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

Abstract

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.

Optimization of the Layer Location of Extraction Wells Based on the Unpressurized Gas Transport Law

“…The use of surface mining wells for coalbed methane extraction has the significant advantages of high drainage concentration and large flow. − The entire construction process is located on the ground and does not affect the coal production. Currently, it has been widely used in the United States, Canada, Australia and other world’s coal-producing countries. − The permeability coefficient of coal reservoirs in China is generally low, which seriously restricts the drainage effect of surface coalbed methane wells. − Through the pressure relief and permeability-enhancing effect of protective layer mining, the gas permeability of coal and rock mass can be increased thousands of times compared with before pressure relief. At this time, the effect of pressure-relief gas drainage using surface drilling is significant.…”

“… 20 − 23 The permeability coefficient of coal reservoirs in China is generally low, which seriously restricts the drainage effect of surface coalbed methane wells. 24 − 26 Through the pressure relief and permeability-enhancing effect of protective layer mining, the gas permeability of coal and rock mass can be increased thousands of times compared with before pressure relief. At this time, the effect of pressure-relief gas drainage using surface drilling is significant.…”

Impact of Crossflow and Two-Phase Flow on Gas Production from Stacked Deposits

“…The production of coal measure gas can complement the depletion of conventional gas reservoirs after long-term exploitation . The stacked gas deposits with coalbed, shale, and tight sandstone reservoirs are typical coal measure gas reservoirs. , Gas from the stacked deposits in the coal measure includes coalbed gas mainly in the adsorbed state, tight sandstone gas mainly in the free state, and shale gas in the coexistence of adsorption and free states. , Coalbed methane, tight sandstone gas, and shale gas are called three gas in the coal measure and widely distributed in the Cooper Basin of Australia, Guizhou of China, and the eastern edge of the Ordos Basin of China. − These reservoirs are rich in gas content and have great potential for extraction. Natural gas is usually extracted from one target coalbed in the coal measure, but the extraction is of low production, short production duration, and high cost .…”