Injecting carbon dioxide (CO2) into deep geological formations is recognized worldwide as the only realistic mitigation technology that can reduce current anthropogenic CO2 emissions to meet national targets by 2050. However, Carbon Capture and Storage (CCS) have aroused public concerns over potential surface leakage of CO2 from geological reservoirs, limiting the number of potential storage sites. European countries, including the UK, have considered depleted oil and gas fields and saline aquifers for CO2 storage (e.g. Sleipner field, North Sea), while a number of projects in the United States have focused on CO2 injection for enhanced oil recovery (EOR) in depleted or unconventional hydrocarbon reservoirs.
Geological reservoirs are complex systems. Their geomechanical integrity can be affected by CO2-fluid-rock interactions following injection of CO2, which can be quantified in terms of Thermal, Hydrological, Mechanical and Chemical coupled phenomena (THMCs). THMCs induced by CO2 injection can lead to detrimental enhanced seismicity and CO2 leakage to the surface. So, the advent of CCS and EOR-CCS operations has triggered the need for more research to preserve the geomechanical integrity of reservoirs during the whole CO2 storage cycle (lasting 100s years). To date, researchers have focused on the induced changes in the physical properties of the reservoir during CO2 injection (transition from brine-bearing to CO2-bearing formations) and associated overpressure effects. It is generally assumed that following the interruption of CO2 injection, the reservoir pressure decreases, the CO2 plume migrates and natural imbibition leads to aquifer recharge. However, CO2 injection is a drying process that triggers complex salt precipitation phenomena in brine saturated formations. Several studies have focused on the risks associated with porosity and permeability reduction with respect to injection efficiency and storage capacity. A less appreciated fact is that salt crystals growing under confinement have the potential to damage the rock by exerting enormous pressures (haloclasty). After ceasing the CO2 injection, the aquifer recharge leads to salt dissolution and reservoir compaction.
The hypothesized reversibility of salt precipitation in CO2 storage contexts has yet to be investigated. Which phenomena do we expect to affect reservoir integrity during the natural aquifer recharge post-CO2 injection? Can we control them? The energy industry is transforming as we move to a lower carbon world; CCS and EOR-CCS are becoming essential practices for the oil and gas industry, a vital sector for the UK economy. Addressing these questions is crucial for the safe CCS operation at the scales needed to mitigate greenhouse gas emissions for the UK, and at the same time improving recovery rates from hydrocarbon reservoirs (e.g., UK North Sea fields). This project seeks to address the UK Industrial Strategy's clean energy agenda by reducing CCS risks, and providing a possible new EOR method.
Geomechanical Assessment of CO2 Storage Reservoir Integrity Post-closure (GASRIP) is a project primarily designed to study how CO2-brine induced-salt precipitation/dissolution affects geomechanical integrity of CO2 storage reservoirs. By looking at changes in the elastic, mechanical and transport properties of natural sandstones in the laboratory, GASRIP will assess variations in the mechanical properties in saline siliciclastic reservoirs post-CO2 injection. By analysing carbonate-rich sandstones, GASRIP will determine the mechanical and chemical post-CO2 injection effects on chemically reactive reservoirs. This information is needed for the potential use of salt precipitation in a controlled manner to improve the transport properties and the viable production of oil and gas from tight reservoirs (EOR alternative). By integrating the results in a numerical model, GASRIP will offer a valuable tool for risk assessment of CO2 storage reservoirs post-closure.
