Method for volumetric fracturing compactability characterization

Abstract

The application discloses a volumetric fracturing compressibility characterization method, which comprises the following steps: measuring the horizontal stress of each well, calculating the stress difference coefficient D, observing the development degree K of the natural fracture of the reservoir at the well site, obtaining the elastic modulus and Poisson's ratio distribution of the reservoir section, calculating the rock brittleness index B from the elastic modulus and Poisson's ratio distribution, and finally calculating the compressibility index F according to the above three parameters, and selecting the reservoir area with the highest F value as the target sweet spot area of the volumetric fracturing reconstruction. The application solves the problem that the existing reservoir compressibility characterization method cannot accurately describe the reservoir compressibility by only using the brittleness index.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of oil and gas field exploration and development method, and particularly relates to a volumetric fracturing pressureability characterization method. BACKGROUND

[0002] Volumetric fracturing is a main technical means for the reconstruction of difficult-to-produce reservoirs such as shale oil and tight gas, and the pressureability is a property reflecting whether the reservoirs such as shale oil and tight gas can be effectively reconstructed to form a complex fracture network through volumetric fracturing. At present, the pressureability in the field of the petroleum industry is mainly characterized by the brittleness index of the reservoir rock. Under the condition that other conditions are similar, the higher the brittleness index, the higher the complexity of the fracture system after fracturing. However, the main factors controlling the complexity of the fracture include not only the brittleness index, but also the horizontal stress and the development degree of natural fractures. When the difference between the maximum horizontal stress and the minimum horizontal stress is smaller, the main fracture characteristics of the hydraulic fracture are less obvious, and it is easier to form branch fractures; when the natural fractures are more developed, it is easier to form a complex fracture network in which the hydraulic fractures and the natural fractures communicate with each other. If the reservoir only has a high brittleness index, but the horizontal stress difference is large and the natural fractures are not developed, it is also difficult to form a complex fracture network. SUMMARY

[0003] The purpose of the present application is to provide a volumetric fracturing pressureability characterization method, which solves the problem that the existing reservoir pressureability characterization method cannot accurately describe the reservoir pressureability only by using the brittleness index.

[0004] The technical scheme adopted by the present application is as follows: a volumetric fracturing pressureability characterization method, which measures the horizontal stress of each well, calculates the stress difference coefficient D, then obtains the development degree K of the natural fractures of the reservoir at the observation well site, and finally obtains the elastic modulus and Poisson's ratio distribution of the reservoir section, calculates the reservoir rock brittleness index B from the elastic modulus and Poisson's ratio distribution, and finally calculates the pressureability index F according to the above three parameters, and selects the reservoir region with the highest F value as the target sweet spot area for volumetric fracturing reconstruction.

[0005] The technical scheme adopted by the present application is as follows:

[0006] Further, the volumetric fracturing pressureability characterization method is implemented according to the following steps:

[0007] Step 1: Use rock mechanics acoustic emission experiments to obtain the horizontal stress of each well in the block, and calculate the horizontal stress difference coefficient D of the reservoir section of each well;

[0008] Step 2: According to the observation results of the natural fractures of the oil layer cores in the block, obtain the development degree K of the natural fractures of the reservoir at the observation well site;

[0009] Step 3, according to the dynamic measurement method of elastic modulus and Poisson's ratio, the elastic modulus and Poisson's ratio distribution of the reservoir section are obtained by using the P-wave and S-wave velocity of the reservoir section measured by the logging curve, and then the reservoir rock brittleness index B is calculated from the elastic modulus and Poisson's ratio;

[0010] Step 4, the crushability index F of the reservoir section is calculated according to the horizontal stress difference coefficient D, the natural fracture development degree K and the reservoir rock brittleness index B obtained in steps 1, 2 and 3, the greater the value of the crushability index F, the more easily the complex fracture network is formed in the reservoir volume fracturing;

[0011] Step 5, according to the crushability index F distribution of the whole block, the reservoir with the highest F value is selected as the target sweet spot area of volume fracturing reconstruction.

[0012] Further, in step 1, drilling coring is performed for each exploration well, and the maximum horizontal stress σH and the minimum horizontal stress σh of each well are obtained by using indoor rock mechanics experiment.

[0013] Further, in step 1, the stress difference coefficient D is calculated according to formula (1):

[0014]

[0015] Wherein, σH is the maximum horizontal stress, and σh is the minimum horizontal stress.

[0016] Further, in step 2, the natural fracture development degree K is specifically the development number of natural fractures per meter of oil layer length.

[0017] Further, in step 3, the reservoir rock brittleness index is calculated by formula (2):

[0018]

[0019] Wherein, E is the elastic modulus, is the Poisson's ratio.

[0020] Further, in step 4, the crushability index F is calculated according to formula (3):

[0021]

[0022] Wherein, D is stress difference coefficient, K is natural fracture development degree, B is reservoir rock brittleness index; The value of a1 is determined according to the size of σH-σh, if σH-σh≤5, a1 takes 0.25, if 5<σH-σh≤10, a1 takes 0.14, if σH-σh>10, a1 takes 0.08;The value of a2 is determined according to the size of K, if K≤0.6, a2 takes 0.5, if 0.6<K≤1.0, a2 takes 0.62, if K>1.0, a2 takes 0.7;The value of a3 is determined according to the size of B, if B≤0.35, a3 takes 0.12, if 0.35<B≤0.5, a3 takes 0.25, if B>0.5, a3 takes 0.38.

[0023] The beneficial effects of the present application are:

[0024] The present application provides a volume fracturing compressibility characterization method, solves the problem that the existing reservoir compressibility characterization method cannot accurately describe the reservoir compressibility only with the brittleness index, when the difference between the horizontal maximum stress and the horizontal minimum stress is smaller, the main fracture characteristics of the hydraulic fracture are less obvious, and branch fractures are more likely to be formed;When the natural fractures are more developed, the hydraulic fractures and the natural fractures are more likely to form a complex fracture network, if the reservoir only has high brittleness index, and the horizontal stress difference is large and the natural fractures are not developed, it is also difficult to form a complex fracture network, the method adopts three parameters, can more accurately characterize the reservoir compressibility, and the method is easy to operate, has strong reliability, is simple and intuitive, and provides important guidance and basis for optimizing the block fracturing reconstruction sweet spot area. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The method flowchart of the present application. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the present application more clear, specific, and complete, the technical scheme of the present application will be described below in conjunction with the drawings.

[0027] The present application discloses a volume fracturing compressibility characterization method, by measuring the horizontal ground stress of each well, calculating the stress difference coefficient D, observing the natural fracture development degree K of the well site reservoir, obtaining the elastic modulus and Poisson's ratio distribution of the reservoir section, calculating the reservoir rock brittleness index B from the elastic modulus and Poisson's ratio distribution, and finally calculating the compressibility index F according to the above three parameters, selecting the reservoir area with the highest F value as the target sweet spot area for volume fracturing reconstruction.

[0028] As shown in the figure, the specific implementation is as follows: Figure 1

[0029] ​Step 1, the horizontal stress of each well in the block is obtained by using rock mechanics acoustic emission experiment, drilling core is carried out for each exploration well, the maximum horizontal stress σH and the minimum horizontal stress σh of each well are obtained by using indoor rock mechanics experiment, the horizontal stress difference coefficient D of each well reservoir section is calculated, so as to obtain the distribution of the stress difference coefficient in the block, the stress difference coefficient D is calculated according to formula (1):

[0030]

[0031] Wherein, σH is the maximum horizontal stress, σh is the minimum horizontal stress;

[0032] Step 2, according to the observation results of the core natural fracture of the oil layer in the block, the natural fracture development degree K of the reservoir in the observation well is obtained, the natural fracture development degree K is specifically the development number of natural fractures per meter of oil layer length, so as to obtain the distribution of the natural fracture development degree K of the reservoir in the whole block;

[0033] Step 3, the longitudinal and transverse wave velocity of the reservoir section is measured by using logging curve, the elastic modulus and Poisson's ratio distribution of the reservoir section is obtained according to the dynamic measurement method of elastic modulus and Poisson's ratio, then the reservoir rock brittleness index B is calculated according to the elastic modulus and Poisson's ratio, the reservoir rock brittleness index is calculated by formula (2):

[0034]

[0035] Wherein, E is the elastic modulus, is Poisson's ratio;

[0036] Step 4, the compressibility index F of the reservoir section is calculated according to the horizontal stress difference coefficient D, the natural fracture development degree K and the reservoir rock brittleness index B obtained in steps 1, 2 and 3, the greater the value of the compressibility index F, the more easily the complex fracture network is formed in the reservoir volume fracturing, the compressibility index F is calculated according to formula (3):

[0037]

[0038] Wherein, D is stress difference coefficient, K is natural fracture development degree, B is reservoir rock brittleness index; The value of a1 is determined according to the size of σH-σh, if σH-σh≤5, a1 takes 0.25, if 5<σH-σh≤10, a1 takes 0.14, if σH-σh>10, a1 takes 0.08;The value of a2 is determined according to the size of K, if K≤0.6, a2 takes 0.5, if 0.6<K≤1.0, a2 takes 0.62, if K>1.0, a2 takes 0.7;The value of a3 is determined according to the size of B, if B≤0.35, a3 takes 0.12, if 0.35<B≤0.5, a3 takes 0.25, if B>0.5, a3 takes 0.38;

[0039] Step 5, according to the compressibility index F distribution of the whole block, the reservoir with the highest F value is selected as the target sweet spot area of volume fracturing reconstruction.

[0040] In order to make the purpose, technical scheme and gist of the present application more clear, the specific implementation of the present application is further explained below in combination with the drawings and examples.

[0041] Example 1

[0042] The characterization method of volume fracturing compressibility:

[0043] Step 1, in order to find the reservoir volume fracturing sweet spot area of a shale oil reservoir, 5 straight wells are deployed as evaluation wells at different positions in the whole block, and the well numbers of the 5 straight wells are X-1, X-2, X-3, X-4 and X-5 respectively; after drilling to the target layer, the cores are taken, the maximum horizontal stress σH and the minimum horizontal stress σh of each well are obtained by using the indoor rock mechanics acoustic emission experiment, and then the stress difference coefficient D of each well reservoir section is calculated according to the calculation formula of stress difference coefficient , as shown in table 1.

[0044] Step 2, the oil layer core taken from each evaluation well drilled in the block is observed for natural fracture, the development degree K of natural fracture of each evaluation well site reservoir is obtained by calculating the development number of natural fracture per meter of oil layer length, as shown in table 1.

[0045] Step 3, the longitudinal and transverse wave velocities measured by logging curve are used to calculate the elastic modulus and Poisson's ratio distribution of the reservoir section according to the dynamic measurement method of elastic modulus and Poisson's ratio, and then the brittleness index of each well is obtained according to the brittleness index formula defined by elastic modulus and Poisson's ratio , as shown in table 1.

[0046] Step 4, the compressibility index F of each evaluation well in the block is calculated by using the characterization formula F = a1 x (1-D) + a2 x K + a3 x B;

[0047] wherein D is the stress difference coefficient, obtained from step 1; K is the natural fracture development degree, obtained from step 2; B is the reservoir rock brittleness index, obtained from step 3; a1, a2, a3 are weight coefficients, since the horizontal two-way stress difference σH-σh of each well is less than 5, a1 is 0.25; the natural fracture development degree K is less than 0.6, a2 is 0.5; the interval of the brittleness index B is between 0.35-0.5, a3 is 0.25; the compressibility index F of each well is obtained from the above parameters, F = 0.25 x (1-D) + 0.5 x K + 0.25 x B, and the specific calculation values are shown in Table 1; the greater the value of the compressibility index F, the easier the complex fracture network is formed in the reservoir volume fracturing at the position of the well;

[0048] Step 5, by comparing the compressibility index F of the 5 evaluation wells, it is found that the compressibility index of X-4 well is the largest, and the position of the well is preferred as the target sweet spot area for volume fracturing reconstruction.

[0049] Table 1 compressibility index calculation parameter detail table

[0050]

[0051] Example 2

[0052] Step 1, a certain tight oil reservoir needs to be developed by horizontal well volume fracturing, in order to find the sweet spot area, 4 straight wells are first deployed as exploration wells in the block plane, the well numbers of the 4 exploration wells are N-1, N-2, N-3, N-4; drilling core is carried out for each exploration well, the maximum horizontal ground stress σH and the minimum horizontal ground stress σh of each well are obtained by using indoor rock mechanics experiment, the ground stress difference coefficient D of the reservoir section of each well is calculated according to the calculation formula of the ground stress difference coefficient , as shown in Table 2;

[0053] Step 2, the natural fracture development degree K of each evaluation well is obtained by observing the natural fractures of the cores of the 4 exploration wells and calculating the number of natural fractures developed per meter of oil layer length, as shown in Table 2;

[0054] Step 3, the elastic modulus and Poisson's ratio distribution of the reservoir section are calculated according to the dynamic measurement method of the elastic modulus and Poisson's ratio by using the vertical and horizontal wave velocities measured by the logging curve, and then the brittleness index of each well is obtained according to the brittleness index formula defined by the elastic modulus and Poisson's ratio , as shown in Table 2;

[0055] Step 4, the compressibility index F of each well in the block is calculated using the characterization formula F = a1 x (1-D) + a2 x K + a3 x B; wherein D is the stress difference coefficient, obtained from step 1; K is the natural fracture development degree, obtained from step 2; B is the reservoir rock brittleness index, obtained from step 3; a1, a2, a3 are weight coefficients, since the value range of the horizontal two-way stress difference σH-σh of each well is between 5-10, a1 is 0.14; the range of the natural fracture development degree K is between 0.6-1.0, a2 is 0.62; the value of the brittleness index B is less than 0.35, a3 is 0.12; the compressibility index F of each well is obtained as F = 0.14 x (1-D) + 0.62 x K + 0.12 x B from the above parameters, and the specific calculation values are shown in Table 2;

[0056] Step 5, by comparing the compressibility index F of the 4 wells, it is found that the compressibility index of well N-4 is the largest, and the position region of the well is preferred as the target sweet spot area for volume fracturing reconstruction.

[0057] Table 2: Compressibility index calculation parameter details

[0058]

[0059] Example 3

[0060] Characterization method of volume fracturing compressibility:

[0061] Step 1, a shale gas reservoir needs to be developed by horizontal well volume fracturing, in order to find the sweet spot area, 4 straight wells are deployed as skeleton wells in the block plane, the well numbers of the 4 skeleton wells are G-1, G-2, G-3, G-4; drilling core sampling is carried out for each skeleton well, the maximum horizontal ground stress σH and the minimum horizontal ground stress σh of each well are obtained by using indoor rock mechanics experiment, and the ground stress difference coefficient D of the reservoir section of each well is calculated according to the calculation formula of the ground stress difference coefficient

[0062] Step 2, the natural fracture development degree K of the reservoir of each evaluation well site is obtained by observing the natural fractures of the cores of the 4 skeleton wells and calculating the number of natural fractures developed per meter of oil layer length, as shown in Table 3;

[0063] Step 3, the elastic modulus and Poisson's ratio distribution of the reservoir section are calculated according to the dynamic measurement method of the elastic modulus and Poisson's ratio based on the vertical and horizontal wave velocities measured by the logging curve, and then the brittleness index of each well is obtained according to the brittleness index formula defined by the elastic modulus and Poisson's ratio

[0064] ​​Step 4, the compressibility index F of each skeleton well in the block is calculated using the characterization formula F = a1 x (1-D) + a2 x K + a3 x B; wherein D is the stress difference coefficient, obtained from step 1; K is the natural fracture development degree, obtained from step 2; B is the reservoir rock brittleness index, obtained from step 3; a1, a2, a3 are weight coefficients, since the value of the horizontal two-way stress difference σH-σh of each well is greater than 10, a1 is 0.08; the value of the natural fracture development degree K is greater than 1, a2 is 0.7; the value of the brittleness index B is greater than 0.5, a3 is 0.38; the compressibility index F of each well is obtained as F = 0.08 x (1-D) + 0.7 x K + 0.38 x B from the above parameters, and the specific calculation values are shown in Table 3;

[0065] Step 5: By comparing the compressibility index F of the 4 shale gas skeleton wells, it is found that the compressibility index of well G-2 is the largest, and the position region of the well is preferably selected as the target sweet spot area for volume fracturing reconstruction.

[0066] Table 3: Compressibility index calculation parameter details

[0067]

[0068] Example 4

[0069] Characterization method of volume fracturing compressibility:

[0070] Step 1, a certain tight oil reservoir needs to be developed by horizontal well volume fracturing, in order to find the sweet spot area, 4 straight wells are deployed as evaluation wells in the block plane, the well numbers of the 4 exploration wells are Q-1, Q-2, Q-3 and Q-4; drilling core sampling is carried out for each evaluation well, the maximum horizontal ground stress σH and the minimum horizontal ground stress σh of each well are obtained by using indoor rock mechanics experiment, and the ground stress difference coefficient D of the reservoir section of each well is calculated according to the calculation formula of the ground stress difference coefficient , as shown in Table 4;

[0071] Step 2, the natural fractures of the cores of the 4 evaluation wells are observed, the development number of natural fractures per meter of oil layer length is calculated, and the natural fracture development degree K of the reservoir of each evaluation well site is obtained, as shown in Table 4;

[0072] Step 3, the elastic modulus and Poisson's ratio distribution of the reservoir section are calculated according to the dynamic measurement method of elastic modulus and Poisson's ratio using the vertical and horizontal wave velocities measured by the logging curve, and then the brittleness index of each well is obtained according to the brittleness index formula defined by the elastic modulus and Poisson's ratio , as shown in Table 4;

[0073] Step 4, the compressibility index F of each evaluation well in the block is calculated by using the characterization formula F = a1 x (1-D) + a2 x K + a3 x B of the compressibility index F; wherein, D is the stress difference coefficient, obtained from step 1; K is the natural fracture development degree, obtained from step 2; B is the reservoir rock brittleness index, obtained from step 3; a1, a2, a3 are weight coefficients, since the value range of the horizontal two-way stress difference σH-σh of each well is between 5-10, a1 is 0.14; the range of the natural fracture development degree K is between 0.6-1.0, a2 is 0.62; the value of the brittleness index B is less than 0.35, a3 is 0.12; the compressibility index F of each well is F = 0.14 x (1-D) + 0.62 x K + 0.12 x B, and the specific calculation value is shown in Table 4;

[0074] Step 5, by comparing the compressibility index F of the 4 evaluation wells, it is found that the compressibility index of Q-4 well is the largest, and the position region of the well is preferred as the target sweet spot area of volume fracturing reconstruction.

[0075] Table 4 compressibility index calculation parameter detail table

[0076]

[0077] Comparative Example 1

[0078] Step 1, in order to find the reservoir volume fracturing sweet spot area of a shale oil reservoir, 5 straight wells are deployed as evaluation wells at different positions in the whole block plane, and the well numbers of the 5 wells are Y-1, Y-2, Y-3, Y-4 and Y-5; after the completion of the straight wells, logging is carried out, the longitudinal and transverse wave velocities measured by the logging curve are used, the elastic modulus and Poisson's ratio distribution of the reservoir section are calculated according to the dynamic measurement method of the elastic modulus and Poisson's ratio, and then the brittleness index of each well is obtained according to the brittleness index formula defined by the elastic modulus and Poisson's ratio , as shown in Table 5;

[0079] Step 2, the brittleness index of the shale oil reservoir evaluation well is directly used as the evaluation basis of the compressibility; by comparing the brittleness index B of the 5 evaluation wells, it is found that the brittleness index of Y-1 well is the largest, and the position region of the well is preferred as the target sweet spot area of volume fracturing reconstruction.

[0080] Table 5 compressibility index calculation parameter detail table

[0081]

[0082] Comparative verification analysis:

[0083] In the embodiment 1, the proposed compressibility index F is used to select the target sweet spot area for volume fracturing reconstruction. By comparing the compressibility index F, the compressibility index of X-4 well is the largest, and the area where the well is located is selected as the sweet spot area. As shown in Table 6, the oil production of X-4 well after implementation is the highest, which proves the correctness of the technical method of the present application.

[0084] Table 6 Comparison table of implementation effect of the method of the present application and conventional method

[0085]

[0086] In the comparative example 1, the conventional method in the early stage is used to select the sweet spot area according to the brittleness index. It is considered that the brittleness index of Y-1 well is the largest by comparison, and the area where the well is located is selected as the target sweet spot area for volume fracturing reconstruction. However, the actual implementation result is that the production of Y-4 well is the highest, which shows that the method of selecting the sweet spot area according to the brittleness index has low accuracy.

[0087] It is considered that the technical method proposed in the present application has more accurate and reliable results, and has stronger guidance to the actual mine.

Claims

1. A method for characterizing the compressibility of volumetric fracturing, characterized in that, By measuring the horizontal geostress of each well, the horizontal geostress difference coefficient D is calculated, and then the degree of natural fracture development K of the reservoir at the observation well location is obtained. The elastic modulus and Poisson's ratio distribution of the reservoir section are then obtained. The brittleness index B of the reservoir rock is calculated from the elastic modulus and Poisson's ratio distribution. Finally, the compressibility index F is calculated based on the three parameters: the horizontal geostress difference coefficient D, the degree of natural fracture development K of the reservoir at the observation well location, and the brittleness index B of the reservoir rock. The reservoir area with the highest F value is selected as the target sweet spot for volumetric fracturing. The specific steps are as follows: Step 1: Obtain the horizontal geostress of each well in the block using rock mechanics acoustic emission experiments, and calculate the horizontal geostress difference coefficient D of the reservoir section of each well; In step 1, core samples were taken from each exploration well, and the maximum and minimum horizontal in-situ stresses σH and σh of each well were obtained using laboratory rock mechanics experiments. Step 2: Based on the observation results of natural fractures in the oil reservoir core of the block, obtain the degree of natural fracture development K of the reservoir at the observation well location; Step 3: Using the P-wave and S-wave velocities of the reservoir section obtained from the logging curves, the elastic modulus and Poisson's ratio distribution of the reservoir section are obtained according to the dynamic measurement method of elastic modulus and Poisson's ratio. Then, the brittleness index B of the reservoir rock is calculated based on the elastic modulus and Poisson's ratio. Step 4: Calculate the compressibility index F of the reservoir section based on the horizontal stress difference coefficient D, the degree of natural fracture development K, and the brittleness index B of the reservoir rock obtained in steps 1, 2, and 3. The larger the value of the compressibility index F, the easier it is for the reservoir volume to form a complex fracture network. In step 4, the compressibility index F is calculated according to formula (3): Where D is the horizontal stress difference coefficient, K is the degree of natural fracture development, and B is the reservoir rock brittleness index; the value of a1 is determined according to the magnitude of σH-σh. If σH-σh≤5, then a1 is 0.25; if 5<σH-σh≤10, then a1 is 0.14; if σH-σh>10, then a1 is 0.08; the value of a2 is determined according to the magnitude of K. If K≤0.6, then a2 is 0.5; if 0.6<K≤1.0, then a2 is 0.62; if K>1.0, then a2 is 0.7; the value of a3 is determined according to the magnitude of B. If B≤0.35, then a3 is 0.12; if 0.35<B≤0.5, then a3 is 0.25; if B>0.5, then a3 is 0.

38. Step 5: Based on the distribution of the compressibility index F of the entire block, select the reservoir with the highest F value as the target sweet spot for volumetric fracturing.

2. The method for characterizing the compressibility of volumetric fracturing according to claim 1, characterized in that, In step 1, the horizontal stress difference coefficient D is calculated according to formula (1): Where σH is the maximum horizontal ground stress and σh is the minimum horizontal ground stress.

3. The method for characterizing the compressibility of volumetric fracturing according to claim 1, characterized in that, In step 2, the degree of natural fracture development K is specifically the number of natural fractures developed per meter of oil layer length on average.

4. The method for characterizing the compressibility of volumetric fracturing according to claim 1, characterized in that, In step 3, the brittleness index B of the reservoir rock is calculated by formula (2): Where E is the elastic modulus. It is Poisson's ratio.

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