Abstract: The strain state is assumed to be one dimension strain, while the stress state appears 3D one in process of rock sedimentation. However, during oil-gas production in reservoir rocks, it is shown that both the strain state and the stress one are of 3D features. Therefore, the rock compressibility coefficients measured in core labs represent the values under the 3D strain state, and can be directly applied to study of related reservoir engineering without necessary to be converted. It is invalid for traditional practice to convert rock compressibility coeffi-cient from 3-D strain state to a dimensional one, which may allow the elastic energy in an oil-gas reservoir rock to be underestimated badly.

Key words: rock, reservoir, stress, strain, compressibility coefficient