Extraction of natural gas from the Groningen gas field in the Netherlands has resulted in surface subsidence and induced seismicity, strongly impacting the environment and society. These phenomena are largely caused by compaction at the reservoir level, due to the decrease in gas pressure in the porous reservoir rock (Slochteren sandstone). However, in-situ strain measurements show that the overlying Ten Boer claystone and the underlying Carboniferous shales are also affected by these pressure changes, though at a longer timescale, due to their low permeability. This means that slow pressure equilibration in the over- and underburden rocks is contributing to the overall compaction of the gas field. To make accurate predictions for future surface subsidence and induced seismicity in the area, the active deformation processes in all formations need to be understood.
We performed rock mechanical experiments at in-situ conditions on clay-rich rocks (Opalinus claystone) similar to the formations enveloping the Groningen gas field to identify the main deformation mechanisms that operate in these formations. The experiments demonstrate that a significant portion of the compaction in the clay-rich formations is time-dependent and inelastic. Once a workflow is established, these experiments will be continued on the Ten Boer claystone and Carboniferous shale, over- and underlying the Groningen gas field.
To be able to couple the mechanical observations made for the Groningen clay-rich rocks to petrographical characteristics, we also used quantitative XRD-analyses to characterise the cm-scale heterogeneities in their mineralogy. Obtaining a detailed understanding of the microscale variations in mineralogy and texture in these heterogeneous rocks aid in our understanding of the active deformation mechanisms, which are operating at the level of the individual grains. The results will not only improve our predictive capabilities for future surface subsidence and induced seismicity for the Groningen gas field, but will also provide crucial information for similar geological settings that may be considered for storage of natural gas, hydrogen and/or CO2.
