Conditions for high production and enrichment of coalbed methane in China

The basic conditions for high production and enrichment of coalbed methane are controlled by many factors. Taking the geological background of the formation of CBM as the main line, combined with the domestic and foreign CBM exploration practice, the preliminary summary of CBM high-yield enrichment factors, mainly in the following eight aspects:

1. Wide distribution of coal beds and large thickness are the fundamental conditions of CBM enrichment

The abundance of CBM resources is positively correlated to the distribution of the coal strata, the thickness and the gas content of coal strata, and the greater the parameters, the richer the CBM is. The larger these parameters are, the richer the coalbed methane is. The abundance of coalbed methane in a coal gathering basin (area) is an important indicator for evaluating whether it has coalbed methane exploration value or not. It is generally believed that a continuous distribution of coal seams over an area of more than 200km2, a total thickness of coal seams greater than 10m in the concentrated section of coal seams, a continuous and stable distribution of the main coal seams in the plane, a gas content of more than 10m3/t, and a methane content of more than 80% are conducive to industrial coalbed methane exploration.

2.Coal rock with high content of specular group and low ash content is favorable for CBM generation, adsorption and exploitation

The microscopic content of coal and ash content not only affect the hydrocarbon potential of coal, but also influence the adsorption capacity of coal beds for methane and the exploitation capacity of CBM. The higher the microscopic group content of coal, the lower the ash content, the stronger the angry ability, the higher the methane adsorption, the more developed the coal seam cuttings, the better the permeability, the easier the coalbed methane mining.

3. Good capping layer is a necessary condition for the preservation of CBM reservoirs

A good capping layer can reduce the outward seepage and transportation of CBM and diffusion loss, maintain high formation pressure, maintain the maximum adsorption capacity, and weaken the loss of CBM caused by formation water. Different types of capping layers formed under different depositional environments have different capping abilities. Mudstone is a good capping rock with developed micropores, strong capping ability and stable performance. Dense sandstone and limestone have strong or weak capping performance, depending on the influence of later diagenesis, and if they are dense before the peak of anger, they are effective in capping coalbed methane. In addition, the capping performance of the same type of cap layer is not the same due to the different regions and the different degree of influence of geological action. Therefore, it should be treated differently and analyzed specifically according to the geological conditions of specific regions to evaluate the degree of its influence on the preservation and enrichment of coalbed methane.

4. Magmatic thermal metamorphism is favorable to CBM enrichment

The high-yield CBM wells in the south of Qingshui Basin, Fuxin of Liaoning, and Tifa Basin in China are mainly distributed in the regional magmatic thermal metamorphic areas. The reason is that under the action of magmatic activity and other thermal events, which increase the amount and intensity of coalbed gas, coupled with the good configuration of hydrocarbon and tectonic histories, the gas-bearing nature of these areas is generally better, and the gas saturation is generally higher. Moreover, due to the thermal baking of magma, the organic matter in the coal is volatilized, leaving a lot of dense groups of rounded or tubular pores, which improves the porosity of the reservoir; the superposition of exogenous fissures and endogenous fissures (cuttings) produced by the power extrusion and cooling contraction of magma intrusion enhances the permeability of the coal seams, and thus the coalbed methane gas wells tend to be high-yielding. However, if natural coke is formed by baking action near the contact zone between the volcanic intrusion and the coal seam, it will be unfavorable for coalbed methane exploration.

5. Groundwater stagnation zone is favorable to the preservation of CBM in middle and high coal rank

Groundwater dynamics state is also one of the factors affecting the high production and enrichment of CBM. Weak runoff-retention area has higher mineralization of formation water and weaker hydrodynamics, which is conducive to the preservation and enrichment of CBM. Statistical results show that in general, the gas content of coal beds in the middle and high coal rank increases with the increase of stratum water mineralization (Figure 8-2), therefore, the high mineralization of middle and high coal rank is conducive to the preservation of coal bed methane.

Figure 8-2 Correlation between CBM gas content and mineralization in the southern part of the Qingshui Basin

6. Suitable hydrogeological conditions are conducive to the generation of biogenic gas in the low coal rank area

Thermogenic gas in the low coal rank coal beds has a relatively small amount of gas, but if there is sufficient atmospheric freshwater recharge, low mineralization, low sulphate, low temperature, anoxicity, and suitable hydrodynamic conditions are conducive to the large growth of methanogens, a large number of bio-gas can be generated, and a large number of bio-gas can be produced. However, if there is sufficient atmospheric freshwater recharge, low sulfate, low temperature, hypoxia, suitable hydrodynamic conditions are conducive to the growth of methane bacteria, with a large number of biogas generation of geological conditions, with good preservation conditions, it is easy for the formation of biogenic coalbed methane reservoir.

7. The permeability of coal beds in the area of low geostress is good, which is favorable for coalbed methane mining

Geostress is the most important factor controlling the permeability of coal beds, and the permeability of coal beds is generally negatively correlated with the geostress (Figure 8-3). The low stress distribution area in the ground stress field is often a high density distribution zone of cracks, and the degree of development of the coal cuttings system determines the permeability of the coal seam, which affects the production of the coal bed methane wells as well as the design of the well network in the later stages of exploration and the implementation of the enhanced treatment program. The development of coal cuttings and high permeability are conducive to the desorption of gaseous adsorbed hydrocarbons adsorbed on the surface of the coal particle matrix over a large area.

Figure 8-3 Relationship between permeability of coal reservoir and original ground stress in Daning-Jixian area

8. Basin marginal slope zone or tectonic high part is favorable for high CBM production

Marginal slope zone and tectonic high part tend to be the low-potential area, which is the pointing area for hydrocarbon transportation. Early coal seams in the marginal slope zone are y buried, with good conditions of anger, and the coal seams are uplifted in the late stage, forming a low-potential zone, which is a hydrocarbon transportation pointing area, and together with the regional distribution of stable direct cover, it is easy to form a high-gas-bearing, highly saturated coalbed methane reservoirs.

The marginal slope zone and the tectonic high part of the cuttings are developed, and the permeability of the coal seam is good. The marginal slope zone is in the late tectonic amplitude of the basin, the coal beds are relatively shallow, in the relatively low value of the geostress area, the tensile fissure is developed, and the permeability of the coal beds is good, which is favorable for the high production of coal bed methane wells.

Marginal slope zones and high tectonic areas have input-type gas production characteristics of coalbed methane wells under overall depressurization. In the case of overall depressurization, the high part is easy to prioritize the formation of area depressurization, and the coalbed methane wells in the high part of the structure are generally characterized by low water production and high gas production.