Coalbed methane mining test well construction and drainage

After analysis, the coalbed methane resources in the Hancheng mining area are rich and have certain extractability. Development tests must be carried out for commercial development. First, the well location of the first well must be selected, and then drilling construction-logging-well testing-well completion-perforating-fracturing engineering-drainage test can be made to evaluate the recoverability. Construction of Well Hanshi 1 began in October 1995.

9.5.1 Test hole and drainage test

9.5.1.1 Selection of test hole location

Based on domestic and foreign data, combined with the geological structure of the area Characteristics, the principles for selecting test hole locations are: ① The gas content of the coal seam is >8m3/t coal; ② The coal seam burial depth is between 400 and 800m; ③ The structure is simple and the coal seam has not been damaged by the structure; ④ The thickness of a single coal layer is greater than 1m; ⑤Convenient transportation conditions.

The structural characteristics of the Hancheng Mining Area are: the shallow edge structure is complex and faults are developed, and the strata quickly become gentle toward the middle and deep parts, and faults are rare. There are five pairs of production mines in the shallow part, from southwest to northeast: Xiangshan Coal Mine, Magouqu Coal Mine, Liaoyuan Coal Mine, Xiayukou Coal Mine, and Sangshuping Coal Mine. The Xiangshan Coal Mine in the south has a large amount of gas gush, but no gas outburst accidents have occurred, indicating that the coal seam has good air permeability; while in the Xiayukou Coal Mine and Sangshuping Coal Mine in the north, the coal seams have been affected by compressive fractures, and the coal seams have been affected by compressive fractures. It is relatively broken and scaly, and the coal seam has poor air permeability. Gas explosions and coal and gas outburst accidents have occurred many times.

Coal seam conditions: *** contains 11 coal layers, with 3 main mineable coal seams in the south and 2 in the north.

Coal seam gas content: The content is lower in the shallow parts and gradually increases toward the middle and deep parts.

Based on the above situation, combined with the principle of hole distribution, the middle and deep part of the southern part of the mining area was selected. The Xuefeng Reservoir in Xuefeng Township, where the coal seam is about 600m deep, is adjacent to the Korean Test Well 1. This location not only has convenient transportation, but also has a total thickness of the coal seam. Relatively thick.

9.5.1.2 Drilling construction

(1) Construction purpose

The main purpose of this construction is: ① Obtain complete and accurate parameters related to coal bed methane And conduct gas testing; ② Based on commercial development, strive to produce commercially developed coal bed methane; ③ The target layers are 3#, 5#, and 11# coal seams.

(2) Overview of the exploration area

1) Exploration well location and traffic conditions

Hanshi No. 1 Well is located in Xuefeng Township, Hancheng City, Shaanxi Province, 100 meters away from Hancheng City. 20km away from Xi'an City, 260km away from Xi'an City. There are highways and railways directly leading to Xi'an, and the transportation conditions are very convenient.

2) Stratigraphic overview

The formations drilled by this well are as follows:

a. Quaternary, Neogene (Q+N)

0~24m, with a layer thickness of 24m. It is modern alluvial, diluvial and slope deposits. The lithology is light yellow, yellowish brown loess, sub-sandy soil and sub-clay. Lithology drillability is grade 1 to 3.

b. Permian (P)

Upper Shiqianfeng Formation (P2s): 24-208m, layer thickness 176m, mainly purple-red mudstone and gray-green medium-coarse sandstone , rock drillability level 4 to 5.

Shangshihezi Formation (P2sh) of the Upper System: 205~508m, layer thickness 303m, lithology is mainly gray-green, gray-white coarse sandstone, rock drillability grade 4~6.

The Lower Shihezi Formation (P1sh): 508-560m, with a layer thickness of 52m. The lithology is mainly gray and purple siltstone and mudstone, and the rock drillability is grade 4-6.

Lower Shanxi Formation (P1s): 560~615m, with a layer thickness of 55m. The lithology is mainly dark gray, fine sandstone, siltstone and mudstone. This group contains the No. 2 locally mineable coal seam and The No. 3 mineable coal seam has rock drillability levels of 3 to 6.

c. Carboniferous (C)

Upper Taiyuan Formation (C3t): 615-676m, layer thickness 61m, the upper part is dominated by siltstone, sandy mudstone, and clay layers , containing No. 5 minable coal seam, the middle part is dominated by limestone and calcareous shale, the lower part is dominated by clay layer, sandy mudstone and siltstone, the top part contains No. 11 coal, and the rock drillability is grade 3 to 6.

d. Ordovician (O)

Middle Series Fengfeng Formation (O2f): 676~710m, layer thickness 34m, mainly dark gray limestone, rock drillability Level 4~6.

(3) Drilling construction requirements

1) Well depth: 710m.

2) Target layers: the 3# coal seam of the Shanxi Formation in the lower Permian system and the 5# and 11# coal seams of the Taiyuan Formation of the upper Carboniferous system. To ascertain the content of coalbed methane and base on commercial development.

3) Completion principles: 40m below the 11# coal seam floor or limestone final well.

4) Well inclination: The completion well inclination shall not be greater than 5°, and the well inclination shall not change by more than 1° for every 50m increase in well depth.

5) Well logging and coring: Carry out rock cuttings logging at 0-560m, take out one pack of cuttings and sand samples every 5m, and carry out core logging at 560-676m. The core collection rate is required to be no less than 75 %, of which the coal core length recovery rate is not less than 90% and the weight recovery rate is not less than 75%.

6) Simple hydrology: ① Observe the water level once per footage; ② Observe the consumption of drilling fluid once every hour during drilling, and observe the proportion of the inlet and outlet of the drilling fluid once every hour in the coal-bearing strata; ③ Drilling If water leakage occurs during the process, it should be observed and recorded in time.

7) Well depth calibration: Drilling tools must be measured every 100 meters, before running pipes, before coring, at coal seam locations, in drilling water leakage sections and during well completion. The error shall not exceed 0.15%, otherwise it should be reasonably balanced. Difference.

(4) Preliminary drilling project

1) Drilling method

This construction uses comprehensive drilling and rope coring drilling, that is, in the loess layer and non-coal-bearing bedrock formations, using roller cone bits for comprehensive drilling to reduce auxiliary operation time and improve drilling efficiency; in coal-bearing strata, diamond rope core drilling is used to improve the extraction rate and quality of the coal cores.

2) Drilling parameters

According to the selected drill bit, medium pressure, medium speed and large pump capacity are required during well expansion drilling.

3) Well structure:

0~26m, well diameter 311mm, run 26m of Φ245mm surface casing, cement cement back to the surface; 26~652m, well diameter 215mm , run 652.3m of Φ140mm technical casing (the technical casing is 0.3m higher than the surface casing), and cement the well back to the surface; 652~710m, well diameter 215mm, open hole.

4) Drill tool assembly

According to our investigation and understanding, the drill tool assembly used is as follows:

Loess layer drilling: Φ108mm square drill pipe + 310×311 joint + Φ89mm drill pipe + 310×311 joint + Φ121mm drill collar + 310×620 joint + Φ311mm roller bit.

Bedrock drilling: Φ108mm Kelly pipe + 310×311 joint + Φ89mm drill pipe + 310×311 joint + Φ121mm drill collar + 310×420 joint + Φ215mm roller bit.

Rope coring drilling: Φ108mm kelly + 310×311 joint + Φ89mm drill pipe + Φ81mm double-tube corer + Φ130mm roller bit.

When expanding the well, the Φ121mm drill collar is connected to the Φ215mm well expansion bit.

5) Drilling fluid selection and configuration

This well is a combined exploration and production well. In order to ensure construction safety and reduce pollution to the coal seam, KP polymer low-carbon There are no hard requirements for solid phase drilling fluids and loess layer drilling fluids.

a. Low solid phase drilling fluid formula and performance

Formulation: water + 30% artificial sodium soil + 0.5% ~ 0.8% KP polymer + 0.5% ~ 0.8%CMC＋0.4%HSP.

Performance: relative density 1.03~1.08, viscosity 18~22s, water loss <10mL/30min, sand content <1%, pH value 8~9.

b. Maintenance and purification of drilling fluid

Equip necessary drilling fluid testing instruments on site; assign a dedicated person to manage the drilling fluid; add treatment agents regularly and quantitatively to maintain the performance of the drilling fluid; Use a solids control system to purify the drilling fluid and, if necessary, a desander and desilder.

6) Protective wall plugging

Based on past construction conditions, water leakage may occur during drilling. ① For mild water leakage, adjust the drilling fluid performance in time to achieve the purpose of plugging the leakage; ② For medium water leakage, use 8012 leakage plugging agent to plug the leakage; ③ For serious leakage formation, use geological survey cement to plug the leakage.

7) Drilling procedure

a. Use a Φ311mm roller bit to drill from 0 to 25m, and then switch to a Φ215mm roller bit to drill to 25m for electrical logging.

b. Φ245mm surface casing is extended to 25m, and mortar is used for cementing. The mortar is required to return to the surface and wait for 72 hours to set.

c. Use a Φ215mm roller drill bit to drill to 560mm.

d. Replace the drilling tool and use a wireline coring bit to drill to 676m.

e. Use a comprehensive alloy drill bit to drill to 710m.

f. Carry out electrical logging.

(5) Well completion technology

The target layers for this exploration and mining are the 3#, 5#, and 11# coal seams. According to the formation conditions, the completion technology adopts open hole-casing completion. Well method, Φ140mm technical casing is lowered to 2m above the roof of the 11# coal seam, and fracturing of the 3# and 5# coal seams is performed. In view of the high technical and equipment requirements for testing, cementing, perforating, fracturing, etc., professional professionals are hired The company is done.

9.5.1.3 Parameter testing

Before coalbed methane extraction, effective fracturing of the coal seam must be carried out. In order to fully understand the permeability, initial pressure, reservoir pressure and other reservoirs of the coal seam, Characteristics provide a basis for fracturing design, and well testing must be carried out.

(1) Well testing method

The injection/pressure drop type well testing commonly used in China is adopted.

(2) Well testing technology

Open hole layered well testing is testing while drilling, that is, a well test is performed every time a layer of coal is drilled, and drilling continues after the well test is completed. .

(3) Selection of well testing team

We must insist on selecting a testing team with high quality, fast speed and rich experience to ensure the smooth progress of the testing work and the quality of the testing data.

(4) Well testing technology

1) The rock coal core extraction rate complies with the national special-grade hole standards. A 5m pocket is left at the bottom of the coal seam to prepare for sediment, but it cannot be connected with the underlying The coal seams are connected.

2) The well diameter of the test layer and its upper 20m and lower 5m is required to be 110-120mm, and the well wall should be flat to facilitate setting.

3) Geophysical well logging must be carried out before well testing to obtain accurate data such as coal seam thickness, depth and well diameter.

4) Geologists make detailed observations and descriptions of the coal seam and upper and lower rock strata, and provide the well testing team with accurate information on the thickness, depth, lithology, gangue conditions, and well diameter of the coal seam and upper and lower rock strata.

5) It is best to use clean water or activated water (2% KCl) for drilling coal seams and their roof and floor plates. Treatment agents such as PHP and CMC can be added to the clean water with a viscosity of 20 to 23 seconds.

6) Conduct simple hydrological observations in accordance with coalfield geological specifications.

7) 2 to 3 days before the test, set up a platform with a canopy at the drill floor site. The foundation of the platform is 250mm×250mm wooden beams, with a platform on top, covering an area of ??5×5m2. Provide 3 ~A 4m3 water tank for testing water.

8) The well team prepares several meters of Φ73mm API standard N80 or J-55 oil pipe (the number of meters is determined according to the hole depth), and prepares tools such as pipe wrench, thread oil, lifting rings and other tools to unscrew the Φ73mm oil pipe.

(5) Test items

Including coal reservoir permeability, reservoir pressure, pressure gradient, skin coefficient, rupture pressure, closure pressure, pressure and time relationship curve, etc., test The results are shown in Table 9.8.

Table 9.8 Reservoir parameter test results of Well No. 1 in Korea

The permeability of coal seams is relatively low. The permeability tested in my country is generally less than 1×10-3μm2, while in Korea Well 1 was tested, and the three seams of coal were all larger than 1×10-3 μm2. The roof of the No. 3 coal seam was sandstone with developed cracks, so the measured permeability was higher. The purpose of surface drainage of coalbed methane is to find high permeability and good gas permeability. Therefore, Well Hanzhai 1 attracted the attention of relevant departments at that time.

9.5.1.4 Fracturing Project

The surface drainage process of coalbed methane is the same as that of oil extraction, so fracturing must be carried out.

1) Fracturing unit: North China Petroleum Geology Bureau of the Ministry of Geology and Mineral Resources.

2) Purpose of fracturing: to relieve possible formation blockage, improve fluid flow conditions in deep coal seams, and understand coal seam productivity.

3) Construction plan: ①Pump injection method, annulus fracturing; ②Select 50t of 20/40 mesh quartz sand as proppant; ③Select HT-21 original glue liquid as fracturing fluid; ④Fracturing pipe Column (from bottom to top), the 3#+5# coal seam fracturing uses oil pipe hanging + 2-7/8″ oil pipe 620m; the 11# coal seam fracturing uses oil pipe hanging + 2-7/8″ oil pipe 660m.

4) Main technical issues and countermeasures

a. Use low-damage fracturing fluid to reduce damage to coal seam permeability.

When fracturing the b.11# coal seam, only the upper 4m of the coal seam is perforated to control the entrance to the formation, use low-viscosity fracturing fluid, use sand deposition to prevent the crack from developing and extending downward, and use reasonable displacement. Through the above Measures should be taken to control/block the downward development of cracks as much as possible, and try to avoid opening the water-bearing limestone in the lower part of coal #11.

c. When fracturing the 3#+5# coal seam, only the 5# coal seam is shot, and the crack initiation entrance is controlled to help the crack expand in the 5# coal seam as much as possible.

d. Prepare sand-carrying slugs to deal with the impact of cracks encountering bends.

5) Construction steps and requirements

a. First, shoot the 11# coal seam, run the fracturing string according to the design requirements, run the oil pipe to a depth of 660m, install the wellhead, and connect it pipeline.

b. Set up the construction vehicles, connect the high and low pressure pipelines and various sensors, test the pressure of the high pressure pipelines to 30MPa, maintain the pressure for 5 minutes, and pass the test without puncture or leakage.

c. Slowly add the fracturing fluid, circulate the fracturing fluid to the wellhead, and discharge the air from the tubing.

d. Install the wellhead gate.

e. Conduct a small-scale fracturing test on the 11# coal seam, and monitor the pressure until the crack closes (about 90 minutes).

f. Based on small-scale fracturing, evaluate and make necessary corrections to the fracturing design (takes about 60 minutes).

g. According to the revised/determined pump injection table, fracturing the fractured coal seam, and the maximum treatment pressure is controlled within 24.5MPa.

h. After fracturing, the well shut-in pressure measurement drops to the point where the fracture is closed, and the fracturing equipment and personnel evacuate the site.

i. Use a throttle valve to control the blowout and control the blowout volume to prevent sand production.

j.11# After fracturing and blasting the coal seam, the pipe string is lowered to explore the sand surface. The depth of the sand surface is required to be 680m. If the depth is not reached, sand is filled to 680m and the pipe string in the well is raised.

k. Throw a rubber plug at a depth of 680m and fill it with sand to a depth of 670m.

l. Pull out the pipe string in the well and shoot the #5 coal seam.

m. Run the 3#+5# coal seam fracturing pipe string to a depth of 620m to prepare for the 3#+5# coal seam fracturing.

n. Repeat the above steps b to g to fracturing the 3#+5# coal.

9.5.1.5 Drainage test

On April 18, 1996, the perforating, fracturing, and installation of Christmas trees and ground supporting facilities were completed, and drainage began. From the end of 2001 to the end of 2001, it lasted more than 5 years, and the pure drainage time was 1,400 days. 7,142 various types of original drainage data were obtained, and the cumulative gas production was about 100×104m3.

(1) Drainage and Mining Work

On June 27, 1996, during the first blowout test, an ignition was successful with a flame height of 0.88m. After the drainage and mining, the ignition flame was as high as 4.92m, the initial gas output reaches 2989~3995m3/d, and the highest is 4035m3/d. After one year of drainage, the stable airflow is 300~500m3/d. Due to the large amount of pulverized coal, the oil pump is easily blocked, so the gas output changes greatly. , the gas volume is larger after the well workover. The original record statistics of the drainage test lasted for three and a half years from May 3, 1996 to the end of 2000, and 5,390 original drainage record data of various types were obtained, including a cumulative water production of approximately 7,000 m3 and a cumulative gas production of nearly 50×104 m3. The initial daily water production is 0.2-28.5m3, the daily gas production is 0-2536m3, and the peak daily gas production is 4035m3. After that, the stable daily water production is about 5m3, and the daily gas production is 800~1500m3.

Coalbed methane production data in the Black Warrior Basin in the United States show that in many wells, the maximum gas production stage occurs three or three years later. The length of time to achieve maximum gas production increases with decreasing permeability and increasing distance between wells. Figure 9.9 is a typical coalbed methane production curve.

In the early stages of mining, a large amount of water is discharged. As the reservoir pressure decreases, water production decreases while gas production increases. Hanzhiyi Well has a similar pattern to the Black Warrior Basin.

Figure 9.9 Typical coalbed methane well production curve

By analyzing a large amount of drainage test data from a well, the drainage work can be divided into two stages, namely: dehydration and gas production stage , Stable gas production stage, corresponding to the first and second stages in the above figure. The drainage process of the first well has the following characteristics:

Dehydration and gas production stage. In the first 4 days, the drainage volume was large, but the gas production was less than 1m3, indicating that only free gas was produced. Starting from the fifth day, the gas production gradually increased until the daily gas production reached 1114m3. The amount of methane desorption gradually increases with the increase of drainage volume.

Stable gas production stage. Frequent well cleaning is caused by spitting out sand and coal dust in the well. After each well cleaning, the time interval between drainage and gas production is getting shorter and shorter, from the first 2 to 3 days of gas production to the same day, indicating that the critical desorption pressure of the reservoir in this well is high, and there is a positive correlation between water production and gas production, that is, the gas production in the well The deeper the liquid level (the less pressure the reservoir bears), the faster the methane desorption rate and the greater the gas production. As long as the frequency of flushing is increased, the gas volume will gradually increase. The cracks in the coal seam are not closed due to the migration of groundwater or reservoir pressure relief, but are relatively unobstructed, thus ensuring the production of gas in a wider range. From the current well Judging from the gas production situation, the gas production of this well is on the rise.

The reservoir water discharged from the well has been tested: the total dissolved solids are 7.404g/L, the water quality type is Na-Cl type, the total hardness is 22.6 degrees, the total alkalinity is 12.49mg/L, and the free carbon dioxide is 58mg. /L. In 2000, we discovered at the well cleaning site that it was not coal dust that blocked the filter screen of the pump body, but calcium carbonate crystals. A 0-2mm layer of carbonate crystals was also deposited on the outside of the pump body. It can be seen that the water in the coal seam is moving. During the process, under the action of CO2, a large amount of minerals originally deposited in the original cracks of the coal seam are carried out, causing the porosity in the coal to increase and the cracks to grow, creating a good channel for the desorption of coal bed methane.

(2) Tests during the drainage and production process

During the drainage and production process, multiple pressure-holding tests were conducted. Under the conditions of equipment leakage and air leakage, the pressure in the well was measured to reach At 0.9MPa, the desorption amount of coalbed methane is significantly reduced. If the pressure of the water head in the well on desorption of coalbed methane is added, it is estimated that the degassing pressure of coalbed methane in the first well is greater than 1MPa. Another characteristic of several similar pressure-holding tests is that the pressure in each test well rose faster than the previous test, and the gas-holding period was gradually shortened (Figure 9.10), indicating that the reservoir degassing amount increased after the pressure was held. , the study believes that it is caused by pressure holding and pressure relief, which makes the gas conduction channel in the coal seam smoother.

Figure 9.10 Pressure-holding test curve of Well No. 1 in Korea Test

After the pressure-holding test, a shut-in test was conducted. The water level slowly recovered after 300 hours. When the liquid level depth was 406.51m, no flammable gas escaped from the well and flameout occurred. The flameout water level is 224.64m higher than the 3# coal roof, indicating that the minimum methane desorption pressure of each coal seam is 2.2MPa. We believe that this is the main reason why gas is produced quickly after restarting after a long shutdown.

9.5.2 Commercial development and utilization

Figure 9.11 Hancheng coalbed methane drainage test area (photographed in March 2006)

Hancheng Mining Area "Han After the "Test No. 1" well obtained the coalbed methane industrial gas flow, it attracted attention from all walks of life. Many domestic and foreign experts and investors came to inspect it. China United Coalbed Methane Co., Ltd. began entering the Hancheng mining area to conduct coalbed methane development tests in 2001. It has constructed 11 exploration test holes, all of which produced gas (Figure 9.11). The maximum gas production of a single well is 3500m3/d, and the stable gas production is 500-2000m3/d. The average daily gas production of a single well is predicted to be around 1800m3/d. In September 2007, the first coalbed methane reserves report in Shaanxi Province was submitted. The reserves are 50×108m3, of which the technically recoverable reserves are 25.05×108m3 and the economically recoverable reserves are 22.55×108m3, which can achieve an annual production capacity of 1.5×108m3.

On October 24, 2007, residents of Hancheng City Finance Bureau Community, Xincheng District Century Garden Community, and Erdian Community used "coalbed methane" for the first time, becoming an example of the commercial utilization of coalbed methane in Shaanxi Province.

It is reported that the first phase of the planned project in Hancheng City will benefit a population of 42,000 people, using 884×104m3/a of coalbed methane, and the maximum flow rate during the peak gas supply period is 4000m3/h.