Reservoir compaction during and after fluid production: A case study of the Groningen Gas Field

Prolonged hydrocarbon production often leads to subsidence and seismicity in offshore and onshore hydrocarbon fields. In the Netherlands, tens of centimetres of subsidence has occurred above the Groningen Gas Field, with widespread induced seismicity during the 60+ years of its lifetime. These phenomena are driven by reservoir compaction at depth, resulting from gas extraction. Modelling the reversible, elastic component of compaction is straightforward. However, permanent deformation can also occur, the rate and effects of which are very poorly constrained. Furthermore, from smaller fields in the vicinity, it has already become clear that compaction may continue even now that production has stopped in 2023. To be able to confidently forecast the long-term surface impact of fluid production, for fields such as Groningen, and many other fields around the world, models are required that include the physical mechanisms responsible for reservoir compaction. These mechanisms are still poorly known and quantified at true reservoir conditions. Combining microstructural observations, obtained from field material and experimental work, and novel experimental mechanical data, obtained at simulated stress changes relevant to the reservoir, enabled us to identify the main grain-scale deformation mechanisms operating in the reservoir sandstone of the Groningen Gas Field. A key role is played by the thin intragranular clay layers present between the quartz grains making up the load-bearing framework. Compaction of and slip along these thin clay films has accommodated the permanent deformation accumulated during the production stage. After production is halted, experiments suggest that slow, time-dependent grain breakage will start to play a role as well. Microphysical models describing rate-insensitive compaction were implemented in Discrete Element models to assess sandstone compaction behaviour at the cm-dm scale. These numerical models can be used to evaluate reservoir compaction in different locations on the field due to pressure equilibration or repressurisation, with rate-sensitive mechanisms, such as stress corrosion cracking, to be added at a later stage, as their descriptions are still be developed. Eventually such small-scale numerical models should form the basis to upscale the sandstone behaviour to the reservoir scale.