Abstract: Shale reactivity is evaluated by testing a preserved test plug of shale sample in a triaxial test machine, the test plug being prepared by collecting a downhole shale sample and keeping it all times immersed in a preserving mineral oil so as to avoid dehydration, then applying radial and axial pressure on the test plug surrounded by mineral oil up to equilibration to overburden pressure, the test fluid being then contacted with the sample and the interaction of fluid and sample being evaluated by axial and radial deformations as measured by a triaxial detector apparatus sensitive to vertical and radial strains occurring across the shale sample, while the shale sample is subjected to any of a set of different conditions including a temperature or thermal potential, a hydraulic potential and/or a chemical potential. Only one fluid is tested on each sample.