A multi-round exploitation method for enhanced drainage and re-increasing permeability of low-permeability unconsolidated reservoirs

Abstract

The present invention discloses a multi-round exploitation method for enhanced drainage and re-increasing permeability in low-permeability unconsolidated reservoirs. The method comprises the following steps: Step 1: Exploit the low-permeability unconsolidated reservoir around the wellbore in a pressure-reducing manner, pump out the gas and water around the wellbore, and form a drainage-enhanced area around the wellbore. A gas and water accumulation area for the second-stage exploitation is formed around the drainage-enhanced area; Step 2: Perform a permeability-increasing operation on the drainage-enhanced area to form a permeability-increasing channel in the drainage-enhanced area. The permeability-increasing channel communicates with the gas and water accumulation area for the second-stage exploitation, and then pump out the gas and water in the gas and water accumulation area for the second-stage exploitation by using a pressure-reducing method; Repeat Step 2 to finally form a progressive exploitation mode centered on the wellbore, from near to far, maximizing the excavation of the resource potential of the low-permeability unconsolidated reservoir and improving the exploitation efficiency of resources such as gas and hydrates.

Description

Technical Field

[0001] The present invention relates to the exploitation of natural gas hydrates and low-permeability unconsolidated gas reservoirs, and particularly to a multi-round exploitation method for enhanced drainage and re-increasing permeability of low-permeability unconsolidated reservoirs. Background Art

[0002] Typical representatives of low-permeability unconsolidated reservoirs are natural gas hydrate (hereinafter referred to as hydrate) reservoirs. Hydrates have the characteristics of high gas storage density and high calorific value of combustion. They are a clean and efficient energy resource and chemical raw material, mainly distributed in terrestrial permafrost zones and coastal continental shelves. More than 230 hydrate mining areas have been discovered globally, with a total amount of 7.6×10 18 m 3 , which is twice the sum of known carbon-containing compounds (including coal, petroleum, and conventional natural gas, etc.).

[0003] The natural gas hydrate resources in the sea areas of our country are mainly stored in strata with a water depth exceeding 1000m and a buried depth below the mud line exceeding 100m. Due to the relatively late geological age of the reservoir formation, hydrate reservoirs generally face two problems. One is that the reservoir is shallow, soft, and unconsolidated, with a high mud content, generally reaching 30%-40%. This leads to great difficulties in the stimulation and transformation of the reservoir. After the muddy silt reservoir is subjected to a high pore pressure, not only the extension law of the fractures is affected by the formation heterogeneity and formation stress in a disorderly manner, but also the compressibility of the reservoir is poor. At the same time, even if fractures are formed, they will close in a short time due to the muddy water generated by the decomposition of hydrates and the action of in-situ stress. The proppant has a poor supporting effect in the unconsolidated formation and is easy to embed into the formation, unable to play the role of supporting the pores of the formation. At the same time, in the fracturing construction, the extension of fractures is not easy to control, and the hydrates are buried relatively shallow, making it extremely easy to communicate with the seawater layer, resulting in poor safety. The other is the low permeability of the reservoir, generally with a permeability <10md, which belongs to an ultra-low permeability reservoir in the oil and gas industry. In the exploitation stage, the low-permeability reservoir will also be blocked by the water, gas, and sediment carried by the migration of the water and gas generated by the decomposition of hydrates in the pores, resulting in a more compact pore structure and lower permeability. Therefore, the exploitable property is even worse.

[0004] The characteristics of hydrate reservoirs determine that the existing exploitation methods cannot effectively improve the production capacity and extend the stable production period. In the prior art, a research team represented by CNOOC proposed an exploitation method of "solid fluidization". This method is similar to the mining of minerals, breaking all the hydrate-containing reservoirs into the separation equipment on the sea surface, then separating the gas, and re-injecting the mud and water into the formation. This method has two problems. One is the high cost and high requirements for the processing equipment capacity. The other is that it causes great damage to the original structure of the reservoir and is prone to geological disasters such as submarine landslides. Therefore, its application and popularization value still need to be further studied. Low-permeability unconsolidated reservoirs represented by hydrate reservoirs all face the problems of poor stimulation effect and short stable production period. Summary of the Invention

[0005] To solve the problems of difficult production increase and stable production faced in the exploitation of low-permeability unconsolidated reservoirs in sea areas, the present invention provides a multi-round exploitation method for drainage enhancement, re-permeability enhancement and multi-round exploitation of low-permeability unconsolidated reservoirs.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] A multi-round exploitation method for drainage enhancement, re-permeability enhancement and multi-round exploitation of low-permeability unconsolidated reservoirs, the method comprising the following steps:

[0008] Step 1: Exploit the low-permeability unconsolidated reservoir around the wellbore in a pressure-reducing manner, pump out the gas and water around the wellbore, and form a drainage enhancement area around the wellbore. A gas and water accumulation area for the second-stage exploitation is formed around the drainage enhancement area;

[0009] Step 2: Perform a permeability enhancement operation on the drainage enhancement area to form a permeability enhancement channel in the drainage enhancement area. The permeability enhancement channel communicates with the gas and water accumulation area for the second-stage exploitation, and then pump out the gas and water in the gas and water accumulation area for the second-stage exploitation in a pressure-reducing manner. The drainage enhancement area is expanded to form a drainage enhancement area for the second-stage exploitation, and a gas and water accumulation area for the third-stage exploitation is formed around the drainage enhancement area for the second-stage exploitation;

[0010] Step 3: Repeat the above Step 2, perform a permeability enhancement operation on the drainage enhancement area for the second-stage exploitation again, and finally form a progressive exploitation mode centered on the wellbore, from near to far, to realize multi-round repeated production until the entire ore body can be completely exploited.

[0011] Further, the permeability enhancement operation on the drainage enhancement area is performed in the following manner:

[0012] Mix the liquid and the proppant to form a sand-containing solution, and then use an injection pump to inject the sand-containing solution into the wellbore. After the injection pump increases the pressure, the liquid pressure acts on the drainage enhancement area through the sand-containing solution to form different types of permeability enhancement channels.

[0013] Further, the permeability enhancement channels are in the form of fractures and holes, and the permeability enhancement channels are filled with proppants.

[0014] Further, the proppant is one or more of ceramsite, quartz sand, coated sand, CaO composite particles, and magnetic particles.

[0015] Further, the liquid and the proppant are mixed by a mixing skid.

[0016] Further, the injection pump injects the sand-containing solution into the wellbore through an injection pipeline and into a coiled tubing.

[0017] Further, the liquid is stored in an injection liquid storage chamber, and the proppant is stored in a proppant storage chamber.

[0018] Furthermore, whether to perform permeability enhancement operation on the drainage enhancement area is determined according to production parameters, wherein the production parameters include daily water production, gas production, sand production and bottom hole pressure.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] By adopting the method of drainage enhancement and permeability enhancement for multiple rounds of exploitation of low-permeability unconsolidated reservoirs of the present application, it is only necessary to install the wellbore tools once, and the shallow soft unconsolidated strata are self-enhanced by drainage and gas production, and finally a wellbore-centered, near-to-far, progressive exploitation mode is formed, which maximizes the resource potential of low-permeability reservoirs without large-scale fracturing transformation, and improves the exploitation efficiency of gas reservoirs, hydrate reservoirs and other resources in low-permeability unconsolidated reservoirs. At the same time, since permeability enhancement operations will be carried out in the second stage and subsequent exploitation, permeability enhancement channels will be generated, that is, gas reservoirs and hydrate reservoirs at different distances will be connected through permeability enhancement channels, then in the second round and each subsequent exploitation, gas reservoirs, hydrate reservoirs and other areas at different distances will be repeatedly pumped and exploited, thereby greatly improving the hydrate recovery rate, breaking the traditional concept of production increase effect and stable production cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the initial state of the first stage of mining;

[0022] Figure 2 This is a schematic diagram of the end state of the first stage of mining;

[0023] Figure 3 This is a schematic diagram of the second stage of infiltration operation;

[0024] Figure 4 This is a schematic diagram of the end state of the second stage of mining;

[0025] Figure 5 This is a schematic diagram of the second stage of infiltration construction process;

[0026] In the figure: 1. Formation; 2. Water, hydrate, associated gas; 3. Drilling and completion contaminated zone; 4. Well wall; 5. Permeability enhancement zone; 6. Proppant; 7. Casing; 8. Casing opening; 9. Screen; 10. Tensile stress action zone; 11. Gas and water accumulation zone; 12. Compressive stress action zone; 13. Drainage enhancement zone; 14. Permeability enhancement channel; 15. Second stage drainage enhancement zone; 16. Injection pump; 17. Mixing skid; 18. Proppant storage chamber; 19. Injection liquid storage chamber; 20. Sand-containing solution; 21. Continuous oil pipe; 22. Derrick; 23. Injection pipeline. Detailed implementation mode

[0027] Example:

[0028] The technical solution of the present invention will be further described below in conjunction with the drawings and examples.

[0029] In order to solve the problems of difficult production increase and stable production faced in the exploitation process of low-permeability unconsolidated reservoirs, the present invention innovatively proposes a multi-round exploitation method for drainage enhancement and re-permeability increase in low-permeability unconsolidated reservoirs, specifically including the following innovative points:

[0030] In order to address the problems of low permeability and low strength in low-permeability unconsolidated reservoirs, an innovative method for enhancing formation drainage is proposed. During the normal production in the first exploitation stage, the gas and water near the wellbore are discharged, while the wellbore pressure is reduced. The reservoir near the wellbore gradually becomes compact due to the loss of water, gas, and some fine sand, and at the same time, the water cut decreases and the strength increases. Therefore, when using transformation methods such as fracturing and squeeze packing for transformation, it can show good compressibility, and at the same time, the pore and fracture morphology formed by the transformation can also be maintained for a relatively long time.

[0031] In order to address the problem of short stable production periods in the exploitation of resources such as low-permeability unconsolidated gas reservoirs and hydrate reservoirs, an innovative multi-round permeability increase exploitation method is proposed. After the resources such as gas, natural gas hydrates, and associated gas in the reservoir near the wellbore are exploited, permeability increase operations are immediately carried out. The channels artificially created by permeability increase transformation can communicate with resources such as gas and hydrates in relatively far areas, and then production operations are carried out again. When the gas, hydrates, and associated gas resources in this area are exploited, the above process is repeated for production. Eventually, a progressive exploitation mode centered on the wellbore, from near to far, is formed to maximize the resource potential of low-permeability unconsolidated reservoirs. It breaks the concept of the traditional stable production period and proposes a multi-round permeability increase exploitation process concept for low-permeability unconsolidated reservoirs.

[0032] The following describes the horizontal wellbore exploitation of a hydrate reservoir, taking the wellbore profile as an example. Figure 1 Shown is a schematic diagram of the initial state in the first stage of exploitation. At Figure 1In it, formation 1 is a hydrate reservoir, which has characteristics such as a fine median particle size, a high shale content, a low permeability, and an unconsolidated reservoir; water, hydrate, and associated gas 2. For the convenience of description, the gas, liquid, and solid that can flow during the production stage, such as water, hydrate, and associated gas, are aggregated to describe the migration state of gas and water during the production stage. In a real hydrate reservoir, the associated gas, water, and hydrate are distributed unevenly in the reservoir pores; the drilling and completion pollution zone 3 is the area where the gas, water, and hydrate in this area are prematurely released due to artificial construction during drilling and completion, making this area have no production potential; the wellbore wall 4 represents the wall surface formed in the formation during the formation of the hole during the drilling stage. Generally, the wellbore wall presents an irregular cylindrical shape; the permeability enhancement area 5 represents the cavity formed in the near-wellbore reservoir by means of hydraulic fracturing, hydraulic perforation, slotted, explosive transformation, laser transformation, chemical dissolution, etc. at the initial stage of exploitation. The cavity can increase the contact area with the formation, increase the permeability of the reservoir, and has an effect of increasing production; the proppant 6 can be different types of filling or supporting particles such as ceramsite, quartz sand, coated sand, CaO composite particles, magnetic particles, etc. Its main functions are two. One is to provide a supporting effect to maintain the stability of the cavity structure in the permeability enhancement area 5, and the other is to be able to block a part of the formation sand from entering the wellbore, playing a role in sand control; the casing 7 is made of steel structure, mainly playing a role in supporting the wellbore wall, and at the same time providing space for the placement of the screen pipe 9; the casing opening 8 can be pre-seamed on the ground according to the exploitation requirements, or after entering the wellbore, different forms such as holes and slotted can be generated on the casing according to different transformation methods, which is the necessary passage for gas, liquid, and sand to enter the wellbore; the screen pipe 9 is a high-permeability porous structure, mainly used for sand control, and at the same time can allow liquids and gases to enter. Generally, it is a composite screen pipe, wire-wound screen pipe, pre-packed screen pipe, slotted screen pipe, GEOFORM screen pipe, etc.

[0033] The exploitation in the first stage mainly focuses on pressure reduction exploitation. As the wellbore pressure decreases, the hydrate will undergo a chemical reaction due to the pressure reduction, and the hydrate decomposes into water and methane gas, and migrates to the wellbore through the reservoir pores and is pumped out. In this stage, as the water and the original pore water in the formation are gradually pumped out with the decomposition of the hydrate, the pressure reduction effect in the wellbore gradually weakens. It is manifested that although the liquid level in the wellbore is very low, the daily gas production does not increase significantly. This is also related to the decrease in porosity and permeability after the formation drains water. As the gas production progresses, the drainage area gradually moves away from the wellbore until a low-permeability high-strength zone, that is, the drainage enhancement zone 13, is formed near the wellbore. At this time, the external hydrate and associated gas cannot enter or are difficult to enter the wellbore, and the water and gas production significantly decreases. The formation drainage enhancement effect in the current stage ends. As Figure 2 shown, it is a schematic diagram of the end state of the exploitation in the first stage, Figure 2In it, the tensile stress action area 10 is affected by the first-stage pressure reduction production. Outside the edge that can be affected by the pressure, the reservoir will be subjected to tensile stress from the wellbore direction, manifested as an increase in the pores of the reservoir in this area; the gas and water accumulation area 11 is affected by the pressure reduction of wellbore drainage. In the area at a certain distance from the wellbore, due to the tensile force, the porosity increases, so it can accumulate gas and water in the nearby area, forming an important production capacity supply area for the second-stage production; the compressive stress action area 12 is affected by the pressure reduction of wellbore drainage. The reservoir around the wellbore will be subjected to the suction force from the wellbore, so a certain compressive stress will be generated from the outer formation to the inner side; in the drainage strengthening area 13, after the reservoir in the near-well zone drains gas and produces gas, part of the water in the reservoir pores loses water, causing the pores to collapse and close. The reservoir skeleton is strengthened by the continuous action of the wellbore suction force, but at the same time the permeability decreases. After the first-stage production ends, after drainage gas production, the permeability of the reservoir in the near-well zone decreases and the strength increases, forming the drainage strengthening area 13. Therefore, the compressibility of the reservoir is improved. Therefore, an operation to increase the permeability is carried out on the drainage strengthening area 13 before the second-stage production.

[0034] As Figure 3 shown, it is a schematic diagram of the second-stage permeability increasing operation. In the figure, the permeability increasing channel 14 is generally in the form of a fracture. When the construction displacement is different, it may also be in the form of pore throats. The permeability increasing channel is filled with proppants, which can play the role of a high-conductivity channel, communicate the formation hydrates and associated gas outside the drainage strengthening area 13 involved in the first-stage gas production, and support the gas production in the second stage. The second stage also adopts the method of pressure reduction production. As Figure 4 shown, it is a schematic diagram of the end state of the second-stage production. In the figure, the second-stage drainage strengthening area 15 undergoes the same method as the first-stage drainage gas production. After the drainage strengthening area is expanded, the overall permeability of this area decreases and the strength increases. Therefore, the transformability is improved. It provides a basis for the drainage gas production in the third stage and strengthens the formation.

[0035] Taking the horizontal well production of marine natural gas hydrates as an example below, the above-mentioned drainage strengthening and permeability increasing multi-round production method is elaborated, mainly including the following steps:

[0036] S1. Select a suitable offshore operation area and formulate a reasonable offshore production operation implementation plan, including determining the transformation area in the first stage, the transformation method, the type of sand control screen pipe, etc. It also includes the prediction of the duration of the first gas production stage and the construction plan for the second-stage permeability increasing operation, such as using squeeze packing to increase permeability or formation micro-fracturing to increase permeability. At the same time, prepare the construction materials used in the first two rounds of production, including sand-carrying fluid, filling particles, etc.

[0037] S2. In the first stage of production, pressure reduction is mainly adopted. As the wellbore pressure decreases, hydrates will undergo chemical reactions due to the pressure drop. The hydrates decompose into water and methane gas, which migrate through the reservoir pores to the wellbore and are discharged. In this stage, as the water and the original formation pore water are gradually discharged during the decomposition of hydrates, the pressure reduction effect in the wellbore gradually weakens. This is manifested as although the liquid level in the wellbore is very low, the daily gas production does not increase significantly. This is also related to the decrease in porosity and permeability after the formation drains water. As gas production progresses, the drainage area gradually moves away from the wellbore until a low-permeability and high-strength zone, that is, a drainage intensification zone, is formed near the wellbore. The external hydrates and associated gas cannot enter or can hardly enter the wellbore, and the water and gas production significantly decreases, marking the end of the formation drainage intensification effect in the current stage.

[0038] S3. Based on the production parameters in the first stage, mainly including the daily water production, gas production, sand production, and bottom-hole flowing pressure, when the daily gas production significantly decreases to the point where it is not economically viable for exploitation, generally at 100,000 m 3 per day, refracturing operations are immediately carried out. The operations mainly rely on surface equipment and the wellbore. The construction materials mainly include refracturing fluids and proppants. As Figure 5 shown, the specific steps include mixing the proppants in the proppant reservoir chamber 18 and the liquid in the injection liquid storage chamber 19 through the mixing skid 17. Using the injection pump 16, they enter the coiled tubing 21 through the injection pipeline 23 and then enter the wellbore. After the injection pump 16 increases the pressure, the liquid pressure acts on the drainage intensification zone through the sand-containing solution 20, and different types of refracturing channels 14 are generated simultaneously, connecting the hydrates and associated gas in the far-well formation to achieve the drainage gas production and formation intensification operations in the next stage. In Figure 5 , the derrick 22 provides the working space and power for the coiled tubing 21 and the injection pipeline 23.

[0039] S4. When the production in the second stage continues until the daily gas production cannot meet the economic requirements, repeat step S3 to perform refracturing operations on the drainage intensification zone in the second stage, thereby achieving multiple rounds of repeated production until the entire ore body can be completely exploited.

[0040] Thus, by adopting the multi-round exploitation method for enhancing drainage and re-increasing permeability in low-permeability unconsolidated reservoirs of the present application, only one installation of wellbore tools is required. The shallow soft unconsolidated formation is self-enhanced by means of gas production by drainage, and finally a progressive exploitation mode centered on the wellbore and from near to far is formed. Without the need for large-scale fracturing transformation, the resource potential of the hydrate reservoir is maximally exploited, and the exploitation efficiency of hydrates is improved. At the same time, since permeability-increasing operations will be carried out during the second stage and subsequent exploitation, permeability-increasing channels are generated. That is to say, hydrate regions at different distances will be connected through the permeability-increasing channels. Then, during each exploitation in the second round and subsequent rounds, hydrate regions at different distances will be repeatedly pumped and pressured, thereby greatly improving the exploitation rate of hydrates and breaking the concept of the stable production period in the traditional sense.

[0041] The above embodiments are only used to illustrate the technical concept and characteristics of the present invention, and the purpose is to enable those of ordinary skill in the art to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A multi-round exploitation method for enhanced drainage and re-increasing permeability in low-permeability unconsolidated reservoirs, characterized in that, The method includes the following steps: Step 1: The low-permeability unconsolidated reservoir around the wellbore is exploited in a pressure-lowering manner to pump out the gas and water around the wellbore. After the first-stage exploitation and gas production by water drainage, the permeability of the reservoir near the wellbore decreases and the strength increases, forming a drainage-enhanced area around the wellbore. A gas and water accumulation area for the second-stage exploitation is formed around the drainage-enhanced area. Step 2: An osmotic-increasing operation is carried out on the drainage-enhanced area to form an osmotic-increasing channel in the drainage-enhanced area. The osmotic-increasing channel communicates with the gas and water accumulation area for the second-stage exploitation, and then the gas and water in the gas and water accumulation area for the second-stage exploitation are pumped out by a pressure-lowering method. The drainage-enhanced area is expanded to form a drainage-enhanced area for the second-stage exploitation, and a gas and water accumulation area for the third-stage exploitation is formed around the drainage-enhanced area for the second-stage exploitation. Step 3: Repeat the above Step 2 to carry out an osmotic-increasing operation on the drainage-enhanced area for the second-stage exploitation, finally forming a progressive exploitation mode centered on the wellbore, from near to far, to achieve multiple rounds of repeated production until the entire ore body can be completely exploited. Determine whether to carry out an osmotic-increasing operation on the drainage-enhanced area according to production parameters, and the production parameters include daily water production, daily gas production, sand production, and bottom-hole flowing pressure. Carry out an osmotic-increasing operation on the drainage-enhanced area in the following manner: Mix a liquid and a proppant to form a sand-containing solution, and then use an injection pump to inject the sand-containing solution into the wellbore. After the injection pump increases the pressure, the liquid pressure acts on the drainage-enhanced area through the sand-containing solution to form different types of osmotic-increasing channels.

2. The enhanced multi-round exploitation method for drainage and re-increasing permeability of low-permeability unconsolidated reservoirs according to claim 1, characterized in that, The osmotic-increasing channels are in the form of fractures and holes, and the osmotic-increasing channels are filled with proppants.

3. The method for multi-round exploitation of enhanced drainage and re-increasing permeability in low-permeability unconsolidated reservoirs according to claim 2, characterized in that The proppant is one or more of ceramsite, quartz sand, coated sand, CaO composite particles, and magnetic particles.

4. The method for multi-round exploitation of enhanced drainage and re-increasing permeability in low-permeability unconsolidated reservoirs as claimed in claim 1, wherein Mix the liquid and the proppant through a mixing skid.

5. The method for enhanced drainage and multiple - round production enhancement of low - permeability unconsolidated reservoirs according to claim 1, wherein, The injection pump injects the sand-containing solution into the wellbore through an injection pipeline and into a coiled tubing.

6. The enhanced drainage and re-infiltration multi-round exploitation method for low-permeability unconsolidated reservoirs according to claim 1, characterized in that The liquid is stored in an injection liquid storage chamber, and the proppant is stored in a proppant storage chamber.

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