Time-dependent deformation of clay-rich rocks enveloping reservoirs exploited for geo-energy purposes

Though the energy transition aims to phase out fossil fuels while continuing to exploit the subsurface for other storage solutions (e.g. geological CO₂ storage, temporary hydrogen storage), natural gas, as a low-carbon energy carrier, will continue to play a role in our energy mix for the foreseeable future. In general, human activities in the subsurface change the physical and chemical environment, which in turn can lead to surface subsidence and induced seismicity. These phenomena may continue even after activities have stopped, as observed for natural gas extraction from the giant Groningen Gas Field in the Netherlands. They are largely caused by deformation in the reservoir rock, driven by fluid pressure changes. However, in-situ strain measurements from the Groningen Gas Field demonstrate that the clay-rich over- and underburden formations of the reservoir are also affected by these fluid pressure changes, displaying slow compaction. To make accurate predictions of reservoir deformation and to allow reliable assessment of the associated surface subsidence and induced seismicity, a detailed understanding of the deformation processes controlling deformation in these clay-rich formations is needed. Understanding which processes caused deformation in past (hydrocarbon) operations will help in understanding what may happen now that we plan to store other fluids in the subsurface.

We performed rock mechanical experiments at in-situ conditions on the Opalinus Claystone (Switzerland), an analogue to the Groningen over- and underburden claystones, to assess the grain-scale mechanisms responsible for deformation. We designed an innovative and comprehensive multi-step experimental procedure that provides new, coherent data on the time-dependent deformation of clay-rich rocks. The experiments were performed in a triaxial compression apparatus, applying systematic steps of constant stress while controlling the pore fluid pressure in the sample. These steps were either stepped up or down in differential stress during an experiment. At each differential stress we systematically analyzed the instantaneous and time-dependent deformation.

We observed general compaction of the samples upon increasing stress, and time-dependent expansion of the sample when stepping down in stress. Our results demonstrate that deformation in clay-rich rocks is strongly affected by the fluid-transport properties of the rock. We infer that sorption of fluids to the clay-rich matrix plays an important role in the deformation of clay-rich rocks, along with frictional slip controlling grain rearrangement. However, matters are complicated by slow diffusion of pore fluid pressure, which leads to an additional time-dependent component. Overall, our results demonstrate that over half of the observed deformation is permanent, even at low differential stresses. A detailed understanding of the time-dependent deformation of clay-rich rocks is crucial for accurate predictions of the impact of human activities in the subsurface, as sorption of fluid to the clay material may also be important during CO₂ and hydrogen storage.