A method for transforming natural gas hydrate reservoirs by filling porous framework materials

Abstract

The present invention provides a method for transforming a natural gas hydrate reservoir by filling a porous framework material, which injects the porous framework material into the natural gas hydrate reservoir in a way of creating fractures first and then filling, so as to form stable fracture channels with high flow conductivity, and improve the stability and permeability of the hydrate reservoir. Compared with the existing hydrate reservoir transformation technologies, the present invention can create fractures in advance by means of hydraulic slotting, hydraulic fracturing, and high-energy gas blasting, and then fill a large amount of porous framework material into the fractures to form stable fracture channels with high flow conductivity. The porous framework material has the characteristics of high strength, high porosity, and high permeability, and does not need to disintegrate or be refractured in the fractures. Even if the fractures close, the porous framework material itself can serve as a high-permeability flow channel to provide conditions for the high-speed migration of natural gas. The present invention can strengthen the reservoir framework and enhance the reservoir seepage capacity at the same time, so as to maintain production.

Description

Technical Field

[0001] The present invention belongs to the field of exploitation of muddy silt-type natural gas hydrates, and particularly relates to a method for reforming a natural gas hydrate reservoir by filling a porous framework material. Background Technique

[0002] Natural gas hydrate is an ice-like compound formed by natural gas molecules and water molecules under high pressure and low temperature conditions, commonly known as "flammable ice", which is mainly distributed in submarine sediments and high-latitude permafrost zones. Its resource volume is equivalent to twice the total reserves of known or unknown coal, oil, and natural gas globally.

[0003] At present, the technical means for exploiting marine hydrates are not yet mature. Although successful trial exploitation of marine hydrates has been achieved in recent years, there are a series of problems. On the one hand, as the exploitation progresses, the reservoir permeability decreases, resulting in the inability to timely discharge the water generated by the decomposition of hydrates, and the sand production phenomenon is relatively serious, leading to a decrease in the gas production rate per well and the production cycle. On the other hand, along with the decomposition of hydrates, the reservoir structure changes, causing accidents such as formation collapse and submarine landslides. Therefore, to achieve the safe and efficient development of natural gas hydrates, it is necessary to reform the natural gas hydrate reservoir in combination with the characteristics of weak cementation and low permeability of the natural gas hydrate reservoir, so as to achieve the purpose of increasing permeability and enhancing stability.

[0004] Hydraulic fracturing is a conventional method for reservoir reform at present. To a certain extent, it can improve the gas production efficiency per well. However, during the decomposition process of natural gas hydrates, the mechanical properties of the reservoir change, and the strength of the reservoir framework gradually decreases, resulting in the closure of fractures and the decline of the diversion ability, and then the failure of reservoir reform. Therefore, the application prospect of the existing hydraulic fracturing reform technology in increasing the production of natural gas hydrates is not very clear, and a reform method that can provide a stable high-permeability fracture channel for the hydrate reservoir needs to be constructed. Summary of the Invention

[0005] In view of the technical problems existing in the above background technique, the present invention proposes a method for reforming a natural gas hydrate reservoir by filling a porous framework material, aiming to form a stable and wide fracture channel with high diversion ability, improve the permeability of the hydrate reservoir, further improve the single-well exploitation efficiency, extend the exploitation cycle, and achieve the purpose of commercial exploitation.

[0006] To solve the above technical problems, a method for reforming a natural gas hydrate reservoir by filling a porous framework material provided by the present invention mainly includes the following steps:

[0007] (1) By setting up an offshore drilling platform, drilling a horizontal well in the middle of the hydrate layer, then completing the cementing work, and performing a fracture operation in the natural gas hydrate reservoir;

[0008] (2) When the cracks formed in the crack creation operation in step (1) reach the set value, prepare the sand-carrying fluid. After the sand-carrying fluid is prepared, it is stored in the liquid slurry tank.

[0009] (3) After the preparation of the sand-carrying fluid is completed, carry out the fracturing and packing work of the porous skeleton material, that is, use the low-viscosity sand-carrying fluid to fill the small-particle-size porous skeleton material into the cracks formed in step (1). By controlling the amount of the mixture of the sand-carrying fluid and the porous skeleton material, transport the slurry composed of the low-viscosity sand-carrying fluid and the small-particle-size porous skeleton material to the crack end, so that the porous skeleton material is blocked and separated at the crack front, preventing the subsequent porous skeleton material from being filtrated, thereby increasing the amount of the porous skeleton material in the crack and the crack width, forming a blockage to prevent the crack from continuing to extend along the crack length direction.

[0010] (4) Increase the viscosity of the sand-carrying fluid and continuously fill the porous skeleton material into the crack, increase the filling amount of the porous skeleton material and gradually upgrade the particle size of the porous skeleton material, so that the large-particle-size porous skeleton material is fully filled in the crack and the crack width is further increased.

[0011] (5) When the injection amount and particle size of the porous skeleton material reach the design value, stop the filling operation and the hydrate reservoir transformation is completed.

[0012] (6) Finally, carry out the gel-breaking backflow of the sand-carrying fluid and the operation of depressurizing and producing natural gas hydrate.

[0013] The method for reforming a natural gas hydrate reservoir by filling a porous skeleton material, wherein: in step (1), a horizontal well is drilled in the middle of the hydrate layer by a mechanical drilling method; in step (1), a crack creation operation is carried out in the hydrate reservoir by means of hydraulic slotting, hydraulic fracturing, and high-energy gas blasting.

[0014] The method for reforming a natural gas hydrate reservoir by filling a porous skeleton material, wherein the sand-carrying fluid in step (2) includes, by mass percentage, 0.25%-0.5% of a thickening agent, 0.02%-0.03% of a crosslinking agent, 0.05%-0.08% of a gel-breaking agent, 1%-2% of a drainage aid, 3%-5% of a clay stabilizer, and the rest is seawater; after the sand-carrying fluid in step (2) is prepared, it is stored in the liquid slurry tank.

[0015] The method for reforming a natural gas hydrate reservoir by filling a porous skeleton material, wherein: the viscosity of the sand-carrying fluid in step (2) is controlled at 200-500 mPa·s.

[0016] The method for reforming a natural gas hydrate reservoir by filling a porous framework material, wherein: in step (3), a low-viscosity sand-carrying fluid in a liquid slurry tank and a porous framework material in a porous framework material storage tank are first mixed by a mixing device to form a slurry, and then the slurry is injected into the fracture by a high-pressure grouting pump through a grouting pipe.

[0017] The method for reforming a natural gas hydrate reservoir by filling a porous framework material, wherein: for the slurry injected into the fracture in step (3), a part of the small porous framework material particles are stuck at the front edge of the fracture and no longer advance, forming a blockage. As the slurry continues to be injected, a sand separation zone is formed at the front edge of the fracture to prevent the fracture from continuing to extend along the length of the fracture. The filtration loss of the sand-carrying fluid begins to decrease, and it becomes more difficult for the subsequently injected porous framework material to pass through this sand separation zone. When the pumping speed of the high-pressure grouting pump remains unchanged, the pressure of the hydrate reservoir begins to increase, and the fracture begins to expand longitudinally and the width increases.

[0018] The method for reforming a natural gas hydrate reservoir by filling a porous framework material, wherein: a packer and a grouting hole are provided on the pipe body of the grouting pipe at the fracture, and the end of the pipe body is blocked by a plug.

[0019] The method for reforming a natural gas hydrate reservoir by filling a porous framework material, wherein: the porous framework material uses natural pumice, the designed maximum particle size is about 4-5 mm, the porosity reaches 71%-80%, the water absorption rate reaches 40%-50%, the pore size range is distributed between 7 μm and 140 μm, the cube compressive strength is 4 MPa-5 Mpa, and the density is 2.5 g / cm 3 , and the permeability reaches 10-20 darcy after treatment.

[0020] Adopting the above technical solution, the present invention has the following beneficial effects:

[0021] The present invention adopts the method of pre-creating fractures + graded continuous filling of porous framework materials. Fractures are pre-created in the hydrate reservoir, which is beneficial for the injection of porous framework materials and increases the scope of reservoir reform. The graded continuous injection of porous framework materials can promote the secondary expansion of the width of the prefabricated fractures, forming a wide fracture channel filled with porous framework materials. The porous framework materials have the characteristics of high strength, high porosity, and high permeability. They do not need to disintegrate or be refractured in the fracture. Even if the fracture closes, the porous framework materials themselves can serve as high-permeability channels, providing conditions for the high-speed migration of natural gas. The present invention can enhance and maintain the seepage capacity of the reservoir during the hydrate decomposition process, expand the mining range, and improve the gas production per well and the mining efficiency.

[0022] The present invention forms a stable high-permeability fracture channel by first creating fractures and then filling them with a porous framework material. The porous framework material has the characteristics of high strength, high porosity, and high permeability. It does not need to disintegrate or be refractured in the fractures. Even if the fractures close, the porous framework material itself can serve as a high-permeability flow channel, providing conditions for the high-speed migration of natural gas. The present invention can enhance the seepage capacity of the reservoir, effectively reduce the sand production phenomenon of the reservoir, maintain the formation stability to a certain extent during the exploitation process, expand the exploitation range, and improve the gas production per well and the exploitation efficiency.

[0023] The sand-carrying fluid of the present invention is composed of seawater + thickening agent + cross-linking agent + flowback aid + clay stabilizer, etc. The sand-carrying fluid needs to have the characteristics of less filtration loss, strong sand-carrying capacity, low friction resistance, good stability, strong compatibility, and easy flowback; the present invention adopts pre-fracturing + staged continuous filling of the porous framework material, which is conducive to the injection of the porous framework material and the expansion of the fracture width, and can ensure that a large amount of porous framework materials with larger particle sizes fill the fractures to form a stable and high-permeability fracture channel; the injected porous framework material itself has the characteristics of high porosity and permeability. When fully filled into the reservoir, it can not only play a supporting role and improve the reservoir stability, but also still have a high flow conductivity after the fractures close due to the decomposition of hydrates, improving the gas production per well and the exploitation efficiency; during the reservoir transformation process of the present invention, packers are used to carry out segmented operations on the hydrate reservoir in horizontal wells, which can achieve a large-scale reservoir transformation; by injecting porous framework materials with different shapes and gradations, the present invention can form a high-density and stable high-conductivity fracture channel. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0025] Figure 1 It is a schematic diagram of the method for reforming a natural gas hydrate reservoir by filling a porous framework material according to the present invention;

[0026] Figure 2 It is a schematic diagram of filling a porous framework material in a horizontal well in the method for reforming a natural gas hydrate reservoir by filling a porous framework material according to the present invention;

[0027] Figure 3 It is a partial enlarged view of the porous framework material filling the hydrate formation in the method for reforming a natural gas hydrate reservoir by filling a porous framework material according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS:

[0029] 1-overlying stratum; 2-hydrate reservoir; 3-underlying stratum; 4-porous skeleton material in hydrate reservoir; 5-plugging; 6-packer; 7-grouting hole; 8-grouting pipe; 9-wellhead; 10-high-pressure grouting pump; 11-mixing device; 12-sand-carrying fluid; 13-porous skeleton material storage tank; 14-crack. DETAILED DESCRIPTION

[0030] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] like Figures 1 to 3 As shown, the method of filling porous skeleton materials to transform natural gas hydrate reservoirs of the present invention specifically comprises the following steps:

[0032] S100. Install a drilling platform on the sea surface, use mechanical drilling to drill a horizontal well in the middle of the hydrate layer 2, and then complete the cementing work. Perform fracture creation operations in the hydrate reservoir 2 by means of hydraulic fracturing, hydraulic fracturing, and high-energy gas blasting (the purpose is to create fractures in the hydrate reservoir 2, which is conducive to filling the porous skeleton material 4 to form fractures); wherein, the horizontal well is arranged in the middle of the hydrate reservoir 2 for reservoir transformation and exploitation.

[0033] S200, when the effect of the seam-making operation in the above step S100 reaches the set value, prepare a sand-carrying liquid; wherein the sand-carrying liquid mainly includes, by mass percentage, 0.25%-0.5% thickener (hydroxypropyl guar gum), 0.02%-0.03% crosslinking agent (borax), 0.05%-0.08% gel breaker (ammonium persulfate), 1%-2% drainage aid (surfactant), 3%-5% clay stabilizer (KCl), and the rest is seawater; the prepared sand-carrying liquid is stored in a liquid slurry tank 12; since the particle size of the proppant is constantly changing during the filling process, in order to ensure the sand-carrying capacity and facilitate gel breaking, the viscosity of the sand-carrying liquid is also constantly changing, and the viscosity of the sand-carrying liquid is controlled at 200-500mpa·s.

[0034] After the preparation of the sand-carrying fluid is completed, the fracturing and packing operation of the porous skeleton material is carried out. In the initial stage of packing, first, the low-viscosity sand-carrying fluid in the liquid slurry tank 12 and the porous skeleton material 4 in the porous skeleton material storage tank 13 are mixed by the mixing device 11 to form a slurry, and then the high-pressure grouting pump 10 injects the slurry into the fracture 14 through the grouting pipe 8 (a packer 6 and a grouting hole 7 are provided on the pipe body of the grouting pipe 8 at the fracture 14, and the end of the pipe body is blocked by a plug 5;). By controlling the construction displacement, that is, the amount of the mixture of the sand-carrying fluid and the porous skeleton material 4, the porous skeleton material 4 is carried by the sand-carrying fluid to the front edge of the fracture 14. Among them, a part of the small-particle-size porous skeleton material is stuck at the front edge of the fracture 14 and no longer advances and forms a blockage. With the continuous injection of the slurry, a sand separation zone is formed at the front edge of the fracture 14, which prevents the fracture 14 from continuing to extend along the fracture length direction. The filtration loss of the sand-carrying fluid begins to decrease, and it is more difficult for the later-injected porous skeleton material 4 to pass through this position. When the pumping speed of the high-pressure grouting pump 10 remains unchanged, the pressure of the hydrate reservoir 2 begins to increase, and the fracture 14 begins to expand longitudinally and the width increases.

[0035] S400. As the width of the fracture 14 expands, increase the viscosity of the sand-carrying fluid and continuously fill the porous skeleton material 4 into the fracture 14, increase the filling amount of the porous skeleton material 4 and gradually upgrade the particle size of the porous skeleton material 4, so that the large-particle-size porous skeleton material 4 is fully filled into the fracture 14, and the width of the fracture 14 will further increase, thereby enhancing the diversion capacity of the fracture 14;

[0036] S500. Until the injection amount and particle size of the porous skeleton material 4 reach the design value, stop the filling operation. Finally, a large amount of porous skeleton materials 4 with different particle sizes are pressed into the hydrate reservoir 2. A stable and wide fracture channel with high diversion capacity can be formed inside the hydrate reservoir 2, and the transformation of the hydrate reservoir 2 is completed, as Figure 2 shown.

[0037] S600. After the transformation of the hydrate reservoir 2 is completed, carry out the gel-breaking backflow of the sand-carrying fluid and the pressure-reducing production of natural gas hydrate by the pressure-reducing method, the CO2 injection method, and the injection inhibitor method.

[0038] Among them, the selected porous skeleton material 4 can be materials such as natural pumice, sintered metal porous material, permeable cement concrete, etc.; the particle size of the porous skeleton material 4 can be proportioned according to the design value and construction requirements, and the maximum particle size is designed to be 4-5 mm. After the porous skeleton material 4 is filled into the crack, there is no need for disintegration and secondary fracturing, and it has high porosity and high permeability itself. Among them, natural pumice and sintered metal porous materials have characteristics such as high porosity (70%-80% or higher), high strength, and low density, which improve the stability of the hydrate reservoir 2 while also increasing the permeability of the hydrate reservoir 2. The porosity of natural pumice can reach 71%-80%, the water absorption rate can reach 40%-50%, the pore size range is distributed between 7μm and 140μm, the cube compressive strength is 4MPa-5Mpa, and the density is 2.5g / cm 3 , and the permeability can reach 10-20 darcy after treatment. The high water absorption rate indicates good connectivity of the pores.

[0039] The present invention can form a stable wide crack channel with high diversion ability, improve the permeability of the hydrate reservoir, further improve the single-well production efficiency, extend the production cycle, and achieve the purpose of commercial production.

[0040] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for reforming a natural gas hydrate reservoir by filling a porous framework material, characterized in that, It mainly includes the following steps: (1) By setting up an offshore drilling platform, drill a horizontal well in the middle of the hydrate layer, then complete the cementing work, and perform a fracture operation in the hydrate reservoir; (2) When the cracks formed by the fracture operation in step (1) reach the set value, prepare a sand-carrying fluid, and store the prepared sand-carrying fluid in a liquid slurry tank; (3) After the preparation of the sand-carrying fluid is completed, carry out the fracturing and filling work of the porous skeleton material, that is, use a low-viscosity sand-carrying fluid to fill small-particle-size porous skeleton material into the cracks formed in step (1). By controlling the mixing amount of the sand-carrying fluid and the porous skeleton material, transport the slurry composed of the low-viscosity sand-carrying fluid and the small-particle-size porous skeleton material to the crack end, so that the porous skeleton material is blocked and separated at the front end of the crack, preventing the subsequent porous skeleton material from filtering out, thereby increasing the amount of porous skeleton material in the crack and the crack width, forming a blockage to prevent the crack from continuing to extend along the crack length direction; (4) Increase the viscosity of the sand-carrying fluid and continuously fill the porous skeleton material into the crack, increase the filling amount of the porous skeleton material and gradually upgrade the particle size of the porous skeleton material, so that the large-particle-size porous skeleton material is fully filled in the crack, further increasing the crack width; (5) When the injection amount and particle size of the porous skeleton material reach the design value, stop the filling operation, and the transformation of the hydrate reservoir is completed; (6) Finally, carry out the gel-breaking backflow of the sand-carrying fluid and the operation of depressurizing and producing natural gas hydrate.

2. The method for reforming a natural gas hydrate reservoir by filling a porous framework material as claimed in claim 1, wherein: In step (1), a mechanical drilling method is used to drill a horizontal well in the middle of the hydrate layer; in step (1), a fracture operation is carried out in the hydrate reservoir by any one of hydraulic slotting, hydraulic fracturing, and high-energy gas blasting.

3. The method for reforming a natural gas hydrate reservoir by filling a porous framework material as claimed in claim 1, wherein The sand-carrying fluid in step (2) includes, by mass percentage, 0.25%-0.5% thickening agent, 0.02%-0.03% crosslinking agent, 0.05%-0.08% gel-breaking agent, 1%-2% drainage aid, 3%-5% clay stabilizer, and the rest is seawater; the prepared sand-carrying fluid in step (2) is stored in a liquid slurry tank.

4. The method for reforming a natural gas hydrate reservoir by filling a porous framework material as claimed in claim 1 or 3, characterized in that: The viscosity of the sand-carrying fluid in step (2) is controlled at 200-500 mPa·s.

5. The method for reforming a natural gas hydrate reservoir by filling a porous framework material according to claim 1, characterized in that: In step (3), first mix the low-viscosity sand-carrying fluid in the liquid slurry tank and the porous skeleton material in the porous skeleton material storage tank through a mixing device to form a slurry, and then inject the slurry into the crack through a high-pressure grouting pump through a grouting pipe.

6. The method for reforming a natural gas hydrate reservoir by filling a porous framework material as claimed in claim 5, wherein: For the slurry injected into the crack in step (3), some small porous skeleton material particles are stuck at the front edge of the crack and no longer move forward to form a blockage. As the slurry continues to be injected, a sand separation zone is formed at the front edge of the crack to prevent the crack from continuing to extend along the crack length direction. The filtration loss of the sand-carrying fluid begins to decrease, and it is more difficult for the subsequently injected porous skeleton material to pass through the sand separation zone. When the pumping speed of the high-pressure grouting pump remains unchanged, the pressure of the hydrate reservoir begins to increase, and the crack begins to expand longitudinally and the width increases.

7. The method for reforming a natural gas hydrate reservoir by filling a porous framework material as claimed in claim 5, wherein: The pipe body of the grouting pipe at the crack is provided with a packer and a grouting hole, and the end of the pipe body is blocked by a plug.

8. The method for reforming a natural gas hydrate reservoir by filling a porous framework material according to claim 1, characterized in that: The porous framework material is natural pumice, with a designed maximum particle size of 4 - 5 mm, a porosity of 71% - 80%, a water absorption rate of 40% - 50%, a pore size range distributed between 7 μm and 140 μm, a cube compressive strength of 4 MPa - 5 Mpa, and a density of 2.5 g / cm 3 , and after treatment, the permeability reaches 10 - 20 darcy.

Images