Oil and gas production from reservoir rock strata in the deep subsurface causes vertical compaction of the reservoir, which at the surface is expressed as land subsidence. Compaction can also lead to earthquakes by causing small movements on pre-existing faults. These processes have been occurring for more than a decade in the Groningen gas field in the Northeast Netherlands, Europe’s largest field, causing major economic impact and public concern. Using unique samples drilled from the Groningen reservoir sandstone, geologists from Utrecht University investigated the processes responsible for compaction, suggesting that ultra-thin clay-films between sand grain contacts play a key role. The results are now published in the internationally renowned journal Geology.
“The gas is trapped in sandstone layers deep underground. We wanted to learn about the changes that occur in the Groningen reservoir rock during gas production, and what this can tell us about the causes of compaction, subsidence, and earthquakes”, explains geologist Bart Verberne. “To this end we analysed the internal, microscopic structure of drill core samples recovered from the reservoir in 1965, before production started, and of samples recovered for research purposes by the field operator (NAM) in 2015, when the field was almost empty.” The drill cores were recovered from the centre of the field near Loppersum, where subsidence and seismicity due to gas production has been the most severe.
Compaction in the lab
Verberne and colleagues also conducted experiments at the High Pressure and Temperature Lab at the Faculty of Geosciences in Utrecht. “In the lab, we can simulate gas production and rock compaction in Groningen”, co-author Suzanne Hangx adds. “We use specialized equipment to replicate the conditions deep underground.” The team used samples of the oldest drill core; rock that had suffered few of the effects of gas production. “By exposing the samples to high pressure and temperature, we caused artificial compaction.”
The role of clay layers
In both the microscopic studies and the lab experiments, the researchers observed that fractures occurred in potassium (K-)feldspar grains; a mineral found in the sandstone samples. “But that can’t explain the compaction, because these grains make up only a tiny proportion of the Groningen sandstone”, Hangx explains. “The majority of the rock is made up of relatively strong quartz grains. But we didn’t observe any visible effect in those.” However, the Groningen reservoir sandstone is also made up of another important component: microscopically thin layers of clay located between the quartz grains. “And when those layers of clay are compressed, this has an important effect on the rock as a whole.”
The study was part of a project financed by NAM aimed at improving our understanding of the physical processes controlling subsidence and earthquakes in Groningen. The results of the study by Verberne et al. are being used in modelling studies of the physics of compaction, subsidence and fault movement/seismicity.
Verberne, B. A., Hangx, S. J. T., Pijnenburg, R. P. J., Hamers, M. F., Drury, M. R., and Spiers, C. J. ‘Drill core from seismically active sandstone gas reservoir yields clues to internal deformation mechanisms’, Geology 49 (2021) https://doi.org/10.1130/G48243.1.