This paper presents a study of heterogeneous caprock fracturing processes during underground CO2 injection into a brine aquifer. We simulate CO2 injection and fracturing using a combination of theTOUGH2 multiphase flow simulator and the RDCA rock discontinuous cellular automaton code. Our analysis shows that fluid pressure evolution,CO2 saturation, fracture opening, propagation and fracturing path are strongly dependent on the heterogeneity of caprock. The obvious risk that the fracture propagates upwards to provide a new flow path toward shallow ground water aquifers or released to the atmosphere and thereby reducing the efficiency of the CO2 sequestration and potentially contaminating groundwater resources is demonstrated.
The caprock mechanical integrity associated with deep underground injection of CO2 is very important since the caprock is a natural barrier to prevent CO2 from migrating upwards towards shallow potable ground water or ground surface. When CO2 is injected into a brine aquifer, the reservoir pore pressure increases and creates loading and straining of the overlying caprock. If there is an initial fracture in the caprock, the fluid pressure gradient induced byCO2 injection could more easily lead to fracturing of the caprock.
Hydraulic fracturing is widely used in reservoir engineering applications such as the exploration and development of hydrocarbons or geothermal reservoirs (Murphy et al. 1981; Mandl and Harkness 1987; Legarth et al. 2005), estimation of in situ stress in rock masses (Haimson and Fairhurst 1969; Bredehoeft et al. 1976), and for deep underground injection disposal of hazardous liquid and solid wastes (Dusseault et al. 1996). In the case of underground CO2 injection, different geological formations in the caprock may have great influence on the potential for fracturing. Furthermore, this influence is dynamic, i.e. the fracture may propagate upwards due to the increase of fluid pressure and CO2-in-brine buoyancy effects. Although many studies have been conducted to consider the influence of geological formations on the caprock integrity (Rutqvist et al. 2010; Morris et al. 2011; Rutqvist 2012), this study is focused on such influence on potential caprock fracturing.