Geologic carbon storage is required for achieving negative CO2 emissions to deal with the climate crisis. The classical concept of CO2 storage consists in injecting CO2 in geological formations at depths greater than 800 m, where CO2 becomes a dense fluid, minimizing storage volume. Yet, CO2 has a density lower than the resident brine and tends to float, challenging the widespread deployment of geologic carbon storage. Here, we propose for the first time to store CO2 in supercritical reservoirs to reduce the buoyancy-driven leakage risk. Supercritical reservoirs are found at drilling-reachable depth in volcanic areas, where high pressure (p>21.8 MPa) and temperature (T>374 ºC) imply CO2 is denser than water. We estimate that a CO2 storage capacity in the range of 50-500 Mt yr-1 could be achieved for every 100 injection wells. Carbon storage in supercritical reservoirs is an appealing alternative to the traditional approach.

With the urgent necessity of geo-energy resources to achieve carbon neutrality, fluid injection and production in the fractured media will significantly increase. Applications such as enhanced geothermal systems, geologic carbon storage, and subsurface energy storage involve pressure, temperature, and stress changes that affect fracture stability and may induce microseismicity. To eventually have the ability to control induced seismicity, it is first necessary to understand its triggering mechanisms. To this end, we perform coupled thermo-hydro-mechanical (THM) simulations of cold water injection and production into a rock containing two fracture sets perpendicular between them. The permeability of fractures being four orders of magnitude higher than the one of the rock matrix leads to preferential pressure and cooling advancement, which induce stress changes that affect fracture stability. We find that the fracture set that is oriented favorably to undergo shear slip in the considered stress regime becomes critically stressed, inducing microseismicity. In contrast, the fracture set that is not favorably oriented for shear remains stable. These results contrast with those obtained for an equivalent porous media that does not explicitly include fractures in the model, which fails to reproduce the direction-dependent stability of fractures present in the subsurface. We contend that fractures should be directly embedded in the numerical models when inhomogeneities are of the spatial scale of the reservoir to enable reproducing the THM coupled processes that may lead to induced microseismicity.

The Underground Gas Storage (UGS) project of Castor, Spain, was strategically conceived to guarantee the Spanish gas demand during 50 days. Yet, the project did not enter into operation because it was cancelled as a result of a sequence of felt earthquakes induced after cushion gas injection. The project cancellation implied an investment compensation to the operating company that may cost up to 4.73 billion euros to Spanish citizens. The sequence contained the three largest earthquakes (M4.08, M4.01 and M3.97) ever induced by any of the more than 640 UGS facilities around the world. The largest earthquakes were induced some 20 days after stopping injection, which lasted for 15 days. The focal depth of these earthquakes was between 4 to 10 km, far deeper than the 1.7 km injection depth. To understand the causes of this induced seismicity, we have performed coupled two-phase flow and geomechanical numerical simulations and we have employed Okada’s solution to analyze the shear slip stress transfer. We analyzed four seismicity-inducing mechanisms (pore pressure build-up, stress transfer, destabilizing buoyancy, and recovery of pressure drops that ensure transient stability in regions that are mechanically destabilized after a microseism). We found that the onset of seismicity was induced by gas injection, which reactivated the critically stressed Amposta fault through pore pressure buildup and buoyancy. The Amposta fault, a mature fault bounding the storage formation, crept, accumulating aseismic slip. Destabilization of the fault continued even after the stop of injection because of the permanent effect of buoyancy caused by the low density of the injected gas. The progressive accumulation of slip perturbed the stress around the rupture area of the Amposta fault and eventually reactivated a critically stressed unmapped fault located in the crystalline basement. Once this deep fault was reactivated, the sequence of earthquakes was induced by shear slip stress transfer, with transient slip-driven pore pressure changes likely controlling the delay between earthquakes. We contend that an analysis of fault stability prior to gas injection would have identified the high risk of inducing seismicity at Castor.