Influence of Well Types on Optimizing the Co-production of Gas from Coal and Tight Formations

Co-production of gas from both coalbeds and tight formations is considered a viable means to improve well productivity. Most previous studies focused on the geology and resource estimates for gas production viability with little attention to the effectiveness of gas co-production with regard to well types. To make up for this weakness, a two-phase flow and reservoir deformation coupled model is proposed together with an anisotropic permeability model. The coupled model is first verified using gas and water production data from a vertical well from the Linxing block in the Ordos Basin, China. Then a reservoir model is built, including one coal seam and one tight gas formation separated by a low-permeability stratum with four simulation scenarios designed. Based on the results, the impacts of the crossflow between different reservoirs are addressed and the mechanisms of the gas co-production rate profile types observed in the Linxing block are analyzed. It is also found that high water-saturated adjacent reservoirs would keep the water relative permeability of the gas-rich reservoir at a high level, impeding the gas flow. The use of a horizontal well is strongly recommended when most gases are stored in a specific thin reservoir and the life of the well is short; however, a vertical well is favored when two or more gas-rich and high permeability reservoirs co-exist and the well life is relatively long. For the application of vertical wells, the hydraulic fractures should extend in the horizontal planes and interact with the pre-existing natural fracture. For horizontal wells, the hydraulic fracture should extend in the host reservoir and penetrate into the adjacent strata. This work can shed new light on the co-exploitation of coal measure methane.