A novel method of microscale gas migration and production characteristics simulation considering adsorption characteristics

One challenge to the effective development of coalbed methane and shale gas is the complexity of their migration mechanism and production characteristics. Recognizing this, our study has devised an indoor experimental simulation system for production characteristics of coalbed methane or shale gas with a consideration of the effect of adsorbed gas. Firstly, we carried out a theoretically-informed quantitative identification of adsorbed gas, pore-confined gas, and free gas in the production process. Then, the theory was combined with experiments to quantitatively evaluate the production rates of these gases in different occurrence states and their contribution to the yield. This paper presents an application of this new method to coal-seam samples from the Da Ningji County Block. Our research suggests that during the extraction process, a higher gas production rate would lead to a higher pressure in the eventually abandoned formation, and ultimately a lower recovery rate. Specific findings include, firstly, that before extraction, the proportion of adsorbed gas in the samples is 80%, which then gradually increases during the extraction process, eventually reaching 90% at the end of production, with an average contribution of 70% to yield. Secondly, a greater contribution of adsorbed gas to production leads to a higher ultimate recovery rate. Thirdly, the desorption rate shows an upward trend in the stable production phase, gradually decreasing during the depletion production period, but the rate of decrease slows down as the development progresses. Fourthly, a higher desorption rate in the later stage of extraction is positively correlated with a higher ultimate recovery rate. Lastly, the contribution of pore-confined gas and free gas to production exceeds 70% in the early stage before gradually declining as extraction progresses, and no relationship of significance has been found between the production rate and recovery rate in the later stage. It is hoped that our study can provide theoretical underpinning for the optimization of coalbed methane and shale gas development.