A fracturing pump injection process for dry hot granite construction artificial thermal reservoir

Abstract

A fracturing pump injection process for dry hot granite formation artificial heat reservoir, step 1, using plant acid to wash the fracturing wellbore; step 2, using variable viscosity slick water fracturing fluid to carry out first displacement, first time length fracturing, to generate first stage hydraulic fracture in dry hot granite reservoir; step 3, reducing the fracturing pump injection displacement and the viscosity of slick water fracturing fluid, to carry out second time length fracturing to the reservoir, to generate second stage hydraulic fracture in dry hot granite reservoir; step 4, using plant acid to pump acid liquid with second displacement; step 5, using crosslinked fracturing fluid to pump 2000-3000m 3 of liquid volume with third displacement; step 6, using variable viscosity slick water fracturing fluid to carry out fourth displacement, third time length pump injection fracturing; step 7, repeating steps 2-6 for two to three times, and experiencing time intervals of different time lengths between adjacent steps in steps 2-6.

Description

Technical Field

[0001] This invention belongs to the field of geothermal energy development technology, and in particular relates to a fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite under extrusion tectonic stress field conditions. Background Technology

[0002] Hot dry rock geothermal energy, as an emerging environmentally friendly and large-scale clean energy source, is gradually influencing the global energy landscape and is expected to become a new direction for energy structure transformation. Developing hot dry rock requires the establishment of enhanced geothermal systems (EGS), the core of which involves drilling wells into the reservoir and fracturing them to form a fracture network of a certain scale, constructing an inter-well circulation loop to extract heat for power generation. Since the 1970s, many countries have successively attempted to develop hot dry rock resources, and the number of EGS projects under construction has been steadily increasing. However, due to limitations in key technologies such as hot dry rock hydraulic fracturing, only a handful of EGS projects have been successfully operational. In recent years, the superiority and feasibility of hot dry rock resources have gradually gained international recognition, and the efficient development of hot dry rock resources has become a critical scientific issue that urgently needs to be addressed.

[0003] Under the influence of various tectonic forces, the stress distribution within the components of the lithosphere constitutes the tectonic stress field. Compressive tectonic stress is a common type of tectonic stress field. Compared to extensional stress environments, its main characteristics for development are: hot dry rock reservoirs are less prone to forming opening fractures, and excessive construction pressure can easily induce large-energy microseismic events. To adapt to the construction environment under these stress conditions and to form a complex network of heat exchange fractures, the fracturing displacement in hot dry rock operations internationally is typically significantly lower than that in oil and gas reservoir fracturing. Artificial fractures are primarily formed through shear failure, a process known as low-displacement shear fracturing.

[0004] Chinese invention patent application number 202010120642.0 discloses a screen string structure and its construction method for fracturing hot dry rock, including pumping measures for clean water, slickwater, and acid. The patent uses a screen string for well completion and specifies a fracturing process flow for this string. However, this invention is highly specific and lacks universality; this screen string well completion and fracturing method can be considered a branch of general fracturing. Furthermore, the invention does not elaborate on the fracturing time or the compatibility between different fluids.

[0005] Chinese invention patent application CN201910411072.8 discloses a method for improving the connectivity of microfractures in hot dry rock. It employs high-flow-rate, multi-stage temporary plugging fracturing to redirect and extend fractures at different locations. Then, acidic slickwater is injected to dissolve the filling material in the hot dry rock, connecting microfractures, redirected microfractures, and dissolution fractures to form a microfracture network system. However, this invention focuses on creating a fracture network system but does not describe the fracturing and reservoir creation process or provide detailed construction parameters.

[0006] The above examples of technical background analysis show that, in the current research process of developing dry hot granite, the construction of artificial thermal reservoirs and the enhancement of the complexity of hydraulic fractures in dry hot granite reservoirs are still in the exploratory stage, and many key technical issues need to be solved. Summary of the Invention

[0007] In view of the above problems, the purpose of this invention is to provide a fracturing pump injection process for constructing artificial thermal reservoirs in dry hot granite. Based on a full analysis of the characteristics of the fracturing process and the geological properties of the reservoir in dry hot granite, this invention reveals a method for the formation of complex fracture networks during the hydraulic fracturing process of constructing artificial thermal reservoirs in dry hot granite under the stress field of compression tectonics. This method can improve the complexity of hydraulic fractures in dry hot granite during the fracturing process.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite includes the following steps:

[0010] Step 1: Use 10%~12% plant acid to wash the fractured wellbore to remove cement slurry residue left in the natural fractures around the well due to drilling and cementing operations.

[0011] Step 2: Use variable viscosity slick water fracturing fluid to perform fracturing at the first displacement and first duration to generate first-stage hydraulic fractures in the dry hot granite reservoir. From starting the fracturing truck to increasing the displacement to the main working displacement of this stage, there are 8 to 10 steps of increasing the displacement. After the fracturing of this stage is completed, the displacement will decrease from the main working displacement to 0, which will also go through the same number of steps of decreasing the displacement.

[0012] Step 3: Reduce the fracturing pump injection rate and the viscosity of the slickwater fracturing fluid to perform fracturing on the reservoir for a second duration, generating secondary hydraulic fractures in the dry hot granite reservoir. The process of increasing and decreasing the injection rate also adopts a step-like change process.

[0013] Step 4: Use plant acid with a concentration of 15%~20% and pump the acid solution at the second flow rate to increase the surface roughness of the primary and secondary hydraulic fractures and reduce the pumping pressure.

[0014] Step 5: Use cross-linked fracturing fluid of the first viscosity at the third pumping rate to inject 2000~3000m³. 3 The volume of liquid continuously increases the length and width of the first-stage hydraulic fracture;

[0015] Step 6: Use variable viscosity slick water fracturing fluid for fourth-volume, third-duration pumping fracturing to increase the length and width of primary hydraulic fractures, as well as the number and scale of secondary hydraulic fractures in the dry hot granite reservoir.

[0016] Step 7: Repeat steps 2 to 6 two to three times, with varying time intervals between adjacent steps 2 to 6.

[0017] Furthermore, in step 2, the variable viscosity slickwater fracturing fluid is made by adding additives to clean water to achieve a viscosity range of 15-25 mPa·s; the first displacement range is 3.5-5 m³ / s. 3 / min, the first duration ranges from 36 to 48 hours; the amplitude distribution of each step of the stepped increase and decrease in emission rate ranges from 0.4 to 0.8m. 3 / min.

[0018] Furthermore, in step 3, the fracturing pump injection rate is reduced to 2~3.5m³. 3 The flow rate is adjusted at a rate of 0.5 m / min to reduce the viscosity of the slickwater fracturing fluid to 0–10 mPa·s; the second duration is 20–24 hours; the step amplitude of the flow rate adjustment process is 0.4–0.8 m. 3 / min, with 4 to 6 steps.

[0019] Furthermore, in step 4, the second displacement is 1.0~2.0m. 3 / min, the total amount of acid solution is no more than 300 m³ 3 pH < 4.5; the pumping rate of acid solution is adjusted using a stepped method.

[0020] Furthermore, in step 5, the first viscosity range is 180~200 mPa·s; the third displacement is 3.5~5m³. 3 / min.

[0021] Furthermore, in step 6, the viscosity range of the variable viscosity slickwater fracturing fluid is 15~25 mPa·s; the fourth discharge rate is 2~3.5 m³ / s. 3 / min, the third duration is 60~72 hours.

[0022] Furthermore, in step 7, after pumping the acid and crosslinking fluid, the pump is stopped for 10 to 12 hours, and the interval after pumping the slickwater fracturing fluid of different viscosities is 24 to 36 hours.

[0023] The beneficial effects of this invention are:

[0024] (1) For the construction of artificial thermal reservoirs in dry hot granite reservoirs under fracturing conditions, this invention provides specific pumping parameters and the reasons for using such parameters.

[0025] (2) The present invention describes a relatively detailed process flow, and adopts a combination of multi-liquid and multi-pumping processes to obtain complex hydraulic fracture morphology in dry hot granite reservoirs step by step. Detailed Implementation

[0026] The following will use specific embodiments to illustrate the structure of the present invention and the desired technical effects. However, the selected embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0027] It should be noted that, in the description of the embodiments of the present invention, the terms "upper," "lower," "front," "rear," "left," "right," "vertical," and "horizontal," indicating orientation or positional relationships, and the terms "larger," "smaller," and "medium," indicating quantity or length relationships, are only for the convenience of describing the present invention and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the present invention. The terms "first-level," "second-level," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0028] Taking a fracturing well in a dry, hot granite reservoir as an example, the fracturing operation is rationally arranged based on well logging interpretation, construction requirements, HSE (Health, Safety, and Environment) and well control requirements, as well as reservoir properties, structural surface development, and differences in horizontal in-situ stress. The fracturing pump injection process of this invention for constructing artificial thermal reservoirs in dry, hot granite reservoirs involves injection through Φ88.9mm tubing, with a packer used to isolate the casing. During construction, the annulus is pressurized according to the tubing pressure to protect the wellhead and upper casing.

[0029] The fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite, provided by this invention, includes the following steps:

[0030] Step 1: Use 10%~12% plant acid to wash the fractured wellbore to clean the cement slurry remaining in the natural fractures around the well due to drilling and cementing operations. The cement slurry remaining in the natural fractures is cement stone used to bond the well wall to the casing. Considering the environmental protection requirements in the geothermal development process, a higher proportion of plant acid is used to corrode it.

[0031] Step 2 involves using variable viscosity slickwater fracturing fluid for initial fracturing at the first displacement and duration, generating primary hydraulic fractures in the dry, hot granite reservoir. The fracturing truck is activated until the displacement reaches the main working displacement for this stage, undergoing 8-10 step-by-step increases. Similarly, after this stage of fracturing, the displacement decreases from the main working displacement to zero through the same number of step-by-step decreases. In Step 2, the variable viscosity slickwater fracturing fluid is made by adding additives to clean water to achieve a viscosity range of 15-25 mPa·s. The first displacement range is 3.5-5 m. 3 / min, the first duration ranges from 36 to 48 hours; the amplitude distribution of each step of the stepped increase and decrease in emission rate ranges from 0.4 to 0.8m. 3 / min; The stepped increase and stepped decrease in discharge rate are to reduce pressure fluctuations in hydraulic fractures and thus reduce the probability of induced earthquakes; Since the wellhead pressure increases with the increase in discharge rate during pumping, and an upper limit is set on the wellhead pressure during fracturing operations, generally the pressure limit for a 105-type fracturing wellhead is 90~95 MPa, and the pressure limit for a 140-type wellhead is 110~120 MPa. Therefore, the increase in discharge rate and the number of steps depend on the wellhead pressure.

[0032] Step 3 involves reducing the fracturing pump injection rate and the viscosity of the slickwater fracturing fluid to perform a second-duration fracturing operation on the reservoir, generating secondary hydraulic fractures in the dry, hot granite reservoir. The injection rate is also adjusted in a step-like manner. In this step, the fracturing pump injection rate is reduced to 2-3.5 m³ / h. 3 The flow rate is adjusted at a rate of 0.5 m / min to reduce the viscosity of the slickwater fracturing fluid to 0–10 mPa·s; the second duration is 20–24 hours; the step amplitude of the flow rate adjustment process is 0.4–0.8 m. 3 / min, with 4-6 steps;

[0033] Step 4 involves injecting a 15%–20% concentration of plant acid at a second pumping rate to increase the surface roughness of the primary and secondary hydraulic fractures and reduce the pumping pressure. In this step, to ensure the acid can etch the hydraulic fracture surface for a longer period, the pumped acid needs to maintain a low pH value for an extended time. Therefore, the acid concentration is appropriately increased, and the second pumping rate is 1.0–2.0 m³ / min. 3 / min, the total amount of acid solution is no more than 300 m³ 3 pH < 4.5; the pumping rate of acid solution is adjusted using a stepped method.

[0034] Step 5: Use cross-linked fracturing fluid of the first viscosity at the third pumping rate to inject 2000~3000m³. 3The liquid volume continuously increases the length and width of the first-stage hydraulic fracture; in step 5, the first viscosity range is 180~200 mPa·s; the third discharge rate is 3.5~5m³ / s. 3 / min;

[0035] Step 6 involves pumping fracturing with a fourth flow rate and third duration using variable viscosity slickwater fracturing fluid to increase the length and width of primary hydraulic fractures, as well as the number and size of secondary hydraulic fractures, in the dry, hot granite reservoir. In this step 6, the viscosity of the variable viscosity slickwater fracturing fluid is in the range of 15–25 mPa·s; the fourth flow rate is 2–3.5 m³ / s. 3 / min, the third duration is 60~72 hours;

[0036] Step 7: Repeat steps 2 to 6 two to three times, with varying time intervals between adjacent steps in steps 2 to 6. In step 7, after pumping in the acid and crosslinking fluid, the pump is stopped for 10 to 12 hours, and the interval after pumping in fracturing fluids of different viscosities is 24 to 36 hours.

[0037] The pump injection process described herein is for a specific stage of the fracturing process. The same method can be used for other stages of fracturing, or it can be finely adjusted according to the actual situation.

[0038] Based on the above main principles of phased, hierarchical, small-volume, long-cycle, multi-liquid, slow-start, and soft-stop pumping, the fracturing pumping process for constructing artificial geothermal reservoirs in dry hot granite has been established. This process promotes the establishment of enhanced geothermal systems in deep dry hot granite reservoirs under compressional tectonic stress fields by increasing fracture complexity, improving heat exchange efficiency of the reservoir, and ensuring the smooth and successful completion of fracturing operations. It can safely and effectively form shear-tension complex fracture networks in granite-type dry hot rocks under compressional stress fields.

[0039] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural and method transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A fracturing pump injection process for constructing artificial thermal reservoirs using dry-hot granite, characterized in that, Includes the following steps: Step 1: Use 10%~12% plant acid to wash the fractured wellbore to remove cement slurry residue left in the natural fractures around the well due to drilling and cementing operations. Step 2: Use variable viscosity slick water fracturing fluid to perform the first flow rate and first duration fracturing to generate first-level hydraulic fractures in the dry hot granite reservoir. The process of starting the fracturing truck and increasing the flow rate to the working flow rate involves 8 to 10 steps. After the fracturing is completed in this stage, the flow rate will decrease from the working flow rate to 0, which also involves the same number of steps to decrease the flow rate. Step 3: Reduce the fracturing pump injection rate and the viscosity of the slickwater fracturing fluid to perform fracturing on the reservoir for a second duration, generating secondary hydraulic fractures in the dry hot granite reservoir. The process of increasing and decreasing the injection rate also adopts a step-like change process. Step 4: Use 15%~20% plant acid solution and pump the plant acid solution at the second flow rate to increase the surface roughness of the primary and secondary hydraulic fractures and reduce the pumping pressure. Step 5: Use cross-linked fracturing fluid of the first viscosity at the third pumping rate to inject 2000~3000m³. 3 The volume of liquid continuously increases the length and width of the first-stage hydraulic fracture; Step 6: Use variable viscosity slick water fracturing fluid for fourth-volume, third-duration pumping fracturing to increase the length and width of primary hydraulic fractures, as well as the number and scale of secondary hydraulic fractures in the dry hot granite reservoir. Step 7: Repeat steps 2 to 6 two to three times, with varying time intervals between adjacent steps 2 to 6.

2. The fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite according to claim 1, characterized in that: In step 2, the variable viscosity slickwater fracturing fluid is made by adding additives to clean water to achieve a viscosity range of 15–25 mPa·s; the first displacement range is 3.5–5 m³ / s. 3 / min, the first duration ranges from 36 to 48 hours; the amplitude distribution of each step of the stepped increase and decrease in emission rate ranges from 0.4 to 0.8m. 3 / min.

3. The fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite according to claim 1, characterized in that: In step 3, the fracturing pump injection rate is reduced to 2-3.5 m³ / h. 3 The flow rate is increased by 0.5 m / min, reducing the viscosity of the slickwater fracturing fluid to 0-10 mPa·s; the second duration is 20-24 hours; the step amplitude of the flow rate increase / decrease process is 0.4-0.8 m. 3 / min, with 4 to 6 steps.

4. The fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite according to claim 1, characterized in that: In step 4, the second displacement is 1.0~2.0m. 3 / min, the total amount of plant acid solution is no more than 300 m³ 3 pH < 4.5; the pumping rate of the plant acid solution is adjusted using a stepped method.

5. The fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite according to claim 1, characterized in that: In step 5, the first viscosity range is 180~200 mPa·s; the third displacement is 3.5~5m³. 3 / min.

6. The fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite according to claim 1, characterized in that: In step 6, the viscosity range of the variable viscosity slickwater fracturing fluid is 15~25 mPa·s; the fourth discharge rate is 2~3.5 m³ / s. 3 / min, the third duration is 60~72 hours.

7. The fracturing pump injection process for constructing artificial thermal reservoirs using dry hot granite according to claim 1, characterized in that: In step 7, after pumping the plant acid solution and cross-linked fracturing fluid, the pump is stopped for 10 to 12 hours, and the interval after pumping the slickwater fracturing fluid of different viscosities is 24 to 36 hours.