Facilitating sustainable geo-resources exploitation: A review of environmental and geological risks of fluid injection into hydrocarbon reservoirs

Natural gas is an important low-carbon geo-resource for sustaining future energy demand. However, production is currently impeded by the negative effects of reservoir compaction, i.e. induced seismicity and surface subsidence. Fluid injection into producing or depleted hydrocarbon reservoirs is one of the strategies to mitigate compaction, though it may introduce other negative consequences. This study aims to identify lessons and potential knowledge gaps on the causes and mechanisms of consequences of such injection operations. An overview of the environmental and geological hazards and risks is developed by examining literature on four commonly injected fluids, i.e. CO2, methane, nitrogen and wastewater. The well-recognised hazards are leakage, reservoir deformation and induced seismicity, which have consequences for several environmental receptors, e.g. the atmosphere, surface sediments and water, subsurface resources and groundwater. Generally, in defining the risk, there is a consensus on the probability of hazards occurrence, while a lack of knowledge on the hazard impacts exists. The assessment approaches analysis also indicates that consequence magnitude evaluations and comparisons to thresholds are often missing from the risk assessments. For all examined injection fluids, knowledge on hazard occurrence, hazard exposure and receptor affectability is insufficient. Furthermore, in complex subsurface systems with high uncertainty, more insight in the probability of multiple hazards occurrence and the corresponding cumulative risks is needed.