Creep and Hysteresis Effects on the Marcellus Shale Matrix Permeability Behavior

Pulse-decay permeability measurements on a Marcellus shale sample at a constant differential pressure of 12.4 MPa [1800 psi] demonstrate minor hysteresis in permeability behavior. Permeability measured at the same confining and pore pressure conditions over 90 days do not decline significantly suggesting that creep also plays a minor role. The sample has ~3% porosity. Permeability measurements were made in two repeated cycles. In each cycle, the sample is first held at a high confining and pore pressure—27.6 MPa [4000 psi] and 15.2 MPa [2200 psi], respectively—where we complete multiple permeability tests to analyze creep behavior. We then incrementally decrease confining and pore pressure to 13.8 MPa [2000 psi] and 1.4 MPa [200 psi], respectively, while completing three to five permeability tests prior to reaching the final pressure. During the first creep stage, the permeability decreases from 4.7×10^-21 m² [4.7 nd] to 4.2×10^-21 m² [4.2 nd] over a two week period while during the second creep stage permeability remains constant at ~3.4×10^-21 m² [3.4 nd]. We observe hysteresis in permeability values between the two cycles. The permeability decreases from 13.2×10^-21 m² [13.2 nd] to 8.7×10^-21 m² [8.7 nd] during the first cycle at the lowest confining and pore pressure condition, but decreases only marginally to 7.7×10^-21 m² [7.7 nd] at the end of the second cycle. A better understanding of how permeability changes as stress evolves may illuminate production behavior of these gas shales.