Frictional and transport properties of simulated faults in CO2 storage reservoirs and clay-rich caprocks

Abstract

In order to mitigate and meet CO2 emission regulations, long-term CO2 storage in hydrocarbon reservoirs is one of the most attractive large-scale options. To ensure save anthropogenic storage, it is important to maintain the sealing integrity of potential storage complexes. It is therefore particularly important to preserve the sealing integrity of any (pre-)existing faults present in these complexes, as these might start to leak due to fault re-activation upon any physical or chemical perturbation resulting from CO2 injection or exposure. In addition, reactivation of such faults might also lead to induced seismicity. Potential storage complexes may consist of quartz-rich sandstones capped by clay-rich seals. Therefore, in order to gain a better insight in the seismic risk and the risk of leakage due to geological CO2 storage, a quantitative understanding of the coupling between the chemical-mechanical effects of CO2 injection and exposure on the frictional strength and stability as well as the transport properties of faults intersecting quartz-rich sandstones and/or clay-rich caprocks is needed. In this study, the coupled chemical-mechanical effects of long-term CO2-fluid-rock interactions on the frictional and transport properties of simulated fault gouges composed of sandstone reservoir rock and carbonate-bearing clay-rich caprocks are tested. In order to do that, friction experiments are conducted at conditions similar to those of potential CO2 storage reservoirs at a depth of ~2-4 km depth. The experimental results show that simulated fault gouges derived from natural sandstones with small amounts of reactive minerals are hardly affected by long-term CO2-exposure and as a result, the frictional behaviour and transport properties of these gouges will not be affected significantly. On the other hand, the presence of >40% of carbonates in a sandstone fault gouge, e.g. occurring naturally or via precipitation due to CO2 leakage along the fault, may result in a small increase in the frictional strength and transport properties. However, this carbonate increase may also make the fault gouge become more prone to (micro)seismicity when at an in-situ temperature >100°C. When testing systematically the effect of CO2 exposure or leakage by gradually increasing the calcite concentration in simulated quartz-calcite, clay-calcite and clay-quartz-calcite mixtures a similar increase in frictional strength with increasing calcite content is observed. Moreover, these mixtures also start to become more prone to (micro)seismicity for calcite concentrations >90% in clay(-quartz)-calcite mixtures and ~50% in quartz-calcite mixtures and at in-situ temperatures of 150°C. More complex CO2-fluid-rock interactions in clay-quartz-calcite mixtures are predicted when using geochemical modelling. Friction results for multiple CO2 exposure scenarios show that the frictional strength increases with respect to the unexposed composition, while the gouges exhibited stable (aseismic) behaviour at in-situ temperature of 100°C. At a temperature of 150°C, however, the compositions may become prone to unstable (seismogenic) behaviour. Shear experiments testing the leakage potential of a simulated, naturally derived clay-quartz-calcite fault gouge composition show that the potential for CO2 leakage along a fault is higher than across a fault, but that the leakage potential decreases with increasing shear displacement of the fault (maturity) and increasing effective normal stress (depth).