Quantitative dynamic analysis of gas desorption contribution to production in shale gas reservoirs

Unlike in conventional gas reservoirs, gas in shale reservoirs is stored mainly as free gas and adsorbed gas, and a small amount of dissolved gas. Well production from shale gas reservoirs usually exhibits sharply decline trend in the early period of production and then turns to long-term stable production at a relatively low rate, for which gas desorption contribution has been considered as a possible explanation.

This study aims at providing an accurate evaluation of the contribution from gas desorption to dynamic production. Through incorporation of artificial component subdivision in a numerical simulator, the production contributions of the free and adsorbed gas can be obtained separately. This analysis approach is validated firstly and then applied to two case studies based on conceptual models of Barnett and Antrim Shale. The results show that desorbed gas dominates the production in Antrim Shale, while it only plays a small role in the production in Barnett Shale. The impact of permeability and initial gas saturation are also analyzed.

In previous studies, numerical and analytical simulators were used to investigate the difference between the production performances with or without desorption, attributing the production increase to gas desorption. However, our study shows this treatment overestimates the contribution from gas desorption.

This work provides a simple but accurate method for the dynamic analysis of desorption contribution to total production, contributing to reservoir resource assessment, the understanding of production mechanisms, and shale gas production simulation.