Method for realizing uniform stimulation for the oil and gas well by low-cost multi-stage fracturing

Abstract

The present invention discloses a method for realizing uniform stimulation for the oil and gas well by low-cost multi-stage fracturing, comprising the following steps: first calculate and predict the inlet widths of the fracture created in the planned hydraulic fracturing; then design the plugging scheme for blocking the fracture inlet after each fracturing treatment; the selected plugging particles are composed of filling particles and skeleton particles, with designed plugging scheme; after complete the design of plugging scheme, perform the fracturing treatment to create hydraulic fracturing in the reservoir; skeleton particles and filling particles are added into the fractures successively according to plan. The present invention provides a multi-stage single-cluster fracturing method, with the advantages of controllable fracture sizes and low cost. The present invention can realize a good uniformity of stimulation for the oil and gas wells.

Claims

1. A method for realizing uniform stimulation for an oil and gas well comprising a wellbore by multi-stage fracturing, comprising the following steps: Step 1: Calculating and predicting the inlet widths of a fracture w.sub.i created in a planned hydraulic fracturing treatment, with Equation (1); $\begin{array}{cc}{w}_i=2.18\left[\frac{Q^2\left(1-v^2\right)\mu }{Eh}\right]{ }_{}^{0.2}{t}^{0.2}& \left(1\right)\end{array}$ Where: w.sub.i is an inlet width of the fracture, in m; E is Young's modulus of reservoir rock, in MPa; Q is a pump rate of a fracturing fluid, in m.sup.3/s; μ is a viscosity of the fracturing fluid, in MPa.Math.s; v is the Poisson's ratio of the reservoir rock, dimensionless; h is a reservoir thickness, in m; and t is a fracturing time, in s; Step 2: Designing a plugging scheme for blocking the fracture inlet after each fracturing treatment; wherein selected plugging particles are composed of skeleton particles and filling particles; the filling particles are a soluble fluid diverting agent, and the filling particles diameter is less than 1/3 of that of the skeleton particles; the filling particles and the skeleton particles are respectively taken for fine screening to determine their particle diameter composition and classify as different kinds; and calculating the harmonic mean of diameters for the particles according to Equation (2), as the average diameter of the plugging particles: $\begin{array}{cc}a=\frac{\left(v_1+v_2+L+v_n\right)T+\left(g_1+g_2+L+g_m\right)\left(1-T\right)}{\left(\frac{v_1}{b_1}+\frac{v_2}{b_2}L+\frac{v_n}{b_n}\right)T+\left(\frac{g_1}{c_1}+\frac{g_2}{c_2}L+\frac{g_m}{c_m}\right)\left(1-T\right)}& \left(2\right)\end{array}$ Where: a is an average diameter of the plugging particles, in m; b.sub.n is a particle diameter of n-th kind of skeleton particles, in m; v.sub.n is a volume fraction of n-th kind of skeleton particles among all the skeleton particles, dimensionless; c.sub.m is a particle diameter of m-th kind of filling particles, in m; g.sub.m is a volume fraction of m-th kind of filling particles among all the filling particles, dimensionless; and T is a volume fraction of the filling particles in the plugging particles, dimensionless; Step 3: Checking whether the average particle diameter a is greater than 1/5 of the inlet width of fracture calculated in Step 1: $\begin{array}{cc}a>\frac{1}{5}{w}_i& \left(3\right)\end{array}$ If the conditions in Equation (3) are met, the average particle diameter a is selected as an optimized average particle diameter for plugging, and its corresponding particle composition is selected as an optimized particle composition; if the conditions in Equation (3) are not met, larger skeleton particles will be reelected, and the average particle diameter of plugging particles will be recalculated according to Step 2 until the conditions in Equation (3) are met; Step 4: Calculating an optimized volume fraction C.sub.o of plugging particles required for blocking the fractures: $\begin{array}{cc}{C}_o\ge \frac{3}{10}\left(1-\frac{1}{2}{e}^{-\frac{w_o}{3.3a}}\right){\left(1-\frac{a}{w_i}\right)}^2& \left(4\right)\end{array}$ Where: α is the average diameter of plugging particles, in m; and w.sub.i is the inlet width of fracture, in m; Step 5: Based on the optimized particle composition and the optimized volume fraction C.sub.o , using a hydraulic fracturing model to simulate a fracturing and plugging process and checking whether the volume fraction of particles in the fracture near the wellbore can reach 60%; if the volume fraction fails to reach 60%, larger skeleton particles will be reelected, and Steps 2-5 are repeated until the volume fraction of particles in the fracture near the wellbore can reach 60% in the simulation; Step 6: After completing the design of the plugging scheme, performing the planned fracturing treatment to create hydraulic fracturing in the reservoir; Step 7: When the injection amount of fracturing fluid reaches 80-85% of the designed volume, pumping the plugging particles according to the optimized design to block the fracture; After successful plugging, performing a next fracturing treatment for a subsequent perforation cluster; Step 8: Repeat Steps 1-7 until fracturing of all perforation clusters in the oil and gas well is completed; Step 9: If necessary, treatment fluid is injected into the wellbore to dissolve the filling particles at the fracture inlet to recover a flow channel between each fracture with the wellbore.

2. The method for realizing uniform stimulation for the oil and gas well by multi-stage fracturing according to claim 1, wherein in the Step 2, proppant particles within a particle diameter range of 30-40 mesh are initially selected as skeleton particles.

3. The method for realizing uniform stimulation for the oil and gas well by multi-stage fracturing to claim 1, wherein in the Step 5, the hydraulic fracturing model used is a three-dimensional hydraulic fracturing model: $\begin{array}{cc}\frac{\partial w}{\partial t}-\frac{\partial q_s}{\partial s}+i=0,\frac{\partial wc}{\partial t}-\frac{\partial q_a}{\partial s}+ic=0& \left(5\right)\end{array}$ $q_d=1.2{c\left(1-c\right)}^{\frac{1}{10}}{q}_s-\frac{a^{2}w}{48\mu }2.3{c\left(1-c\right)}^2\Delta \rho g$ Where: w is the width of the fracture, in m; t is the time, in s; s is the spatial distance, in m; i is the injection rate of fracturing fluid, in m/s; q.sub.s is the flow rate of sand-suspended fracturing fluid, in m/s; q.sub.d is the particle transport speed, in m/s; C is the volume fraction of particles among the fracture; c is the normalized volume fraction of particles among the fracture; μ is the viscosity of fracturing fluid, in Pa.Math.s; a is the particle diameter; μρ is the density difference between fracturing fluid and particles, in kg/m.sup.3; and g is the acceleration of gravity, in m/s.sup.2.

4. The method for realizing uniform stimulation for the oil and gas well by multi-stage fracturing according to claim 1, wherein in the Step 7, the method for identifying the success of plugging is as follows: after the completion of plugging, enabling bypass of a sand mixer truck and injecting a displacing fluid into the wellbore to force the particles to enter the fracture, wherein if the wellbore pressure continues to rise and exceeds a predesignated safe pressure, it is assumed that the fracture is successfully plugged by the particles.

5. The method for realizing uniform stimulation for the oil and gas well by multi-stage fracturing according to claim 1, wherein in the Step 6, the specific method is as follows: using a coiled tubing to transport a perforating tool to an aim depth, creating perforation clusters for the hydraulic fracturing, opening a wellhead valve without removing the coiled tubing from the wellbore after perforation, closing a circulating emptying valve, activating a fracturing pump, inject the fracturing fluid for a fracture test, selecting a pump rate based on friction along the downhole and a fracture initiation pressure to create and extend the hydraulic fracture, and recording a fracture pressure at the moment of the fracturing treatment.

6. The method for realizing uniform stimulation for the oil and gas well by multi-stage fracturing according to claim 5, wherein pre-fracturing preparations are required before Step 6, including preparation of fracturing equipment and materials, well site layout and construction inspection.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is the process diagram of optimization calculation for the optimized parameters of plugging particle of the present invention.

(2) FIG. 2 is the distribution diagram of sand ratio for single fracture in low-cost single cluster fracturing.

(3) FIG. 3 is the process diagram of low-cost single cluster fracturing.

(4) FIG. 4 is the diagram of three simulated hydraulic fractures in low-cost single-cluster fracturing.

(5) FIG. 5 is the diagram of three simulated hydraulic fractures in traditional multi-cluster fracturing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(6) The preferred embodiments of the present invention are described in combination with the attached drawings. It should be understood that the preferred embodiments described here are only used for describing and explaining the present invention instead of limiting the present invention.

(7) FIG. 1 gives the process diagram of optimization calculation for the optimized parameters of plugging particle corresponding to Steps 1-5 of the present invention.

(8) Take a certain stage of shale gas well B in a certain zone of Sichuan as an example. This well is located in a favorable reservoir for shale gas with stable formation structure. In order to make each fracture grow evenly and obtain better fracturing performance, the low-cost single-cluster fracturing process provided by the present invention is selected for fracturing. The specific geological and engineering parameters of the reservoir are shown in Table 1.

(9) TABLE-US-00001 TABLE 1 Key geological and engineering parameters Maximum horizontal 60 Minimum horizontal 50 principal stress (MPa) principal stress (MPa) Vertical principal stress (MPa) 75 Formation porosity (%) 3 Young's modulus (MPa) 30000 Poisson's ratio 0.38 Fracture toughness of rock 1.5 Combined leak-off coefficient 2 × 10.sup.−5 (MPa .Math. m.sup.1/2) (natural fracture/matrix) (m.sup.3/s.sup.1/2) Interlayer stress barrier 5 Reservoir thickness (m) 100 (MPa) Pump rate of fracturing fluid 5 Viscosity of fracturing 5 × 10.sup.−9 (m.sup.3/min) fluid (MPa .Math. s) Biot coefficient 0.6 Density of fracturing fluid 1011 Perforation density (l/m) 6 (kg/m.sup.3) Effective fracturing time (s) 1800 Perforation aperture (m) 0.008 Stage spacing (m) 90

(10) Step 1: The collected geological and engineering parameters of the target block (Table 1) are substituted into Equation (1), and the predicted value of inlet width of hydraulic fracture calculated by the fracture model is 0.00302 m, that is, 3.02 mm.

(11) Step 2: Two groups of particles (20/40 meshes) and (40/70 meshes) are selected and respectively defined as Group 1 (20/40 meshes) and Group 2 (40/70 meshes). Group 1 (20/40 meshes) is selected as the skeleton particles, while Group 2 (40/70 meshes) is selected as the filling particles. The average diameter of particles with 20/40 meshes is 0.759 mm, while the average diameter of particles with 40/70 meshes is 0.274 mm. The average diameter of Group 2 (40/70 meshes) is greater than 1/3 of the average diameter of Group 1 (20/40 meshes), which meets the requirements. The two groups of proppant samples are subject to further fine screening to obtain specific particle composition, with the ratio of skeleton particles with 20/40 meshes being 70%. For the particle diameter corresponding to mesh during screening, see Table 2 for the correspondence between particle diameter and mesh.

(12) TABLE-US-00002 TABLE 2 Correspondence between particle diameter and mesh Particle 2.38 2.00 1.68 1.41 1.68 1.19 1.00 diameter (mm) Mesh 8 10 12 14 12 16 18 Particle 0.841 0.707 0.400 0.297 0.250 0.210 0.177 diameter (mm) Mesh 20 25 40 50 60 70 80

(13) The average plugging particle diameter a of the two groups of mixed particles, calculated by Equation (2), is 0.613 mm.

(14) Step 3: The average diameter of plugging particles (a=0.613 mm) is greater than 1/5 of the fracture width (3.02 mm), that is, 5α>w.sub.i, so the size of plugging particles meets the requirements.

(15) Step 4: The volume fraction of particles C.sub.o required for realizing the single-cluster fracturing with high-concentration plugging particles, calculated by Equation (4), is at least 0.2821.

(16) Step 5: The 3D hydraulic fracturing model shown in Equation (5) is used to simulate and calculate the volume fraction of particles in the fracture near the wellbore (greater than 0.6) (as shown in FIG. 2), meeting the required volume fraction.

(17) Step 6: First perform the pre-fracturing preparations, including: construction preparations: (1) Preparing the fracturing equipment and materials according to the geological conditions of the reservoir, the design parameters and construction requirements for fracturing, and allocate the personnel matching the scale of treatment. (2) Well site layout and wellbore preparation: according to the standardized scheme of fracturing of the oil and gas well, install fracturing equipment and set the threshold value of maximum pumping pressure for safe, transport the coiled tubing into wellbore and clean up the well. (3) Construction inspection: before fracturing treatment, inspect the performance of fracturing truck via fluid circulating, ensure that pipelines for high and low pressures on the ground are unblocked, carry out pressure tests for the wellhead valve and the ground pipelines, with maximum pressure being 1.2 to 1.5 times of the predicted pumping pressure, and keep the pressure for 5 min to ensure that the pressure tests are qualified.

(18) Perform the fracturing: use a coiled tubing to carry the perforating tool to the corresponding depth, perform the cluster of perforations at the target location, open the wellhead valve without removing the coiled tubing from the wellbore after perforation, close the circulating emptying valve, start the fracturing pump trucks one by one, slightly pump the fracturing fluid into the formation till the pressure becomes stable to check that the downhole strings and tools work normally, conduct injection into annulus after successful mini-fracturing test, select a reasonable maximum pump rate based on the friction resistance of downhole string and the formation fracture pressure to initiate and extend the hydraulic fracture, and record the fracture pressure at the moment of fracturing.

(19) Step 7: Blocking fracture: after the fracture is formed, the pumping pressure and the pump rate of fracturing pump become stable to ensure stable pressure and pump rate, add the sand gradually and evenly. When the injection amount of fluid reaches 80-85% of the designed fluid volume, pump the plugging particles based on the optimized volume fraction of particles C.sub.o. After the high-concentration plugging materials used for blocking enter the formation holes, reduce the injection pump rate until all these plugging particles enter the fracture.

(20) Identifying the success of plugging: after adding the sand, enable the bypass of the sand mixer truck and inject the displacing fluid into the wellbore to force all the particles enter the fracture. If the wellbore pressure continues to rise and exceeds the predesignated safe pressure, it is reasonable to assume that the fracture is successfully plugged by the particles. Then prepare the construction materials for the fracturing of next fracture.

(21) Step 8: Proceed to the subsequent steps in sequence from Step 1 according to the basic geological conditions where the next cluster of fractures are located until the fracturing of all fractures in the target well is completed. The fracturing construction process is as shown in FIG. 3.

(22) Step 9: Plugging removal: If necessary, treatment fluid is injected into the wellbore to dissolve the filling particles at fracture inlet to recover the flow channel between each fracture with the wellbore. So far, the low-cost multi-stage single-cluster fracturing of the well has been completed.

(23) To compare the performance of traditional multi-cluster fracturing with the single-cluster fracturing in the present invention, 3D simulated fracture geometries creating by two methods are respectively presented. By the method provided in the present invention, all fractures are fractured in turn, and the fracture lengths are controllable. After the fracturing of whole well is completed, it can be found that the difference of fracture lengths are small, and the fracturing performance is good (as shown in FIG. 4). After the traditional multi-cluster fracturing method is used to carry out the fracturing operation, there are significant differences in the lengths of three fractures. The fracture lengths are relatively uncontrollable and highly uneven, resulting in a poor fracturing performance (as shown in FIG. 5).

(24) The above are not intended to limit the present invention in any form. Although the present invention has been disclosed as above with embodiments, it is not intended to limit the present invention. Those skilled in the art, within the scope of the technical solution of the present invention, can use the disclosed technical content to make a few changes or modify the equivalent embodiment with equivalent changes. Within the scope of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention are still regarded as a part of the technical solution of the present invention.