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

The gas production from stacked gas deposits with coalbed,shale,and tight sandstone reservoirs in the coal measure is an effectivenew technology to enhance per well gas production. However, previouslydeveloped numerical models for stacked gas deposits usually targetone or two reservoir types. The contribution and impact mechanismof crossflow and two-phase flow on gas production from stacked gasdeposits are unclear for the stacked gas deposits with coalbed, shale,and tight sandstone reservoirs in the coal measure. This paper developsa numerical simulation model for the gas production from stacked depositswith shale, coalbed, and tight sandstone reservoirs. This numericalsimulation model considers the two-phase flow, gas diffusion in differentporosity, pore deformation, and interlayer crossflow. After verificationwith available data from the literature, this numerical simulationmodel is used to explore the effects of crossflow across interlayersand two-phase flow on gas transport and production efficiency. Thesimulation results show that the main sources of gas production fromstacked deposits are shale and coalbed reservoirs. The gas in thesetwo reservoirs can be rapidly transported to the wellbore throughthe tight sandstone reservoir. The gas production from stacked depositswill be underestimated without the interlayer crossflow and overestimatedif the two-phase flow is ignored. The gas production efficiency canbe effectively improved through the enhancement of the crossflow rateat the interlayer and the decrease of the water saturation in thereservoir.