Water-bearing characteristics of coal reservoirs and its control mechanism on coalbed methane production

Water-bearing characteristics of original coal reservoir not only determine the application of coalbed methane(CBM) adsorption and accumulation theory,but also affect the selection of reservoir reconstruction measures, and the understanding of the CBM production process. Regarding the water-bearing properties of coal seams, in the coalbed methane geology,it believes that coal reservoirs are basically saturated with water,while in the coal gas geology,it basically does not consider the influence of water in gas content calculation and gas seepage,which is equivalent to thinking that coal reservoirs are saturated with gas. From the perspective of coal seam water source,coal reservoir burial and thermal evolution history, the changes of water bearing in coal reservoirs are expounded. According to the analysis, there may be four states in the present coal reservoir, i. e., saturated water, ultra-low saturated water,unsaturated water and no water. The influence of pre-existing gas on later water invasion,the change of reservoir wettability reduced by the thermal evolution and hydrocarbon generation of coal reservoir are the key factors affecting the water bearing of the reservoir. Due to the complexity of tectonic movement in China and the frequent occurrence of late water intrusion,"saturated water in large pores and fractures and saturated gas in small pores" may be the occurrence state of most coal reservoir gas and water. This state of occurrence of gas and water can effectively explain the argument that " saturated water in coal reservoir is not applicable to the Lan's adsorption theory" in the geological theory of coalbed methane,and it also determines that the influence of capillary force should be considered in the analysis of gas content of coal reservoir. In addition,the coal reservoir is a low-porosity and low-permeability reservoir,and the early stage of drainage and production is generally single-phase flow,so the drainage and production process of coalbed methane needs to determine and consider the influence of start-up pressure gradient. To sum up,it should carry out the study on the wettability of coal reservoir under the condition of coal-water-methane, study the influence of hydrocarbon generation and pre-existing gas on reservoir water content,and re-recognize the water bearing capacity of coal reservoir. Based on the studies above,to further describe the occurrence and production process of coalbed methane may be a key to solve the problem of low production of coalbed methane,especially medium and high rank coalbed methane in China.