Carbonate rock hydraulic sand fracturing combined depressurization design method

Abstract

The invention discloses a carbonate rock hydraulic sand fracturing combined pressure reduction design method. The method comprises the steps that 1, low-displacement injection of conventional acid is adopted for pretreatment, and the construction pressure near a shaft is reduced; 2, high-viscosity fracturing fluid is injected, and the artificial fracture is opened and extended; 3, gelled acid is injected, and by means of the characteristics that the reaction rate of the gelled acid and the carbonate rock reservoir is low and the like, the construction pressure of the far end of the crack is reduced; 4, high-viscosity fracturing fluid is injected and carries a small-particle-size proppant slug, fracturing fluid filtration loss is reduced, and meanwhile main seam extension is promoted; 5, high-viscosity fracturing fluid carrying a large-particle-size propping agent is injected, and the flow conductivity of the artificial fracture is improved; sixthly, high-viscosity fracturing fluid is injected to replace sand-carrying fluid in the shaft, and sand setting of the shaft is avoided; by the adoption of the combined pressure reduction design method, the construction pressure near a shaft can be reduced, the construction pressure at the far end of a crack can also be reduced, and therefore it is guaranteed that sand is smoothly added into a carbonate reservoir.

Description

Technical Field

[0001] This invention relates to the field of low-permeability tight carbonate reservoir stimulation technology, specifically a combined hydraulic fracturing and sand fracturing depressurization design method for carbonate rocks. Background Technology

[0002] Currently, for carbonate reservoirs, acid fracturing is the main method used both domestically and internationally to connect fractures and cavities. Simultaneously, non-uniform etching is used to create artificial fracture channels with high conductivity, thereby increasing single-well production. For low-permeability tight carbonate reservoirs, where fracture and cavity development is highly heterogeneous, hydraulic fracturing with proppant is currently being tested to further increase the stimulation volume and improve the probability of connecting fractures and cavities.

[0003] However, carbonate reservoirs have higher Young's modulus and extended pressure gradient than sandstone reservoirs, narrower fracture widths, and are more difficult to add sand, making fracture extension more challenging. Therefore, reducing construction pressure and successfully adding sand are key to ensuring the effectiveness of fracturing operations in tight carbonate reservoirs.

[0004] Currently, existing technologies disclose various methods to reduce fracturing pressure. For example, Chinese Patent CN113062732A discloses a method and application for reducing fracturing pressure and extension pressure in hot dry rock fracturing. This method reduces fracturing pressure and extension pressure in hot dry rock fracturing by injecting cold fluid with a variable displacement pulse. This method can reduce fracturing pressure by more than 20% and reduce fracture extension by more than 10%. Another example is Chinese Patent CN107446566A, which discloses a pre-acid fracturing method for deep shale gas horizontal well fracturing. One method involves pretreatment with acid and liquid, which reduces the construction pressure near the perforation holes by injecting 10-20% local acid, thereby controlling the fracture propagation morphology and improving the effectiveness of staged fracturing. Another Chinese patent, CN107842353A, discloses a method for optimizing acid pretreatment for shale reservoir fracturing. Through tests of dissolution rate, acid permeability improvement, and tensile strength reduction, the method selects the acid system with the highest fracturing pressure reduction capacity for specific shale reservoirs, thereby reducing the power and load-bearing capacity requirements of fracturing equipment and minimizing construction risks.

[0005] In summary, existing technologies mainly reduce the construction pressure during fracturing operations in hot dry rock and shale reservoirs, but no research has been found on reducing the construction pressure during hydraulic sand fracturing operations in carbonate reservoirs. Summary of the Invention

[0006] The purpose of this invention is to provide a combined pressure reduction design method for hydraulic fracturing of carbonate rocks, aiming to improve the lack of effective methods to reduce the fracturing pressure and the pressure at the distal end of artificial fractures in hydraulic fracturing of carbonate rock reservoirs, and to ensure successful sand injection.

[0007] This invention is implemented as follows: a combined hydraulic fracturing and sand-filled depressurization design method for carbonate rocks, comprising:

[0008] Step 1: Pretreatment with low-volume injection of conventional acid to reduce construction pressure near the wellbore;

[0009] Step 2: Inject high-viscosity fracturing fluid to open and extend artificial fractures;

[0010] Step 3: Inject gelling acid. Utilize the low reaction rate of gelling acid with carbonate reservoirs to reduce construction pressure at the far end of the fracture.

[0011] Step 4: Inject high-viscosity fracturing fluid with small-diameter proppant slugs to reduce fracturing fluid loss and promote main fracture extension.

[0012] Step 5: Inject high-viscosity fracturing fluid carrying large-diameter proppant to improve the conductivity of artificial fractures;

[0013] Step 6: Inject high-viscosity fracturing fluid to replace the sand-carrying fluid in the wellbore and prevent sand from settling in the wellbore.

[0014] Preferably, the conventional acid in step one is hydrochloric acid with a mass fraction of 20%; the injection rate of the conventional acid is 1.5-2 cubic meters per minute.

[0015] Preferably, in step one, a retarder is also added, which includes a corrosion inhibitor, an iron ion stabilizer, and an acid decompression aid.

[0016] Preferably, the amount of fracturing fluid used in step two should be based on the effective distance, fracture length, and discharge rate of the gelling acid.

[0017] Preferably, the fracturing fluid dosage follows the volume balance equation and is calculated using the following formula.

[0018]

[0019] Where Q(t): injection volume; t: construction time; δ; A: crack area; w: crack width; C: filtration coefficient; erfc(x): error compensation function, which is obtained by looking up the function table.

[0020] Preferably, the distance of the gelling acid interaction in step three is calculated using the following convection-diffusion partial differential equation based on the experimental test results of the acid-rock reaction rate.

[0021]

[0022] In the formula: C is C(x, y), the acid concentration at any location in the crack; u x u represents the acid velocity component in the x-direction. y D represents the acid velocity component in the y-direction; H+The effective mass transfer coefficient.

[0023] Preferably, when using convection-diffusion partial differential equations, boundary conditions must be given based on the actual flow and reaction conditions. The boundary conditions for the reaction between hydrochloric acid and rock are as follows:

[0024]

[0025] Preferably, under the boundary conditions of equation (4), for the partial differential equation (1), the analytical solution given in the field of heat conduction under the same boundary conditions is used to draw it into a layout form.

[0026] Preferably, in the layout form, the vertical axis represents the number of Pictets without a specific value. The x-axis represents the distance without any coordinates. Each curve corresponds to a different concentration of acid C. D =C / C0.

[0027] Preferably, in step four, the small-particle-size proppant is made of 70 / 140 mesh ceramsite; in step five, the large-particle-size proppant is made of 40 / 70 mesh ceramsite, and 30 / 50 mesh proppant is used at the seam opening to improve the seam opening's drainage capacity; in step six, the displacement liquid is a 10 cubic meter high-viscosity liquid, and the remainder is a backflow liquid to reduce costs.

[0028] Compared with the prior art, the beneficial effects of the present invention are: by adopting the combined pressure reduction design method of the present invention, not only can the construction pressure near the wellbore be reduced, but also the construction pressure at the far end of the fracture can be reduced, thereby ensuring the smooth sand addition to the carbonate reservoir. Attached Figure Description

[0029] Figure 1 This is a flowchart of the combined pressure reduction design method for near-wellbore and fracture distal ends;

[0030] Figure 2 This is a curve diagram of fracturing operations after combined pressure reduction;

[0031] Figure 3 It is a map-based method for calculating the effective action distance of acid in the case of filtration loss;

[0032] Figure 4 This is a top view of the reaction of acid flowing along a flat plate. Detailed Implementation

[0033] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0034] The following description, in conjunction with the accompanying drawings and specific embodiments, provides further details:

[0035] Example 1

[0036] The technical solution steps of this invention are as follows:

[0037] (1) Inject conventional acid for pretreatment to reduce construction pressure near the wellbore;

[0038] (2) Inject high-viscosity fracturing fluid to open and extend artificial fractures;

[0039] (3) Inject gelling acid to reduce the pressure of far-well construction by taking advantage of the low reaction rate of gelling acid with carbonate reservoirs;

[0040] (4) Inject high-viscosity fracturing fluid and carry small-particle-size proppant slugs to reduce fracturing fluid loss and promote main fracture extension.

[0041] (5) Injecting high-viscosity fracturing fluid carrying large-diameter proppant improves the conductivity of artificial fractures;

[0042] (6) Inject high-viscosity fracturing fluid to replace the sand-carrying fluid in the wellbore to prevent sand from settling in the wellbore.

[0043] The conventional acid mentioned in step 1 is 20% hydrochloric acid, and it is accompanied by the addition of corrosion inhibitors, iron ion stabilizers and acid pressure relief agents.

[0044] The aforementioned conventional acid is injected at a low flow rate of 1.5-2 cubic meters per minute.

[0045] The optimization steps for fracturing fluid usage in step 2 are as follows: Determine the required fracture length based on the effective distance of the gelling acid in step 3. With the required fracture length determined, calculate the fracturing fluid usage required to achieve this fracture length at a suitable flow rate.

[0046] The fracturing fluid dosage follows the volume balance equation and is calculated using the following formula:

[0047]

[0048] Where Q(t): injection volume; t: construction time; δ; A: crack area; w: crack width; C: filtration coefficient; erfc(x): error compensation function, which is obtained by looking up the function table.

[0049] In step 3, the distance of the gelling acid reaction is determined based on the experimental results of the acid-rock reaction rate.

[0050] Calculate using the following formula:

[0051] ① Treat the cracks as having equal width And an ideal vertical fracture with a uniform fracture height h. Simplify the two-dimensional flow reaction of acid between permeable parallel rock slabs, and based on the law of conservation of mass, establish the convection-diffusion partial differential equations for the acid flow reaction along the fracture. For example... Figure 4 As shown.

[0052] The partial differential equation for convection-diffusion is:

[0053]

[0054] In the formula: C is C(x, y), the acid concentration at any location in the crack; u x u represents the acid velocity component in the x-direction. y D represents the acid velocity component in the y-direction; H+ The effective mass transfer coefficient.

[0055] The solution C(x, y) to this partial differential equation represents the distribution of acid concentration during the acid flow and reaction in the crack. To solve this partial differential equation, boundary conditions must be given based on the actual flow and reaction conditions. Assume the coordinate axes are chosen as follows... Figure 1 As shown, boundary conditions are given at the fracture inlet and the reaction surface, and sometimes also at the fracture centerline. The acid concentration at the fracture inlet is C0. On the fracture wall, for the reaction of hydrochloric acid with rock, the reaction rate can be considered infinitely large compared to the H+ mass transfer coefficient, therefore the acid concentration on the wall is C≈0.

[0056] At the center of the crack and in the direction perpendicular to the wall, the acid concentration gradient is zero.

[0057] In summary, the boundary conditions for the reaction between hydrochloric acid and rock are:

[0058]

[0059] The reaction of acid flowing between rock fissures is similar to the heat conduction convection equation of heat carrier flowing between permeable plates. Under the boundary conditions of equation (4), for the partial differential equation (1), we borrow the analytical solution already given in the field of heat conduction under the same boundary conditions. However, it is inconvenient to use, so when dealing with practical problems, we generally use the approximate solution obtained by computer and draw it in the form of a diagram. Figure 3 For ease of use.

[0060] Figure 3 In the diagram, the vertical axis represents the number of Pictets without any known cause. The x-axis represents the distance without any coordinates. Each curve corresponds to a different concentration of acid C. D =C / C0.

[0061] Formula for calculating average filtration rate:

[0062]

[0063] In the formula: t is the construction time, in minutes.

[0064] Formula for calculating the average crack width:

[0065]

[0066] In the formula: is the viscosity of the acid solution, mPa·s; G is the rock elasticity model, MPa.

[0067] The formula for calculating Pictite numbers:

[0068]

[0069] In the formula: D H+ H+ mass transfer coefficient, cm 2 / s.

[0070] After obtaining the above physical parameters, the Pictrite number N can be calculated. p Given the position x of any cross-section within the crack, the corresponding infinite distance L can be calculated. D .

[0071] Formula for calculating dynamic crack length:

[0072]

[0073] In the formula: Q is the displacement, m 3 / min; H is the reservoir thickness, in meters.

[0074] Formula for calculating the average flow velocity at the crack inlet:

[0075]

[0076] Formula for calculating the effective action distance of acid:

[0077]

[0078] The amount of gelling acid used in step 3 is calculated using the following formula:

[0079]

[0080] In the formula: Q is the amount of gelling acid injected, m 3 ; q represents the injection displacement, m 3 / min; t is the construction time, min; h is the reservoir thickness, m.

[0081] In step 4, the small-particle-size proppant is made of 70 / 140 mesh ceramsite.

[0082] In step 5, 40 / 70 mesh ceramic granules are used as the large-particle support, while 30 / 50 mesh support is used at the seam opening to improve the seam's drainage capacity.

[0083] In step 6, the displacement fluid is 10 cubic meters of high-viscosity liquid, and the remainder is replaced with backflow fluid to reduce costs.

[0084] The beneficial effects of this invention are: by adopting the combined pressure reduction design method of this invention, not only can the construction pressure near the wellbore be reduced, but also the construction pressure at the far end of the fracture can be reduced, thereby ensuring the smooth sand addition to the carbonate reservoir.

[0085] Example 2

[0086] This embodiment is a specific operation based on embodiment 1.

[0087] (1) Taking the hydraulic fracturing of carbonate rock with a depth of 3000m and a length of 160m as an example, in the pumping procedure, 40 cubic meters of conventional acid are first injected at a flow rate of 2 cubic meters / minute. The conventional acid formula is: 20% HCl + 1.8% corrosion inhibitor + 0.3% iron ion stabilizer + 0.3% acid pressure aid to reduce the construction pressure near the wellbore.

[0088] (2) Inject 150 cubic meters of high-viscosity fracturing fluid at a flow rate of 12 cubic meters / minute to open and extend artificial fractures;

[0089] (3) According to the formula, the amount of gelling acid is 100 cubic meters under the limited action distance of 80 meters of acid. Injecting 100 cubic meters of gelling acid reduces the construction pressure in the far well.

[0090] (4) Inject high-viscosity fracturing fluid and carry small-particle-size proppant slugs to reduce fracturing fluid loss and promote main fracture extension.

[0091] (5) Injecting high-viscosity fracturing fluid carrying large-diameter proppant improves the conductivity of artificial fractures;

[0092] (6) Inject high-viscosity fracturing fluid to replace the sand-carrying fluid in the wellbore to prevent sand from settling in the wellbore.

[0093] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and variations. 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 design method for combined hydraulic fracturing and sand fracturing depressurization in carbonate rocks, characterized in that, include Step 1: Pre-treatment with low-volume injection of conventional acid to reduce construction pressure near the wellbore; Step 2: Inject high-viscosity fracturing fluid to open and extend artificial fractures; Step 3: Inject gelling acid. Utilize the low reaction rate of gelling acid with carbonate reservoirs to reduce construction pressure at the far end of the fracture. Step 4: Inject high-viscosity fracturing fluid with small-diameter proppant slugs to reduce fracturing fluid loss and promote main fracture extension. Step 5: Inject high-viscosity fracturing fluid carrying large-diameter proppant to improve the conductivity of artificial fractures; Step 6: Inject high-viscosity fracturing fluid to replace the sand-carrying fluid in the wellbore and prevent sand from settling in the wellbore.

2. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 1, characterized in that, The conventional acid used in step one is hydrochloric acid, with a mass fraction of 20%; the injection rate of the conventional acid is 1.5-2 cubic meters per minute.

3. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 2, characterized in that, Step one also requires the addition of a retarder, which includes a corrosion inhibitor, an iron ion stabilizer, and an acid decompression aid.

4. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 1, characterized in that, The amount of fracturing fluid used in step two should be based on the effective distance of the gelling acid action, the fracture length, and the discharge rate.

5. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 4, characterized in that, The fracturing fluid dosage follows the volume balance equation and is calculated using the following formula. Where Q(t): injection volume; t: construction time; δ; A: crack area; w: crack width; C: filtration coefficient; erfc(x): error compensation function, which is obtained by looking up the function table.

6. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 5, characterized in that, The distance of the gelling acid interaction in step three is calculated using the following convection-diffusion partial differential equation based on the experimental results of the acid-rock reaction rate. In the formula: C is C(x, y), the acid concentration at any location in the crack; u x u represents the acid velocity component in the x-direction. y D represents the acid velocity component in the y-direction; H+ The effective mass transfer coefficient.

7. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 6, characterized in that, When using the convection-diffusion partial differential equation, boundary conditions must be given based on the actual flow and reaction conditions. The boundary conditions for the reaction of hydrochloric acid with rock are as follows:

8. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 7, characterized in that, Under the boundary conditions of equation (4), for the partial differential equation (1), the analytical solution given in the field of heat conduction under the same boundary conditions is used to draw it into a layout form.

9. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 8, characterized in that, In the map format, the vertical axis represents the number of Pictets without any definite origin. The x-axis represents the distance without any coordinates. Each curve corresponds to a different concentration of acid C. D =C / C0.

10. The combined hydraulic fracturing and depressurization design method for carbonate rocks according to claim 9, characterized in that, In step four, 70 / 140 mesh ceramsite is used as the small-particle proppant; in step five, 40 / 70 mesh ceramsite is used as the large-particle proppant, and 30 / 50 mesh proppant is used at the seam opening to improve the seam's drainage capacity; in step six, the displacement liquid is a 10 cubic meter high-viscosity liquid, and the remainder is a backfill liquid to reduce costs.

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