Fracturing method for improving sand adding strength of high-stress shale reservoir

Abstract

The invention discloses a fracturing method for improving the sand adding strength of a high-stress shale reservoir. The method comprises the steps that reservoir parameters are collected, and fracturing process parameters are simulated and optimized; the upper limit pressure value of the fracturing unit is set step by step; acid liquid is injected in a large-displacement mode to pretreat the reservoir, and preposed slickwater is injected to form a fracture; injecting slick water carrying a small-particle-size ultra-low-density proppant; switching and carrying a small-particle-size conventional-density proppant; the sand mold is repeatedly switched for two rounds, and the optimal sand-liquid ratio in different stages is tested; different-density proppants carrying small and medium particle size combinations are switched to be mixed and added with sand; switching to carry a medium-particle-size ultralow-density proppant for sealing; fracturing fluid displacement operation is carried out, and single-section construction is completed; and repeating the steps. The method has the advantages that the fracturing transformation effect of a high-stress reservoir is improved, the economic loss that the operation time is long due to low sand adding efficiency and a fracturing unit is easy to damage under long-time high-pressure pumping injection is avoided, the sand adding efficiency is greatly improved, the material cost and the construction cost are reduced, and the purposes of reducing cost and increasing efficiency are achieved.

Description

Technical Field

[0001] The invention relates to a shale reservoir sand fracturing method, in particular to a fracturing method for improving the sand strength of a high-stress shale reservoir, and belongs to the technical field of petroleum exploitation. Background Art

[0002] As the exploration and development process deepens, the burial depth becomes deeper and deeper, which brings about a substantial increase in wellhead construction pressure, which in turn leads to a relatively small size of fracturing cracks, which brings great difficulties to the smooth addition of proppants. Shale resources with a burial depth of more than 3,800m are abundant, but there are problems such as high fracturing construction pressure, difficulty in adding sand, and low production after fracturing, and commercial development has not yet been achieved. Shale reservoirs show the characteristics of high overburden pressure, high fracture pressure, high stress difference, and high closure pressure as the burial depth increases, which brings great challenges to the fracturing transformation of shale reservoirs. With high overburden pressure, the rock confining pressure increases, the plastic characteristics become stronger, and it is difficult for cracks to start and extend; with high fracture pressure and construction pressure, the construction displacement is limited, and the fracture-making ability is weakened; with high closure pressure, the original fractures of various scales are narrow, the sand-liquid ratio is difficult to increase, and sand addition is difficult.

[0003] When shale high-stress formations fracture, the wellhead pressure is often close to the limit pressure, and the construction pressure is generally as high as 120MPa. During fracturing construction, due to the relatively narrow fracture width under high stress, it is sensitive to sand and liquid, and the construction window is small. The change in construction pressure is somewhat sudden. Once the pressure rises suddenly, the reaction time is extremely short (generally only 10 to 20 seconds), and sand plugging directly leads to construction interruption. The high wellhead pressure is extremely unfavorable for the injection of medium-sized and large-sized proppants. Although the designers use professional software indoors to model and simulate the fracture width of multi-scale fractures at different pumping stages, the actual construction situation is often complex and changeable, and stable sand addition cannot be achieved. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a fracturing method for increasing the sanding strength of shale oil and gas wells, realizing long-term filling of cracks in high-stress reservoir transformation, and improving the production capacity of a single well.

[0005] In order to solve the above problems, the fracturing method for improving the sand strength of high-stress shale reservoirs of the present invention comprises the following steps:

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

[0007] S2. According to the construction pressure limit design value of the fracturing well, the upper limit pressure value of the fracturing unit is set in multiple steps;

[0008] S3, inject a large amount of pre-acid solution to pre-treat the reservoir, and then inject low-viscosity slick water to create fractures;

[0009] S4, preferentially injecting slippery water carrying small-size ultra-low-density ceramsite proppant;

[0010] S5, continuously injecting slick water carrying small-size conventional density proppant to add sand;

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

[0012] S7, continuously injecting slick water carrying proppants of different densities with small and medium particle sizes for mixing and sanding;

[0013] S8, continuously injecting slick water carrying medium-sized ultra-low-density proppant to add sand and tail-end seal;

[0014] S9, fracturing fluid displacement operation, completing single-stage construction;

[0015] S10, repeat the above steps to complete the remaining fracturing construction.

[0016] In step S2, the step difference of the fracturing unit is set to 1MPa; when the construction dynamic pressure suddenly rises to the protection pressure value, the fracturing pump with a lower upper limit pressure setting will promptly remove the throttle, run in neutral, stop pumping and pressurizing, and actively reduce the construction pressure to prevent multiple fracturing units from removing the throttle at the same time, avoid a sharp decrease in displacement, and cause sand blockage caused by sand settling in the wellbore.

[0017] In step S4, the sand-to-liquid ratio of the slick water carrying small-size ultra-low-density expanded clay is a step-by-step sand extraction; the small-size ultra-low-density expanded clay injected at this stage is 30% of the total amount of small-size ultra-low-density expanded clay designed for fracturing; preferably, in step S4, the sand ratio is increased in a step-by-step manner with an increase of 1%-2%, and at least 3 step-by-step sand ratios are increased to explore the pressure response characteristics of adding sand at different sand ratios.

[0018] The particle size of small-size ultra-low-density ceramsite proppant is 70-140 mesh and the volume density is 1.4g / cm 3 ; The particle size of small-size conventional density proppant is 70-140 mesh, and the bulk density is 1.48g / cm 3 ; The particle size of conventional medium-sized proppant is 40-70 mesh, and the bulk density is 1.48g / cm 3 The particle size of medium-sized ultra-low-density proppant is 40-70 mesh, and the volume density is 1.4g / cm 3 .

[0019] In step S1, reservoir parameter evaluation is performed before fracturing, and process parameter simulation optimization and fracturing operation parameter optimization are performed based on the reservoir parameters; the reservoir parameters include one or more of the drilling strata, rock mechanics parameters, and natural fracture conditions.

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

[0021] In step S4, the sand-to-liquid ratio of the slick water carrying small-size ultra-low-density ceramsite proppant is increased stepwise from 4% to 6-9% with an increase of 1%-2%. The sand ratio is applicable to the width of the hydraulic fracture in the trial stage. In each sand ratio cycle, the injection amount of the slick water carrying the proppant is 150%-200% of the wellbore volume.

[0022] In step S5, the slick water carrying small-size conventional density proppant is switched to control the change of sand ratio during proppant delivery, and the value of the switched sand ratio is obtained by the change of proppant density and the change of sand-carrying fluid mass:

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

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

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

[0026] (4) Calculate the proppant mass flow rate W by the volume flow rate of the sand-carrying fluid and the sand ratio s ;

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

[0028]

[0029] Where δ is the density difference between proppants; V s1 is the volume flow rate of the sand-carrying fluid in the previous stage, m 3 / min; V s2 is the volume flow rate of the sand-carrying fluid in the next stage, m 3 / min; ρ s1 is the bulk density of the proppant in the previous stage, kg / m 3 ρ s2 is the bulk density of the proppant in the next stage, kg / m 3 ; W s is the proppant mass flow rate; S1 is the highest sand ratio of the proppant in the previous stage, %; S2 is the sand ratio of the proppant in the next stage, %.

[0030] In step S5, the sand-liquid ratio is adjusted according to the formula to change the sand type and add sand; if the pressure of the new sand type is relatively stable after entering the reservoir, the current sand-liquid ratio is continued to add sand; if the pressure rises, the sand-liquid ratio decreases by 1%-2% in steps; after the pressure stabilizes, the sand-liquid ratio is increased by 1%-2% in steps until the highest sand ratio of the stage is 9%. The injection amount of sand-carrying slick water for each sand-liquid ratio is 70-150% of the wellbore volume; the small-particle conventional proppant injected at this stage is 20% of the total amount of small-particle conventional proppant designed for fracturing.

[0031] In step S6, the first round of S4-S5 is repeated, the proppant is switched to small-size ultra-low-density ceramsite, low-viscosity slippery water is added with sand, and the sand-liquid ratio is increased stepwise by 1%-2%. In this stage, the adaptability of the hydraulic fracture width under the sand ratio of 10-12% is tested; when the sand ratio reaches 12%, the proppant is switched to conventional density quartz sand;

[0032] The small-size ultra-low-density ceramsite injected at this stage is 20% of the total amount of small-size ultra-low-density ceramsite designed for fracturing; the small-size conventional proppant injected at this stage is 30% of the total amount of small-size conventional proppant designed for fracturing;

[0033] In the second round, S4-S5 were repeated, and the proppant was switched to small-size ultra-low-density ceramsite proppant. The sand-liquid ratio was increased stepwise by 1%-2%. In this stage, the adaptability of the seam width under the sand ratio of 13-14% was tested; when the sand ratio reached 14%, the proppant was switched to conventional density quartz sand;

[0034] The small-size ultra-low-density ceramsite injected at this stage is 20% of the total amount of small-size ultra-low-density ceramsite designed for fracturing; the small-size conventional proppant injected at this stage is 50% of the total amount of small-size conventional proppant designed for fracturing;

[0035] In step S7, preferably, after 60% of the total sand volume of a single stage is completed, slick water carrying a 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; at this stage, the total amount of sand added by the small particle size ultra-low density ceramsite is completed; that is, 30% of the total amount of the small particle size ultra-low density ceramsite;

[0036] In step S8, preferably, slick water carrying medium-sized ultra-low-density proppant is continuously injected to add sand until the total amount of sand added in a single stage is completed;

[0037] The advantages of the present invention are:

[0038] In view of the characteristics of high pressure and difficult sand addition in fracturing construction of high stress reservoirs, innovative methods such as the dual mechanism of coordinating the control of fracturing units and process parameters, and the density and particle size of proppants are adopted. The low density advantage of ultra-low density ceramsite is brought into play, and the alternating and mixed sand addition mode with conventional density quartz sand is formed to form the idea of ​​ultra-low density powder ceramsite to improve the sand ratio and test the critical high sand ratio to add sand quickly, and conventional density quartz sand to stabilize the sand ratio to add sand, so as to achieve efficient and continuous sand addition, solve the technical problems of large pressure fluctuations, great difficulty in adding sand, and low sand laying strength during fracturing of high stress reservoirs, meet the requirements of high-strength sand addition, ensure the fracturing transformation effect of high stress reservoirs, and realize the idea of ​​stable pressure and strong sand addition for high stress reservoirs. The method of the present invention avoids the situation that under the condition of high construction pressure of high stress reservoirs, the sand addition fluctuates greatly and cannot be added continuously, resulting in low sand addition efficiency, long operation time, and economic losses caused by easy damage of the fracturing unit under long-term high-pressure pumping; compared with the conventional sand addition mode, the sand addition efficiency is greatly improved, the cost of fracturing materials is reduced, and the purpose of reducing costs and increasing efficiency is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a fracturing and sanding curve diagram of a certain section of a shale gas high stress reservoir in the present invention; DETAILED DESCRIPTION

[0040] The fracturing method for improving the sanding strength of high-stress shale reservoirs of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0041] The fracturing method for improving the sand-adding strength of a high-stress shale reservoir of the present invention comprises the following steps:

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

[0043] S2. According to the construction pressure limit value of the fracturing well, the upper limit pressure value of the fracturing unit is set in steps;

[0044] In this step, the use of the fracturing unit is coordinated, and the upper limit pressure (protection pressure) of the fracturing unit is set in multiple steps, with a step difference of 1MPa; when the construction dynamic pressure suddenly rises to the protection pressure value, the fracturing pump with a lower upper limit pressure setting is promptly removed from the throttle, operated in neutral, and pumping and pressurization are stopped, so as to actively reduce the construction pressure and prevent overpressure; this method avoids the simultaneous removal of the throttles of multiple fracturing pumps, which causes a sharp reduction in displacement in an instant and affects the construction quality;

[0045] S3, inject a certain amount of pre-acid solution at a large displacement to pre-treat the reservoir, and then inject low-viscosity slick water to create fractures;

[0046] S4, preferentially injecting slippery water carrying small-size ultra-low-density ceramsite proppant;

[0047] In this step, the sand-liquid ratio of the slick water carrying small-size ultra-low-density ceramsite is a step-by-step sand extraction; preferably, the small-size ultra-low-density ceramsite injected at this stage is 30% of the total amount of small-size ultra-low-density ceramsite designed for fracturing; preferably, the sand ratio is increased stepwise by 1%-2%, and at least 3 step-by-step sand ratios are increased to test the pressure response characteristics of adding sand at different sand ratios;

[0048] S5, continuously injecting slick water carrying small-size conventional density proppant to add sand;

[0049] In this step, the slick water carrying small-size conventional density proppant is switched to control the change of sand ratio during proppant delivery. The value of the switched sand ratio is obtained by the change of proppant density and the change of the mass of the wellbore sand-carrying fluid:

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

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

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

[0053] (4) Calculate the proppant mass flow rate Ws by the sand-carrying fluid volume flow rate and the sand ratio;

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

[0055]

[0056] Where δ is the density difference between proppants; V s1 is the volume flow rate of the sand-carrying fluid in the previous stage, m3 / min; V s2 is the volume flow rate of the sand-carrying fluid in the next stage, m3 / min; ρ s1 is the bulk density of the proppant in the previous stage, kg / m3; ρ s2 is the bulk density of the proppant in the next stage, kg / m3; W s is the proppant mass flow rate; S1 is the highest sand ratio of the proppant in the previous stage, %; S2 is the sand ratio of the proppant in the next stage, %.

[0057] In this step, the sand-changing type is added according to the converted sand-liquid ratio; preferably, if the construction pressure is relatively stable after the sand-changing, the sand is added continuously at the current sand-liquid ratio; if the pressure rises, the sand-liquid ratio decreases in a step-by-step manner by 1%-2%; after the pressure is stable, the sand-liquid ratio increases in a step-by-step manner by an increase of 1%-2% until the highest sand ratio of the ultra-low-density ceramsite in the previous stage is reached; the injection amount of sand-carrying slick water for each sand-liquid ratio is 70-150% of the wellbore volume;

[0058] S6, repeat steps S4-S5 twice, try 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, the proppant was switched to small-size ultra-low-density ceramsite, low-viscosity slippery water was added with sand, and the sand-liquid ratio was increased stepwise by 1%-2%. In this stage, the adaptability of the hydraulic fracture width under the sand ratio of 10-12% was tested; when the sand ratio reached 12%, the proppant was switched to conventional density quartz sand;

[0060] In the second round, S4-S5 were repeated, and the proppant was switched to small-size ultra-low-density ceramsite proppant. The sand-liquid ratio was increased stepwise by 1%-2%. In this stage, the adaptability of the seam width under the sand ratio of 13-14% was tested; when the sand ratio reached 14%, the proppant was switched to conventional density quartz sand;

[0061] Switch to slick water carrying small-size conventional density proppant, and the initial sand-liquid ratio is determined by the conversion formula; after the sand-carrying liquid enters the formation, the sand-liquid ratio is adjusted in steps with an increase or decrease of 1%-2% according to the pressure fluctuation, until the highest sand ratio of ultra-low density ceramsite in the previous stage is reached;

[0062] S7, continuously inject slick water carrying proppants of different densities with small and medium particle sizes to mix and add sand; in this stage, test the adaptability of the seam width under the sand ratio of 14-15%;

[0063] Preferably, slick water carrying a combined proppant (the ratio of small-size ultra-low-density ceramsite to medium-size proppant with conventional density = 1:1) is injected to add sand, and according to the pressure fluctuation, the sand-liquid ratio is increased or decreased in steps of 1%-2% to a maximum sand ratio of 15% in the stage;

[0064] S8, continuously injecting slick water carrying medium-sized ultra-low-density proppant to add sand and tail-end seal;

[0065] Preferably, in this stage, the adaptability of the seam width under the sand ratio of 15-16% is tested. According to the pressure fluctuation, the sand-to-liquid ratio is increased in steps of 1%-2% or decreased in steps of 1%-2% to the highest sand ratio of 16% in the stage; until the single-stage sand addition is completed.

[0066] S9, fracturing fluid displacement operation, completing single-stage construction;

[0067] S10, repeat the above steps to complete the remaining fracturing construction.

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

[0069] Furthermore, before step S1, it is also possible to include a pre-fracturing reservoir parameter evaluation, and perform process parameter simulation and fracturing construction parameter optimization based on the reservoir parameters; the reservoir parameters include one or more of the drilled layers, rock mechanics parameters, and natural fracture conditions; use ECLIPSE to optimize process parameters, and use Fracpro PT or GOFHER to optimize fracturing construction parameters, which are methods well known to industry technicians.

[0070] Furthermore, in step S4, the sand-liquid ratio of the slick water carrying the small-size ultra-low-density ceramsite proppant is increased stepwise from 4% to 6-9% with an increase of 1%-2%, and the sand ratio is adapted to the width of the hydraulic fracture in the efficient trial stage;

[0071] In each sand ratio cycle, the injection volume of slick water carrying proppant is 150%-200% of the wellbore volume; preferably, after the sand-carrying fluid enters the reservoir, the pressure drops and the displacement is gradually increased, or the pressure rises and the displacement is reduced to avoid the pressure exceeding the construction pressure limit.

[0072] The slippery water is a powder-type drag reducer prepared and stored in a flexible three-dimensional tank with a viscosity of 9-12mPa·s. It has sufficient swelling time from being pumped into the tank to entering the well. The liquid performance is relatively stable, and the performance of low-viscosity slippery water is consistent. The pressure fluctuation during the sand addition process can eliminate the influence of the liquid performance and directly reflect the sand feeding capacity of the formation.

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

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

[0075] Through the above methods, innovative dual control mechanisms of pressure window and displacement, alternating multiple types of proppants and combined sand adding modes were formulated to achieve efficient and continuous sand adding, which solved the technical problems of high construction pressure, great difficulty in sand adding and low sand laying strength during fracturing of high stress reservoirs, met the requirements of high-strength sand adding and strong support for high stress reservoirs, ensured the fracturing and transformation effect of high stress reservoirs, realized the idea of ​​stable pressure and strong sand adding for high stress reservoirs, and solved the problems of large pressure fluctuations, inability to continuously add sand, extended pumping time, and easy fatigue damage of the unit. Compared with the conventional sand adding mode, the sand adding efficiency was greatly improved, the cost of fracturing materials was reduced, and the purpose of reducing costs and increasing efficiency was achieved.

[0076] The following is a specific application example to verify its effect:

[0077] The fracturing method of the present invention for increasing the sanding strength of high-stress shale reservoirs was applied in the volume fracturing reconstruction construction of a well in Nanchuan, Chongqing. The vertical depth of the fracturing section of the well was 3914-4576m, the oblique depth was 5808m, and the horizontal section length was 1806m. The steps and results of fracturing and sanding were as follows:

[0078] Construction preparations were made according to the engineering design parameters. Perforating parameters: large-aperture perforating bullet BH42RDX28-2, spiral hole arrangement, aperture 16mm, 60° phase angle, 3-5 clusters of perforations in a single stage, 0.6-1m per cluster; designed construction displacement 18-20m 3 / min, equipped with 15 6000-type electric pumps, with a construction pressure limit of 125MPa.

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

[0080] 1) Inject 40m 3 Acid, quickly increase the displacement under the pressure limit of 125MPa, and the displacement is increased to 7.0m 3 / min acid is delivered into the formation, the construction pressure is reduced by 6MPa, and the acid etching volume is greatly increased.

[0081] 2) Inject low-viscosity slippery water, increase the displacement in steps, control the pressure window to be greater than 8.75MPa (7% of the construction pressure limit value), and control the construction displacement to be less than 18m 3 / min(design maximum displacement 20m 3 / min); displacement increased to 15.1m 3 / min, construction pressure 116MPa, completed injection of pre-pad low-viscosity slippery water 210m 3 ;

[0082] 3) Inject the slippery water carrying sand liquid with small particle size ultra-low density ceramsite, using 9-12mPa·s low viscosity slippery water carrying 70-140 mesh and volume density of 1.4g / cm 3 Ultra-low density ceramsite proppant injection, the injection sand-liquid ratio is 4%-6%-8%-9%, and the volume of each sand-liquid ratio fracturing fluid is about 80m 3 ;

[0083] 4) Switch the proppant to small-size quartz sand, convert the initial sand-liquid ratio to 8%, inject 9-12mPa·s low-viscosity slippery water with 70-140 mesh and a bulk density of 1.48g / cm 3 Quartz sand proppant injection, the injection sand-liquid ratio is 8%-9%;

[0084] The pressure fluctuates slightly during this stage and is controllable. The volume of each sand-liquid ratio fracturing fluid is 150m 3 ;

[0085] 5) Switch the proppant to small-size ultra-low-density ceramsite, convert the initial sand-liquid ratio to 10%, inject 9-12mPa·s low-viscosity slippery water with 70-140 mesh and a bulk density of 1.4g / cm 3 Ultra-low density ceramsite proppant injection, the injection sand-liquid ratio is 10%-11%-12%;

[0086] The pressure is relatively stable at this stage, and the volume of each sand-liquid ratio of the fracturing fluid is 70m 3 ;

[0087] 6) Switch the proppant to small-size quartz sand, convert the initial sand-liquid ratio to 11%, inject 9-12mPa·s low-viscosity slick water carrying 70-140 mesh and 1.48g / cm3 quartz sand proppant, and inject the sand-liquid ratio of 11%-10%-11%-12%;

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

[0089] 8) Switch the proppant to small-size ultra-low-density ceramsite, convert the initial sand-liquid ratio to 13%, inject 9-12mPa·s low-viscosity slick water carrying 70-140 mesh and ultra-low-density ceramsite proppant with a volume density of 1.4g / cm3, and inject the sand-liquid ratio of 13%-14%; the pressure is relatively stable at this stage, and the fracturing fluid volume for each sand-liquid ratio is 70m3;

[0090] 9) Switch the proppant to small-size quartz sand, convert the initial sand-liquid ratio to 13%, inject 9-12mPa·s low-viscosity slippery water with 70-140 mesh and a bulk density of 1.48g / cm 3Quartz sand proppant injection; during this stage, the pressure rises slightly, and the sand-to-liquid ratio is reduced by 1% to add sand; after the pressure stabilizes, the sand-to-liquid ratio is gradually increased by 1% to 14%;

[0091] 10) Switch the proppant to small-size ultra-low-density ceramsite and medium-size quartz sand (volume ratio 1:1), convert the initial sand-liquid ratio to 14%, inject 9-12mPa·s low-viscosity slick water carrying 70-140 mesh ultra-low-density ceramsite with a volume density of 1.4g / cm3 and 40-70 mesh quartz sand with a volume density of 1.48g / cm3. The pressure fluctuates slightly during this stage, and the sand-liquid ratio is reduced by 1% to add sand; after the pressure stabilizes, the sand-liquid ratio is gradually increased by 1% to 15%; the sand-liquid ratio injected in this stage is 14%-13%-14%-15%;

[0092] 11) Switch the proppant to medium-sized ultra-low-density ceramsite, convert the initial sand-liquid ratio to 15%, inject 9-12mPa·s low-viscosity slippery water with 40-70 mesh, and the volume density is 1.4g / cm 3 Ceramic proppant injection;

[0093] During this stage, the construction pressure dropped slightly, the displacement was increased to 17.5 cubic meters, and the construction pressure was 116.2MPa; the construction pressure was relatively stable, and the sand-to-liquid ratio was gradually increased by 1% to 16%; the single-stage sand addition was completed.

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

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

[0096] The results are as follows: The sand addition completion rate of the whole well fracturing design is 101.96%, the average sand-liquid ratio is 9.86%, and the average single-stage liquid volume is 2240.8m 3 The average sand volume per stage is 188.4m 3 . The proportion of quartz sand used in adjacent wells increased from 20% to 60%, and the material cost was reduced by 24%. After the fracturing, the flowback, production test and formal production were carried out according to the conventional process. After the fracturing, the well was tested with a 10.0mm nozzle, and the maximum daily gas production was 17.2×104m 3 , an increase of 29% compared with the adjacent wells on the platform, achieving good transformation effects and economic benefits.

[0097] Through the verification of the above specific applications, it can be known that under high-stress construction conditions, the idea of ​​stabilizing the pressure and strengthening the sand addition in high-stress reservoirs is realized by coordinating the setting and control mechanism of the unit, changing the density and particle size of the proppant, etc., and solving the problems of high construction pressure, difficulty in adding sand, large pressure fluctuations, inability to add sand continuously, extended pumping time, and easy fatigue damage of the unit during the fracturing of high-stress reservoirs. Compared with the conventional sand addition mode, this method greatly improves the sand addition efficiency, reduces the cost of fracturing materials, reduces the difficulty of adding sand for fracturing of high-stress reservoirs, and achieves the purpose of reducing costs and increasing efficiency at the same time, and has a wide range of application value.

Claims

1. A fracturing method for improving the sand strength of high-stress shale reservoirs, characterized in that: The steps include: S1. Collect reservoir parameters and optimize fracturing process parameters through numerical simulation; S2. According to the construction pressure limit value of the fracturing well, the upper limit pressure value of the fracturing unit is set in steps; S3, inject a large amount of pre-acid solution to pre-treat the reservoir, and then inject low-viscosity slick water to create fractures; S4, preferentially injecting slippery water carrying small-size ultra-low-density ceramsite proppant; S5, continuously injecting slick water carrying small-size conventional density proppant to add sand; S6. Repeat S4-S5 twice to test the optimal sand-liquid ratio at different stages and complete 60% of the designed sand addition; S7, continuously injecting slick water carrying proppants of different densities with small and medium particle sizes for mixing and sanding; S8, continuously injecting slick water carrying medium-sized ultra-low-density proppant to add sand and tail-end seal; S9, fracturing fluid displacement operation, completing single-stage construction; S10, repeat the above steps to complete the remaining fracturing construction.

2. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 1, characterized in that: In step S2, the step difference of the fracturing unit is optimized to be 1MPa in view of the high construction pressure of the high-stress reservoir; when the construction dynamic pressure suddenly rises to the protection pressure value, the fracturing pump with a lower upper limit pressure setting will promptly remove the throttle, run in neutral, stop pumping and pressurizing, and actively reduce the construction pressure.

3. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 1 or 2, characterized in that: In step S4, the sand-to-liquid ratio of the slick water carrying small-size ultra-low-density ceramsite is increased in a step-by-step manner; the small-size ultra-low-density ceramsite injected in this stage is 30% of the total amount of small-size ultra-low-density ceramsite designed for fracturing; preferably, the sand-to-liquid ratio in step S4 is increased in a step-by-step manner with an increase of 1%-2%, and at least 3 step-by-step sand ratios are increased in this stage.

4. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 3, characterized in that: The particle size of small-size ultra-low-density ceramsite proppant is 70-140 mesh and the volume density is 1.4g / cm 3 ; The particle size of small-size conventional density proppant is 70-140 mesh, and the bulk density is 1.48g / cm 3 ; The particle size of conventional medium-sized proppant is 40-70 mesh, and the bulk density is 1.48g / cm 3 The particle size of medium-sized ultra-low-density proppant is 40-70 mesh, and the volume density is 1.4g / cm 3 .

5. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 1, 2 or 4, characterized in that: In step S1, reservoir parameter evaluation is performed before fracturing, and process parameter simulation optimization and fracturing operation parameter optimization are performed based on the reservoir parameters; the reservoir parameters include one or more of the drilling strata, rock mechanics parameters, and natural fracture conditions.

6. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 5, characterized in that: In step S4, the sand-to-liquid ratio of the slick water carrying small-size ultra-low-density ceramsite proppant is increased stepwise from 4% to 6-9% with an increase of 1%-2%, and the applicable sand ratio of the width of the hydraulic fracture is tested in the trial stage; in each sand ratio cycle, the injection amount of the slick water carrying the proppant is 150%-200% of the wellbore volume, and the pressure response characteristics of adding sand at different sand ratios are tested.

7. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 1, 2, 4 or 6, characterized in that: In step S5, the slick water carrying small-size conventional density proppant is switched to control the change of sand ratio during proppant delivery, and the value of changing the sand ratio is obtained by the change of proppant density and the change of the mass of the wellbore sand-carrying fluid: (1) Volume density of proppant in the previous stage ρ s1 and the bulk density of the proppant in the next stage ρ s2 ; (2) the highest sand ratio S1 of the proppant in the previous stage; (3) Volume flow rate of sand-carrying fluid in the previous stage V s1 ; (4) Calculate the proppant mass flow rate W by the volume flow rate of the sand-carrying fluid and the sand ratio s ; (5) Calculate the sand ratio S after switching the proppant. The specific formula is as follows: ； ； ； ; Where δ is the density difference between proppants; V s1 is the volume flow rate of the sand-carrying fluid in the previous stage, m 3 / min; V s2 is the volume flow rate of the sand-carrying fluid in the next stage, m 3 / min; ρ s1 is the bulk density of the proppant in the previous stage, kg / m 3 ; ρ s2 is the bulk density of the proppant in the next stage, kg / m 3 ; W s is the proppant mass flow rate; S1 is the highest sand ratio of the proppant in the previous stage, %; and S2 is the sand ratio of the proppant in the next stage, %.

8. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 7, characterized in that: In step S5, the sand-liquid ratio is adjusted according to the formula after conversion to perform sand-changing type sand addition; If the pressure of the new sand mold is relatively stable after entering the reservoir, continue to add sand at the current sand-to-liquid ratio; If the pressure rises, the sand-liquid ratio decreases in steps of 1%-2%; After the pressure stabilizes, the sand-to-liquid ratio increases stepwise by 1%-2% to the highest sand-to-liquid ratio of 9% at this stage; the injection volume of sand-carrying slick water for each sand-to-liquid ratio is 70-150% of the wellbore volume.

9. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 1, 2, 4, 6 or 8, characterized in that: In step S6, the first round of S4-S5 is repeated, the proppant is switched to small-size ultra-low-density ceramsite, low-viscosity slippery water is added with sand, and the sand-liquid ratio is increased stepwise by 1%-2%. In this stage, the adaptability of adding sand to the hydraulic fracture width under the sand ratio of 10-12% is tested; when the sand ratio reaches 12%, the proppant is switched to conventional density quartz sand; according to the pressure fluctuation after the conventional density quartz sand enters the reservoir, the sand-liquid ratio is decreased or increased stepwise by 1%-2%, until the highest sand ratio of small-size ultra-low-density ceramsite is 12%; In the second round, S4-S5 were repeated, and the proppant was switched to small-size ultra-low-density ceramsite proppant. The sand-liquid ratio was increased stepwise by 1%-2%. In this stage, the adaptability of adding sand to the seam width under the sand ratio of 12-14% was tested. When the sand ratio reached 14%, the proppant was switched to conventional density quartz sand. The initial sand-to-liquid ratio of ultra-low density proppant switching to conventional density proppant is determined by formula conversion. After the sand-carrying liquid of the switched sand type enters the formation, the sand-to-liquid ratio is adjusted in steps with an increase or decrease of 1%-2% according to the pressure fluctuation, until the maximum sand ratio of small-size ultra-low density ceramsite is 14%.

10. The fracturing method for improving the sand strength of high-stress shale reservoirs according to claim 9, characterized in that: In step S7, preferably, after completing 60% of the total sand volume of a single stage, slick water carrying ultra-low-density small-particle ceramsite and conventional-density medium-particle proppant is injected to add sand. According to the fluctuation of construction pressure, the sand-liquid ratio is increased in steps of 1%-2% or decreased in steps of 1%-2%. The sand ratio is adjusted to ensure a stable construction pressure until the highest sand ratio of the stage is 15%.

11. The fracturing method for improving the sand strength of high-stress shale reservoir according to claim 10, characterized in that: In step S2, in view of the high construction pressure of the high-stress reservoir, a dual control mechanism of displacement and pressure is optimized and formulated. The construction pressure window must be controlled to be greater than 7% of the construction pressure limit value, and the displacement must be controlled to be less than 90% of the maximum displacement.

Images