A screen string structure for hot dry rock fracturing and its construction method

Abstract

A screen pipe string structure for hot dry rock fracturing and a construction method thereof, the screen pipe string structure comprises a top joint and a bottom joint, between which a first screen pipe corresponding to a first layer section of a hot dry rock reservoir, a second screen pipe corresponding to a second layer section of the hot dry rock reservoir, and a non-porous solid pipe corresponding to a third layer section of the hot dry rock reservoir are arranged; the construction method comprises: selecting a fracturing layer according to a natural fracture development section shown by a well logging result; combining screen pipes with different parameters and a non-porous solid pipe into a well; preparing clean water, acid solution, slick water, and a small amount of displacement fluid at the same time; firstly pumping acid solution at a small displacement to remove near-well contamination and corrode rocks, and then continuing to pump clean water at a small displacement to cause thermal fracture of reservoir rocks; successively pumping clean water, slick water, and acid solution in a step-up, constant, and step-down displacement manner; pumping displacement fluid to push the acid solution into the formation; stopping the pump to release pressure after a certain amount of fracturing fluid has been pumped, and repeating the above steps.

Description

Technical Field

[0001] The invention belongs to the technical field of hot dry rock development research, and in particular relates to a screen pipe string structure for hot dry rock fracturing and a construction method thereof. Background Art

[0002] Hot dry rock is a clean renewable energy source. my country has a huge reserve of hot dry rock. According to preliminary estimates, the total amount of hot dry rock resources at a depth of 3 to 10 km in mainland my country is 2.5×10 25 J, equivalent to about 860 trillion tons of standard coal, and its reserves rank among the top in the world. Even if the heat extraction efficiency of about 2% is taken into account, the amount of electricity converted from such a huge amount of thermal energy is considerable. Therefore, the development of hot dry rocks is of vital importance to my country's energy structure adjustment and energy security. The development of hot dry rocks requires the establishment of an enhanced geothermal system (EGS), the core of which is to drill into the reservoir and fracturing to form a fracture network of a certain scale, and to construct a circulation loop of injection wells and production wells to extract heat energy for power generation.

[0003] Different from oil and gas reservoirs, hot dry rock granite not only has physical and mechanical properties of high temperature (≥200℃), high hardness (>HS70), ultra-dense (porosity 0.3%~0.7%), and obvious anisotropy (filled with various altered minerals), but also has the characteristics of large geostress and large difference coefficient, which makes it difficult to carry out fracturing construction. In addition, the segmented cluster fracturing commonly used in oil and gas fracturing often causes the failure of the packer due to the high temperature of the reservoir, which leads to the failure of the entire fracturing construction. Therefore, we need to adopt a variety of methods to overcome the high temperature and reduce the initiation and extension pressure of the fracture. Screen completion and pumping fracturing fluid in the layer section with natural fractures are one of the main measures. The formation rock will form many natural fractures due to the influence of long-term geological action. Due to its relatively low degree of cementation, if hydraulic fracturing is to be carried out in the layer section with natural fractures, the pressure will undoubtedly be greatly reduced. Soultz hot dry rock in France is one of the most successful EGS projects in the world. It encountered natural fracture zones during the construction of artificial heat storage, which reduced the initiation and extension pressure of the fractures. Therefore, accurately identifying the natural fracture development zone of the reservoir and adopting appropriate fracturing technology are the keys to hot dry rock fracturing. At present, there are relatively few studies on hot dry rock fracturing methods. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a screen string structure for hot dry rock fracturing, so as to propose a completion plan in a targeted manner based on the development of natural fractures in the hot dry rock reservoir, so that the fracturing fluid is concentrated in the strata with a high degree of natural fracture development, which is beneficial to reducing the initiation and extension pressure of the fractures and increasing the fracture scale.

[0005] Another object of the present invention is to provide a construction method for a screen string structure for hot dry rock fracturing, which is beneficial to reducing the initiation and extension pressure of the fracture and increasing the size of the fracture.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A screen pipe string structure for hot dry rock fracturing comprises a top joint and a bottom joint, between which a first screen pipe corresponding to a first layer section of a hot dry rock reservoir, a second screen pipe corresponding to a second layer section of the hot dry rock reservoir, and a non-porous solid pipe corresponding to a third layer section of the hot dry rock reservoir are arranged; the fracture density of the first layer section is less than 1.8# / m, the fracture density of the second layer section is ≥1.8# / m, and the third layer section has no fracture density; the opening density of the first screen pipe is less than the opening density of the second screen pipe.

[0008] Furthermore, the first screen tube has spiral holes, a phase angle of 30° between holes, a hole diameter of 15 mm, and an axial hole spacing of 80 mm; the second screen tube also has spiral holes, a phase angle of 22.5° between holes, a hole diameter of 15 mm, and an axial hole spacing of 60 mm.

[0009] A construction method of a screen string structure for hot dry rock fracturing comprises the following steps:

[0010] S1. Select the fracturing layer according to the hot dry rock reservoir shown by the logging results;

[0011] S2, the top joint, the bottom joint and the first screen tube, the second screen tube and the non-porous solid pipe therebetween are assembled by threads and lowered into the well, so that the first screen tube, the second screen tube and the non-porous solid pipe correspond to the first layer section, the second layer section and the third layer section of the hot dry rock reservoir respectively, and the well is cemented above the target layer;

[0012] S3. Fracturing fluid preparation: prepare clean water, and mix acid, slick water, and displacement fluid;

[0013] S4, chemical stimulation and thermal rupture: first use less than 0.5m 3 / min displacement pump to inject acid to remove near-well pollution and corrode the rock, and then continue to use less than 0.5m 3 / min pump to inject clean water to cause thermal cracking of reservoir rocks;

[0014] S5. Variable displacement multi-liquid pumping: Use step-up displacement, constant displacement and step-down displacement to successively pump clean water, slippery water and acid liquid. The initial displacement is 0.3-0.5m 3 / min, the displacement increase and decrease is less than 50%, and the maximum displacement is not more than 3.5m 3 / min;

[0015] S6, pumping displacement fluid: pumping displacement fluid at a constant displacement rate to push the acid liquid pumped in S5 into the formation;

[0016] S7, intermittent pumping: after the fracturing fluid is pumped, the pump is stopped to release the pressure, and steps S5, S6, and S7 are repeated.

[0017] Furthermore, there is no hole within 1.5 m of the connection between the first screen tube, the second screen tube and the solid pipe without holes; the lowest end of the bottom joint is two solid pipes serving as sand settling pipes.

[0018] Furthermore, the step S1 is to screen out the randomly distributed natural fractures in the hot dry rock reservoir by different logging methods, and the probe tube for performing the logging is resistant to a high temperature of 260°C.

[0019] Furthermore, the acid solution in step S3 is high temperature resistant earth acid, comprising 10% hydrochloric acid, 10% hydrofluoric acid and high temperature resistant materials, and the slippery water is ordinary sticky slippery water.

[0020] Furthermore, in step S5, the step-up displacement stage pumps 80 to 100 m 3 The fracturing fluid is pumped at a constant displacement stage of 150 to 200 m 3 The fracturing fluid is pumped for 40 to 60 m 3 fracturing fluid; in step S6, the displacement fluid pumping volume is 40m 3 , the displacement fluid is an oil-based fracturing fluid.

[0021] Furthermore, in the step-by-step displacement increase process of step S5, when the displacement is less than 2.5m 3 / min, the displacement increase is 40% to 50%; when the displacement increases to more than 2.5m 3 / min, the displacement increase is 20% to 30%.

[0022] Furthermore, steps S5, S6 and S7 are the contents of one day's pumping. Pumping is carried out during the day and the pump is stopped to release pressure at night, forming intermittent and slug-type pumping. The daily pumping liquid volume is 1000m 3 The pump should be stopped for pressure relief for no less than 10 hours, and the pumping cycle should be repeated until the designed liquid volume is reached.

[0023] The beneficial effects of the present invention are as follows: the screen string structure and construction method for hot dry rock fracturing of the present invention formulate a completion plan in combination with the actual conditions of natural fracture development in the reservoir, so that the fracturing fluid is concentrated in the strata with a high degree of natural fracture development, which is beneficial to reducing the fracture initiation and extension pressure of the fracture and increasing the fracture scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1It is a structural schematic diagram of a screen string structure for hot dry rock fracturing according to the present invention.

[0025] Figure 2 It is a structural schematic diagram of the first screen pipe of the screen pipe string structure for hot dry rock fracturing of the present invention.

[0026] Figure 3 It is a schematic structural diagram of the second screen pipe of the screen pipe string structure for hot dry rock fracturing of the present invention.

[0027] Figure 4 It is a schematic diagram of the state of the screen string structure of the present invention when it is actually applied for hot dry rock fracturing.

[0028] Figure 5 It is a schematic diagram of the steps of the construction method of the screen string structure for hot dry rock fracturing of the present invention. DETAILED DESCRIPTION

[0029] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0030] like Figure 1-Figure 4 As shown, the present invention provides a screen pipe string structure for hot dry rock fracturing, including a top joint 1 and a bottom joint 2, between which are arranged a first screen pipe 4 corresponding to the first layer section 3 of the hot dry rock reservoir, a second screen pipe 6 corresponding to the second layer section 5 of the hot dry rock reservoir, and a non-porous solid pipe 8 corresponding to the third layer section 7 of the hot dry rock reservoir; the crack density of the first layer section 3 is less than 1.8# / m, the crack density of the second layer section 5 is ≥1.8# / m, and the third layer section 7 has no crack density; the opening density of the first screen pipe 4 is less than the opening density of the second screen pipe 6. Further, the first screen pipe 4 is a spiral hole arrangement, with a phase angle of 30° between holes, a hole diameter of 15mm, and an axial hole spacing of 80mm; the second screen pipe 6 is also a spiral hole arrangement, with a phase angle of 22.5° between holes, a hole diameter of 15mm, and an axial hole spacing of 60mm. In the present invention, the non-porous solid pipe 8 is a common petroleum casing (diameter 177.8 mm, length 11.51 m, steel grade P110), and the size and material parameters of the first and second screen pipes are also consistent with those of the non-porous solid pipe (diameter 177.8 mm, length 11.51 m, steel grade P110). The two screen pipes are set with different perforation methods and hole densities according to the different degrees of natural fissure development in the reservoir. Through the combination of the two screen pipes and a solid pipe, conditions are created for hot dry rock fracturing.

[0031] The present invention also provides a construction method of a screen string structure for hot dry rock fracturing, comprising the following steps:

[0032] S1. Select the fracturing layer according to the hot dry rock reservoir shown by the logging results;

[0033] S2, the top joint 1, the bottom joint 2 and the first screen tube 4, the second screen tube 6 and the non-porous solid pipe 8 therebetween are assembled by threads and lowered into the well, so that the first screen tube 4, the second screen tube 6 and the non-porous solid pipe 8 correspond to the first layer section 3, the second layer section 5 and the third layer section 7 of the hot dry rock reservoir respectively, and the well is cemented above the target layer;

[0034] S3. Preparation of fracturing fluid: prepare sufficient clean water, and configure acid of a certain concentration, slippery water, and a small amount of displacement fluid; the acid is high temperature resistant earth acid, including 10% hydrochloric acid, 10% hydrofluoric acid and high temperature resistant materials, and the slippery water is ordinary viscous slippery water;

[0035] S4, chemical stimulation and thermal cracking: In view of the high temperature, high hardness and high quartz content of dry hot granite, first use less than 0.5m 3 / min displacement pump to inject acid to remove near-well pollution and corrode the rock, and then continue to use less than 0.5m 3 / min pump to inject clean water to cause thermal cracking of reservoir rocks;

[0036] S5. Variable displacement multi-liquid pumping: Use step-up displacement, constant displacement and step-down displacement to successively pump clean water, slippery water and acid liquid. The initial displacement is 0.3-0.5m 3 / min, the displacement increase and decrease is less than 50%, and the maximum displacement is not more than 3.5m 3 / min;

[0037] S6, pumping displacement fluid: pumping displacement fluid at a constant displacement rate to push the acid liquid pumped in S5 into the formation;

[0038] S7, intermittent pumping: after a certain amount of fracturing fluid is pumped, the pump is stopped to release the pressure, and steps S5, S6, and S7 are repeated.

[0039] The present invention increases the volume of hot dry rock reservoir construction and realizes artificial heat storage construction on the basis of fully considering the existing technical conditions and the characteristics of hot dry rock reservoirs.

[0040] Furthermore, to ensure the strength of the pipe string, the first screen tube 4, the second screen tube 6 and the non-porous solid pipe 8 have no holes within 1.5m, and the lowest end of the bottom joint is two solid pipes used as sand settling pipes. Figure 4 , Figure 5As shown, step S2 is to lower the first screen pipe 4 into the discontinuous fracture section (first layer section 3) with secondary development according to the natural fractures shown by the logging results, and lower the second screen pipe 6 into the continuous fracture section with relatively developed fractures (second layer section 5). The non-porous solid pipe 8 corresponds to the layer section without developed fractures in the target layer (third layer section 7). The standard of development and secondary development is the quantitative parameter of natural fractures tested by electrical imaging. The fracture density ≥1.8# / m is considered to be relatively developed natural fractures, and the fracture density <1.8# / m is considered to be secondary development. In this way, the fracturing fluid is concentrated in the layer with high degree of natural fracture development, which is conducive to reducing the fracture initiation and extension pressure of the fracture and increasing the fracture scale.

[0041] It should be understood that, due to the limitation of high temperature directional drilling technology, the hot dry rock fracturing well described in the present invention is a vertical well. Since natural fissures are one of the prerequisites for the initiation of hot dry rock fracturing cracks, the step S1 is to screen out the natural fissures randomly distributed in the hot dry rock reservoir by different logging methods. The temperature of the hot dry rock reservoir is often greater than 200°C, and with the increase of depth, the temperature increase gradually accelerates, so the probe tube for implementing the logging must withstand a high temperature of 260°C. The main logging items include: resistivity logging, acoustic imaging logging, electrical imaging logging, acoustic remote detection, and radioactive logging, which can effectively identify natural cracks in the well wall.

[0042] Considering that the friction resistance gradually increases during the crack expansion process, in step S5, the fracturing fluid should be pumped in the order of clean water, slippery water, and acid solution. The pumping process of each fracturing fluid should follow the sequence of step-up displacement, constant displacement, and step-down displacement. The step-up displacement stage is pumped for 80 to 100 m 3 The constant displacement stage pumps 150 to 200 m 3 The fracturing fluid is pumped for 40 to 60 m3 in this step-down stage. 3 In step S6, the displacement fluid pumping rate is 40m 3 The displacement fluid is a low-concentration ordinary oil-based fracturing fluid. Considering the risk of inducing higher-energy micro-earthquakes due to high displacement increase, during the step-by-step displacement increase in step S5, when the displacement is less than 2.5m 3 / min, the displacement increase is 40% to 50%. When the displacement increases to more than 2.5m 3 / min, the displacement increase is 20% to 30%.

[0043] Considering the actual situation of hot dry rock fracturing construction, steps S5, S6, and S7 are the pumping content of one day. Pumping construction is carried out during the day, and the pump is stopped to release pressure at night, forming intermittent and slug pumping. The daily pumping liquid volume is 1000m 3The pumping time should not be less than 10 hours, and the pumping cycle should be repeated until the designed liquid volume is completed. Small displacement, long cycle, intermittent, multi-liquid, and variable displacement pumping are the main characteristics of hot dry rock fracturing.

[0044] It should be noted that the above is only a preferred specific implementation mode of the present invention, and its purpose is to enable those skilled in the art to better understand the present invention. It should not be used to limit the scope and application of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit or principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A construction method for a screen string structure for hot dry rock fracturing, characterized in that: The following steps are involved: S1. Select the fracturing layer according to the hot dry rock reservoir shown by the logging results; S2, the screen string structure includes a top joint and a bottom joint, The top joint, the bottom joint and the first screen tube, the second screen tube and the non-porous solid pipe therebetween are assembled through threads and lowered into the well, so that the first screen tube, the second screen tube and the non-porous solid pipe correspond to the first layer, the second layer and the third layer of the hot dry rock reservoir respectively, and the well is cemented above the target layer; the fracture density of the first layer is less than 1.8# / m, the fracture density of the second layer is ≥1.8# / m, and the third layer has no fracture density; the opening density of the first screen tube is less than the opening density of the second screen tube; the first screen tube is a spiral hole arrangement, the phase angle between holes is 30°, the hole diameter is 15mm, and the axial hole spacing is 80mm; the second screen tube is also a spiral hole arrangement, the phase angle between holes is 22.5°, the hole diameter is 15mm, and the axial hole spacing is 60mm; S3. Fracturing fluid preparation: prepare clean water, and mix acid, slick water, and displacement fluid; S4, chemical stimulation and thermal rupture: first use less than 0.5m 3 / min displacement pump to inject acid to remove near-well pollution and corrode the rock, and then continue to use less than 0.5m 3 / min pump to inject clean water to cause thermal cracking of reservoir rocks; S5. Variable displacement multi-liquid pumping: Use step-up displacement, constant displacement and step-down displacement to successively pump clean water, slippery water and acid liquid. The initial displacement is 0.3-0.5m 3 / min, the displacement increase and decrease is less than 50%, and the maximum displacement is not more than 3.5m 3 / min; S6, pumping displacement fluid: pumping displacement fluid at a constant displacement rate to push the acid liquid pumped in S5 into the formation; S7, intermittent pumping: after the fracturing fluid is pumped, the pump is stopped to release the pressure, and steps S5, S6, and S7 are repeated.

2. The construction method of the screen string structure for hot dry rock fracturing according to claim 1, characterized in that: There is no hole within 1.5m of the connection between the first screen tube, the second screen tube and the solid pipe without holes; the lowest end of the bottom joint is two solid pipes used as sand settling pipes.

3. The construction method of the screen string structure for hot dry rock fracturing according to claim 1, characterized in that: The step S1 is to screen out the randomly distributed natural fractures in the hot dry rock reservoir by different logging methods, and the probe for performing the logging is resistant to a high temperature of 260°C.

4. The construction method of the screen string structure for hot dry rock fracturing according to claim 1, characterized in that: The acid solution in step S3 is high temperature resistant earth acid, comprising 10% hydrochloric acid, 10% hydrofluoric acid and high temperature resistant materials, and the slippery water is ordinary sticky slippery water.

5. The construction method of the screen string structure for hot dry rock fracturing according to claim 1, characterized in that: In step S5, the step-by-step displacement stage pumps 80 to 100 m 3 The fracturing fluid is pumped at a constant displacement stage of 150 to 200 m 3 The fracturing fluid is pumped in the step-down stage with 40 to 60 m3 of fracturing fluid; in step S6, the displacement fluid pumping volume is 40 m 3 , the displacement fluid is an oil-based fracturing fluid.

6. The construction method of the screen string structure for hot dry rock fracturing according to claim 1, characterized in that: In the step-by-step displacement increase process of step S5, when the displacement is less than 2.5m 3 / min, the displacement increase is 40% to 50%; when the displacement increases to more than 2.5m 3 / min, the displacement increase is 20% to 30%.

7. The construction method of the screen string structure for hot dry rock fracturing according to claim 1, characterized in that: Steps S5, S6 and S7 are the contents of one day's pumping. Pumping is carried out during the day and the pump is stopped to release pressure at night, forming intermittent and slug-type pumping. The daily pumping liquid volume is 1000m 3 The pump should be stopped for pressure relief for no less than 10 hours, and the pumping cycle should be repeated until the designed liquid volume is reached.

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