Present situation of drainage and production of DCMT 3 well and analysis of development prospect of coalbed methane in Sichuan

Yin Zhongshan Jianxin Xiaowei Wang Wei

About the author: Yin Zhongshan, born in 1967, is a senior engineer engaged in coalfield geology and coalbed methane exploration and development; Address: Geological Exploration Department of Sichuan Coalfield Geology Bureau, No.38 Qingnian Road, Chengdu, Sichuan (6 1072), Tel: 1598 2349009, E-mail: yzs-1@163.com.

(Chengdu 6 10072, Sichuan Coalfield Geology Bureau)

Abstract: The ground drainage test of coalbed methane in Dacun mining area of southern Sichuan coalfield has made a historic breakthrough. The cumulative gas production of DCMT 3 coalbed methane test well has exceeded 500,000 m3 from high and stable production to self-production. The drainage curve characteristics of well DCMT 3 show that the well is a low and stable coalbed methane well. Through economic evaluation, it is preliminarily considered that the coalbed methane in Dacun mining area has a good commercial development prospect; Under the guidance of the theory of coalbed methane gas-bearing system, it is suggested that in the exploration and development test of Permian and Triassic coalbed methane in Sichuan Basin, the scope of exploration and test should be expanded, wells should be arranged on the same horizontal line, the clean fracturing fluid test should be accelerated, the productivity of single well or well group should be improved, and the comprehensive social and economic benefits should be brought into play as soon as possible, so as to realize the safe exploitation of Permian (P) wells in Guxu mining area and Triassic (T) wells in Wei Zi coalfield.

Key words: clean fracturing fluid for low-yield and stable coalbed methane reservoir in well DCMT 3 with linear well layout.

Characteristics of coalbed methane production curve and analysis of development prospect in DCMT 3 well

Yin Zhongshan, Jianxin Xiao, Wang Wei

(Sichuan Coalfield Geological Bureau, Chengdu, China 6 10072)

Abstract: A historic breakthrough has been made in block trial drainage in Dacun coalbed methane mining area. The DCMT 3 test well has gone through the stages of high and stable production and self-production, and the cumulative gas production exceeds 500,000 cubic meters. This paper summarizes the reconstruction methods of coalbed methane wells from the aspects of reservoir characteristics, reconstruction scheme, gas-liquid production characteristics and effective drainage methods, especially referring to the drainage effect of well DCMT 3. Through economic evaluation, it is preliminarily considered that the coalbed methane in Dacun coal seam has a good commercial development prospect. Under the guidance of coal-bed methane-bearing system and overall development technology, the exploration, development and test of Permian and Triassic coal-bed methane in Sichuan Basin are carried out. It is suggested to expand the scope of exploration and test, arrange wells along the strike, speed up the test of clean fracturing fluid, improve the fracturing capacity of single wells or well groups as soon as possible, and give play to the role of comprehensive social and economic benefits, so as to realize the construction of wells in Permian (P) and Triassic (T) and the safe mining of coal mines in the mining area.

Key words: well DCMT 3; CMB bearing system; Integrated development; Well pattern along strike; Clean fracturing fluid

1 preface

He Weigang [1] and Ni Xiaoming [2] made a comprehensive study on the production curve of coalbed methane, and divided the typical production curve of coalbed methane wells in China into four types: attenuation type, rising type, low and stable production type and high and stable production type.

DCMT 3 well [3] is located in the northwest wing of Erlangba syncline of Gu Lin compound anticline, and it is a parameter well organized and implemented by Sichuan Coal Geology Bureau in 2007. Three sets of main coal seams (C 19, C23 and C25) of Longtan Formation (P3l) of Upper Permian started to drain water in February 16 after drilling, well testing, cementing, perforation and fracturing. Frequency conversion and speed regulation pumping unit+tubular pump are used to exploit the combined layer in the whole well. The drilling horizon is Middle Permian Maokou Formation (P2m).

Well DC- 1 and well DC-2 were put into production at the same time. Well DCMT 3 discharged 4 1 day gas production, and the daily gas production was above 1000m3 for 330 days. On February 26th, 2065, 438+0 decided to stop pumping and enter the stage of natural gas production because there was no water. The daily water production of DC- 1 well and DC-2 well is 1.5~5.5m3, with low gas production and slow production reduction.

Yin Zhongshan and others [3] made a detailed summary of the reservoir characteristics and reconstruction methods in this well area. This paper summarizes and discusses the changes of gas production, water production and pressure during the drainage process of well DCMT 3, and puts forward the reasons and suggestions for continuing to expand the exploration and test area.

2 production basic situation and curve characteristics analysis

2. 1 Basic production situation

As of April 20th, 20 1 1 year, the trial production of well DCMT 3 has been going on for 17 months, and it has gone through overflow stage before gas production, drainage and depressurization stage, production increase-high production stage after gas production and production reduction stage. See table 1 for the specific production situation. These include:

2. 1. 1 drainage system in drainage stage

Pump diameter is 44mm, stroke 1.44m, stroke time is 0.5~ 1.5n/min, and scouring distance is 0.3m m. ..

2. 1.2 high-yield stage

20 10.2. 17~6.25, and the daily gas production 1500~ 1692m3. At this time, the water production drops sharply, and the flushing times drop from 1.5n/min to 0.5n/min.

2. 1.3 gas production decline stage

From 20 10.6.25 to 12.9, the casing pressure drops to zero, and the daily gas production drops to 1000m3, with an average decrease of 2.62 m3/d;

From 20 10. 12.9 to 20 1 1.4.20, the daily gas production decreased to 600m3, with an average decrease of 3.03 m3/d;

2010.12.9 ~ 2011.2.26, the daily water production is below 0.005m3, and the washing times are kept at 0.5n/min. There was no water production in February 18, and the pumping unit stopped on February 26th, and the gas production was basically stable.

Scientific and reasonable drainage management is the guarantee of maximizing the productivity of coalbed methane wells [2][4][5]. In the drainage management of well DCMT 3, an effective working system is adopted to control the drainage speed. Before pumping, flush the pump with high-frequency mixed water reflux circulation to ensure the normal operation of the pump and avoid the liquid level from falling too fast.

2.2 Drainage and production curve analysis

According to the drainage curve of Well DCMT 3 (Figure 2), this well belongs to a stable medium and low yield coalbed methane well.

2.2. 1 Analysis of gas production characteristics

After the gas breakthrough, the high-yield stage came quickly, with the gas breakthrough on February 2nd at 20 10, and the daily gas production in February was 1500m3, with the highest daily gas production 1692m3. At this stage, the output can be kept above 1600m3/d by controlling casing pressure. In July 1, the bottom hole flowing pressure dropped to 0.248MPa, and the output decreased.

Table 1 DCMT 3 Well Drainage and Production Statistics Table

Figure/Changes of chloride ion content in produced fluid of Well Kloc-0/DCMT-3.

See figure 1 for the change of chloride ion during production.

2.2.2 Characteristic analysis of pressure curve

At the beginning of dehydration and depressurization, the flowing pressure at the bottom of the well has slowly decreased to the reservoir desorption pressure of 3.48MPa, and the casing pressure has increased. After the ignition started at 2.20MPa, the casing pressure has started to decrease, and the daily gas production has gradually decreased. The substantial change is that after 0.248MPa, the desorption amount of reservoir is lower than 1500~ 1600m3/d, and the production decreases gradually.

Fig. 2 Drainage and Production Curve of Well DCMT 3

2.2.3 Analysis of water production characteristics

At the initial stage, under the condition of low-speed pumping, the liquid level drops at a speed of 5.6m/d, and the water output is more than 2m3/d.. After gas production, the water yield decreases to about 0.5m3/d until the water yield reaches 0.005m3/d. ..

3 existing problems and thinking

3. 1 Is there any development prospect?

Whether the coalbed methane resources in Dacun area have the prospect of ground-scale development needs to consider the comprehensive indicators such as coalbed methane resources, reservoir characteristics, development input-output and predictable comprehensive benefits.

3. 1. 1 Dacun area is rich in coalbed methane resources.

The initial estimate of coalbed methane resources in Sichuan Province is 3480× 108m3, in which the Permian coalbed methane resources in Guxu mining area exceed 1000× 108m3, and the coalbed methane resources in Dacun mining area exceed 100× 108m3[3], with concentrated horizons and regions.

3. 1.2 Coal reservoir characteristics (development conditions) are suitable for surface development.

The gas content of coal seam in Dacun area is high, and the average gas content exceeds 20m3/t, which is a gas-rich area. The buried depth of coal seam is moderate; The coal structure is well preserved; The characteristics of coal reservoir, such as high critical desorption pressure, high gas saturation, high ground-solution ratio and short desorption time, are all beneficial to the surface development of coalbed methane [3][4], but effective measures should be taken in view of the present situation of low permeability, thin coal seam, large dip angle and high stress.

3. 1.3 input-output analysis

The investment in coalbed methane development mainly includes pre-drilling, drilling, reservoir reconstruction, drainage management, daily maintenance and various pipelines. According to the time value method of funds, the preliminary calculation is as follows: the annual interest rate of bank loans within five years is 5.65%, and the annual interest rate of loans over five years is 6.85%; The annual gas production of each well is 50× 104m3 (daily gas production 1500m3). If 200 wells are drilled, the annual gas production will be 1× 108m3. The sales price of natural gas is 1.2 yuan /m3, and the annual income is1.200 million yuan.

Key premise: (1) The daily gas production of a single well reaches1500m3; (2) The construction time is 1~2 years; (3) Strive for national policy subsidies, and the price of coalbed methane should be consistent with the price of natural gas; (4) The single well investment in Table 2 can be revised with reference to the market price. If the investment is balanced within five years, the mining period is 10 year and the net profit period is about five years, and the economic benefits are obvious.

Table 2 Summary of Economic Benefits (Saveable)

3.2 Can single well productivity be improved?

Well DCMT 3 has produced high-quality coalbed methane, which has realized the surface development of coalbed methane in the same type of reservoirs in Sichuan. However, from the effect, the single well stable production is not ideal. In order to obtain better economic and social benefits, it is necessary to further deepen research and refine measures to improve the productivity of coalbed methane wells in this area.

3.2. 1 Reasonable well arrangement to form linear production well group [7]

The conventional layout of coalbed methane wells is mainly rhombic and square. In view of the large dip angle of coal seam in southern Sichuan, wells can be arranged along the coal seam strike in different horizontal coal seams to form small linear well group mining, expand the control area of pressure drop funnel and the desorption range of coal seam, and improve the single well output and recovery ratio. Before determining the well pattern and well group layout density, it is necessary to use COMET3 software for numerical simulation, and predict the productivity according to the reservoir parameters and the trial production data of production wells such as DCMT 3. The trapezoidal well arrangement scheme proposed by Yang Shuguang and others needs mechanical research [8], and the method of multi-branch horizontal wells and well arrangement along the dip angle of coal seam [9] needs to move from theory to practice.

3.2.2 Adopting new drilling technology is beneficial to protect coal reservoir.

Because the coal seam is very sensitive to speed, water, acid, salt and pressure, conventional drilling with low solid, no solid or clean water will cause damage to the coal seam, which will reduce the permeability of coal and rock and cannot be restored through transformation. Underbalanced drilling technology is the best choice for coal seams such as well DCMT 3, which are dominated by primary fault structures and have poor water productivity [4].

3.2.3 Optimization of Reservoir Reconstruction Scheme

Based on the advanced version of COMET3 which is widely used in China at present, the coal reservoir reconstruction scheme is formulated, and it is appropriate to establish a clean fracturing fluid laboratory at the construction site. According to the characteristics of low permeability of coal seams in our province, nitrogen and foam fracturing fluid [7] with low friction, easy backflow and little damage to coal seams were selected for coiled tubing fracturing technology [10] and other new technology tests to form large-scale artificial fractures with high conductivity.

3.2.4 Expand mining series.

According to the theory of sequence stratigraphy, it is considered that Longtan Formation (P3l) of Upper Permian is a "composite coalbed methane system" for developing coalbed methane. The idea of "coalbed methane system" put forward by Ni Xiaoming [2] and Zhu Zhimin [1] has important reference value for the development of coalbed methane with developed coal seams, thin thickness and large dip angle in our province. Longtan Formation (P3l) of Upper Permian in Dacun area is divided into coal-bearing series A (C25~C2 1), which is composed of fine sandstone, mudstone and coal seam mixed with thin limestone. B Coal-bearing system (C20~C 1 1) is characterized by gray-dark gray limestone, mudstone and coal seam. The system takes coalbed methane, shale gas in shale and free gas in sandstone reservoir as the whole development research objects, so as to increase the total exploitable resources and improve the single well productivity.

4 understanding and suggestions

4. 1 Understanding

The drainage curve of (1)DCMT 3 well belongs to low-yield and stable coalbed methane well, and the gas well should be produced under a certain casing pressure. Through economic evaluation, it is preliminarily considered that the coalbed methane in Dacun mining area has a good commercial development prospect.

(2) Reasonable well arrangement to form linear production well groups; Adopt new drilling technology to protect coal reservoir; Promote overall development.

4.2 Suggestions

(1) Strengthen the basic geological research of coalbed methane accumulation law in various coal-bearing zones in Sichuan Province.

(2) The development of coalbed methane should rely on new technologies. For example, scanning electron microscope, physical simulation of coalbed methane reservoir formation and thermal change simulation, etc., the pore characteristics, permeability and adsorption/desorption characteristics of Permian and Triassic coal reservoirs in Sichuan Province are systematically studied in order to control the transformation process of coal reservoirs and realize the best effect and benefit of coalbed methane development. Through three-dimensional seismic exploration, the spatial distribution law of coal seam occurrence and the influence of small faults on reservoirs are revealed, and the distribution law of karst water in limestone strata such as T 1J, P3C and P2M is studied by using transient electromagnetic technology, which creates conditions for large-scale development.

(3) Expand the test scope, speed up the test, and strive to realize the rapid development of coalbed methane exploration and development in our province during the Twelfth Five-Year Plan period.

refer to

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[1 1] Zhu Zhimin, Chen Cen, Yin Zhongshan. Analysis of late Permian coal-bearing system in southern Sichuan coalfield [J]. Coal Science and Technology, 38(7): 105~ 108.