Static and dynamic reservoir rock compressibility at high pressure

The paper reports measurements of rock compressibility performed by means of an equipment capable to operate up to 150 MPa confining pressure. Measurements have been performed on 32 samples from 3 sandstone and 3 carbonatic reservoirs. The experimental apparatus was designed to perform both static investigations (deformation tests, to calculate bulk and pore compressibilities) and dynamic investigations (acoustic tests, for the evaluation of undrained compressibility). Results show that compressibility is not constant, but is a function of reservoir pressure. In particular, compressibility of sandstones follows an exponential trend, decreasing rapidly in the first part of the loading cycle, and remaining almost constant at high pressure. Compressibility of carbonatic samples shows the same behaviour, but increases for stresses close to the yield point. The attempt to correlate dynamic and static tests reveals the necessity of additional investigations; in fact, it has been noticed that undrained compressibility measured by dynamic tests yields inconsistent results for non-elastic rocks, and, generally, dynamic measurements do not correspond to the static ones.