Meitan xuebao (Aug 2023)
Phase change and mass transfer process of coalbed water in anthracite reservoir and its engineering inspiration for coalbed methane development
Abstract
The scientific understanding of the fluid flow and mass transfer mechanism in pores and fractures of coal seam are the fundamental strategy to achieve the efficient production of coalbed methane (CBM). During CBM development, the occurrence, migration, and production characteristics of coalbed water not only determine the drainage and depressurization effect of CBM well, but also significantly affect the desorption, migration, and production process of CH4, which is crucial to the efficient drainage and production control of CBM well. Taking the Fanzhuang block in the southern Qinshui Basin as an example, based on the occurrence form, production process, and flow form of coalbed water, the research on the mechanism of phase change and mass transfer process of coalbed water has been carried out. The threshold pore diameter of phase change and the mass transfer process of coalbed water in anthracite reservoir under the engineering background have been quantified, and its engineering implications for CBM production have been discussed. The results show that the occurrence state of coalbed water can be changed dynamically, and the fluid pressure difference is the main mechanism of its dynamic change. In the single-phase flow stage of water, the migration of movable water is mainly affected by the surface tension. Under the engineering background, the migration and production of movable water in pores and fractures with macropore (pore diameter > 50 nm) is the precondition of the overall release of coal seam pressure, and lays the foundation for a high and stable production of CBM well. In the gas-water two-phase flow stage, the transformation of bound water (capillary water) to movable water is greatly affected by the wettability of coal, and the interface property and interaction between gas and water. The pressure condition requirements of the transformation of bound water (capillary water) to movable water increase due to the capillary effect. In the bubbly flow and plug flow stage, CH4 desorption promotes the transformation of bound water (capillary water) to movable water in some pores and fractures with mesopore (2−50 nm) scale, which determines the overall release of coal seam pressure. This also indicates that after the CBM well has casing pressure, the overall release of the coal seam pressure is the result of the joint action of the gas and water, and the slow transition from the water production stage to the depressurization and gas production stage is necessary for the high production of the CBM well. The continuous casing pressure holding may lead to early formation of slug flow in pores and fractures with mesopore scale, which is not conducive to the high and stable production of CBM well. Moreover, with the decrease of coal seam pressure, the liquid water in pores and fractures with the equivalent pore size > 40 nm can be vaporized and separated to the gaseous free water, which forms competitive adsorption/desorption with the adsorbed CH4. However the water in pores and fractures with the equivalent pore size < 40 nm is preserve with liquid or transported by wetting migration, and has little influence on the transformation of bound water (capillary water) to movable water in the gas-water two-phase flow stage.
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