A fracturing method for improving sanding strength of high stress shale reservoir

Abstract

The application discloses a fracturing method for improving sanding strength of high-stress shale reservoirs. The steps are: collecting reservoir parameters, simulating and optimizing fracturing process parameters; setting the upper limit pressure value of the fracturing unit in steps; pretreating the reservoir by injecting acid liquid with large displacement; creating a fracture by injecting preflush slickwater; injecting slickwater carrying small-diameter ultra-low-density proppant; switching to small-diameter conventional-density proppant; repeating the two rounds of sand switching to probe the optimal sand-liquid ratio at different stages; switching to mixed sanding of different-density proppant carrying small and medium-diameter combined proppant; switching to medium-diameter ultra-low-density proppant for sealing; performing fracturing fluid displacement operation to complete single-stage construction; and repeating the above steps. The method has the advantages of improving the fracturing reconstruction effect of high-stress reservoirs, avoiding the economic loss caused by long operation time and easy damage of the fracturing unit under long-time high-pressure pumping due to low sanding efficiency, greatly improving the sanding efficiency, reducing the material cost and construction cost, and achieving the purpose of cost reduction and benefit increase.

Description

Technical Field

[0001] This invention relates to a fracturing method for adding sand to shale reservoirs, specifically a fracturing method for enhancing the sand addition strength of high-stress shale reservoirs, belonging to the field of petroleum extraction technology. Background Technology

[0002] As exploration and development progresses and the burial depth increases, the wellhead operating pressure increases significantly, leading to relatively small fracture sizes and posing significant challenges to the smooth addition of proppant. Shale resources at depths greater than 3800m are abundant, but face challenges such as high fracturing operating pressure, difficulties in proppant addition, and low post-fracturing production, preventing commercial development. Shale reservoirs exhibit high overburden pressure, high fracturing pressure, high stress differential, and high closure pressure with increasing depth, posing significant challenges to shale reservoir fracturing stimulation. High overburden pressure increases confining pressure, strengthens plasticity, and hinders fracture initiation and propagation; high fracturing and operating pressures limit the flow rate and weaken fracture creation capacity; high closure pressure results in narrow initial fractures of various sizes, making it difficult to increase the proppant-fluid ratio and adding proppant.

[0003] When shale formations under high stress fracture, the wellhead pressure often approaches the pressure limit, with the operating pressure typically reaching 120 MPa. During fracturing operations, the fracture width is relatively narrow under high stress, making it highly sensitive to proppant and resulting in a small operating window. Pressure changes are often sudden; a sudden pressure surge has an extremely short reaction time (generally only 10-20 seconds), and sand blockage can directly lead to operational interruption. The persistently high wellhead pressure is extremely detrimental to the injection of medium and large-diameter proppant. Although designers use specialized software to model and simulate the fracture width at different pumping stages, actual operational conditions are often more complex and variable, making stable proppant injection impossible. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a fracturing method for improving the sand addition intensity of shale oil and gas wells, achieving long-term filling of fractures in high-stress reservoirs, and increasing single-well productivity.

[0005] To address the aforementioned problems, the present invention provides a fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs, comprising the following steps:

[0006] S1. Collect reservoir parameters and optimize fracturing process parameters through numerical simulation;

[0007] S2. Based on the design pressure limit of the fracturing well, set the upper limit pressure value of the fracturing unit in multiple steps;

[0008] S3. Pre-treat the reservoir with acid solution before injecting large volume, and then inject low viscosity slickwater to create fractures.

[0009] S4. Prioritize injecting slickwater carrying small-particle-size ultra-low-density ceramic proppant;

[0010] S5. Continuously inject slickwater carrying small-particle-size conventional density proppant to add sand;

[0011] S6. Repeat S4-S5 twice to explore the optimal sand-liquid ratio at different stages and complete 60% of the designed sand addition.

[0012] S7. Continuously inject slickwater carrying proppants of different densities with small and medium particle sizes for mixing and sand addition;

[0013] S8. Continuously inject slickwater carrying medium-sized ultra-low density proppant to add sand, and then seal the tail end.

[0014] S9. Fracturing fluid displacement operation to complete single-section construction;

[0015] S10. Repeat the above steps to complete the fracturing construction of the remaining sections.

[0016] In step S2, the step difference value of the fracturing unit is set to 1MPa. When the dynamic pressure during construction suddenly rises to the protection pressure value, the fracturing pump with the lower upper limit pressure setting should promptly disconnect the throttle, run in neutral, stop pumping and pressurizing, and actively reduce the construction pressure to prevent multiple fracturing units from disconnecting the throttle at the same time, thus avoiding a sharp decrease in discharge volume and causing sand blockage in the wellbore.

[0017] In step S4, the sand-liquid ratio of the slickwater carrying small-diameter ultra-low-density ceramsite is increased in a stepped manner; the small-diameter ultra-low-density ceramsite injected in this stage is 30% of the total amount of small-diameter ultra-low-density ceramsite designed for fracturing; preferably, in step S4, the sand ratio is increased in a stepped manner with an increase of 1%-2%, at least 3 steps, to explore the pressure response characteristics of sand added with different sand ratios.

[0018] The particle size of the small-particle-size ultra-low-density ceramic proppant is 70-140 mesh, and the bulk density is 1.4 g / cm³. 3 The particle size of the small-particle-size conventional density proppant is 70-140 mesh, and the bulk density is 1.48 g / cm³. 3 Medium-sized conventional proppant has a particle size of 40-70 mesh and a bulk density of 1.48 g / cm³. 3 The medium-sized ultra-low density proppant has a particle size of 40-70 mesh and a bulk density of 1.4 g / cm³. 3 .

[0019] In step S1, reservoir parameters are evaluated before fracturing, and process parameters are simulated and optimized based on the reservoir parameters, as well as fracturing operation parameters are optimized; the reservoir parameters include one or more of the drilled layers, rock mechanical parameters, and natural fracture conditions.

[0020] In step S3, a dual control mechanism for discharge and pressure is established. The pressure window must be controlled to be 7% greater than the construction pressure limit, and the discharge must be controlled to be less than 90% of the maximum discharge.

[0021] In step S4, the sand-liquid ratio of the slickwater carrying small-diameter ultra-low density ceramic proppant is increased stepwise from 4% to 6-9% with an increase of 1%-2%. The sand ratio is suitable for the width of the hydraulic fracture during the trial phase. In each sand ratio cycle, the injection volume of the proppant-carrying slickwater is 150%-200% of the wellbore volume.

[0022] In step S5, the slickwater carrying small-particle-size conventional density proppant is switched, and the change in sand ratio during proppant delivery is controlled. The value of the switched sand ratio is obtained by the change in proppant density and the change in the mass of the proppant-carrying fluid.

[0023] (1) The bulk density ρ of the proppant in the previous stage s1 and the bulk density ρ of the proppant in the next stage s2 ;

[0024] (2) The highest sand ratio S1 of the proppant in the previous stage;

[0025] (3) Volumetric flow rate V of the sand-carrying liquid in the previous stage s1 ;

[0026] (4) Calculate the proppant mass flow rate W using the volumetric flow rate of the proppant-carrying liquid and the sand ratio. s ;

[0027] (5) Calculate the sand ratio S after switching proppant. The specific formula is as follows:

[0028]

[0029] In the formula, δ is the density difference between the proppant elements; V s1 It is the volumetric flow rate of the sand-carrying liquid in the previous stage, in m 3 / min;V s2 It is the volumetric flow rate of the sand-carrying liquid in the next stage, in m 3 / min; ρ s1 This is the bulk density of the proppant in the previous stage, kg / m³. 3 ;ρ s2 This is the bulk density of the proppant in the next stage, kg / m³ 3 W s S1 is the proppant mass flow rate; S2 is the highest sand ratio of the proppant in the previous stage (%); S3 is the sand ratio of the proppant in the next stage (%).

[0030] In step S5, the sand-fluid ratio is adjusted according to the formula to add sand of the different sand types. If the pressure is relatively stable after the new sand type enters the reservoir, sand is added continuously at the current sand-fluid ratio. If the pressure rises, the sand-fluid ratio decreases in steps of 1%-2%. After the pressure stabilizes, the sand-fluid ratio increases in steps of 1%-2% until the highest sand ratio of 9% is reached. The injection volume of sand-carrying slickwater for each sand-fluid ratio is 70-150% of the wellbore volume. In this stage, 20% of the total amount of small-diameter conventional proppant designed for fracturing is injected.

[0031] In step S6, the first round repeats S4-S5, the proppant is switched to small-particle-size ultra-low-density ceramsite, low-viscosity slickwater is added with sand, and the sand-liquid ratio is increased stepwise by 1%-2%. This stage is to test the adaptability of hydraulic joint width under a sand ratio of 10-12%; when the sand ratio reaches 12%, the proppant is switched to conventional density quartz sand.

[0032] In this stage, the amount of small-diameter ultra-low density ceramic particles injected is 20% of the total amount of small-diameter ultra-low density ceramic particles designed for fracturing; in this stage, the amount of small-diameter conventional proppant injected is 30% of the total amount of small-diameter conventional proppant designed for fracturing.

[0033] The second round repeats S4-S5, switching the proppant to small-particle-size ultra-low-density ceramsite proppant, with the sand-liquid ratio increasing stepwise by 1%-2%. This stage tests the adaptability of the joint width at a sand ratio of 13-14%. When the sand ratio reaches 14%, the proppant is switched to conventional density quartz sand.

[0034] In this stage, the amount of small-diameter ultra-low density ceramic particles injected is 20% of the total amount of small-diameter ultra-low density ceramic particles designed for fracturing; in this stage, the amount of small-diameter conventional proppant injected is 50% of the total amount of small-diameter conventional proppant designed for fracturing.

[0035] In step S7, preferably, after completing 60% of the total sand volume in a single stage, slickwater carrying mixed particle size proppant (the ratio of ultra-low density small particle size to conventional density medium particle size = 1:1) is injected to add sand; this stage completes the designed total sand volume of small-particle-size ultra-low density ceramsite; that is, 30% of the total volume of small-particle-size ultra-low density ceramsite.

[0036] In step S8, preferably, slickwater carrying medium-sized ultra-low density proppant is continuously injected for sand addition until the total amount of sand added in a single stage is completed.

[0037] The advantages of this invention are:

[0038] To address the challenges of high pressure and difficult proppant addition during fracturing operations in high-stress reservoirs, this invention innovatively employs a dual-mechanism approach that coordinates the control of fracturing unit and process parameters, along with variations in proppant density and particle size. Leveraging the low density of ultra-low density ceramsite, it alternates with and mixes with conventional density quartz sand in a proppant addition pattern. This approach achieves efficient and continuous proppant addition by using ultra-low density ceramsite to increase the proppant ratio for rapid, high-ratio proppant addition, while using conventional density quartz sand to stabilize the proppant ratio and ensure proppant addition. This solves the technical difficulties of large pressure fluctuations, difficult proppant addition, and low proppant strength during fracturing in high-stress reservoirs, meeting the requirements for high-strength proppant addition and ensuring the effectiveness of fracturing in high-stress reservoirs. This method avoids the economic losses caused by large pressure fluctuations and inability to continuously add proppant under high-stress reservoir conditions, which leads to low proppant addition efficiency, long operation times, and potential damage to the fracturing unit under prolonged high-pressure pumping. Compared to conventional proppant addition methods, this significantly improves proppant addition efficiency, reduces fracturing material costs, and achieves cost reduction and efficiency improvement. Attached Figure Description

[0039] Figure 1 This is a fracturing and proppant addition curve diagram of a certain section of a high-stress shale gas reservoir in this invention; Detailed Implementation

[0040] The fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs according to the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] The fracturing method of the present invention for improving the sand-addition strength of high-stress shale reservoirs includes the following steps:

[0042] S1. Collect reservoir parameters and optimize fracturing process parameters through numerical simulation;

[0043] S2. Based on the construction pressure limit value of the fracturing well, set the upper limit pressure value of the fracturing unit in stages;

[0044] In this step, the fracturing units are used in a coordinated manner, with the upper limit pressure (protective pressure) of the fracturing units set in multiple steps, with a step difference of 1 MPa. When the dynamic pressure during construction suddenly rises to the protective pressure value, the fracturing pump with the lower upper limit pressure setting is immediately de-energized, runs in neutral, stops pumping and pressurization, and actively reduces the construction pressure to prevent overpressure. This method avoids the situation where multiple fracturing pumps are de-energized at the same time, which would cause a sudden and significant reduction in displacement and affect the construction quality.

[0045] S3. A certain amount of pre-treated acid solution is injected into the reservoir at a large displacement, and then low-viscosity slickwater is injected to create fractures.

[0046] S4. Prioritize injecting slickwater carrying small-particle-size ultra-low-density ceramic proppant;

[0047] In this step, the sand-liquid ratio of the slickwater carrying small-diameter ultra-low density ceramsite is increased in a stepped manner; preferably, the small-diameter ultra-low density ceramsite injected in this stage is 30% of the total amount of small-diameter ultra-low density ceramsite designed for fracturing; preferably, the sand ratio is increased in a stepped manner with an increase of 1%-2%, at least 3 steps, to explore the pressure response characteristics of sand added with different sand ratios.

[0048] S5. Continuously inject slickwater carrying small-particle-size conventional density proppant to add sand;

[0049] In this step, the slickwater carrying small-diameter conventional density proppant is switched, and the change in the sand ratio during proppant delivery is controlled. The value of the switched sand ratio is obtained by the change in proppant density and the change in the mass of the proppant-carrying fluid in the wellbore.

[0050] (1) The bulk density ρ of the proppant in the previous stage s1 and the bulk density ρ of the proppant in the next stage s2 ;

[0051] (2) The highest sand ratio S1 of the proppant in the previous stage;

[0052] (3) Volumetric flow rate V of the sand-carrying liquid in the previous stage s1 ;

[0053] (4) Calculate the proppant mass flow rate Ws by using the volumetric flow rate of the sand-carrying liquid and the sand ratio;

[0054] (5) Calculate the changed sand ratio. The specific formula is as follows:

[0055]

[0056] In the formula, δ is the density difference between the proppant elements; V s1 V is the volumetric flow rate of the sand-carrying liquid in the previous stage, in m³ / min; s2 The next stage is the volumetric flow rate of the sand-carrying liquid, in m³ / min; ρ s1 This is the bulk density of the proppant in the previous stage, kg / m3; ρ s2 This is the bulk density of the proppant in the next stage, kg / m3; W s S1 is the proppant mass flow rate; S2 is the highest sand ratio of the proppant in the previous stage (%); S3 is the sand ratio of the proppant in the next stage (%).

[0057] In this step, sand is added according to the converted sand-liquid ratio. Preferably, if the construction pressure is relatively stable after sand replacement, sand is added continuously at the current sand-liquid ratio. If the pressure increases, the sand-liquid ratio decreases in a stepwise manner of 1%-2%. After the pressure stabilizes, the sand-liquid ratio increases in a stepwise manner of 1%-2% until the highest sand ratio of the ultra-low density ceramsite in the previous stage is reached. The amount of sand-carrying slickwater injected for each sand-liquid ratio is 70-150% of the wellbore volume.

[0058] S6. Repeat steps S4-S5 twice to explore the optimal sand-liquid ratio at different stages and complete 60% of the designed sand addition.

[0059] In the first round, S4-S5 were repeated, and the proppant was switched to small-particle-size ultra-low-density ceramsite. Low-viscosity slickwater was added with sand, and the sand-liquid ratio was increased stepwise by 1%-2%. This stage was to test the adaptability of hydraulic joint width under a sand ratio of 10-12%. When the sand ratio reached 12%, the proppant was switched to conventional density quartz sand.

[0060] The second round repeats S4-S5, switching the proppant to small-particle-size ultra-low-density ceramsite proppant, with the sand-liquid ratio increasing stepwise by 1%-2%. This stage tests the adaptability of the joint width at a sand ratio of 13-14%. When the sand ratio reaches 14%, the proppant is switched to conventional density quartz sand.

[0061] Switching to slickwater carrying small-particle-size conventional density proppant, the initial sand-liquid ratio is determined by a conversion formula; after the sand-liquid enters the formation, the sand-liquid ratio is adjusted stepwise by an increase or decrease of 1%-2% according to the pressure fluctuation, until the highest sand ratio of the ultra-low density ceramsite in the previous stage is reached.

[0062] S7. Continuously inject slickwater carrying proppant of different densities with small and medium particle sizes for mixing and sand addition; this stage tests the adaptability of a sand ratio of 14-15% to the width of the joint.

[0063] Preferably, sand is added by injecting slickwater carrying a combination proppant (the ratio of small-diameter ultra-low density ceramsite and medium-diameter conventional density proppant is 1:1). Depending on the pressure fluctuation, the sand-liquid ratio increases stepwise by 1%-2% or decreases stepwise by 1%-2%, until the highest sand ratio of 15% is reached in the stage.

[0064] S8. Continuously inject slickwater carrying medium-sized ultra-low density proppant to add sand, and then seal the tail end.

[0065] Preferably, this stage tests the adaptability of the joint width with a sand ratio of 15-16%. Depending on the pressure fluctuation, the sand-liquid ratio is increased in steps of 1%-2% or decreased in steps of 1%-2% until the maximum sand ratio of 16% is reached in the stage; until the amount of sand added in a single stage is completed.

[0066] S9. Fracturing fluid displacement operation to complete single-section construction;

[0067] S10. Repeat the above steps to complete the fracturing construction of the remaining sections.

[0068] Among them, the small-particle-size ultra-low-density ceramsite proppant has a particle size of 70-140 mesh and a bulk density of 1.4 g / cm³. 3The particle size of the small-particle-size conventional density proppant is 70-140 mesh, and the bulk density is 1.48 g / cm³. 3 Medium-sized conventional proppant has a particle size of 40-70 mesh and a bulk density of 1.48 g / cm³. 3 The medium-sized ultra-low density proppant has a particle size of 40-70 mesh and a bulk density of 1.4 g / cm³. 3 .

[0069] Furthermore, prior to step S1, pre-fracturing reservoir parameter evaluation may be performed, and process parameter simulation and fracturing operation parameter optimization may be conducted based on the reservoir parameters. The reservoir parameters may include one or more of the drilled strata, rock mechanical parameters, and natural fracture conditions. Process parameter optimization may be performed using ECLIPSE, and fracturing operation parameter optimization may be performed using Fracpro PT or GOFHER, which are methods well known to industry professionals.

[0070] Furthermore, in step S4, the sand-liquid ratio of the slippery water carrying small-particle-size ultra-low-density ceramsite proppant is increased stepwise from 4% to 6-9% with an increase of 1%-2%, which is the sand ratio that adapts to the width of the hydraulic cracks in the efficient testing stage.

[0071] Within each sand ratio cycle, the injection volume of proppant-carrying slickwater is 150%-200% of the wellbore volume; preferably, after the proppant-carrying fluid enters the reservoir, the pressure decreases and gradually increases or the pressure increases and the discharge volume is reduced to avoid the pressure exceeding the construction pressure limit.

[0072] The so-called slickwater is a powdered drag-reducing agent prepared and stored in a flexible three-dimensional tank. It has a viscosity of 9-12 mPa·s and allows sufficient swelling time from pumping into the tank to wellbore entry, resulting in stable liquid properties and consistent performance of low-viscosity slickwater. Pressure fluctuations during sand addition can eliminate the influence of liquid properties, directly reflecting the formation's sand-feeding capacity.

[0073] Furthermore, in step S2, a dual control mechanism for pressure window and discharge rate is established. The construction pressure window must be controlled to be greater than 7% of the construction design pressure limit, and the discharge rate must be controlled to be less than 90% of the maximum discharge rate.

[0074] Under pressure constraints, a large volume of hydrochloric acid is injected to significantly increase the acid etching volume and establish a better near-wellbore fracture channel.

[0075] Through the above methods, an innovative dual control mechanism for pressure window and displacement, and alternating and combined proppant addition modes were developed to achieve efficient and continuous proppant addition. This solved the technical challenges of high construction pressure, difficult proppant addition, and low proppant strength during fracturing in high-stress reservoirs. It met the requirements of high-strength proppant addition and strong support for high-stress reservoirs, ensuring the fracturing effect of high-stress reservoirs. It realized the concept of stable pressure and strong proppant addition in high-stress reservoirs, and solved problems such as large pressure fluctuations, inability to add proppant continuously, prolonged pumping time, and easy fatigue damage to the unit. Compared with conventional proppant addition modes, it significantly improved proppant addition efficiency, reduced fracturing material costs, and achieved the goal of cost reduction and efficiency improvement.

[0076] The following examples demonstrate its effectiveness:

[0077] The fracturing method of the present invention for enhancing the proppant addition strength of high-stress shale reservoirs was applied in the volumetric fracturing stimulation of a well in the Nanchuan area of ​​Chongqing. The vertical depth of the fracturing section in this well was 3914-4576m, the inclined depth was 5808m, and the horizontal section length was 1806m. The fracturing proppant addition was performed using the method provided by the present invention, and the steps and results are as follows:

[0078] Construction preparation should be carried out according to the engineering design parameters. Perforation parameters: large-diameter perforating ammunition BH42RDX28-2, spiral perforation pattern, 16mm diameter, 60° phase angle, 3-5 clusters per section, 0.6-1m per cluster; designed construction displacement 18-20m³ / h. 3 / min, equipped with 15 6000 type electric pumps, construction pressure limit 125MPa.

[0079] Based on the well construction pressure limit of 125MPa, the upper limit pressure protection pressure of the electric pump is set in stages, with one pump at 122MPa, two at 123MPa, two at 124MPa, and the rest at 125MPa.

[0080] 1) Inject 40m 3 The acid solution was rapidly discharged at a pressure limit of 125 MPa, reaching a discharge rate of 7.0 m³. 3 The acid delivery rate was reduced by 6 MPa per minute, while the acid etching volume was significantly increased.

[0081] 2) Inject low-viscosity slickwater, gradually increase the discharge rate, control the pressure window to be greater than 8.75 MPa (7% of the construction pressure limit), and simultaneously control the construction discharge rate to be less than 18 m³. 3 / min (Design maximum displacement 20m) 3 (90% of / min); displacement increased to 15.1m 3 / min, construction pressure 116MPa, completed injection of 210m of low-viscosity slickwater pre-flushing solution. 3 ;

[0082] 3) Inject slickwater carrying sand containing small-diameter, ultra-low-density ceramsite. Use low-viscosity slickwater (9-12 mPa·s) to carry 70-140 mesh sand with a bulk density of 1.4 g / cm³. 3 Ultra-low density ceramic proppant injection, with a sand-to-fluid ratio of 4%-6%-8%-9%, where each sand-to-fluid ratio has a fracturing fluid volume of approximately 80m³. 3 ;

[0083] 4) Switch the proppant to small-diameter quartz sand, converting the initial sand-to-liquid ratio to 8%, and inject low-viscosity slickwater at 9-12 mPa·s carrying 70-140 mesh particles with a bulk density of 1.48 g / cm³. 3 Quartz sand proppant injection, with the sand-to-liquid ratio being 8%-9%;

[0084] During this stage, the pressure fluctuates slightly but remains controllable, with each sand fluid having a volume of 150m³ relative to the fracturing fluid. 3 ;

[0085] 5) Switch the proppant to small-diameter ultra-low-density ceramsite, converting the initial sand-to-liquid ratio to 10%, and inject 9-12 mPa·s low-viscosity slickwater carrying 70-140 mesh particles with a bulk density of 1.4 g / cm³. 3 Ultra-low density ceramsite proppant injection, with the injection sand-to-liquid ratio successively at 10%-11%-12%;

[0086] During this stage, the pressure is relatively stable, and the volume of each sand fluid relative to the fracturing fluid is 70m³. 3 ;

[0087] 6) Switch the proppant to small-particle-size quartz sand, convert the initial sand-to-liquid ratio to 11%, inject 9-12 mPa·s low-viscosity slickwater to carry 70-140 mesh quartz sand proppant with a bulk density of 1.48 g / cm3, and inject the sand-to-liquid ratio in sequence as 11%-10%-11%-12%.

[0088] During this stage, the pressure rises slowly. A 1% reduction in the sand-liquid ratio is adopted, and sand is added. After the pressure stabilizes, the sand-liquid ratio is gradually increased by 1% to 12%.

[0089] 8) Switch the proppant to small-diameter ultra-low density ceramsite, with an initial sand-to-fluid ratio of 13%. Inject 9-12 mPa·s low-viscosity slickwater carrying 70-140 mesh ultra-low density ceramsite proppant with a bulk density of 1.4 g / cm3. The sand-to-fluid ratio is 13%-14% at each injection point. The pressure is relatively stable during this stage, and the fracturing fluid volume is 70 m3 for each sand-to-fluid ratio.

[0090] 9) Switch the proppant to small-diameter quartz sand, converting the initial sand-to-liquid ratio to 13%, and inject 9-12 mPa·s low-viscosity slickwater carrying 70-140 mesh particles with a bulk density of 1.48 g / cm³. 3Quartz sand proppant is injected; during this stage, the pressure rises slightly, and sand is added while reducing the sand-to-liquid ratio by 1%; after the pressure stabilizes, the sand-to-liquid ratio is gradually increased by 1% to 14%;

[0091] 10) Switch the proppant to small-diameter ultra-low density ceramsite and medium-diameter quartz sand (volume ratio 1:1), assuming an initial sand-to-liquid ratio of 14%. Inject 9-12 mPa·s low-viscosity slickwater carrying 70-140 mesh ultra-low density ceramsite with a bulk density of 1.4 g / cm³ and 40-70 mesh quartz sand with a bulk density of 1.48 g / cm³ proppant. During this stage, the pressure fluctuates slightly upwards, and the sand-to-liquid ratio is reduced by 1% while adding sand. After the pressure stabilizes, the sand-to-liquid ratio is gradually increased by 1% to 15%. The sand-to-liquid ratio injected during this stage is successively 14%-13%-14%-15%.

[0092] 11) Switch the proppant to medium-sized ultra-low density ceramsite, with an initial sand-to-liquid ratio of 15%, and inject low-viscosity slickwater at 9-12 mPa·s carrying 40-70 mesh bulk density of 1.4 g / cm³. 3 Ceramsite proppant injection;

[0093] During this stage, the construction pressure dropped slightly, the discharge volume was increased to 17.5 cubic meters, and the construction pressure was 116.2 MPa; the construction pressure was relatively stable, and the sand-liquid ratio was gradually increased to 16% in 1% increments; the sand addition for each section was completed.

[0094] 12) Inject 120 ml of displacement fluid. 3 Low viscosity slippery water.

[0095] 13) Repeat the above steps to complete the fracturing construction of the remaining sections.

[0096] The results are as follows: the overall well fracturing design sand injection completion rate was 101.96%, the average sand-to-fluid ratio was 9.86%, and the average single-stage fluid volume was 2240.8 m³. 3 The average sand volume per section is 188.4 m³. 3 The proportion of quartz sand used increased from 20% to 60% compared to adjacent wells, resulting in a 24% reduction in material costs. After fracturing, the flowback, production testing, and formal production were carried out according to standard procedures. Following fracturing, a 10.0mm nozzle was used for gas testing, achieving a maximum daily gas production of 17.2 × 10⁴ m³. 3 Compared to adjacent wells on the platform, the improvement was 29%, achieving good transformation results and economic benefits.

[0097] Verification through the above specific applications shows that, under high-stress construction conditions, by adopting methods such as coordinating unit setup and control mechanisms, and changing the density and particle size of proppant, the approach of stabilizing pressure and increasing proppant addition in high-stress reservoirs has been successfully implemented. This approach solves problems such as high construction pressure, difficulty in proppant addition, large pressure fluctuations, inability to continuously add proppant, prolonged pumping time, and easy fatigue damage of the unit during fracturing in high-stress reservoirs. Compared with conventional proppant addition methods, this method significantly improves proppant addition efficiency, reduces fracturing material costs, and reduces the difficulty of proppant addition in high-stress reservoir fracturing, while achieving the goal of cost reduction and efficiency improvement, and has broad application value.

Claims

1. A fracturing method for improving the sand-addition strength of high-stress shale reservoirs, characterized in that, Includes the following steps: S1. Collect reservoir parameters and optimize fracturing process parameters through numerical simulation; S2. Based on the construction pressure limit value of the fracturing well, set the upper limit pressure value of the fracturing unit in stages; S3. Pre-treat the reservoir with acid solution before injecting large volume, and then inject low viscosity slickwater to create fractures. S4. Prioritize injecting slickwater carrying small-particle-size ultra-low-density ceramic proppant; S5. Continuously inject slickwater carrying small-particle-size conventional density proppant to add sand; S6. Repeat S4-S5 twice to explore the optimal sand-liquid ratio at different stages and complete 60% of the designed sand addition. S7. Continuously inject slickwater carrying proppants of different densities with small and medium particle sizes for mixing and sand addition; S8. Continuously inject slickwater carrying medium-sized ultra-low density proppant to add sand, and then seal the tail end. S9. Fracturing fluid displacement operation to complete single-section construction; S10. Repeat the above steps to complete the fracturing construction of the remaining section; In step S4, the sand-liquid ratio of the slickwater carrying small-diameter ultra-low density ceramic particles is increased in a stepwise manner. The small-diameter ultra-low density ceramic particles injected in this stage account for 30% of the total amount of small-diameter ultra-low density ceramic particles in the fracturing design. In step S4, the sand ratio is increased in a stepwise manner with an increase of 1%-2%, and this stage increases the sand ratio by at least 3 steps. In step S5, the slickwater carrying small-diameter conventional density proppant is switched to control the change in the sand ratio during proppant delivery. The value of the changed sand ratio is obtained by the change in proppant density and the change in the mass of the proppant-carrying fluid in the wellbore. (1) The bulk density ρ of the proppant in the previous stage s1 and the bulk density ρ of the proppant in the next stage s2 ; (2) The highest sand ratio S1 of the proppant in the previous stage; (3) Volumetric flow rate V of the sand-carrying liquid in the previous stage s1 ; (4) Calculate the proppant mass flow rate W using the volumetric flow rate of the proppant-carrying liquid and the sand ratio. s ; (5) Calculate the sand ratio S after switching proppant. The specific formula is as follows: In the formula, δ is the density difference between the proppant elements; V s1 It is the volumetric flow rate of the sand-carrying liquid in the previous stage, in m 3 / min;V s2 It is the volumetric flow rate of the sand-carrying liquid in the next stage, in m 3 / min; ρ s1 This is the bulk density of the proppant in the previous stage, kg / m³. 3 ;ρ s2 This is the bulk density of the proppant in the next stage, kg / m³ 3 W s S1 is the proppant mass flow rate; S2 is the highest sand ratio of the proppant in the previous stage, %; S3 is the sand ratio of the proppant in the next stage, %; In step S5, the sand-liquid ratio is adjusted according to the formula to add sand of different types. If the pressure is relatively stable after the new sand type enters the reservoir, the current sand-liquid ratio is maintained and sand is added continuously. If the pressure rises, the sand-liquid ratio decreases in a stepwise manner of 1%-2%. After the pressure stabilizes, the sand-liquid ratio increases in a stepwise manner of 1%-2% until the highest sand ratio of 9% in this stage is reached. The amount of sand-carrying slickwater injected for each sand-liquid ratio is 70-150% of the wellbore volume. In step S6, the first round repeats S4-S5, switching the proppant to small-diameter ultra-low density ceramsite, adding sand with low-viscosity slickwater, and increasing the sand-to-liquid ratio in steps of 1%-2%. This stage tests the adaptability of adding sand to hydraulic fracture width at a sand ratio of 10-12%. When the sand ratio reaches 12%, the proppant is switched to conventional density quartz sand. Based on the pressure fluctuation after the conventional density quartz sand enters the reservoir, the sand-to-liquid ratio decreases or increases in steps of 1%-2% until the highest sand ratio of small-diameter ultra-low density ceramsite reaches 12%. The second round repeats S4-S5, switching the proppant to small-particle-size ultra-low-density ceramsite proppant, with the sand-liquid ratio increasing stepwise by 1%-2%. This stage explores the adaptability of adding sand to the joint width at a sand ratio of 12-14%. When the sand ratio reaches 14%, the proppant is switched to conventional density quartz sand. The initial sand-liquid ratio for switching from ultra-low density proppant to conventional density proppant is determined by formula conversion. After the proppant-carrying fluid of the switched sand type enters the formation, the sand-liquid ratio is adjusted stepwise by an increase or decrease of 1%-2% according to the pressure fluctuation, until the highest sand ratio of the small-diameter ultra-low density ceramsite reaches 14%.

2. The fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs according to claim 1, characterized in that: In step S2, considering the high construction pressure of high-stress reservoirs, the step difference of the fracturing unit is optimized to 1 MPa; when the construction dynamic pressure suddenly rises to the protection pressure value, the fracturing pump with a lower upper limit pressure setting promptly disconnects the throttle, runs in neutral, stops pumping and pressurization, and actively reduces the construction pressure.

3. The fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs according to claim 2, characterized in that: The particle size of the small-particle-size ultra-low-density ceramic proppant is 70-140 mesh, and the bulk density is 1.4 g / cm³. 3 The particle size of the small-particle-size conventional density proppant is 70-140 mesh, and the bulk density is 1.48 g / cm³. 3 Medium-sized conventional proppant has a particle size of 40-70 mesh and a bulk density of 1.48 g / cm³. 3 The medium-sized ultra-low density proppant has a particle size of 40-70 mesh and a bulk density of 1.4 g / cm³. 3 .

4. The fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs according to claim 1, 2, or 3, characterized in that: In step S1, reservoir parameters are evaluated before fracturing, and process parameters are simulated and optimized based on the reservoir parameters, as well as fracturing operation parameters are optimized; the reservoir parameters include one or more of the drilled layers, rock mechanical parameters, and natural fracture conditions.

5. The fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs according to claim 4, characterized in that: In step S4, the sand-liquid ratio of the slickwater carrying small-diameter ultra-low-density ceramic proppant is increased stepwise from 4% to 6-9% in increments of 1%-2%, to test the appropriate sand ratio for the width of the hydraulic fracture during the trial phase; within each sand ratio cycle, the injection volume of the proppant-carrying slickwater is 150%-200% of the wellbore volume, to test the pressure response characteristics of sand addition at different sand ratios.

6. The fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs according to claim 5, characterized in that: In step S7, after completing 60% of the total sand volume for a single section, slickwater carrying ultra-low density small-particle-size ceramsite and conventional density medium-particle-size proppant is injected to add sand. Depending on the fluctuation of construction pressure, the sand-liquid ratio is increased stepwise by 1%-2% or decreased stepwise by 1%-2% to adjust the sand ratio to ensure stable construction pressure, up to the highest sand ratio of 15% for the stage.

7. The fracturing method for enhancing the sand-addition strength of high-stress shale reservoirs according to claim 6, characterized in that: In step S2, in view of the high construction pressure of high stress reservoirs, an optimized dual control mechanism of discharge rate and pressure is formulated. The construction pressure window must be controlled to be greater than 7% of the construction pressure limit value, and the discharge rate must be controlled to be less than 90% of the maximum discharge rate.

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