3.3 Cap rock and fault integrity

Background

Gas reservoirs and aquifers, are generally sealed by both cap rocks and laterally bounding faults. Injection of CO2 will change the stress-strain field in such reservoir-seal systems, through a variety of dynamic phenomena.

These include poro-elastic effects caused by changes in pore fluid pressure, thermo-elastic effects caused by changes in pore fluid temperature, dissolution-precipitation reactions caused by changes in pore fluid chemistry, and desiccation effects caused by water uptake by the injected CO2. Buoyancy forces exerted by trapped CO2 will also influence stress-strain evolution, especially within cap rocks and faults.

These changes in stress-strain field will generally lead to vertical expansion or heave of the reservoir, deformation of the cap rock, and to vertical ground movement or surface uplift. Such effects will usually be small and benign. However, if injected CO2 pressures are too high, larger changes in stress and strain may lead to damage of the cap rock or to reactivation of pre-existing, sealed faults, thus reducing seal integrity. To evaluate how the sealing integrity of cap rocks and faults will evolve at a given storage site, these dynamic effects must be considered.

Main objectives

WP 3.3 addresses the dynamic evolution of cap rock and fault integrity via two, integrated lines of attack with the following aims.

Geomechanical evolution of the reservoir-seal system and induced deformation

The objective here is to develop numerical modelling capability allowing prediction of the stress-strain evolution in and around a generic reservoir-seal system. This will be applied to specific sites to evaluate reservoir deformation (heave vs. compaction), cap rock deformation and ground deformation at the surface, as well as the reactivation and seismic risk potential of pre-existing faults.

Sealing capacity of cap rocks, faults and coal seams

The aim of this laboratory-based task is to determine the sealing capacity and mechanical properties of site-representative cap rocks and fault rocks, and to determine how these are influenced by stress changes, by deformation and by chemical interaction with CO2 under in-situ conditions, i.e. by reactive transport.

Alongside baseline work on intact cap rocks, emphasis will be placed on the permeability, mechanical stability (slip/dilation potential) and chemical integrity (chemical degradation vs. healing) of faulted cap rock, clay-smear faults and diagenetically sealed faults. The results will be combined with the geomechanical models developed in this WP to evaluate how the sealing capacity of cap rocks and faults, and the potential for fault activation and induced seismicity, will evolve generically and at specific sites, in the short and long term.

Alongside the clay and evaporite cap rocks relevant for most prospective sites, coal seams will also be considered as a cap rock relevant to the DSM-Heerlen site.