Oil- or gas production can lead to surface subsidence and earthquakes, such as seen in Europe’s largest gas field - the Groningen gas field in the Netherlands (earthquake magnitude up to 3.6). In this field, subsidence (to date, max. 36 cm) is caused by compaction of the 3 km deep, gas-bearing sandstone. In turn, compaction is caused by lowering the gas/fluid pressure in the sandstone pores and with that the back pressure to the weight of the overlying rock on the sandstone grains. Such pressure/stress changes and associated compaction can lead to centimetre-sized movements along old faults, which possibly generate earthquakes.
Understanding the compaction behaviour of the gas-bearing sandstone is therefore essential for understanding such earthquakes. It is often assumed that compaction is fully reversible (elastic), which is relatively easily described. However, compaction can also be permanent, which may imply temporary continuing compaction, even after stopping gas production in 2022. Permanent compaction is understood far less.
Simulations in the laboratory
Against this background, Ronald Pijnenburg performed laboratory experiments on sandstones from the Groningen gas field, in which he simulated gas production at subsurface temperature and pressure. The results show that 30 to 50% of compaction is permanent. Microscope analyses show that permanent compaction is largely caused by compression of micrometre-thick clay rims between sand grains. It is further shown that: a) The pressure/stresses in the sandstone and on faults are better described if permanent compaction is accounted for; b) The energy potentially available in the sandstone for earthquakes is less by 30 to 50% than would be expected if compaction were to be fully reversible; and c) Any temporary continuing sandstone compaction after stopping gas production is expected to be very limited.