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

Abstract: Liulin block is located in the west of Shanxi Province, adjacent to the Yellow River in the west. Preliminary exploration and trial production show that the exploration and development prospects of coalbed methane in this block are broad. In order to realize the commercial production of coalbed methane in this area as soon as possible and meet the local demand for coalbed methane resources to the greatest extent, it is very important to find high-yield areas rich in coalbed methane. According to the coupling relationship between coalbed methane occurrence characteristics and groundwater chemical field and dynamic field in Liulin area, the relationship between hydrogeological conditions and coalbed methane enrichment and integration is discussed. The results show that the groundwater in Liulin block flows westward along the stratum, and the weaker the hydrodynamic force, the more favorable it is for the enrichment and integration of coalbed methane.

Key words: the hydrogeological conditions of coalbed methane in Liulin block are rich and complete.

Fund Project: 62 (201zx05062), a major national science and technology project "Development of large oil and gas fields and coalbed methane".

About the author: Zhang Wenzhong, male, engineer, born in 1979, Ph.D., graduated from China Geo University (Beijing) in 2009, and now works for Zhonglian Coalbed Methane Co., Ltd., tel: 0 1064297957, and email: zwz98413 @.

Hydrogeological conditions in Liulin block, Shanxi Province and their influence on coalbed methane accumulation

Zhou MENG Shangzhi Mo Rihe

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

Abstract: Liulin block is located in the west of Shanxi Province. The preliminary exploration and trial production show that this block has great development potential. In order to realize the commercial production of coalbed methane, it is very important to find rich areas and high production areas of coalbed methane. According to the spatial coupling relationship between coalbed methane enrichment and groundwater geochemical field and dynamic field, the relationship between hydrogeological conditions and coalbed methane enrichment is discussed. The results show that the groundwater in Liulin block flows from northeast to southwest, and the hydrodynamic conditions in the west are weak, which is more conducive to the accumulation of coalbed methane.

Keywords: Liulin block; Coalbed methane; Hydrogeological conditions; accumulate

Coalbed methane (CBM) is an unconventional natural gas with CH4 as the main component, which is produced in the coalification process of coal and mainly exists in the coal seam in the adsorption state. Coalbed methane is a high-quality energy and basic chemical raw material, which has the characteristics of high calorific value, less pollution and high safety, and is an important supplement to conventional geological energy such as oil and natural gas. Coalbed methane is also a harmful and dangerous gas. The greenhouse effect of CH4 in coalbed methane is about 265,438+0 times that of CO2, and its damage to the atmospheric ozone layer is 7 times that of CO2 (Zhao Qingbo et al., 65,438+0,998), which is extremely destructive to the ecological environment. The flammability and explosiveness of coalbed methane also seriously endanger the safety production of coal mines. Therefore, the effective utilization of coalbed methane is of great significance for alleviating the shortage of energy supply in China, reducing greenhouse gas emissions, improving coal mine safety production and stimulating the development of other related industries.

Liulin block is located in the west of Shanxi Province, in the middle of Hedong coalfield, and adjacent to the Yellow River in the west, with an area of about 183km2. Liulin block is located on the Lishi nose structure in the middle of the eastern margin of Ordos basin, and the main structure is an arc fold with a convex arc top. There are few faults in the block, and only in the north of the block is a graben composed of the north-south normal fault of Jucaita and its derived small faults. There are 14 coal seams in Liulin area, including 5 coal seams in Shanxi Formation, numbered from top to bottom as 1, 2, 3, 4(3+4) and 5; There are 9 layers in Taiyuan Formation, numbered from top to bottom as No.6, No.6, No.7, No.7, No.8+9, No.9, 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 for CBM exploration and development (Ren Guangjun et al., 2008).

Overview of hydrogeological conditions in Liulin 1 block

Aquifer type and distribution 1. 1

There are six sets of main aquifer groups in Liulin area, namely: Ordovician limestone karst fissure aquifer group, Carboniferous upper Taiyuan limestone karst fissure aquifer group, Permian lower Shanxi sandstone fissure aquifer group, Permian upper and lower Shihezi formation and Shiqianfeng formation sandstone fissure aquifer group, Triassic sandstone fissure aquifer group and Neogene and Quaternary gravel (rock) pore aquifer group (figure 1). Among them, the limestone karst fissure aquifer group of Taiyuan Formation of Upper Carboniferous and sandstone fissure aquifer group of Shanxi Formation of Lower Permian are two sets of aquifer groups directly related to coalbed methane exploitation.

Middle Ordovician limestone karst fissure aquifer formation consists of Upper, Lower Majiagou Formation and Fengfeng Formation. It is a set of shallow-sea facies deposits dominated by carbonate rocks such as limestone, marl and dolomite, which are exposed in a large area on the eastern edge of Liulin block. The aquifer group is monoclinic structure, with gradually increasing buried depth from east to west and rich in karst water. It is the main aquifer system in this area.

The limestone karst fissure aquifer group of Taiyuan Formation of Upper Carboniferous is mainly composed of five layers of limestone (L 1-L5) sandwiched between Taiyuan Formation of Upper Carboniferous, which are scattered in the Grand Canyon in the east of the block, and the buried depth gradually increases from east to west (Figure 2). The water storage space is mainly composed of structure, solution gap and solution hole, and the water abundance varies greatly in different places. The eastern margin of Liulin block is characterized by karst development, good connectivity, easy recharge and strong water abundance. To the west, with the gradual increase of the buried depth of the stratum, the karst and fracture of limestone gradually develop, and the water abundance gradually becomes worse. Generally speaking, the aquifer group is rich in water.

The sandstone fissure water-bearing formation of Shanxi Formation of Lower Permian is composed of K3 sandstone, which is exposed sporadically at the periphery of the eastern boundary of Liulin block. Sandstone fractures in aquifers are mostly filled with calcite veins or calcareous films, which have poor openness and connectivity, small water storage space and weak water abundance.

The fractured aquifer of sandstone in Upper and Lower Shihezi Formation and Shiqianfeng Formation of Permian consists of K4 sandstone in Lower Shihezi Formation and sandstone in Upper Shihezi Formation and Shiqianfeng Formation. K4 sandstone has developed joints and fissures. Due to poor openness, filling calcite veins or calcareous thin films, and the limitation of recharge conditions, the water abundance is poor. Sandstones of Shangshihe Formation and Shiqianfeng Formation are also weak in water abundance.

Triassic sandstone fractured aquifer formation is dominated by sandstone fractured aquifer, which is exposed in the southwest of Liulin block and Jucaita graben, with poor water yield.

Figure 1 division of water-bearing system in Liulin block

Pores developed in Neogene and Quaternary gravel (rock) pore aquifer groups, and were replenished by atmospheric precipitation, forming pore phreatic water. Limited by topography, recharge conditions and distribution areas, the water abundance is generally not strong. After short-distance runoff, it is discharged into rivers or ditches to replenish surface water or penetrate into underlying rock cracks, and a descending spring is formed when concentrated discharge.

1.2 flow field characteristics

1.2. 1 groundwater recharge, diameter and drainage conditions

The strata in Liulin block are generally inclined to the west, and the regional hydrogeological conditions are simple. It is a monoclinic water storage structure inclined to the west, wide and gentle. Atmospheric precipitation and lateral recharge of eastern limestone are the main recharge sources of all groundwater in this block, and sometimes atmospheric precipitation becomes the only recharge source of the main aquifer. Most of the surface rivers are seasonal rivers, which is not conducive to groundwater recharge or small recharge. Controlled by monoclinic structure, the main aquifer in Liulin block is basically replenished by atmospheric precipitation in the exposed area of eastern strata, and then flows from shallow to deep.

Fig. 2 Simple hydrogeological profile of Liulin block

The elevation of groundwater level in Liulin block is high in the north and low in the south, high in the east and low in the west. Groundwater mainly flows along the bedding to the deep, and with the extension of lateral distance, the runoff intensity gradually weakens. When the depth of Shanxi Formation exceeds 500 meters, the groundwater runoff speed is already very slow, and the groundwater runoff is basically in a state of stagnation. The Ordovician strata at the bottom of coal seam also show a consistent trend of higher in the east and north, and lower in the west and south.

According to the water supply intensity of surrounding rock aquifer to coal seam, the water content of coal seam in Liulin block can be divided into three types. (1) the roof of coal seam is limestone karst cave aquifer, with sufficient water supply and large water production. No.8 coal seam of carboniferous Taiyuan formation belongs to this type; (2) The roof or floor of the coal seam is sandstone pore and fissure aquifer, which has limited water supply to the coal seam, and the water yield of the coal seam is generally small. No.4 coal seam of Permian Shanxi Formation belongs to this type; (3) The roof and floor of the coal seam are argillaceous rocks, and the water supply is poor, so little water permeates into the coal seam, which can only be provided to the coal seam in the parts where faults or cracks develop. No.5 coal seam of Shanxi Formation belongs to this type.

1.2.2 chemical characteristics of groundwater

Hydrochemical composition is a true record of groundwater movement. The study of coal seam hydrochemistry is to find out the characteristics of groundwater circulation. HCO-3 is the main anion in the chemical composition of coal seam water in Liulin block, and its content is generally 2100 ~ 2400 mg/L. The HCO-3 content in No.8 coal seam water is slightly higher than that in No.4 and No.5 coal seam water. Followed by Cl-, and a small amount of CO-3 and SO2-4. The cation is mainly Na+, with the content of 1300 ~ 1800 mg/L, and a small amount of K+, Ca2+, Mg2+ and NH+4. The pH value is 6.7 ~ 8.2.

The salinity of water in No.4 and No.5 coal seams in Liulin Block is higher than that in No.8 and No.9 coal seams, which indicates that the water in No.8 and No.9 coal seams is highly active, and the hydrogeological conditions for coalbed methane enrichment in No.4 and No.5 coal seams are better than those in No.8 and No.9 coal seams.

2 Relationship between hydrogeological conditions and coalbed methane enrichment

2. 1 Hydrogeological gas control

Hydrogeological conditions of coal measures strata are one of the important geological factors affecting the enrichment, preservation, reservoir formation and development of coalbed methane. Under different hydrogeological conditions, the enrichment conditions and gas saturation of coal seam gas are different, which leads to great differences in coal seam gas content. Some hydrogeological conditions are beneficial to coalbed methane preservation, while others are very unfavorable to coalbed methane preservation. Hydrogeological gas control can be summarized into three characteristics: ① hydraulic migration dispersion gas control; (2) Hydraulic sealing and gas control; ③ Hydraulic gas plugging (Ye Jianping, 200 1). Hydraulic sealing and hydraulic plugging are beneficial to the preservation of coalbed methane, and hydraulic migration and dispersion cause the loss of coalbed methane. Generally speaking, the groundwater pressure is high and the coalbed methane content is high, and vice versa. The content of coalbed methane in the strong groundwater runoff zone is low, and the content of coalbed methane in the stagnant zone is high.

2. 1. 1 Hydraulic migration and gas control

In the fault structure development area with strong water conductivity, hydraulic migration and gas control are common. The coal seam and aquifer are connected by water-conducting faults or fractures, and the recharge, diameter and drainage system of hydrogeological units are perfect. The aquifer is rich in water and hydrodynamic force, and the aquifer has good hydraulic contact with the coal seam. In the process of groundwater movement, groundwater carries the gas in the coal seam to escape.

2. 1.2 hydraulic seal gas control function

Hydraulic plugging and gas control occur in wide gentle syncline or monocline where faults are undeveloped. Fault structures are mainly non-hydraulic faults, especially some boundary faults, which have the properties of compression and overthrust and become water-resisting boundaries. Hydraulic sealing gas control generally occurs in the deep, and coalbed methane is adsorbed in coal through the pressure transfer of groundwater, so that coalbed methane is relatively enriched without migration, and the gas content in coal seam is high.

2. 1.3 Hydraulic plugging and pneumatic control function

Hydraulic blockage and pneumatic control are common phenomena in asymmetric syncline or monocline. Under certain pressure difference, coalbed methane seeps from high pressure area to low pressure area, or from deep to shallow. The pressure drop desorbs coalbed methane, so it is the escape zone of coalbed methane in the outcrop and shallow part of coal seam. If the aquifer or coal seam receives outcrop recharge, the groundwater will go from shallow to deep along the layer, and the upward diffusion gas in the coal seam will be blocked, leading to the accumulation of coalbed methane.

2.2 Influence of groundwater chemical characteristics on coalbed methane reservoir formation

For coal-bearing strata, different types of groundwater reflect different salinity and salinity, and different salinity and salinity have different effects on coalbed methane reservoirs. Therefore, different types of groundwater play different roles in coalbed methane accumulation. According to the chemical properties of formation water, formation water can be divided into three types: CaCl2 _ 2, NaHCO3 _ 3 and Na2SO4. CaCl2 _ 2 _ 2 water is generally deep genetic water, which is often located in the pressure-bearing area with high salinity. The confined water area has good sealing conditions for coal seams and favorable conditions for coalbed methane reservoir formation, but the buried depth of coal seams in the confined water area is often greater than 1000m, and the porosity and permeability conditions are poor, which is favorable for reservoir formation, but unfavorable for coalbed methane development. Low salinity Na2SO4 groundwater is a sign of surface recharge water. Located near the recharge area or drainage area, the buried depth of coal seam is shallow or the coal seam is laterally exposed to the surface. It is the product of hydraulic alteration after surface water seeps into coal seam along outcrop area, which often corresponds to methane weathering zone, and the conditions for coalbed methane accumulation are poor. The groundwater salinity of NaHCO3 _ 3 is between the first two, and the buried depth of coal seam is mainly between 250 ~1000 m. The buried depth of coal seam is moderate, and the hydraulic power slows down alternately. At the contact surface between infiltration water and formation water, the direction of water flow is opposite, resulting in local stagnant zone, poor flow of formation water, resulting in overpressure, thus forming a blocked coalbed methane reservoir.

2.3 Characteristics of CBM Enrichment in Liulin Block

Formation water in Liulin block is mainly of sodium bicarbonate type, with calcium chloride or sodium sulfate type locally (Figure 3). The total salinity is low in the east and high in the west. In the vertical direction, the total salinity gradually increases from shallow to deep. The Cl- content of formation water in Shanxi Formation is obviously higher than that in Taiyuan Formation, which reflects that the sealing property of formation water in Shanxi Formation is better than that in Taiyuan Formation. The hydrochemical characteristics of upper Paleozoic coal measures strata in Liulin block show that the formation water in this block is connected with the surface, but it belongs to a relatively stable confined water dynamic system. The relatively high salinity water type of NaHCO3 shows its good preservation conditions for hydrocarbons, and the Cl- content can reach about 1 1,000 mg/L at the buried depth of about 400m, which is a hydrochemical condition conducive to coalbed methane enrichment (Wang Mingming et al., 65,438+).

Fig. 3 Distribution map of groundwater chemical types of Shanxi Formation in Liulin block

Generally speaking, the relationship between hydrogeological conditions and coalbed methane enrichment can be discussed according to the coupling relationship between coalbed methane occurrence characteristics and groundwater chemical field and dynamic field in Liulin area. Hydrogeological parameters, water level elevation, salinity and influence radius will all have great influence on coalbed methane productivity. The distribution law of high gas-bearing zone has a corresponding relationship with the division of groundwater system, hydrodynamic conditions and salinity distribution law. As far as the hydrodynamic conditions and hydrogeochemical characteristics in Liulin area are concerned, groundwater flows along the stratum to the depth of the west, and the deeper the depth of the west, the greater the salinity and the weaker the hydrodynamic force, which is more conducive to the enrichment of coalbed methane.

Reservoir-forming model of coalbed methane in Liulin block 3

The coal seams of Taiyuan Formation and Shanxi Formation in Liulin Block are mainly coking coal and lean coal, and the coalbed methane enrichment area is the overlapping area of high gas content and thick coal belt. Buried depth and hydrodynamic conditions largely control the enrichment degree of coalbed methane.

3. 1 CBM reservoir-forming model of Taiyuan Formation

8+9 and. Taiyuan Formation 10 coal seam is a monoclinal hydrodynamic closed reservoir (Figure 4). Because the recharge area and runoff area are outside Liulin block, Liulin block is mostly in weak runoff and stagnant flow environment, which is beneficial to the preservation of coalbed methane. From northeast to southwest, with the increase of buried depth, the formation pressure increases and the gas content in coal seam increases accordingly. The groundwater runoff in the southwest of the block is weak, which is beneficial to the enrichment and preservation of coalbed methane in theory. Although the gas content per ton of coal is considerable, it is difficult to reduce the pressure by drainage because of its high water content and high reservoir pressure. In addition, the trend of coal seam thinning at this bifurcation is not conducive to the development of coalbed methane.

Fig. 4 Reservoir-forming model of monoclinic hydraulic plugging of Taiyuan Formation in Liulin block

3.2 CBM reservoir-forming model of Shanxi Formation

The roof of No.3+4 and No.5 coal seams in Shanxi Formation is dominated by mudstone, with siltstone in some areas, and the roof and floor are weak in water-rich property. Although there is a fractured aquifer of roof sandstone, there is no water dynamic migration and escape on the whole, and the gas saturation is over 90%, which is a regional effective caprock gas pressure closed reservoir (Figure 5). The thickness and buried depth of coal seam are the main controlling factors of coalbed methane enrichment. The gas content of coal seam in Liulin block increases gradually with the increase of buried depth, and the development area of thick coal seam zone is a favorable area for coalbed methane enrichment.

Fig. 5 Gas pressure sealing reservoir-forming model of Shanxi Formation caprock in Liulin block

4 conclusion

Hydrodynamic conditions directly affect the formation pressure distribution and fluid migration, thus changing the original balance between adsorbed gas, dissolved gas and free gas, thus affecting the enrichment and preservation of coalbed methane.

No.8 coal seam of Taiyuan Formation in Liulin block and roof limestone are the same hydrodynamic system. Because there is a large concentration gradient between coal matrix and formation water, methane gas in coal and rock continuously escapes upward, and then is carried away by alternate formation water, so it is difficult to store in coal seam. Therefore, the overall gas saturation of Taiyuan Formation coal seam in Liulin block is low, and the water saturation is high, which is not conducive to the later drainage and depressurization.

The roof of No.3+4 and No.5 coal seams of Shanxi Formation in Liulin Block is dominated by mudstone, partly siltstone, and the water-rich property of the roof and floor is weak. Although there is a fractured aquifer of roof sandstone, there is no water dynamic migration and dispersion on the whole, and the gas saturation is over 90%, so the coal bed methane mining conditions are good.

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