The hidden value of insufficiently preserved shale material

Abstract

The research is focused on investigating if insufficiently preserved shale can still be valuable in a correct geo-mechanical characterization of the material. This is achieved by targeted attempts leading to reverse the processes that diminished the quality of the material in the first place, allowing the shale to regain the original rock properties. The focus is limited, for now, to recapture the saturation state by investigating the fluid effect on the mechanical and acoustic response. A considerable number of basic mechanical and acoustic tests were conducted on poorly preserved Opalinus clay to identify the pristine properties of this material. Among these, the strength of the shale was measured by unconfined compressive strength, scratch, and punch tests, whereas the acoustic wave velocity was tested by continuous-wave technique and through-transmission method. The experiments wereperformed for three saturation states: 'as-received', dry and re-saturated. To confront the outcome, the same test campaign was conducted on well-preserved Opalinus clay from a nearby location, at the Mont Terri underground laboratory in Switzerland. Results show that obtaining valuable data on the damaged cores, such as unpreserved shale, is possible by utilizing adequate testing methods. One example are tests on the small-size samples that provide reliable information regarding the acoustic and mechanical properties of the material. It is, however, disputable to what extent one can recover the original properties from the unpreserved material.