Abstract
In tight shales, gas is stored in both free and adsorbed forms where the latter can make a significant or majority part of the gas in place. When the adsorbed layer thickness becomes of similar magnitude as the pore radius the adsorption can also affect the flow performance. We consider a 1D model for shale gas production where we implement adsorption with the adsorbed layer thickness as function of pressure, pore geometry where the pores are n-spherical (for 1 < n < 3). A higher n indicates more spherical pores, while a lower n means more fracture shaped pores. The shale is assumed to be compressible and its porosity and pore radius reduce with pressure depletion. The effective pore radius, which also depends on adsorption layer thickness, controls both intrinsic and apparent permeability. This study will address gas production by natural pressure depletion and the impact of the adsorption layer in the flow-compaction interplay. Marcellus shale data are used as input.
For a given compressibility and hence porosity-pressure relation, the pore radius is less reduced at a high n than for a low n. The adsorbed layer thickness is assumed to be pressure dependent only, and fills a greater volume in the pores when the pores are more spherical (high n). Increasing the maximum adsorption layer thickness makes the adsorbed layer fill more of the pore volume and gas in place. The increased volume fraction of adsorbed gas reduces the free gas saturation and the apparent permeability of the gas, resulting in delayed production compared to systems where the pores are less spherical (lower n) and the adsorbed layer is thinner. Desorption is not very significant until pressure is reduced well below the initial value and mainly free gas is produced at early times. Hence, systems with more adsorbed volume fraction see lower recovery at a given pressure. Pressure depletion causes both the pore radius and the adsorbed layer to be reduced. The change in adsorbed layer with pressure is lower at high pressure and greater at low pressure, while pore radius changes more linearly with pressure. The free gas saturation can increase with pressure depletion (time) for low compressibility cases, or can reach a minimum before increasing in high compressible cases. It was observed that compressibility and production from the adsorbed layer both contributed significantly to recovery. Setting either the porosity or the adsorbed layer thickness constant (same as for the initial pressure condition) resulted in lower recoveries. Setting both constant gave the lowest recovery.
