The Effective Pressure Law for Permeability in Northern Hubei Low Permeability Sandstone Rocks

Abstract eng:
Some new cognition has not been acquired from the experimental research on the effective pressure law for permeability for a long time. Therefore, reports about the experimental research for the effective pressure law have not been seen recent years. To obtain new view on the effective pressure law, laboratory experiments with two modified factorial designs were performed to determine the effective pressure law for permeability of two samples from Northern Hubei low permeability sandstone formation. One modified factorial design for one sample included four cycles from which pore pressure was different. Each cycle was run through loading and unloading confining pressure in constant-pore-pressure condition. Another design for the second sample contained three cycles from which confining pressure was different. Each cycle was run through raising and lowering pore pressure in constant-confining-pressure condition. Permeability data were taken with the steady-state method. The response-surface method was used which supposed that nothing was known about the material behavior, and a model was built empirically by matching an approximate k − pc − p p surface to the data. The coefficients describing the surface reflected information about material behavior and were transformed into α − pc − p p response surface. The α − pc − p p surfaces showed that at intervals along the pressure path “local” values of the effective pressure coefficient α in the effective pressure law varied with pore and confining pressure. At the highest confining pressure 44MPa in the test, values of α of two samples were all significantly less than 1.0 and varied below 0.1 at different pore pressure interval in most cases. Small values of α at high confining pressure are contrary to conventional view α=1.0. The data suggest a decrease of α with increasing confining pressure and small values of α at high confining pressure. This is interpreted in terms of change in the geometry of the micro-cracks during closure with increasing confining pressure in the paper.

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