Compaction of reservoir sandstones: The cause of induced seismicity in the Groningen and other gas fields?

Subsurface exploitation of the Earth’s natural resources, such as oil, gas and groundwater, removes the natural system from its chemical and physical equilibrium. With global energy and water demand increasing rapidly, while availability diminishes, densely populated areas are becoming increasingly targeted for exploitation. Indeed, the impact of our geo-resources needs on the environment has already become noticeable. Deep groundwater pumping has led to significant surface subsidence in urban areas such as Venice and Bangkok. Hydrocarbons production has also led to subsidence and seismicity in offshore and onshore hydrocarbon fields. Here in the Netherlands, tens of centimetres of subsidence occurring above the gasfields of Groningen and Friesland, and associated induced seismicity, are key issues in the news.

Fluid extraction inevitably leads to (poro)elastic compaction of reservoirs. However, associated subsidence and seismicity often exceed what is expected from purely elastic reservoir behaviour and may continue long after exploitation has ceased or show other time-lag effects in relation to changes in production rates. One of the main hypotheses advanced to explain this is time-dependent compaction, or ‘creep deformation’, of such reservoirs, driven by the reduction in pore fluid pressure compared with the vertical rock overburden pressure.

In this contribution, we will consider what processes may control the progress of compaction in gas reservoirs such as Groningen, and how this can lead to induced seismicity. Time-independent compaction of sands and sandstones by brittle grain failure and intergranular sliding is fairly well-understood in terms of grain contact mechanics coupled with equilibrium crack extension criteria. However, the deformation mechanisms controlling the creep behaviour of sediments, such as sands and sandstones, are poorly known and poorly quantified. Indeed, time-dependent growth of subcritically stressed cracks, leading to time-dependent grain failure and hence creep, has barely been addressed. In addition, thermally-activated mass transfer processes, like pressure solution, can cause creep via dissolution of material at stressed grain contacts, grain-boundary diffusion and precipitation on pore walls, and can be accelerated by brittle grain failure. These, and other processes such as intergranular cement failure or dissolution, may operate on the grain scale, but also at the micro-asperity scale within grain boundaries. Attempts to evaluate which of these processes are the most important by means of laboratory experiments are revealing that their rates are strongly affected by fluid-rock interaction, i.e. by pore fluid pH and chemical composition. Interestingly, these effects, along with microstructural and acoustic or microseismic emission studies, offer a new way of probing the mechanisms that control compaction of gas and other reservoirs, and hence of arriving at mechanism-based models for compaction creep and its role in controlling subsidence and associated seismicity.