With a sustained interest in exploration for shale gas, and condensate recovery from ultra-tight reservoirs, for purposes of understanding production decline, the mechanism of transport within the reservoir matrix is of fundamental importance. In our laboratory, a permeameter to measure rock permeability down to a hundred nanoDarcy (0.001 nm2) has been built and tested rigorously. In this device, the experimental protocol calls for a perturbation from an equilibrated pressure. The resulting decay in the imposed perturbation to a final equilibrium value is used to infer the permeability of the sample. The details of the methodology is available elsewhere (paper under review in AIChEJ.)
In this paper, we study effects of various factors such as stress and fluid pressure on the permeability of the sample. We show that two distinct contributions of fluid pressure act in opposing directions. The first is pore-wall slip, which decreases with increasing pressure, and thus reduces permeability with pressure. The second is effective stress which decreases with increasing pressure, and therefore tends to favor permeability increase with pressure. Since the dependence of permeability on stress is path dependent, an experimental protocol to unambiguously determine this variation is quite to difficult to construct, and we discuss possible methods.
For characterization purposes, we also show that some tight rock samples have permeability variation with respect to molecular species. We conjecture that this variation is caused by adsorption. The mechanism of permeability alteration due to adsorption is debatable, and procedures to separate the contribution as that due to effective pore size are discussed.
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