, volumetric fracturing technology has evolved beyond the traditional single tensile fracture pattern to a complex network of fracture initiation and propagation. Failure manifests not as a single opening failure, but rather as shear failure, faulting, and slippage.

[0003] Microseismic technology is usually used to detect the distribution range and morphology of fractures after multi-stage fracturing in horizontal wells. However, microseismic events during hydraulic fracturing are not equivalent to actual opened fractures. For shale reservoirs with developed lamellae and natural fractures, these weak surfaces are easily opened during hydraulic fracturing, and the energy of the resulting microseismic events is relatively small, which can easily lead to omissions in monitoring and positioning. On the other hand, microseismic events formed at the far end of the fracturing point may be non-opened or isolated invalid fractures, which may lead to misjudgment of the scale and distribution of hydraulic fracturing. Therefore, the evaluation of hydraulic fracturing effects is of great significance to the fine development of oil and gas fields. It is necessary to improve the reliability and accuracy of fracturing effect evaluation, and then guide the integrated geological engineering design of multi-stage fracturing in horizontal wells. Summary of the Invention

[0004] The main purpose of the present invention is to provide a method for calculating the effective fracture half-length of multi-stage fracturing in horizontal wells of shale reservoirs. The method of the present invention provides a quantitative evaluation of the fracture half-length of multi-stage fracturing in horizontal wells, thereby providing a basis for the evaluation and analysis of the effect of multi-stage fracturing in horizontal wells, overcoming the above-mentioned difficulties.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention provides a method for calculating the effective fracture half-length of multi-stage fracturing in horizontal wells of shale reservoirs, the method comprising:

[0007] Step 1: Collect daily mining data;

[0008] Step 2, calculating the bottom hole flowing pressure of the horizontal well;

[0009] Step 3, calculate material balance time;

[0010] Step 4, draw a curve of the relationship between normalized pressure and material equilibrium time;

[0011] Step 5, calculating the comprehensive fracture half-length of the multi-stage hydraulic fractures in the horizontal well;

[0012] Step 6, using tracer monitoring to calculate the productivity contribution of different fracturing stages;

[0013] Step 7: Calculate the effective fracture half-length of multi-stage fracturing in the horizontal well.

[0014] To achieve the above object, the present invention may also adopt the following technical solutions:

[0015] In step 1, the daily mine data includes oil production, water production, gas production, water content and wellhead oil pressure.

[0016] In step 2, the formula for calculating the bottom hole oil pressure of the horizontal well is:

[0017] p wf =p t +ρgh

[0018] ρ=(1-f w )×ρ o +f w ×ρ w

[0019] Where p wf is the bottom hole pressure, p t is the wellhead oil pressure, ρ is the density of the mixed fluid in the wellbore, ρ o is the density of crude oil on the ground, ρ o is the ground water density, fw is the water content, g is the acceleration of gravity, and h is the vertical depth of the horizontal section.

[0020] In step 3, the material balance time is calculated as:

[0021]

[0022] Where te is the material balance time, Q is the cumulative oil production at the current time, and q is the current production.

[0023] In step 4, calculate and draw the normalized pressure formula:

[0024]

[0025] Where, is the normalized pressure at time t, p i is the original formation pressure, p wf is the bottom hole flowing pressure, and q(t) is the daily oil production at time t.

[0026] In step 4, a curve of the relationship between normalized pressure and the square root of material balance time is plotted in the Cartesian coordinate system. When the slope of the curve is constant, it means that the shale oil is in the linear flow stage of the formation. The slope m of the straight line segment is obtained by fitting:

[0027]

[0028] Where m is the slope of the line segment and b is the intercept.

[0029] In step 5, the formula for calculating the comprehensive fracture half-length of multi-stage hydraulic fractures in horizontal wells is:

[0030]

[0031] Where k is the matrix permeability, μ is the crude oil viscosity, B is the crude oil volume compressibility, ψ is the matrix porosity, and c t is the comprehensive compression coefficient of the formation, and h is the formation thickness.

[0032] In step 6, the tracer monitoring results are used to calculate the productivity contribution of different fracturing stages:

[0033]

[0034] Where q i is the production of the fracturing stage, i=1,2,....,N, q is the total production of the horizontal well, η i is the capacity contribution rate of the i-th segment (i=1,2,....,N).

[0035] The effective number of fracturing stages n needs to meet the following conditions:

[0036] η i >c

[0037] In the formula, c is a constant to be determined.

[0038] In step 7, the formula for calculating the effective fracture half-length of multi-stage fracturing in horizontal wells is:

[0039]

[0040] Where x eff is the effective fracture half-length of multi-stage fracturing in horizontal wells, x f is the comprehensive fracture half-length of multi-stage fracturing in horizontal wells, and n is the number of effective fracturing stages.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] The method for calculating the effective fracture half-length of multi-stage fracturing in horizontal wells in shale reservoirs, described in this paper, has a clear technical concept, is closely integrated with production practices, and features clear and logical steps. The resulting fracture half-lengths for multi-stage fracturing in horizontal wells are highly accurate and precise, enabling quantitative evaluation of fracture half-lengths for multi-stage fracturing in horizontal wells, thus addressing the shortcomings of existing technologies. This method provides a basis for evaluating and analyzing the effectiveness of multi-stage fracturing in horizontal wells, thereby laying the foundation for integrated geology and engineering design for shale oil production. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0044] Figure 1 This is a flow chart of a method for calculating the effective fracture half-length of multi-stage fracturing in a shale reservoir horizontal well according to a specific embodiment of the present invention;

[0045] Figure 2 A graph showing the relationship between bottom hole flowing pressure and production time of a horizontal well according to a specific embodiment of the present invention;

[0046] Figure 3 This is a graph showing the relationship between normalized pressure and production time according to a specific embodiment of the present invention;

[0047] Figure 4 A graph showing the relationship between normalized pressure and the square root of material balance according to a specific embodiment of the present invention;

[0048] Figure 5 It is the productivity contribution rate of different fracturing stages in the multi-stage fracturing of a horizontal well according to a specific embodiment of the present invention. DETAILED DESCRIPTION

[0049] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0050] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations and / or combinations thereof.

[0051] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0052] like Figure 1 As shown, Figure 1 This is a flowchart of a specific embodiment of the method for calculating the effective fracture half-length of multi-stage fracturing in a shale reservoir horizontal well of the present invention. In a specific example of the application of the present invention, multi-stage fracturing was applied to a domestic shale oil horizontal well to calculate the effective fracture half-length after fracturing. The method includes the following steps:

[0053] In step 101, daily mine data is collected, including oil production, water production, gas production, water content, and wellhead oil pressure.

[0054] In step 102, the bottom hole flowing pressure of the horizontal well is calculated by converting the wellhead oil pressure into the bottom hole flowing pressure.

[0055] p wf =p t +ρgh

[0056] ρ=(1-f w )×ρ o +f w ×ρ w

[0057] Where p wf is the bottom hole pressure, p t is the wellhead oil pressure, ρ is the density of the mixed fluid in the wellbore, ρ o is the density of crude oil on the ground, ρ o is the ground water density, fw is the water content, g is the acceleration of gravity, and h is the vertical depth of the horizontal section.

[0058] In this example, we take the calculation of the bottom hole pressure on the 18th day as an example, the water content is 30%, and the crude oil density is 0.85g / cm 3 , the density of water is 0.9g / cm 3 Calculate the density of the mixed liquid in the wellbore to be 0.865 g / cm 3 The vertical depth h of the horizontal well section is 3495m, the wellhead oil pressure is 18.67MPa, and the bottom hole pressure on the 18th day is calculated to be 48.3MPa. Similarly, the bottom hole pressure under different production times is calculated ( Figure 2 ).

[0059] In step 103, the material balance time is calculated using the formula:

[0060]

[0061] Where, t e is the material balance time, Q is the cumulative oil production at the current time, and q is the current production.

[0062] Taking the data of the 18th day as an example, the cumulative oil production on the 18th day is 263m3 and the daily oil production is 127.2m 3 / day, the material balance time on the 18th day is calculated to be 1.44 days.

[0063] In step 104, the normalized pressure is calculated and plotted using the formula:

[0064]

[0065] Where, is the normalized pressure at time t, p i is the original formation pressure, p wf is the bottom hole flowing pressure, and q(t) is the daily oil production at time t.

[0066] The original formation pressure is 56.4 MPa. Calculate the production pressure difference at different times, i.e., p i -p wf The ratio of the production pressure difference to the daily oil production at a certain moment is the normalized pressure at that moment. Taking the 18th day of production as an example, the bottom hole pressure is 48.26Mpa, the production pressure difference is 8.14MPa, and the daily oil production is 127.2m 3 / day, so 64.1Mpa / m 3 / day. According to this method, the normalized pressure at different times since the start of production is calculated ( Figure 3 ).

[0067] The relationship curve between normalized pressure and the square root of material balance time is drawn in the Cartesian coordinate system. The material balance time ranges from 0 to 299 days, and the curve shows a straight line relationship. The relationship formula of the straight line segment is fitted as follows:

[0068] The correlation coefficient R2 is 0.9899

[0069] The slope of the linear flow stage of the formation is 287.8, as Figure 4 shown.

[0070] In step 105, based on step 104, the comprehensive fracture half-length of the multi-stage hydraulic fractures in the horizontal well is calculated using the formula:

[0071]

[0072] Where k is the matrix permeability, μ is the crude oil viscosity, B is the crude oil volume compressibility, ψ is the matrix porosity, and c t is the comprehensive compression coefficient of the formation, h is the formation thickness, and m is the slope of the linear flow stage of the formation.

[0073] The matrix permeability is 3×10 -6 md, crude oil viscosity is 0.8 mPa.s, volume compressibility coefficient B is 1.1, matrix porosity is 5%, and rock comprehensive compressibility coefficient is 5×10 -7 kpa -1 The formation thickness is 20m, the slope m is 287.8, and the calculated comprehensive fracture half-length of the multi-stage fracturing fractures in the horizontal well is 618.8m.

[0074] In step 106, the productivity contribution of different fracturing stages is calculated using tracer monitoring, and the formula is:

[0075]

[0076] Where q i is the production of the fracturing stage, i=1,2,…,N, q is the total production of the horizontal well, η i is the capacity contribution rate of the i-th segment (i=1,2,…,N).

[0077] When η i >1% is considered as effective fracturing stage, so the number of effective fracturing stages is 16. Figure 5 shown.

[0078] In step 107, the effective fracture half-length of the multi-stage fracturing of the horizontal well is calculated using the formula:

[0079]

[0080] Where x eff is the effective fracture half-length of multi-stage fracturing in horizontal wells, x f is the comprehensive fracture half-length of multi-stage fracturing in horizontal wells, and n is the number of effective fracturing stages.

[0081] The comprehensive fracture half-length of multi-stage fracturing in horizontal wells is 618.8m, and the effective fracture half-length of multi-stage fracturing in horizontal wells is 38.7m.

[0082] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for calculating the effective fracture half-length of multi-stage fracturing in horizontal wells of shale reservoirs, characterized by: The method comprises: Step 1: Collect daily mining data; Step 2, calculating the bottom hole flowing pressure of the horizontal well; Step 3, calculate material balance time; In step 3, the material balance time is calculated as: Where, t e is the material balance time; Q is the cumulative oil production at the current time; q is the current production; Step 4, draw a curve of the relationship between normalized pressure and material equilibrium time; In step 4, calculate and draw the normalized pressure formula: Where, is the normalized pressure at time t; p i is the original formation pressure; p wf is the bottom hole flowing pressure; q(t) is the daily oil production at time t; In step 4, a curve of the relationship between normalized pressure and the square root of material balance time is plotted in the Cartesian coordinate system. When the slope of the curve is constant, it means that the shale oil is in the linear flow stage of the formation. The slope m of the straight line segment is obtained by fitting: Where m is the slope of the straight line segment; b is the intercept; Step 5, calculating the comprehensive fracture half-length of the multi-stage hydraulic fractures in the horizontal well; In step 5, the formula for calculating the comprehensive fracture half-length of multi-stage hydraulic fractures in horizontal wells is: Where k is the matrix permeability; μ is the crude oil viscosity; x f is the comprehensive fracture half-length of multi-stage fracturing in horizontal wells; B is the volume compression coefficient of crude oil; is the matrix porosity; c t is the comprehensive compression coefficient of the formation; h is the formation thickness; Step 6, using tracer monitoring to calculate the productivity contribution of different fracturing stages; In step 6, the tracer monitoring results are used to calculate the productivity contribution of different fracturing stages: Where q i is the production of the fracturing stage; i=1,2,…,N; q is the total production of the horizontal well; η i is the capacity contribution rate of the i-th segment, i=1,2,…,N; In step 6, the number of effective fracturing stages n needs to meet the following conditions: or i >c Where c is a constant to be determined; Step 7, calculating the effective fracture half-length of multi-stage fracturing in the horizontal well; In step 7, the formula for calculating the effective fracture half-length of multi-stage fracturing in horizontal wells is: Where x eff is the half-length of the effective fracture in multi-stage fracturing of horizontal wells; n is the number of effective fracturing stages.

2. The method according to claim 1, characterized in that In step 1, the daily mine data includes oil production, water production, gas production, water content and wellhead oil pressure.

3. The method according to claim 1, characterized in that In step 2, the formula for calculating the bottom hole oil pressure of the horizontal well is: p wf =p t +ρgh p=(1-f w )×ρ o +f w ×r w Where p wf is the bottom hole pressure, p t is the wellhead oil pressure, ρ is the density of the mixed fluid in the wellbore, ρ o is the density of crude oil on the ground, ρ w is the surface water density, f w is the water content, g is the acceleration of gravity, and h is the vertical depth of the horizontal section.

Images