## Abstract

Two general laws are put forward to described the sensitivity to hydrostatic stress observed in the laboratory on a selection of unsaturated reservoir core and outcrop sandstones, with low to moderate porosities. These relations are based on using the concept of excess compliance as a pseudo-function to describe all internal weaknesses, regardless of whether their origin is cracks, contacts or other mechanisms. The bulk modulus, _{?}, is defined by a dimensionless parameter S_{v} lying between 0 and 1, which depends upon the values of the modulus at room pressure and the high pressure asymptote _{?}^{8} of the behaviour. The rate parameter, P_{c}, determines the rapidity of convergence on the final high pressure asymptotic state as pressure is applied. There is a well-defined relationship between _{?}^{8} and porosity, but in contrast S_{v} and P_{c} are almost independent of porosity. S_{v} is weakly correlated to clay fraction. Shear wave properties are specified through the normalized shear modulus ? = µ/(? + 2µ). An additional dimensionless parameter S_{s} determines the magnitude of this particular stress sensitivity. The parameter is predicted to be smaller than S_{v} but can be either positive or negative. For most dry sandstones S_{s} remains negative (this corresponding to an increase in V_{p}/V_{s} with pressure), with its magnitude and sign correlating to clay fraction.

Original language | English |
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Pages (from-to) | 59-81 |

Number of pages | 23 |

Journal | Journal of Seismic Exploration |

Volume | 11 |

Issue number | 1-2 |

Publication status | Published - 2002 |

## Keywords

- Excess compliance
- Laboratory measurements
- Pressure sensitivity
- Sandstone
- Stress sensitivity
- Time-lapse sensitivity