Laboratory investigation of the sealing properties of the Lea Park Shale with respect to carbon dioxide

Abstract

The Intergovernmental Panel on Climate Change (2001) reports that increased anthropogenic greenhouse gas (GHG) emissions, of which carbon dioxide (CO2) is the main component, have caused the Earth’s temperature to rise. Therefore, it is necessary to find ways to reduce GHG emissions and to deal with the emissions that continue to be produced. Carbon capture and storage (CCS) is one method that is being considered to deal with GHG emissions, specifically CO2 emissions. The basic idea behind CCS is that CO2 is captured from a point source, such as a power plant, and is then transported to a storage site (e.g., an oil or gas reservoir), where it is subsequently stored.

The International Energy Agency Greenhouse Gas Programme (IEA GHG) began a CO2 geological sequestration pilot project in 2000 in Weyburn, Saskatchewan as part of an enhanced oil recovery project operatedby Cenovus (formerly EnCana) in the Weyburn Field (White et al. 2004). The research presented in this thesis evaluates the sealing potential of the Lea Park Formation in the Weyburn Field by determining its permeability and CO2 breakthrough pressure. In this context, breakthrough pressure describes the differential pressure between a wetting phase (e.g., formation brine) and a non-wetting phase (e.g., CO2) that is sufficient to enable the non-wetting phase to form a connected flow system across a given volume of porous medium (e.g., a rock sample).

A new system for measuring the permeability and CO2 breakthrough pressure of shales was developed in this research. The development effort included extensive trouble-shooting and, ultimately, the development of sample preparation and testing procedures. The new system was used to conduct permeability and CO2 breakthrough pressure tests on shale samples from the Lea Park Formation (i.e., “Lea Park shale”) and the Colorado Group (i.e., “Colorado shale”). Permeability results for samples from the Lea Park shale ranged from 14 to 35 nd (14•10-21 to 35•10-21 m2), and between eight and 46 nd (8•10-21 to 46•10-21 m2) for the Colorado shale. A CO2 breakthrough pressure for the Lea Park shale was determined to be 0.02 MPa, while values of 0.02 and 2.7 MPa were measured for the Colorado shale.

The CO2 breakthrough pressure test results indicate that the Lea Park shale will not withstand large pressures before allowing CO2 to flow through it. However, the permeabilities are extremely low; hence the rate of flow would be low. In other words, the low permeability of the Lea Park shale will be the controlling factor in terms of the rate of potential CO2 leakage through it. Calculations based on the properties measured in this research suggest that the time required for CO2 to flow from the base to the top of the Lea Park Formation would be on the order of ten thousand years. Based on diffusion coefficients published for other shales, calculations suggest that CO2 leakage via chemical diffusion would be several times slower leakage via hydraulically-driven flow.