Rate-dependent deformation behaviour of porous sandstones at reservoir conditions: An experimental, numerical and microphysical study

Extraction of fluids, like oil/gas or geothermal fluids, from the Earth’s crust (1-5 km depth) often leads to surface subsidence and tremors, such as observed in the Dutch Groningen Gas Field. Fluid production leads to reduced fluid pressure in the pores of the reservoir. Together with the weight of the overlying rock this causes the reservoir to compact slightly, resulting in surface subsidence. While reservoir compaction is often assumed to be fully reversible, a substantial portion of compaction has been shown to be permanent and may even be time-dependent, meaning it may continue temporarily even if production is stopped. To reliably evaluate reservoir behaviour after fluid extraction has stopped, it’s key to understand the physical processes that occur within the rock causing compaction. In my work, I performed laboratory experiments and numerical simulations on various porous sandstones, deformed under conditions relevant to typical gas fields, to identify these processes. Results show that after production is stopped, some limited reservoir compaction may still occur, caused by slow, time-dependent breakage of grains, coupled with grain slip along clay-coated contacts. When lateral displacements are restricted, such as is the case in the crust, compaction is further limited. These observations highlight the importance of including such subtle, grain-scale deformations in models used to assess reservoir deformation. With the energy transition becoming increasingly pressing, this is also relevant to geological storage of CO2, and energy storage (hydrogen, compressed air), which aim to curb anthropogenic CO2 emissions and provide a stable, renewable energy supply.