Analysis of coalbed methane reservoir formation conditions, mining characteristics and development applicable technologies

Zhao Qingbo, Sun Fenjin, Li Wuzhong, Li Guizhong, Sun Bin, Wang Bo, Sun Qinping, Chen Gang, Kong Xiangwen

(Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang 065007)

Abstract: Coalbed methane accumulation The model can be divided into self-generation and self-storage adsorption type, self-generation and self-storage free type, and endogenous and external storage type; the coalbed methane accumulation period can be divided into early accumulation, late structural transformation accumulation and secondary accumulation during mining. It is particularly pointed out that Conditions for secondary accumulation during mining. Sedimentary facies were used to analyze the intra-bed microcycles of thick coal seams, and the gas-rich sections of high-quality coal seams were subdivided; Sedimentary facies were further used to explore the type of coal-forming parent material and its control effect on the high yield and enrichment of coal-bed methane; The structural stress field and hydrodynamic force were explained. The mechanism of action on coal bed methane accumulation. The characteristics of coalbed methane mining are summarized: The three mining stages of obstruction, smoothness and undersaturation in coalbed methane well mining are pointed out, and the undersaturated stage can be divided into multiple ladder-like decreasing stages; It is determined by the structural location and intra-layer heterogeneity. The differences form three types of mining characteristics: self-sufficient type, export type and imported type. Based on the geological conditions, the applicability and domestic application effects of two-dimensional seismic AVO, directional plume horizontal wells, ultra-short radius hydraulic jet, U-shaped wells, and V-shaped well drilling technologies were analyzed.

Keywords: Coalbed methane accumulation model, coal-forming parent material, high-yield and enriched mining characteristics; Applicable technology

About the author: Zhao Qingbo, born in 1950, professor-level senior engineer, China National Petroleum Corporation Senior technical expert of the company, part-time professor at China University of Geosciences (Wuhan); deputy leader of the coalbed methane group of the China Petroleum Society; mainly engaged in coalbed methane exploration and development, writing 17 monographs and publishing more than 50 academic papers. Address: Coalbed Methane Research Institute, Box 44, Wanzhuang, Langfang City, Hebei Province. Telephone: (010) 69213108.

E mail: zhqib@ petrochi-na. com. cn

Coalbed Methane Accumulation Conditions, Production Characteristics and Applicable Technology Analysis

ZHAO Qingbo SUN Fenjin LI Wuzhong LI Guizhong SUN Bin WANG Bo SUN Qinping CHEN Gang KONG Xiangwen

(Reserch Institute of Petroleum Exploration and Development, PetroChina, Langfang Branch, Langfang 065007 China)

Abstract: Accumulation model of coalbed methane can be divided into three types : authigenic reservoir with adsorbed gas, authigenic reservoir with free gas and authigenic source rock with external reservoir. Three accumulation stages are indicated as early stage accumulation, late stage accumulation with tectonic reworking and second-ary accumulation during development. Conditions for secondary accumulation during development are specially in- dicated. Micro-cycle in thick coal are analyzed using sedimentary facies. Coalbed interval with high gas content is classified, and further more, coal-forming sources type and its controling on coalbed methane productive and en- richment is explored. Mechanism of tectonic stess field and hydrodynamic force on coalbed methane accumulation is elaborated. Production characteristics of coalbed methane wells is concluded as follows: blocked, unblocked and unsaturated production stages are indicated, and unsaturated stage is considered to be divided into several deple- tion stages;

structure localization and inner layer heterogeneity result in three production characteristics-self-sup-porting, exporting and importing types. According to geological setting, the applicability and its effect of 2-dimensional seismic AVO (Amplitude versus Offset), pinnate horizontal multilateral well , ultrashort radius hyraulic jetting, U and V type well drilling technique is analyzed.

Keywords: Coalbed methane; accumulation model; coal-forming sources; productive and enrichment; production characteristics; applicable technology

1 Analysis of coalbed methane accumulation conditions

1.1 Coalbed methane accumulation mode and accumulation period

1.1.1 Coalbed methane accumulation mode is divided into three categories

Self-generation and self-storage adsorption type: Most coalbed methane exists in the coal seam in an adsorbed state, and is enriched in slope zones with relatively stable structures. For example, the average daily gas production of a single well in the Panzhuang horizontal well in the southern Qinshui Basin is 30,000 m3; the 3# coal in Well Zhengshi 60 has a burial depth of 1,337 m and a daily gas production of 2,000 m3.

Self-generated and self-storage free type: Coal seam adsorbed gas and free gas are somewhat relative, mostly originating and interacting from the same source. Part of the coal seam methane exists in the coal seam in a free state, and some local structural high points Occupying the main body, the early coal seams are deeply buried and have high gas production. In the later stages, the uplifted coal seams become shallower and less compacted. Secondary cleats develop and have good permeability. The two wings are also the direction of hydrocarbon supply. Local highs form high points under favorable sealing layer conditions. Infiltrated high-yielding enrichment areas. In Well Cai 504 in the Cainan area of ??the Junggar Basin, the coal layer at the high point of the structurally developed fault block has cleat fissures and good physical properties. Free gas and adsorbed gas are stored in the same source. The coal seam is 2575m deep and the daily gas production is 6500m3.

Endogenous and external storage type: Coal seams serve as source rocks, and the generated gas migrates to the upper part or surrounding rocks, forming free gas reservoirs in sandstone and limestone under favorable trap conditions, allowing adsorption Gas and free gas have homologous origin, association, conversion and superposition, and can be superimposed on a plane to form a large area distribution. The roof sandstone of the Shanxi Formation coal seam in Well WL2015 in the Hancheng area on the eastern edge of the Ordos Basin is 14.1m thick. The wellhead pressure after fracturing is 2.32MPa, and the daily gas production is 2400m3.

Figure 1 Coalbed methane accumulation model

1.1.2 The coalbed methane accumulation period is divided into three categories

Early accumulation: with the development of sedimentation As the coal seam depth increases, a large amount of gas continues to be generated. Sufficient gas generation environment, good migration and accumulation potential energy, sufficient adsorption, and favorable sealing, high saturation, and high permeability accumulation conditions have laid the foundation for early accumulation. This type of gas reservoir has relatively heavy δ13C1 (Table 1), showing characteristics of primary gas reservoirs.

Reservoir formation in the later stage of structural reform: Once the dynamic balance of the system is broken by tectonic fault activity, that is, the coalbed methane reservoir will be opened by water and the coal seam cleats will be filled by calcite veins, the energy will be readjusted and the hydrocarbons will be regenerated. distribution, the ancient coalbed methane reservoirs were destroyed, and new high-yield and enriched blocks began to form (Fig. 2).

After tectonic uplift, a fractured anticline structure appears locally. The uplift reduces the pressure of the coal seam and desorbs the gas. The cracks generated by the tectonic movement communicate with the gas in the lower parts, allowing it to move towards the local structural high point. Move and gather. When the basin subsides and accepts deposition, the pressure gradually increases, gas is generated again, and gas in the anticline wings is re-adsorbed and accumulated. Most of these gas reservoirs are secondary type, and δ13C1 is relatively light (Table 1).

Table 1 CH4 content and δ13C1 distribution table of different types of gas reservoirs

Figure 2 Coal bed methane transport and accumulation process

Secondary accumulation during mining: coal seam The original state of gas is an adsorption state. When the pressure drops to a critical point during mining, it breaks the original equilibrium state and transforms into a free state. Gas and water will be redistributed, desorbing and channeling gas into layers or locations, thus forming secondary accumulation in coalbed methane mining. This is a condition that conventional oil and gas does not have. Such gas reservoirs in coal mining areas have low CH4 content due to the adjacent goaf areas.

(1) The channeling position in the secondary accumulation of coalbed methane

The channeling position refers to the migration of gas to high places or high permeability areas during coalbed methane mining, and water to low places It migrates to form a three-phase flow of pulverized coal, gas, and water. After a few years of development, it enters the residual state, and small pores and deep gas are produced in large quantities. In the process of coalbed methane mining, in the same area, some wells have high productivity and some wells have low productivity. This is related to the structural position in which they are located. The desorbed gas flows differentially to the top of the structure or high permeability channels or "free accumulation", and coalbed methane occurs. The position changes, resulting in high air and less water, and even self-eruption in the later period, and less water and air in the syncline. Such as the development example of coalbed methane in Puchi anticline (Fig. 3, Table 2).

The area has single-phase flow with overall drainage and pressure reduction in the early stage, three-phase flow of gas, water and pulverized coal in the middle stage, pressure reduction in low parts in the later period, and single-phase flow in high-position self-injection and high-yield gas wells, which are basically maintained after 4 years. status quo. The 477 vertical wells and 57 horizontal wells in the block have been mined for more than 4 years. Currently, the number of vertical wells and horizontal wells producing gas but not water is 29 and 11 respectively, and the number of water and gas production is 12 and 19 respectively.

(2) The channeling layer in the secondary accumulation of coalbed methane

The channeling layer refers to the desorption of gas along fault fissures or in the collapse of the upper part of the coalbed goaf area during coalbed methane mining or coalbed goaf area collapse. The channels formed in later development will gather upward to other layers. There are five main situations: ① The original fault is closed in the early stage, and opens after the pressure drops to a critical point; ② The horizontal well penetrates the roof and floor and the fault; ③ Fracturing opens the roof and floor; ④ The release of mining stress causes cracks to dissolve. The air inhales through the roof and floor plates and enters the sandstone and limestone to form free gas; ⑤ After the coal seam is mined, the roof collapse stress is released, and a fissure zone appears at the bottom.

Typical case analysis:

① The release of mining stress in the Fuxin coal mine area leads to secondary accumulation

Mining and goaf areas: 7 drilling wells in Fuxin, After the collapse of the empty area, the daily gas production of a single well in the sandstone fissure zone at the top of the coal seam was 15,000 to 21,500 m3, and the CH4 content was greater than 50. After one year of production, the cumulative gas production from a single well reaches a maximum of 2.6 million m3; Yangquan’s annual gas production is 716 million m3, 90 of which is extracted from adjacent layers; Tiefa 70 coalbed methane is extracted from the mining area (Figure 4).

Figure 3 Coalbed methane development characteristics map of Puchi anticline

Table 2 Development well production conditions of Puchi anticline

Note: In the columns of daily gas production and daily water production The numerator is production four years ago, and the denominator is current production.

Figure 4 Schematic diagram of coalbed methane production in mining and goaf areas

②Vertical well fracturing and channeling layer

Fracturing of Well 38 in Punan shows ultra-low fracture pressure , is 9.6MPa, which is more than 10MPa lower than that of adjacent wells. The initial daily water production was 62m3. After 4 years, it is currently 54.8m3. The cumulative gas production is only 38,000m3.

③ Horizontal well channeling layer

The coal seam footage of Well FZP031 is 4084m, the drilling encounter rate is 81, the main and branch drillings encountered 4 faults, and the lower water layer was obviously drilled, and the development Poor effect (Figure 5): The highest intermittent daily gas production is 1366m3, the cumulative gas production is 290,000 m3, the cumulative water production is 43,000 m3, the current daily gas production is 392m3, and the daily water production is 28m3; the structural high point of the original water layer is occupied by desorption gas. Well FZP03-3, which is 75m shallower than this well, has a daily gas production of 3783m3 and a daily water production of 5m3.

In the exploration and development of coalbed methane, a primary development well network should be formed to search for adsorbed gas in the coal seam. During the secondary development well network, due to the pressure drop during mining, the hydrocarbons change from the adsorbed state to the free state, causing the gas and water to re-enter. Distribution, breaking the original equilibrium state, desorbing the gas channeling layer or channeling position to form a secondary accumulation of free gas reservoir exploration and development ideas.

1.2 Favorable coal-forming environment and high-yield enrichment cycle section of coalbed methane

In the past, sedimentary facies were used to analyze the change characteristics of sand bodies in oil and gas exploration. Through the analysis of a large number of coal seam clay minerals, plants Identification, well logging characteristics, especially whole-coal seam coring observations, and coal quality and gas-bearing analysis indicate that the influence of sedimentary environment on the generation, storage, preservation and permeability of coalbed methane is achieved by controlling the material composition of the reservoir. , the heterogeneity within the layer and the microcyclicity of coal quality are controlled by the depositional environment, and control the heterogeneous changes in gas content and permeability within the layer.

On the plane: the Hejianwan facies coal seam is thick, the coal quality is good, the gas content is high, and the single well production is high. The riverside highlands and the lake lagoon facies are opposite (Table 3).

Figure 5 FZP03 1, FZP03 3 horizontal well trajectory schematic diagram

Table 3 Coal quality and production data table of different coal lithofacies zones C-P in the Edong Gas Field

Vertical: Affected by the depositional environment, thick coal seams often form several sedimentary cycles of gangue, dark coal, and bright coal vertically. Bright coal has high vitrinite content, high permeability, and high gas content. Different coal rock components are controlled by the type of coal-forming parent material. Higher plants are abundant. Bright coal formed by gelation has low ash content, high vitrinite, well-developed cleats, and high gas content; clastic materials and water dissolution Ion entrainment or herbaceous coal-forming environment is the opposite of dark coal.

The 9# coal in Well Wuchai 1 can be divided into 4 intra-layer microcycles (Figure 6). Ash content: dark coal 14-15, bright coal 3.7-5.1; vitrinite content: dark coal 23-49, bright coal 66-79.

1.3 The control effect of tectonic stress field on coalbed methane accumulation

The high value area of ??paleostress field has developed faults, active hydrodynamics, severe mineralization of coal seams, and low gas content; low value The area has developed coal seam cleats and is in a closed environment with confined water. The coal bed methane has good preservation conditions and high gas content. Local structural high points are often areas with relatively low stress fields, and have high coal seam permeability, high single well production, good coal seam methane preservation conditions, and the coal seam has not been washed by water and has high gas content.

1.4 The control effect of thermal evolution on the pore structure of coal bed methane

High coal rank is dominated by micropores less than 0. 01 μm and mesopores 0. 01 ~ 1 μm, generally in Above 80, mesopores and micropores are the main adsorption spaces for coalbed methane, which are transported by secondary cleats and fissures;

Figure 6 Sedimentary cycle diagram of coal #9 in Wuchuan 1 Well

Low coal rank is dominated by gt; 1μm macropores and mesopores, with a low degree of evolution and undeveloped fissures. Macropores are the main storage space and diffusion, seepage and production channels for adsorbed gas and free gas;

The middle coal rank is dominated by medium and large pores, which are the diffusion and seepage channels of coalbed methane.

Nuclear magnetic resonance: The T2 relaxation time spectrum of the coalbed methane reservoir has a characteristic bimodal structure. Compared with the T2 relaxation time spectrum of the conventional low permeability reservoir, the coalbed methane reservoir There is an obvious gap between the two peaks, which shows that for coalbed methane reservoirs, bound water and movable fluid cannot communicate effectively. However, the structures of T2 spectra of coal reservoirs with different coal ranks are different, which is due to different pore structures (Figures 7 and 8). Low coal ranks are dominated by macropores, high coal ranks are dominated by micropores and small pores, and high coal ranks are dominated by micropores and small pores. The left peak of the peak coal seam of the rank curve is high and the right peak is low, with a zero value in the middle of the peak. The opposite is true for low coal rank. The left peak is the non-flowable pore and the right peak is the flowable secondary cleat fracture reservoir. The higher the flowable peak of the right peak of the high coal rank is. (the development of cleats), the higher the gas well production (Fig. 9).

1.5 The control effect of hydrodynamic field on coalbed methane reservoirs

Figure 7 Pore structural characteristics of high and low coal ranks

Local structural high points retain water areas with low production Water has high gas production, and syncline pressure-bearing areas have high water production.

Groundwater is generally active in slopes and valleys, which is consistent with the mechanism of water flowing to lower areas and air migrating to higher areas. The stagnation-weak runoff areas in the Fanzhuang block are mostly gt; 2500m3/d high-yield wells; the eastern groundwater recharge area has a gas content of lt; 10m3/t, a gas saturation of 55, slow gas production, and a single well production of 200-500m3/d (Figure 10).

Fig. 8 Pore distribution characteristics of different coal ranks

Fig. 9 T2 relaxation time spectrum of coal reservoirs with different coal ranks

2 Coalbed methane mining characteristics

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For China's medium-low permeability coal seams, coalbed methane wells are generally 300m×300m well spacing, and the stable production period of a single well is 4 to 6 years, and horizontal wells are shorter. The mining process is divided into a rising period, a stable production period, and a declining period. There are three stages, and the decreasing period can be divided into multiple stepped decreasing stages.

2.1 Differences in structural location and intra-layer heterogeneity form three types of mining characteristics

Self-sufficient type: often located in areas with gentle structures and strong homogeneity. The gas production is the gas desorbed from the well within the depressurization radius of the well. Drainage and production wells are generally located in gentle structural parts with strong intra-layer homogeneity. There are three stages of daily gas production: rising - stable production - declining. Most of these wells have low production (Figure 11).

Figure 10 Relationship between groundwater, gas content and coalbed methane high-yield areas in Fanzhuang block

Figure 11 Coalbed methane single well mining characteristics map

Export type : Located in the structural wings and areas with strong heterogeneity. Part of the gas production is produced from the well through the pressure reduction and desorption radius of the well, while most of the gas is produced in other wells through high permeability channels or along the updip diffusion. Drainage and production wells are generally located in the structural wings with strong heterogeneity. Daily gas production is low or no production - rises - slowly decreases. Such wells are mostly low-yield, and the production decreases rapidly.

Wells P111, PN11, PN25, HP110, and HP2113 of the Puchi anticline are located on the wings of the anticline, which is a relatively low part of the structure. There is basically no gas production, but the water production is large. According to analysis, due to The gas desorbed due to depressurization migrates to high structural parts without output, which has the characteristics of export-type mining.

Input type: mostly located at structural high points. In the early stage, the depressurized and desorbed gas of this well was produced from this well along with the depressurized funnel. In the later stage, the desorbed gas from the downdip part of the structure migrated to this well and was produced. Drainage and production wells are located at structural high points. Such wells generally have high production and a long stable production period. Daily gas production increases - stable production - increases - decreases.

Wells PN14, P13, PN27, and P15 located at the structural high point in the Puchi anticline have high gas production and low water production. This is related to the diffusion input of gas at low locations and has typical input-type mining characteristics.

2.2 Different pressure reduction rates result in three types of mining effects

2.2.1 Smooth desorption

The pumping liquid level is reasonably controlled, and the pressure reduction rate is close to the desorption rate , the negative effect caused by effective stress is smaller than the positive effect caused by matrix shrinkage. The permeability increases with the production of irreducible water and gas and is stable. The bubbles bring out part of the irreducible water, and the output is ideal (Figure 12Ⅰ). Taking Well Gu

Figure 12 Characteristic curve of gas production impact of different measures on coalbed methane wells

2.2.2 Supercritical desorption

The desorption rate is less than the pressure reduction rate, and the pressure reduction liquid level If the decline speed is too fast, the cracks and cleats in the coal seam will produce stress closure, the daily gas production will rise sharply - drop sharply, the permeability will drop - stabilize, and the gas production effect will be poor (Figure 12Ⅱ). Taking Well Gu Y2 as an example, after more than 30 days of drainage and depressurization, the liquid level in this well dropped below the coal seam. Due to the excessive drainage speed, the early gas production effect was poor. In July 2010, the secondary fracturing and drainage system was adjusted. After that, the daily gas production reached a maximum of 4000m3/d, and later stabilized at more than 1600m3/d; during the peak period of gas production, the daily liquid level of Well PzP03 was 63-87m, causing the well to have the highest single-well production in the country at the beginning (105,000), but now it is The well with the lowest single well production in the area.

2.2.3 Impeded desorption

The liquid-falling rate is too slow, the desorption rate is greater than the pressure reduction rate, the negative effect caused by effective stress is greater than the positive effect of matrix shrinkage, and it is difficult to deform and desorb the bubbles , in the early stage of pressure reduction, it is blocked by coal powder, desorption is not smooth due to liquid surface resistance, daily gas production is unstable, and the development effect is poor (Figure 12Ⅲ). Well FzP03-3 was shut down more than 26 times in 770 days of production, and the development effect was very poor.

2.3 Types of coal seam water and its mining characteristics

Coal seam water can be divided into intra-layer water, inter-layer water and external source water; high gas production areas include intra-layer and inter-layer water. Areas with external water sources are low gas production areas.

(1) Intra-bed water: water in coal seam cleats and fissures. The daily water production is small. In the middle and late stages of mining, the high areas are almost non-productive and the low areas are decreasing. The water in the layer can be further divided into four categories: movable water (cavities), adsorbed water (coal grain surface), wet water (lt; within 10-5cm capillary tubes), and crystallized water (calcium carbonate).

(2) Interlayer water: Thin interlayer water penetrates into the coal seam. Water production decreases significantly during mining and can be controlled.

Continuous pressure reduction in gas wells with interlayer water can control water production and improve development results. The total coal seam footage of Well FzP111 in Fanzhuang, Qinshui is 4710m. It was put into operation in April 2009, with a maximum daily water production of 175m3. Currently, the daily gas production is 21,436m3, the daily water production is 20.7m3, the casing pressure is 0.15MPa, the liquid level is 4m, the cumulative water production is 37,000 m3, and the cumulative gas production is 8.14 million m3. It can be seen that when interlayer water enters coalbed methane wells, if the drainage volume is increased in the short term, the daily gas production will continue to increase in the later period, and the development effect will be better.

(3) Exogenous water: Faults or cracks communicate with high-permeability ash water and other water layers. The water production is large and difficult to control.

3 Analysis of applicable technologies for coalbed methane exploration and development

3.1 Seismic AVO technology predicts high-yield enrichment areas

The wave impedance difference between coal seams and surrounding rocks is large, and the coal seams themselves are strong reflection. The difference between gas and water content is prominent in local anomalies: after high gas content, the amplitude decreases with the increase of offset, resulting in AVO anomaly (bright spot), which is different from the high impedance amplitude of conventional natural gas, which increases with the increase of offset. The concepts of bright spots are different and have the following characteristics: high-yield wells have strong AVO anomalies (high gas content and low water content), and coal seam sections have large intercepts and large gradient anomalies, that is, strong points among bright spots; low-yield wells have weak AVO anomalies (low gas content and high water content). water content) is characterized by low gas content, low saturation, and low permeability.

The 5# coal in Well Jizhu 1 in the strong AVO anomaly area in the high coalbed methane production area has a gas content of 21m3/t and the daily gas production is 2847m3 (Fig. 13); the 5# well in Jizhu 4 Well in the low production area has weak AVO anomaly. The gas content of coal is 12m3, the daily gas production is 64m3, and the water production is 90m3. According to this theory, seismic AVO technology can be used to predict high-yield and enrichment areas.

Figure 13 AVO characteristic map of coal No. 5 in Well Jitest 1

3.2 Applicable geological conditions for drilling directional plume horizontal wells

Directional plume water has been drilled nationwide There are more than 160 flat wells, and the maximum daily gas production of a single well is 105,000 m3. Directional plume horizontal well technology is suitable for mining coalbed methane in lower permeability reservoirs. It integrates drilling, completion and production stimulation measures, and can maximize the communication of natural fracture systems in coal seams, so that the production of single wells in the same area can be increased. 5 to 10 times, applicable geological conditions include the following 10 points:

(1) The structure is stable and there are no major faults: FzP031 encountered 4 faults, with a maximum daily gas production of 1366m3, currently 687m3, and a daily water production of 32~75m3 ; The daily water production of wells 04, 07 and 09 in Hancheng is 20 to 48m3, and the daily gas production is less than 60m3.

(2) The sealing conditions are good far away from the water layer: the Sanjiao roof mudstone is lt; 2m thick, with less water and atmosphere. The 9# coal in Well SJ61 is 9.4m thick, the roof is 6.8m limestone, and the coal seam footage is 4137m. The drilling rate is 100, the maximum daily water production is 465m3, the water production is 46,000m3 in 19 months, and no gas is produced.

(3) Soft coal structural coal is not developed: the average daily gas production of 12 wells in Hancheng and Heshun is 720m3.

(4) The coal seam depth is less than 1000m: the daily gas production of Wum11 and Fz151 wells with coal seam depths of 800 to 1000m is <500m3.

(5) Coal thickness gt; 5m: The coal seam in Well Liulin CL3 is 4m thick, with a maximum daily gas production of 9,500 m3, and stable production for 160 days, with a daily gas production of 2,807 m3, and a cumulative gas production of 1.21 million m3.

(6) Gas content gt; 15m3/t: 8m3/t in the east of Panzhuang (caprock thickness 2~5m), 15~22m3/t in the north (caprock thickness gt; 10m), although in the east It is 100-200m shallower than that in the north. The average daily gas production of 6 wells in the north is 30,000 m3, and the 7 wells in the east are 1869m3, with a maximum of 3697m3. The production difference of a single well 6km apart is 20 times.

(7) Main branch parallel coal seams or up-dip: analysis of average daily gas production of a single well, stage accumulation and formation drop of 1MPa, horizontal well trajectory: parallel coal seams are the best, followed by up-dip, The downtilt is poor; the "convex" and "concave" types are the worst.

(8) The effective coal seam footage is >3000m: the horizontal coal seam footage is <2000m, the maximum daily gas production of a single well is <800m3, and the cumulative gas production in stages is <20,000m3.

(9) Reasonable branch distribution: the main branch is about 1000m long, the branch spacing is 200~300m, and the included angle is 10°~20°.

(10) The effective coal seam drilling rate is gt; 85: The coal seam drilling rate of 10 wells is lt; 85, and has been put into production for more than 1 year. The average daily gas production of a single well is 800m3, with a maximum of lt; 2000m3, and the stage average The cumulative gas production is 270,000 m3.

3.3 Applicable conditions for ultra-short radius hydraulic jet drilling

my country has drilled more than 23 coalbed methane wells using this technology, but the results are not ideal. The main reasons are low permeability, small diameter of the spray holes, large bends, and blockage after spraying before and after; the diameter of the hydraulic jet window is 28mm, the hole diameter is small, and it is easy to be blocked by coal powder and water during drainage. Rotary large-diameter nozzle and open-hole spray tests can be carried out.

3.4 Applicable conditions for drilling "mountain"-shaped wells, U-shaped wells, and V-shaped wells

Since China's coalbed methane reservoirs have the characteristics of low permeability and are mostly fault block gas reservoirs, U-shaped horizontal wells communicate with coal seams in a small area and have poor application effects. my country has drilled more than 16 U-shaped horizontal wells, but the production increase effect is not obvious.

The staged fracturing of Well SJ12-1 has a stable daily gas production of 1,750 m3, and a cumulative gas production of 191,000 m3, which has declined after three and a half months of production. Both oil pipes and fiberglass pipes have been successfully installed in the horizontal section, but the effect is poor in low-permeability gas reservoirs. The higher permeability zone [(1.0~3.6)×10-3μm2] has good effect: the daily gas production of single wells in Binchang and Sihe is 0.56 million to 14.000 m3.

In the future, a "mountain"-shaped well group test with one horizontal well penetrating multiple vertical wells can be carried out. At present, foreign countries have successfully used this technology to develop salt rocks.

4 Conclusions

(1) According to the practical understanding of coalbed methane exploration and development in China, the coalbed methane accumulation model is divided into self-generation and self-storage adsorption type, self-generation and self-storage free type, and endogenous and external generation. There are three types of storage types; at the same time, it is believed that the coalbed methane accumulation period can be divided into three categories: early accumulation, late structural transformation accumulation and secondary accumulation during mining. Secondary accumulation during mining will be the main secondary well pattern for coalbed methane development. Yield takes over the field.

(2) Use sedimentary facies to analyze thick coal seams, high-quality coal seams and high-yield enrichment areas; analyze the intra-layer microcycles of thick coal seams, the coal-forming parent material controls the coal rock components and single well production, and higher plants are abundant , the bright coal formed by gelation has low ash content, high vitrinite, well-developed cleats, and high gas content, and is a high-yield enrichment section; dark coal is a dark coal that is brought in by clastic materials, water-soluble ions, or herbaceous coal-forming environment on the contrary.

(3) In areas with low values ??of paleostress fields, coal seam cleats are developed and are in a closed environment of confined water, with good coalbed methane preservation conditions and high gas content; in stagnant water areas, there is low water production and high gas production, and syncline bearings High water production in the pressure zone.

(4) The differences in structural locations and intra-layer heterogeneity form three types of mining characteristics: self-sustaining type, export type and input type. The different depressurization rates form unobstructed type, obstruction type and super-excessive type. Critical type three mining effects.

(5) High-yield wells have strong AVO anomalies, which are strong points among bright spots; low-yield wells have weak AVO anomalies, which are characterized by low gas content, low saturation, and low permeability. Directional plume horizontal wells can achieve better development results under applicable geological conditions and drilling methods; ultra-short radius hydraulic jetting should be preferred for coal seams with higher permeability, relatively stable coal seam structure, and high gas content and saturation; U Type and V-shaped horizontal well drilling technology has poor results in low-permeability gas reservoirs, but good results in high-permeability areas.

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