IMPACT of fault rock properties on CO2 storage in sandstone reservoirs

Research on geological storage of CO2 is relatively young, although the technology has matured over the past decade from feasibility to practical storage experiences (e.g. Shipton et al., 2009; Bergmo et al., 2009; Hermanrud et al., 2009). CO2 can be safely stored where three geological characteristics are fulfilled (Bachu, 2008): 1) storage capacity of the reservoir in question, 2) injection capacity for CO2 in relation to its injection rate, 3) confinement to prevent migration and leakage away from the storage space.

The storage capacity of a reservoir is determined by five factors: The formation thickness, the area of the storage site, rock porosity and permeability, CO2 density and the storage efficiency (Cooper, 2009). Within sandstone reservoirs, deformation bands and faults may act as barriers, introduce compartmentalization and hence reduce the injection rate and the total capacity of the reservoir or compartments. CO2 injection in aquifers creates a fluid pressure increase, which again leads to changes in the stress state of the aquifer and the sealing rocks above and below. This might affect and reactivate faults both within and around the reservoir. (Li et al., 2007)

Although previous studies have increased our understanding of the impact of tectonic structures on fluid flow in porous reservoirs (e.g. Antonellini and Aydin, 1994; Jourde et al., 2002, Lothe et al., 2002; Shipton et al., 2002; Sternlof et al., 2004; Torabi et al., 2007), important aspects remain to be explored. Understanding the process of strain localization and faulting at laboratory scale, can increase our knowledge about formation and development of large faults and their associated microstructure (e.g. Wong et al., 1997; Bésuelle, 2001; Olsson and Holcomb, 2002; Rudnicki, 2002; Holcomb and Olsson, 2003; Digiovanni et al., 2007; Haimson and Klaetsch, 2007, Holcomb et al., 2007).

Bifurcation analysis provides an excellent tool for understanding the onset of strain localization as a failure mode of brittle rocks (Rudnicki and Rice, 1975). The bifurcation analysis also allows specifying the strain type within a localization band. There exists a continuous evolution from pure extension bands to pure compaction bands via dilating and compacting shear bands, with respect to the constitutive parameters of the law (Besuelle, 2004).

However, the quantitative comparison between localization theory and experiment is not straightforward (Paterson and Wong, 2005). At present, there is a need for additional field examples of strain localization within sandstone and at the same time for a better understanding of the conditions that gives rise to different types of deformation bands in the laboratory. What controls localization? Where would we expect localization to occur? What controls the number, width and final spatial distribution of deformation bands? When, with respect to strain and time, does the major fault form in the damaged sandstone, and where will it be located? How will petrophysical properties of sandstone change during the process of faulting and how does this affect the storage capacity of the reservoir and the process of CO2 injection? What about the effect of other lithologies and cement, how would they influence faulting and deformation within a sequence of clastic rocks? These are the fundamental questions that will be addressed within the project.