PORE PRESSURE BUILDUP COEFFICIENT IN SYNTHETIC AND NATURAL SANDSTONES

We present laboratory measurements of Skempton's pore pressure buildup coefficient B for synthetic and natural sandstones, and use our resul ts to estimate the unjacketed pore compressibility 1/K(phi). Measured values of B are always near unity for differential pressures below app roximately 2 MPa, regardless of porosity, grain composition, or the pr esence of small amounts of clay in the samples. We observed B near 0.7 -0.8 for differential pressures of about 8-21 MPa in a clay-free synth etic sandstone. Similar values are expected for natural sandstones wit h low crack densities, whereas sandstones containing greater concentra tions of microcracks apparently have values of B near 0.6. Our results suggest that the pore compressibility may have values close to the po re-fluid compressibility rather than the grain compressibility.