Caprock Threshold Pressure Measurements for Hydrogen Storage

In depleted reservoirs, the maximum operative pressure is typically set equal to the discovery pressure so that the ability of the caprock to confine natural gas underground is ensured based on the very existence of the reservoir. However, if this pressure is exceeded the storage performance can be enhanced. When fluids are stored in aquifers, the initial pressure must always be exceeded to displace the water. In these cases, along with any possible interaction between the injected gas and the brine-saturated caprock, the hydraulic integrity of the caprock is one of the main aspects that must be carefully addressed to verify the feasibility and ensure the long-term safety of the storage. This study presents a laboratory investigation aimed at determining the threshold pressure of a clay-rich caprock from an Italian gas field. In perspective, the field could be considered for hydrogen storage. Preserved brine-saturated plugs were obtained from a core retrieved from a well intersecting the caprock at a depth of about 1400 meters. The caprock characterization included grain size and mineralogical analyses using X-ray diffraction (XRD). Step-by-step threshold pressure tests were conducted using nitrogen, methane, and hydrogen to measure the breakthrough pressure, which is the pressure needed for the gas to displace the brine and flow path through the caprock. Effective permeability to gas was also measured. The threshold pressures were found to range from 20 to 35 bars, independently of the gas used during the test, reflecting the caprock ability to confine the fluids underground, including hydrogen. Furthermore, the effective permeability to gas was within the range of nano Darcies, confirming the caprock low transmissibility. These findings provide insights into caprock integrity, supporting the feasibility of converting depleted gas reservoirs into hydrogen storage sites.