A clean sand control completion method for natural gas hydrate reservoirs

Abstract

The invention discloses a method for cleaning and sand-controlling completion of a natural gas hydrate reservoir, which relates to the technical field of natural gas hydrate exploitation. The method comprises a self-cleaning precision composite sand-control screen pipe, which comprises, from the inside to the outside, a fixedly connected base pipe, an inner protective sleeve, a sand-flushing pipe, a multi-layer sand-control filter sleeve and an outer protective sleeve, wherein the sand-flushing pipe is a flat slotted pipe. During production, when the output decreases to half, pressure-flushing is performed. When the output recovers to 90-95% of the initial output, the sand-flushing is stopped and the production is continued for observation. When the output begins to decrease to half again for the second time, pressure-flushing is performed again, and the cycle is repeated to find a reasonable sand-flushing cycle suitable for a production well in a target reservoir. The completion method designs a precision composite sand-control screen pipe with a "self-cleaning" function. By regularly performing surface pressure-flushing, formation sand in the screen pipe is effectively flushed, thereby solving the sand blockage problem to the greatest extent and facilitating the long-term exploitation of natural gas hydrates.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas hydrate exploitation, and in particular to a clean sand control completion method for a natural gas hydrate reservoir. Background Art

[0002] Natural gas hydrates are hailed as the most promising alternative clean energy source after coalbed methane, tight gas, and shale gas. Their vast reserves have made them a strategic frontier in energy competition among major global powers. However, significant challenges remain in their extraction, primarily due to the muddy siltstone reservoirs that comprise over 90% of hydrates in China and globally. Consequently, the selection of completion methods during extraction places high demands on the right well. Currently, research and evaluation of completion methods for natural gas hydrate extraction in marine and permafrost regions has been conducted both domestically and internationally. In 2002, Canada conducted a trial production test on the Mallik 5L-38 well using a combination of heat injection and pressure reduction, employing mechanical screens for sand control. While sand production occurred, it did not cause significant damage. In 2007, a trial production test was conducted on the Mallik 2L-38 well using the same combination of heat injection and pressure reduction, completing the well using cased perforation without any sand control measures. Results showed that within the 30-hour effective production test, the wellbore contained as much as 2 m³ of sand, causing blockage of the electric submersible pump (ESP), reducing pump efficiency and requiring constant start-up and shutdown operations, forcing the termination of the production test. In 2013, during the first production test of hydrates in the Japanese waters, preliminary sand control was implemented. To maximize reservoir production, an openhole gravel packing process was employed. A 139.7 mm Baker Hughes precision screen was run through the 215.9 mm openhole section, covering the entire hydrate reservoir for over 300 m. Lightweight ceramsite with a mesh size of 40-60 was preferred for gravel packing, using the Baker Hughes CS-300™ openhole gravel packing tool. The gravel packing operation was successful. In 2017, during the second production test in Japan, both a pre-expanded Geo-FORM sand control system and a downhole expanded Geo-FORM sand control system were employed. In May 2017, the China Geological Survey conducted a hydrate trial production in the Shenhu area of ​​the South China Sea using the "formation fluid extraction method." Using sand control technology from Stareasy Energy Technology, the trial production avoided significant sand production issues. However, due to the limited time of the trial production, low water production, and rapidly declining gas production, the underlying fluid-solid production mechanism remains unclear. From October 2019 to April 2020, a trial well in the Shenhu area was completed using 9-5 / 8" uncemented casing. This casing was run using innovative power casing running technology, ultimately forming a three-wellbore structure. A three-stage composite sand control process was employed, combining "coarse and fine" gravel packing with high-precision pre-packed screens.

[0003] However, many completion methods have many disadvantages in the application of natural gas hydrate extraction, mainly because they fail to solve the problem of sand control and production capacity balance during hydrate extraction, which increases the difficulty of extraction and limits production capacity. Therefore, there is an urgent need for an effective sand control completion method suitable for natural gas hydrate extraction in muddy siltstone reservoir types. Summary of the Invention

[0004] The purpose of the present invention is to address the defects of the existing technology and provide a clean sand control completion method for natural gas hydrate reservoirs.

[0005] The technical solution of the present invention is: a clean sand control completion method for a natural gas hydrate reservoir, comprising a self-cleaning precision composite sand control screen, wherein the self-cleaning precision composite sand control screen comprises, from the inside to the outside, a fixedly connected base pipe, an inner protective sleeve, a sand flushing pipe, a multi-layer sand control filter sleeve, and an outer protective sleeve, wherein the outer protective sleeve is a slotted pipe, and the multi-layer sand control filter sleeve comprises a support and drainage net and a precision microporous filter screen;

[0006] The sand flushing pipes are provided in a plurality and arranged in a circumferential array between the inner protective sleeve and the multi-layer sand control filter sleeve. The sand flushing pipes are flat slotted pipes, and axial staggered slots are adopted on the wider plane of the pipes, and vertical parallel slots are adopted on the narrower plane of the pipes.

[0007] The clean sand control completion method comprises the following steps:

[0008] 1) lowering the self-cleaning precision composite sand control screen into the well using a special screen running tool;

[0009] 2) Natural gas hydrate wells are put into normal production;

[0010] 3) After production begins, as time passes and muddy siltstone continues to be produced, the screen will gradually become clogged, causing the wellhead gas production to decline. When the production decreases to half of the initial production, perform surface or downhole micro-circulation pump pressure sand flushing. When the sand flushing returns to 90-95% of the initial production, stop the sand flushing and continue production for observation.

[0011] 4) After the initial sand flushing and resumption of production, the gas production gradually decreased due to the continuous production of muddy silt and the blockage of the screen. When the gas production decreased to half of the initial production, the second pressure flushing and sand flushing was carried out to remove the blockage. After the blockage was removed, the production was resumed and the subsequent changes in gas production were observed;

[0012] 5) Repeat the above steps to find a reasonable sand flushing cycle suitable for the target reservoir production well.

[0013] Preferably, the self-cleaning precision composite sand control screen is made of stainless steel.

[0014] Preferably, the precision microporous filter is a three-layer filter structure.

[0015] Preferably, the sand flushing pipe is a variable density slotted pipe, wherein the slot density is low at the initial high pressure portion of the sand flushing pipe, and the slot density gradually increases at the subsequent high pressure portion.

[0016] Preferably, the sand flushing pipes are arranged in a circumferential manner with six pieces arranged evenly.

[0017] Preferably, the slit length of the sand flushing pipe is 1 cm and the slit width is 1 / 5 cm.

[0018] Preferably, the base pipe is a perforated pipe with spiral holes.

[0019] Preferably, the base pipe has a pore density of 10-15 pores / m and a pore diameter of 18-23 mm.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] This completion method designs a precision composite sand control screen with a "self-cleaning" function. By regularly ground pressure-pressing, the formation sand in the screen is effectively flushed, which can solve the sand blockage problem to the greatest extent, contribute to the long-term exploitation of natural gas hydrates, achieve a new breakthrough in the balance between sand control and production, and contribute to the early realization of commercial exploitation of natural gas hydrates. This screen has high permeability and deformation resistance, and is also highly corrosion-resistant. The sand flushing pipe is a flat slotted pipe. Compared with a circular pipe, the flat sand flushing pipe can save a certain amount of annular space and achieve the effect of uniform circumferential sand flushing. There are multiple sand flushing pipes, which are arranged in a circular array between the inner protective sleeve and the multi-layer sand control filter sleeve, which can ensure the effect of uniform sand flushing through ground pressure-pressing. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the self-cleaning precision composite screen tube structure;

[0023] Figure 2 This is a three-dimensional structural diagram of the self-cleaning precision composite screen tube;

[0024] Figure 3 This is a schematic diagram of the cross-sectional structure of a self-cleaning precision composite screen tube;

[0025] Figure 4 This is a schematic diagram of the distribution of the slits in the sand flushing pipe;

[0026] Figure 5 This is the cumulative distribution curve of hydrate reservoir particle size in the Shenhu area;

[0027] Figure 6 This is the production status of natural gas hydrate trial production in Shenhu Sea Area;

[0028] Figure 7 This is the result table of laser particle size test of hydrate reservoir core in Shenhu Sea Area;

[0029] Figure 8 The following table shows the relative content of mud and minerals in the core of the hydrate reservoir in Shenhu Sea Area.

[0030] Figure 9 This is the calculation result table of the sieve hole size and sand retaining accuracy of the precision microporous filter.

[0031] In the figure: 1. Base pipe, 2. Inner protective sleeve, 3. Sand flushing pipe, 4. Multi-layer sand control filter sleeve, 5. Outer protective sleeve. DETAILED DESCRIPTION

[0032] The present invention is further described below with reference to the accompanying drawings and embodiments.

[0033] Example 1

[0034] In 2017, the Shenhu area in the northern part of the South China Sea successfully conducted its first exploratory test production of natural gas hydrates. In 2020, a breakthrough was made in the pilot production of horizontal wells in the same area, achieving a breakthrough from vertical wells to horizontal wells, and taking an extremely critical step in the industrialization of natural gas hydrates. Figure 6 It can be seen that although both test productions were successful, due to the influence of the reservoir type (muddy siltstone type) in the area, the production and timeliness of natural gas hydrate production in the area are difficult to guarantee. Therefore, there is an urgent need for an effective sand control completion method suitable for natural gas hydrate extraction in muddy siltstone reservoir types to achieve a balance between sand control and production capacity.

[0035] First, the target reservoir formation sand particle size was tested, and the results were as follows: Figure 7 As shown, the cumulative distribution curve of the hydrate reservoir particle size in the Shenhu area was drawn based on the test results. The results are shown in Figure 5 As shown in the figure, the test results show that the median particle size (D50) of the five rock samples ranges from 10-30 μm, D10 from 50-70 μm, D40 from 20-35 μm, and D90 from 2-4 μm, indicating that the reservoir rock particles are extremely fine. The median particle size of the reservoir section ranges from 16.156 to 25.162 μm. The heterogeneity coefficient ranges from 7.454 to 10.448, with a homogeneity coefficient greater than 5, indicating significant heterogeneity. The sorting coefficient ranges from 21.681 to 39.334, with a sorting coefficient greater than 20, indicating poor sorting. The formation sand is fine silt, with strong heterogeneity and uneven distribution.

[0036] Cores were taken from some sections of the target reservoir and the mud content was measured using an X-ray diffractometer. Figure 8As shown in the table, due to the limited availability of cores for X-ray diffraction testing, only a preliminary estimate of the reservoir's shale content can be made based on these test results. The table shows that the shale content of the hydrate reservoir ranges from 24.6% to 35.1%. A shale content well above 15% poses a high risk of screen plugging. Therefore, designing a self-cleaning sand control screen has become a key task in natural gas hydrate extraction.

[0037] Combined with the analysis of mud content and particle size data of the hydrate reservoir in the Shenhu area, and using the screen selection method proposed by Tiffin and King, it is recommended that high-quality metal mesh screens or above be used for sand control completion in the Shenhu area. Taking into account the reservoir type and the inevitability of sand production, a "self-cleaning" precision composite screen was designed, which adds a sand flushing pipe 3 to the original precision composite screen structure. This achieves the effect of regular sand flushing, achieving a balance between sand control and production capacity while ensuring sand production.

[0038] Reference Figure 1-2 As shown, the self-cleaning precision composite sand control screen pipe consists of a fixedly connected base pipe 1, an inner protective sleeve 2, a sand flushing pipe 3, a multi-layer sand control filter sleeve 4 and an outer protective sleeve 5 from the inside to the outside. The self-cleaning precision composite sand control screen pipe is made of stainless steel, which can slow down the corrosion rate of the screen pipe and increase the service life of the screen pipe; the outer protective sleeve 5 is a punched seam pipe, and the multi-layer sand control filter sleeve 4 includes an outer support and drainage net and an inner precision microporous filter net; the screen pipe has high permeability and deformation resistance, as well as high corrosion resistance, so that part of the formation sand is blocked outside the punched seam outer protective sleeve 5 to strengthen the protection of the multi-layer sand control filter sleeve 4, and the formation fluid enters the screen pipe through the punched seam gap to achieve the purpose of sand control.

[0039] Reference Figure 3-4 As shown, the sand flushing pipe 3 is a flat slit pipe. Compared with the circular pipe, the flat sand flushing pipe 3 can save a certain amount of annular space and achieve the effect of uniform circumferential sand flushing. There are six sand flushing pipes 3, which are arranged in a circular array between the inner protective sleeve 2 and the multi-layer sand control filter sleeve 4, which can ensure the effect of uniform sand flushing through ground pressure. Axial staggered slits are adopted on its wider plane, and vertical parallel slits are adopted on its narrower plane. This arrangement ensures that the formation sand can be flushed uniformly in the circumferential direction.

[0040] The clean sand control completion method comprises the following steps:

[0041] 1) Use a special screen running tool to lower the self-cleaning precision composite sand control screen into the well;

[0042] 2) Natural gas hydrate wells are put into normal production;

[0043] 3) After production begins, as time passes and muddy siltstone continues to be produced, the screen will gradually become clogged, causing the wellhead gas production to decline. When the production decreases to half of the initial production, perform surface or downhole micro-circulation pump pressure sand flushing. When the sand flushing returns to 90-95% of the initial production, stop the sand flushing and continue production for observation.

[0044] 4) After the initial sand flushing and resumption of production, the gas production gradually decreased due to the continuous production of muddy silt and the blockage of the screen. When the gas production decreased to half of the initial production, the second pressure flushing and sand flushing was carried out to remove the blockage. After the blockage was removed, the production was resumed and the subsequent changes in gas production were observed;

[0045] 5) Repeat the above steps to find a reasonable sand flushing cycle suitable for the target reservoir production well.

[0046] The sand flushing cycle is determined by the production half-life method, which ensures that natural gas hydrate wells can produce at a higher rate while overcoming the problem of unpredictable blockage cycles of downhole screens due to differences in hydrate reservoir properties in individual wells.

[0047] This completion method designs a precision composite sand control screen with a "self-cleaning" function. Through regular surface pressure injection, the formation sand in the screen is effectively flushed. Compared with the previous two completion sand control methods, it can solve the sand plugging problem to the greatest extent, facilitate the long-term exploitation of natural gas hydrates, achieve a new breakthrough in the balance between sand control and production, and help to realize the commercial exploitation of natural gas hydrates at an early date.

[0048] Example 2

[0049] Reference Figure 1-2 As shown, the self-cleaning precision composite sand control screen pipe consists of a fixedly connected base pipe 1, an inner protective sleeve 2, a sand flushing pipe 3, a multi-layer sand control filter sleeve 4 and an outer protective sleeve 5 from the inside to the outside. The self-cleaning precision composite sand control screen pipe is made of stainless steel, which can slow down the corrosion rate of the screen pipe and increase the service life of the screen pipe; the outer protective sleeve 5 is a punched seam pipe, and the multi-layer sand control filter sleeve 4 includes an outer support and drainage net and an inner precision microporous filter net; the screen pipe has high permeability and deformation resistance, as well as high corrosion resistance, so that part of the formation sand is blocked outside the punched seam outer protective sleeve 5 to strengthen the protection of the multi-layer sand control filter sleeve 4, and the formation fluid enters the screen pipe through the punched seam gap to achieve the purpose of sand control.

[0050] Reference Figure 3-4As shown, the sand flushing pipe 3 is a flat slit pipe. Compared with the circular pipe, the flat sand flushing pipe 3 can save a certain amount of annular space and achieve the effect of uniform circumferential sand flushing. There are six sand flushing pipes 3, which are arranged in a circular array between the inner protective sleeve 2 and the multi-layer sand control filter sleeve 4, which can ensure the effect of uniform sand flushing through ground pressure. Axial staggered slits are adopted on its wider plane, and vertical parallel slits are adopted on its narrower plane. This arrangement ensures that the formation sand can be flushed uniformly in the circumferential direction.

[0051] In addition, the difference between this embodiment and the first embodiment lies in the arrangement of the multi-layer sand control filter sleeve 4, specifically:

[0052] The precision microporous filter screen of this embodiment has a three-layer filter screen structure, which ensures that the filter screen has sufficient sand retaining accuracy and meets the sand retaining requirements of muddy siltstone.

[0053] Since the precision composite screen structure is generally composed of three layers, the sand retention accuracy will change greatly during the change from a single layer to a three-layer. Not only will the screen be easily clogged, but the effectiveness of sand control completion will inevitably be greatly affected. Therefore, the size of the screen holes from the single layer to the three-layer must be reasonably adjusted according to the sand retention accuracy. According to the particle size distribution curve of the Shenhu sea area, the sand retention accuracy range is calculated based on the fitting empirical formula to be 19-26μm. Combined with the target reservoir particle size median range of 16-25μm, the upper limit of the particle size median is used as the sand retention accuracy standard. Using the determined sand retention accuracy (25μm), the size of the screen hole of the precision composite screen filter layer is determined when changing from a single layer to a three-layer; the size design and optimization results are shown in Figure 9 .

[0054] Example 3

[0055] Reference Figure 1-2 As shown, the self-cleaning precision composite sand control screen pipe consists of a fixedly connected base pipe 1, an inner protective sleeve 2, a sand flushing pipe 3, a multi-layer sand control filter sleeve 4 and an outer protective sleeve 5 from the inside to the outside. The self-cleaning precision composite sand control screen pipe is made of stainless steel, which can slow down the corrosion rate of the screen pipe and increase the service life of the screen pipe; the outer protective sleeve 5 is a punched seam pipe, and the multi-layer sand control filter sleeve 4 includes an outer support and drainage net and an inner precision microporous filter net; the screen pipe has high permeability and deformation resistance, as well as high corrosion resistance, so that part of the formation sand is blocked outside the punched seam outer protective sleeve 5 to strengthen the protection of the multi-layer sand control filter sleeve 4, and the formation fluid enters the screen pipe through the punched seam gap to achieve the purpose of sand control.

[0056] Reference Figure 3-4As shown, the sand flushing pipe 3 is a flat slit pipe. Compared with the circular pipe, the flat sand flushing pipe 3 can save a certain amount of annular space and achieve the effect of uniform circumferential sand flushing. There are six sand flushing pipes 3, which are arranged in a circular array between the inner protective sleeve 2 and the multi-layer sand control filter sleeve 4, which can ensure the effect of uniform sand flushing through ground pressure. Axial staggered slits are adopted on its wider plane, and vertical parallel slits are adopted on its narrower plane. This arrangement ensures that the formation sand can be flushed uniformly in the circumferential direction.

[0057] In more detail, the precision microporous filter has a three-layer filter structure.

[0058] In addition, the difference between this embodiment and the second embodiment lies in the setting of the slit density of the sand flushing tube 3, which is specifically:

[0059] The sand flushing pipe 3 of this embodiment is a variable-density slotted pipe. The design of the variable-density slotted sand flushing pipe 3 fully considers the problem of uneven unblocking of the screen pipe in the horizontal direction due to the difference in hydraulic jet intensity caused by the friction pressure drop along the flow when the fluid inside the sand flushing pipe 3 moves from the heel end to the toe end under uniform slotting conditions. Through the variable-density slotting treatment, the slot density at the initial high-pressure position is reduced, and the slot density at the high-pressure position along the flow is gradually increased, thereby achieving balanced flushing under the same pumping pressure, achieving balanced unblocking of the blocked screen pipe, and improving the unblocking efficiency.

[0060] Example 4

[0061] As a preferred embodiment of the present invention, this embodiment optimizes the size of the slit of the sand flushing pipe 3 on the basis of the third embodiment, specifically:

[0062] In this embodiment, the slit length of the sand flushing pipe 3 is 1 cm and the slit width is 1 / 5 cm. This setting can ensure that sufficient water is sprayed out during pressure sand flushing to meet the cleaning water volume requirement.

[0063] Example 5

[0064] As a preferred embodiment of the present invention, this embodiment optimizes the structure of the base tube 1 based on the fourth embodiment, specifically:

[0065] In this embodiment, the base pipe 1 is a perforated pipe with spiral holes, a hole density of 10-15 holes / m, and a hole diameter of 18-23 mm. While ensuring sufficient strength, it increases the effective flow area to meet the production capacity flow demand within 500,000 cubic meters per day.

[0066] The present invention is not limited to the above-mentioned embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. The contents after the changes still fall within the scope of protection of the present invention.

Claims

1. A clean sand control completion method for a natural gas hydrate reservoir, characterized by: It includes a self-cleaning precision composite sand control screen pipe, which is composed of a fixedly connected base pipe, an inner protective sleeve, a sand flushing pipe, a multi-layer sand control filter sleeve and an outer protective sleeve from the inside to the outside. The outer protective sleeve is a punched pipe, and the multi-layer sand control filter sleeve includes a support and drainage net and a precision microporous filter net; The sand flushing pipes are provided with six and arranged in a circular array between the inner protective sleeve and the multi-layer sand control filter sleeve. The sand flushing pipes are flat slotted pipes, with axial staggered slots on the wider plane and vertical parallel slots on the narrower plane. The sand flushing pipe is a variable density slotted pipe, the slot density is low at the initial high pressure part of the sand flushing pipe, and the slot density gradually increases at the subsequent high pressure part; The clean sand control completion method comprises the following steps: 1) lowering the self-cleaning precision composite sand control screen into the well using a screen running tool; 2) Natural gas hydrate wells are put into normal production; 3) After production begins, as time passes and muddy siltstone continues to be produced, the screen will gradually become clogged, causing the wellhead gas production to decline. When the production decreases to half of the initial production, perform surface or downhole micro-circulation pump pressure sand flushing. When the sand flushing returns to 90-95% of the initial production, stop the sand flushing and continue production for observation. 4) After the initial sand flushing and resumption of production, the gas production gradually decreased due to the continuous production of muddy silt and the blockage of the screen. When the gas production decreased to half of the initial production, the second pressure flushing and sand flushing was carried out to remove the blockage. After the blockage was removed, the production was resumed and the subsequent changes in gas production were observed; 5) Repeat the above steps to find a reasonable sand flushing cycle suitable for the target reservoir production well.

2. A clean sand control completion method for a natural gas hydrate reservoir according to claim 1, characterized in that: The self-cleaning precision composite sand control screen is made of stainless steel.

3. The method for clean sand control completion of a natural gas hydrate reservoir according to claim 1, characterized in that: The precision microporous filter screen is a three-layer filter screen structure.

4. The method for clean sand control completion of a natural gas hydrate reservoir according to claim 1, characterized in that: The slit length of the sand flushing pipe is 1 cm, and the slit width is 1 / 5 cm.

5. The method for clean sand control completion of a natural gas hydrate reservoir according to claim 1, characterized in that: The base pipe is a perforated pipe with spiral holes.

6. A clean sand control completion method for a natural gas hydrate reservoir according to claim 1 or 5, characterized in that: The base pipe has a pore density of 10-15 pores / m and a pore diameter of 18-23 mm.

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