Humbly Grove gas storage – does the Earth move?

Substantial energy storage capacity, yet to be developed, will be required if the UK is to continue to decarbonize power generation, transport and heating. A variety of storage methods could be used, but given the scale of the challenge it is likely that much of the storage of chemical energy, (potential) kinetic energy, and heat will be in the geological rock formations beneath our feet. Such geostorage can include combustible gases (methane and hydrogen), non-combustible gas mixtures (compressed air), and heat storing fluids (almost exclusively water).

Here we report on an extensive study on the Humbly Grove gas storage site in rural Hampshire in southern England, particularly regarding elevation changes to the Earth’s surface above the storage site. We instrumented and monitored the site for 15 months from March 2016, and used legacy ‘earth-observation’ data over a much longer period, covering the past 35 years , to try to understand the impact of fluid pressure changes due to fluid production and injection on the overburden at the Humbly Grove site.

Humbly Grove began life as an oil discovery in 1982. For some time oil, gas and some associated water were produced, and water was injected to maintain the pressure close to pre-production levels. However, the field underperformed because of the low permeability of the reservoirs. It was thus turned over to gas storage. Methane is purchased and injected when prices are low, and produced and sold when prices are elevated. The field has operated this way since the early part of this millennium.

Legacy InSAR satellite data from the late 1990s to the early 2000s demonstrated that the area above the southern and eastern margins of the field had subsided by about 10cm over a four year period. The subsidence coincided with the pressure blowdown of the field associated with preparation for gas storage. The field operators were concerned that gas cycling (injection and production) during the current gas storage phase of the field might cause similar fluctuations of the land surface.

High fidelity monitoring using InSAR, GNSS and surface LIDAR of the field from March 2016 until mid-2017 demonstrated that no significant surface movement had occurred during a complete ‘full-empty-full’ storage cycle.

Samples of reservoir and overburden were tested under triaxial stress conditions replicating both surface and reservoir-depth confining pressures. These experiments demonstrated that the carbonate reservoir deformed plastically when subject to the pressure change associated with the gas blowdown phase and elastically under the rather narrower pressure fluctuations associated with gas storage operating conditions.

Examination of earthquake records revealed no earthquakes associated with the blowdown but one earthquake nearby in 1982 before any oil or gas production began. Multiple modeling runs were performed to try to simulate the observed surface deformation using the rock properties data obtained from the experimental work. The results of the modeling were consistent with surface deformation occurring by aseismic slip on two bounding faults in response to the pressure drop during blowdown.

The observations form this study indicate that geostorage of fluids for energy storage purposes can lead to surface deformation, and as a consequence locations sought for geostorage need to be thoroughly characterized before storage sites are installed and extensively monitored once commissioned.