Adsorption Characteristics And Production Mechanisms Of Coalbed Methane Reservoirs

Coalbed methane is an important resource to replace conventional oil and gas.There are significant differences in geological and development characteristics of different coalbed methane reservoirs.Due to the characteristics of low permeability,low pressure and low saturation of coal reservoirs in China,the theory of coalbed methane reservoir development abroad cannot be fully applied.On the basis of drawing lessons from the experience of foreign coalbed methane reservoir development,China's coalbed methane reservoir development is still facing great challenges.There is discrepancy between the production prediction and actual output of some coalbed methane blocks,and the microscopic pore characteristics and gas-water occurrence relation in the process of coal-mineralization can cause this contradiction.Unfortunately,the existing research lacks the deep level understanding to this.Therefore,it is necessary to analyze the key factors affecting the occurrence,seepage and output of coalbed methane in China's coalbed methane blocks,objectively evaluate China's coalbed methane mining potential,further improve China's coalbed methane development theory,and promote the rational development of China's coalbed methane resources.In this study,by evaluating the coalbed methane generation mechanism and law,conducting scanning electron microscopy experiments and high-temperature furnaceheating experiments for coal sample from Baode block,clears pore formation and development characteristics of coal with different thermal evolution stages,and further discusses the adsorption ability of metamorphic pore in coal with different thermal evolution stages,constructs pore water saturation model for coal with low/high evolution stages.By molecular simulation and adsorption experiments,the adsorption capacity of methane on coal with different maturity and moisture content is quantified on the nanometer scale.Based on this,a model for the dynamic change of permeability in coal reservoir under "four flow states" is established,and a prediction method for coalbed methane well production with the influence of reservoir water is further proposed.Main achievements and understanding are as follows:(1)It reveals the difference of gas-water distribution characteristics in pores of coal with different maturity.The results show that metamorphic pores are mainly adsorbed at the solid-gas interface,and there is a certain range of adsorbed water in different evolution stages,while primary pores can be adsorbed at the solid-liquid interface.(2)The characterization model of dynamic permeability change under four flow states of coal reservoir considering the influence of reservoir water is established.It is concluded that the reservoir permeability is affected by four effects including "pore-water" in the gas-water two-phase flow near the well zone.The matrix shrinkage effect caused by water desorption in the pores increases the effective pore size and the permeability of coal.The occurrence of water in the pores of coal has a significant influence on the permeability of the reservoir,which can reach about 18.5%.Existing characterization models of coal reservoir permeability usually underestimate the permeability of reservoir in gas-water two-phase flow near well zone.(3)The productivity model of coalbed methane well considering the characteristics of pore water,adsorption gas desorption and gas-water two-phase flow of coal is established.Considering the matrix shrinkage induced by water desorption,the predicted gas well production increased by 11.6%.The prediction accuracy of the newly established diminishing empirical model is higher than that of traditional Duong model and SEDM model.Compared with the prediction results of the new model,the prediction results of Duong model are higher with a relative difference of 4.03%,while the prediction results of SEDM model are lower with a relative difference of -5.81%.In this paper,by studying the gas water distribution characteristics within the coal matrix pore for coal with different maturity,quantitative water of coal on methane occurrence ability,the dynamic permeability of reservoir and well gas production representation model is established incorporating the influence of reservoir water during mining process in coalbed methane wells.In order to provide theoretical support for solving the problem of the reservoir with water that coalbed methane well production forecast and actual production data is poorer.Furthermore,this thus promotes our country's coalbed methane resources to develop reasonably and effectively.