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 coexploitation of coal measure methane.