Laboratory Study of the Effective Pressure Law for Permeability of the Low‐Permeability Sandstones from the Tabamiao Area, Inner Mongolia

Laboratory experiments with two modified factorial designs were performed to determine the effective pressure law for permeability of two low permeability sandstone samples from the Tabamiao area, Inner Mongolia. Two modified factorial schemes were adopted for the experiments. The first included three cycles, of which each run through loading and unloading confining pressure ( P c ) in a constant‐pore‐pressure condition. The second scheme has four cycles, of which each run through raising and lowering pore pressure ( P p ) in a constant‐confining‐pressure condition. Permeability data were collected by 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 – P c – P p surface to the data. The coefficients describing the surface reflected information about material behavior and were transformed into κ – P c – P p response surface. The κ – P c – P p surfaces show that at intervals along the pressure path the κ values in the effective pressure law ( P eff = P c – κP p ) varied with pore and confining pressure. At the highest confining pressure 44 MPa in the test, the values of κ of two samples were all significantly less than 1.0 and even below zero at some low pore pressure. The data suggest that values decrease with increasing confining pressure and lowering pore pressure, and are very small at high confining pressure and low pore pressure. This can be interpreted in terms of the changes in the geometry of the micro‐cracks during closure and the deformation of the rock particles after the closure of micr‐cracks with increasing confining pressure and lowering pore pressure.