Effectiveness of drainage, depressurization and gas production of coalbed methane wells based on production data: A case study of the Zhijin Block in Western Guizhou

The process of drainage, depressurization, and gas production in coalbed methane wells is a progressive process that can be decomposed into two steps: the effectiveness of drainage inducing depressurization, and the effectiveness of depressurization inducing gas production. Failure in either step can lead to limited gas production. Discriminating the effectiveness of the two steps based on the analysis of production data and identifying the main constraints on gas production are of great significance for promoting the economic and efficient development of CBM. An analysis method was constructed for the effectiveness of CBM recovery based on three relationship curves: cumulative water production and bottom-hole flow pressure, cumulative gas production and bottom-hole flow pressure, and cumulative water production and cumulative gas production. It has the functions of water invasion identification, water source splitting, gas reservoir parameter acquisition, and key production step distinction. It has been applied to the dynamic analysis of CBM production in the Zhijin block in western Guizhou, and has achieved certain results. Based on the relationship curve between cumulative water production and bottom-hole flow pressure, the up-concave type represents water-invaded wells, while the down-concave type represents non-water-invaded wells. Facing the development demand, the concept of gas-bearing system is extended to gas-producing system, which refers to gas-bearing system and their combinations with similar hydrogeological conditions, development dynamics, and production rules. The compatibility of different gas-producing systems is poor. The coal-bearing strata in the Zhijin block can be divided into two gas-producing systems: the upper system (Coal Seams 1-9) and the lower system (Coal Seams 10-35). The upper system has a high degree of hydrogeological opening and is prone to water intrusion, while the lower system has relatively closed hydrogeological conditions and no significant water intrusion, which is conducive to efficient pressure reduction and gas production. The calculation results of gas reservoir parameters show that water-invaded wells have characteristics of low start pumping pressure and high elastic water production index, while non-water-invaded wells have the opposite characteristics. The water-invaded wells are characterized by producing more intrusion water than elastic water, resulting in a high total water production. The non-water-invaded wells mainly produce elastic water with low total water production. The difference in the type of energy driving the water production determines the effectiveness of drainage and depressurization, which in turn determines the productivity potential of the gas well. Limiting or avoiding water intrusion and improving the effectiveness of drainage inducing depressurization will be the main ideas for the production increase of old wells and the design of new wells in the Zhijin Block. The research results can provide methodological references for the dynamic analysis of CBM production and the diagnosis of main factors controlling productivity in the context of the development of superimposed CBM systems.