Preliminary study on coalbed methane extraction technology in Liulin area

Mo Ri and Guo Benguang Meng Shangzhi Zhang Wenzhong

About the author: Mo Rihe, born in 1969, male, Han nationality, born in Gaozhou, Guangdong Province, master, senior engineer, Zhonglian Coalbed Methane Co., Ltd., majoring in oil and gas wells, engaged in drilling and drainage engineering technology and management, No.88A Anwai Street, Beijing, (0 10) 64299374, 65438+.

(China United Coalbed Methane Co., Ltd., Beijing 1000 1 1)

Based on the technical analysis of geological and reservoir characteristics in Liulin area, according to the different geological characteristics in different locations in Liulin area, the corresponding drainage equipment and drainage scheme are designed by numerical simulation method, and three different types of pumps, namely electric submersible pump, screw pump and beam pump, are tried. It is the first time to test the production mode of coalbed methane with cluster wells in this area, which has achieved a breakthrough in coalbed methane production in this area. The output of horizontal wells exceeds 15000m3/d, and the maximum gas production of vertical wells reaches1800m3/d. The application shows that this drainage technology can meet the needs of coalbed methane wells in Liulin area, which has accumulated valuable experience for large-scale exploitation of coalbed methane in this area.

Keywords: the application of drainage and mining technology in Liulin area

Discussion on dewatering technology of coalbed methane wells in Liulin area

Mo Ri and Guo Benguang MENG Shangzhi Zhang Wenzhong

(China United Coalbed Methane Co., Ltd., Beijing 1000 1 1, China)

Abstract: The geological and reservoir characteristics in Liulin area are analyzed by numerical simulation. According to the different geological characteristics in different locations in Liulin area, corresponding drainage equipment, schemes and three different types of pumps are designed, including electric submersible pump, screw pump and beam pumping unit. As the first experimental test, the use of cluster coalbed methane production wells has made a major breakthrough in the production history of this area. The production of horizontal wells exceeds 15000 m3/d, and the highest production of vertical wells reaches1800m3/d. The application shows that the dehydration technology meets the dehydration demand of coalbed methane in Liulin area, and also accumulates experience for large-scale production of coalbed methane in the future.

Key words: Liulin area; Dehydration technology; Dehydration result, application

1 preface

Most coal seams in China belong to low porosity, low permeability and low pressure coal seams, so it is very important to determine a reasonable working system to ensure the maximum output of coalbed methane. Drainage quality often determines the output of coalbed methane, which is an important factor to ensure continuous, stable and economical drainage of coalbed methane wells. The permeability of coal seam is much lower than that of ordinary oil and gas reservoirs. If the drainage system is not properly selected, it is easy to cause damage to the coal seam, close the fracturing cracks, and even lead to gas production failure in gas wells in serious cases.

The Liulin demonstration area in the eastern margin of Ordos Basin is rich in coalbed methane resources and has good coal seam physical properties. The research on drainage system and equipment is the premise of ensuring continuous, stable and economical drainage of coalbed methane wells, which is of great significance to the formation of commercial mining scale in Liulin demonstration area. At the same time, the study of this block is also of great significance to the development of coal-bed methane in China.

2 CBM drainage mechanism

Coalbed methane, also known as coalbed methane, is called coalbed methane in coal industry. It is an unconventional natural gas that is formed in the process of coal formation and exists in coal seams. Most of this natural gas (70%~90%) exists in the inner surface of coal pores, and a small amount exists in pores and fractures such as coal cleat in a free state. Mining coalbed methane can provide important energy for industry and civil use. At the same time, it can also reduce gas explosion accidents during coal mining [1~4]. Natural cracks or fissures in coal seams are usually saturated with water, and coalbed methane is adsorbed on coal. To extract coalbed methane, it must first be desorbed from coal. Only when enough water is discharged and the pressure of coal seam drops to the desorption pressure of coal can the desorption of coalbed methane begin. Therefore, unlike natural gas production, coalbed methane must discharge a large amount of water from the coal seam before it starts to produce gas [5].

3 Geological survey

3. 1 Coal-bearing strata and coal seams

There are coal seams 14 in this block, including 5 layers in Shanxi Formation, numbered from top to bottom as 1, 2, 3, 4 (3+4) and No.5 coal seam; Taiyuan Formation consists of 9 floors, numbered from top to bottom as 6, 6, 7, 7, 8+9, 9, 10, 10,1. Among them, No.2, No.3, No.4 (3+4) and No.5 coal seams of Shanxi Formation and No.8+9 and 10 coal seams of Taiyuan Formation are the main coal seams, and the thickness of No.3+4 coal seam is 0.04~6.05m, with an average of 2.81m. The whole area is developing. The coal seam is simple in structure, with 1~3 layers of carbonaceous mudstone or mudstone inclusions locally, with the thickness of 0.05 ~ 0.50m. No.5 coal seam is relatively stable, with the thickness of 0~5.04m and the average thickness of 2.70mm. No.8+9 coal seam is 0.79~ 10.30m, with the average thickness of 5.

3.2 Coal seam adsorption characteristics

The coal seam in this block has high metamorphic degree and strong adsorption capacity. According to the Langmuir volume of No.3+4 coal seam in Shanxi Formation of coalbed methane well in this block is 18.34~22.45m3/t, with an average of 20.70m3/t, and the Langmuir pressure is 1.49~3.52MPa, with an average of 2.27MPa.

3.3 gas saturation

The Langmuir volume (maximum adsorption capacity) of coal in Liulin demonstration area is 18.34~24.43m3/t, with an average value of 21.38m3/t. The test results show that the coal reservoir has strong adsorption capacity. The gas saturation of coal seam is generally 60.22%~75. 10%, with an average of 66.73%. Most coal reservoirs in Liulin demonstration area are under-saturated.

3.4 Permeability

The permeability of No.4 (3+4) coal seam in Shanxi Formation is between 0.0 1 1~2.80mD, and that of No.5 coal seam is between 0.06~2.26mD: The permeability of No.8+9 8+9+ 10/0 coal seam in Taiyuan Formation is between 0.005-24.80 MD, with an average permeability of 3.93 MD It can be seen that the permeability of coal seam in this block is relatively high, with a large change range, which is not obviously related to the degree of coal metamorphism and buried depth, and the anisotropy and heterogeneity are obvious.

3.5 Reservoir pressure

The reservoir pressure of No.4 (3+4) coal seam in this block is 2.58~8.33MPa, with an average of 5.79MPa, and the pressure gradient is 0.46 ~1.12mpa/100m, with an average of 0.84mpa/100m. The reservoir pressure of No.5 coal seam is 2.92~8.4 1MPa, with an average of 6.0 1MPa, and the pressure gradient is 0.60 ~1.1MPa//00m, with an average of 0.83 MPa/1000. The reservoir pressure of No.8+9 (8+9+ 10) coal seam is 3.3 1~7.46MPa, with an average of 6.47MPa, and the pressure gradient is 0.53 ~1.174 MPa/100m. It can be seen that the reservoir pressure in this block is relatively high, and the pressure gradient is generally less than the hydrostatic pressure gradient (0.98MPa/ 100m), which belongs to the abnormal low pressure state.

3.6 Regional hydrogeological conditions

The main aquifers in this area are Ordovician and Carboniferous limestone karst and fissure aquifers; Permian and Triassic sandstone fractured aquifers; Tertiary and Quaternary gravel (rock) porous aquifers.

The thickness of middle and lower Ordovician limestone, marl and dolomite is 400 ~ 600 meters, mainly exposed in the periphery of coal field. Ordovician is shallow-sea sedimentary facies, in which Shangmajiagou Formation has the highest karst development degree and the strongest water abundance, followed by Fengfeng Formation and Xiamajiagou Formation. Generally speaking, the karst fractures in Lower Yeli Formation and liangjiashan Formation are undeveloped and have weak water-rich property, but the karst in some fault zones is developed and has strong water-rich property. This layer is rich in karst water and is the main regional aquifer. The main types of water are sodium bicarbonate and sodium chloride. The aquifer is covered with well-developed shale and bauxite aquifuge, which is far from the coal seam and has little influence on the coal seam.

The karst fissure aquifer of limestone in Taiyuan Formation of Upper Carboniferous consists of five layers of limestone, with a total thickness of about 20m. The exposed area is small, and karst and cracks are generally undeveloped. Karst is mainly dissolved crevices and small dissolved pores, mostly filled with calcite, with weak water abundance. The shallow buried area on the eastern edge of the block is developed with a honeycomb shape, good connectivity, easy recharge and strong water abundance. Due to the uneven development of karst fractures, the water abundance varies greatly from place to place. The water level elevation is between 789.3 1~8 14.74m, the water quality types are mostly NaHCO3 and NaCl, and the salinity is1190 ~ 3210 mg/L.

3.7 coal seam water content

The source of coal seam water in Liulin experimental area is restricted by regional hydrogeological conditions, mainly including surface water and aquifer water, and fault water is not developed. The surface water source is mainly the flowing water of Sanchuan River. In the eastern and upper reaches of the experimental area, river water is injected or infiltrated into coal measures strata, which plays a supplementary role in coal seam water. Regional aquifer is the main source of coal seam water in the experimental area, and its strength determines the size of coal seam water. The water output of production wells in Liulin area changes greatly, with large water output in the north block and small water output in the south block. Generally speaking, it is high in the north and low in the south, high in the east and low in the west, which is basically consistent with the structural trend. The roof and floor of coal seams in southern China are all argillaceous rocks, and the water supply is poor, and little water penetrates into coal seams.

4 Selection of drainage and production equipment

According to the characteristics of coalbed methane in Liulin area, the optimization of drainage and production methods mainly considers the following three points: first, reduce the bottom hole flowing pressure as much as possible and fully reduce the reservoir pressure; Second, consider the influence of gas on the pump and other factors; The third is to determine the liquid supply capacity of coal seam.

The method of equipment selection is to select a variety of drainage and coal mining methods in the production operation system. For example, for coalbed methane wells with low water production in the south of the block or low water production in the later period, variable speed adjustable pumping units and numerical control pumping units with easy adjustment of working system and good liquid level control are selected [6]. However, in the northern region, coalbed methane wells have large water production and strong liquid supply capacity (usually the daily water production is above1100m3). In the early stage, drainage was mainly considered, and large pumps were selected to strengthen drainage and decompression. Screw pump, large-diameter beam pump and electric submersible pump are usually used.

4. 1 beam pump

The beam pump (pumping unit) has stable production, long pump inspection period and mature technology and management. But the displacement should not be too high, and the influence factors of gas should be considered. The reservoir in Yangjiayu area in the south of Liulin is short of water supply and water production. The beam pump drainage technology has been used for more than 6 months, and the pump condition is still good at present. In many domestic coalbed methane exploration and development operations, the commonly used drainage and production method is beam pump gas drainage and production technology, and the application effect is very good. The use of type 5 pumping unit in the south of this area can completely meet the production needs.

4.2 Screw pump

Screw pump is mainly composed of ground driving device and underground pump. The advantage of screw pump is that gas, pulverized coal and fracturing sand have relatively little influence on screw pump. Compared with beam pump, screw pump has the advantages of low cost, simple installation and small floor space. When the screw pump is in production, the suction port usually goes below the coal seam, so that only water can come out of the tubing and gas production can be minimized. Its disadvantage is that in the early stage of production, too much pulverized coal in the formation will jam the screw pump, causing the sucker rod to break, and downhole accidents are prone to occur when the torque is high, and the pump inspection cycle is generally short. If the daily water production is below 60m3/d, glb 600-23； can be used; If the daily water production is close to 150m3/d, GLB900- 18 pump is better; If it exceeds 150m3/d, GLB900-23 pump should be selected.

The water production in the northern part of Liulin is generally 50~200m3/d, so the screw pump is widely used in the northern part, which proves that the screw pump can well complete the task of drainage and gas production.

4.3 Electric submersible pump

When the output exceeds 200m3/d, the electric submersible pump can be used, and the selected model depends on the displacement and the depth of pumping. In addition, the use of ESP in highly deviated directional wells can effectively prevent tubing accidents caused by eccentric wear of tubing and sucker rod. At present, there are three kinds of ESPs in use: QYB 98-200/700, GQYB 1M0 1-220/700 and QYB 98-300/700-N8. In the northern area, individual vertical wells and horizontal wells produce a large amount of water, so we choose the electric submersible pump, and the displacement of horizontal wells reaches 300m3/d, which meets the needs of drainage and pressure reduction.

5 Selection of downhole string and tools [7~8]

(1) The selection of tubing and sucker rod meets the needs of load. 89mm tubing and 22mm or 25m sucker rod are suitable for wells with large water production in the north (Figure 1), and 73mm tubing and 22mm sucker rod are suitable for the south (Figure 2).

(2) Selection of pump diameter: Try to meet the requirements of maximum liquid production when draining, and the pump diameter should not be too large, because the larger the pump diameter, the greater the hanging point load, and the higher the requirements for the sucker rod and the whole drainage gas production system. In the south of Liulin, a 38mm tubular combined pump is generally used, with a stroke of 2. 1m and a stroke number of 1~ 1.5 times /min, which can meet the construction requirements of displacement less than 10m3/d in this area.

6 drainage and mining system selection [9]

Reasonable drainage speed is the guarantee of high yield of coalbed methane. If the drainage speed is too high and the liquid level drops too fast, the drainage radius of potential coalbed methane wells will be shortened, resulting in speed-sensitive effect, proppant particles will be embedded in the coal seam, the fracture will close faster, and the permeability will drop rapidly, resulting in low gas production of single wells. If the drainage speed is too small, it will not meet the requirements economically. With the help of the model established by ECLIPSE, the influence of pressure-sensitive effect and speed-sensitive effect is fully considered.

Figure 1 Downhole String Structure of Screw Pump

Fig. 2 downhole string structure of beam pump

The simulation results show that with the increase of downcomer speed, the peak output and cumulative output gradually increase and finally tend to be flat. It is suggested that the liquid level in the 3rd, 4th and 5th floors should drop at a speed of 6m per day. The calculated results are consistent with the actual droplets of 5 ~10m in Yangjiayu area.

7 application of coalbed methane drainage technology

7. 1 gas prevention and control measures

Place the pump under the coal seam. A settling air anchor or a spiral air anchor is installed below the drainage pump.

7.2 Measures to prevent pulverized coal

Sand stuck pump (solid plunger pump) is used to install wire wound screen, grit chamber, "small pump slow pumping" and "intermittent drainage" under the pump.

7.3 Drainage and mining scheme

Meet the technical requirements of production well drainage, and make corresponding adjustments with the dynamic changes of the well. Constant pressure drainage is adopted at the initial stage, and constant production drainage is adopted during production.

(1) Debug the pump and metering process to normal working conditions, and try to maintain the continuity of drainage and production.

(2) Determine the desorption pressure, and divide the drainage into three stages according to the desorption pressure:

Initial drainage stage: start drainage. When the liquid level drops to about 200m above the desorption pressure point, it is mainly for drainage and liquid dropping, and the liquid dropping speed can be controlled within 15m/ day, which takes about 1~2 months.

Steady drainage stage: 200m above desorption pressure point to100m above coal seam. At this stage, the drainage speed can be further reduced to 5~ 10m per day, which takes about 2 months.

Stable production stage: from above the coal seam 100 meters to the coal seam, which is the stable production stage, to ensure the smooth operation of pumping units and other equipment and the stability of liquid level, thus ensuring stable and continuous gas production.

Figure 3 Borehole Trajectory of Cluster Well Group

7.4 Cluster Well Group Test

Cluster well is an oil and gas development technology of drilling multiple wells in the same well site, which has the advantages of saving land, saving pre-drilling engineering investment and facilitating production management. In view of the characteristics that the ground in Liulin CBM exploration area is mostly alpine forest land and fertile soil, on the basis of repeated demonstration, pilot, summary and continuous improvement, wellbore technologies such as highly deviated wells and horizontal wells are vigorously applied to try to popularize the application of cluster well groups. Five wells are deployed in the same site as cluster wells (Figure 3), and each well group is equipped with a set of wastewater pool and sewage cleaning and diversion system, which effectively protects the cultivated land area, effectively promotes the company's intensive, clean and economical development, and comprehensively improves the comprehensive benefit of investment.

The drainage and production equipment of cluster wells are basically the same as those of ordinary vertical wells. In the case of small well deviation and low water production, the beam pump is selected. If the deviation is greater than 40, consider using ESP. In our well group, the beam pump is used for 4 wells, and the ESP is used for 1 well. The test shows that the selected drainage equipment has achieved the purpose of durability and drainage decompression.

7.5 Application effect

A set of drainage system and technology suitable for vertical and horizontal wells of bituminous coal is formed, and the drainage effect is good. The single-well output of horizontal wells in this area exceeded 15000 m3/d for the first time (Figure 4), and the single-well output of vertical wells was above 1000 m3/d, with the highest output of 1800 m3/d (Figure 5).

Fig. 4 Drainage curve of horizontal well

8 conclusion

(1) According to the production needs of coalbed methane drainage, the research, matching and preliminary attempt of coalbed methane drainage technology were carried out. Formed a set of drainage equipment and supporting technology suitable for different areas and different pay zones in Liulin area.

Fig. 5 Drainage curve of vertical well

Fig. 6 Cluster CBM production well group

(2) According to the drainage characteristics of coalbed methane well, through the design of underground pipe string and surface flow of Liulin coalbed methane well, the stepless numerical control pumping unit and permanent pressure monitoring are introduced, and the drainage construction and data acquisition requirements are well completed, which lays the foundation for large-scale development in this area.

(3) The drainage and production equipment and technology suitable for the cluster well group in this area (Figure 6) are tried, which creates conditions for the large-scale application of the cluster well group in this area. Cluster well group occupies less land and is easy to manage, which will significantly improve the overall benefit of coalbed methane development in Liulin area with complex terrain.

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