Laboratory Testing to Evaluate the Effectiveness of Caprocks for CO2 Storage

Abstract

The increased anthropogenic emission of carbon dioxide (CO2) is a serious concern due to its effects on global climate change. Capture of CO2 from point sources and storage in the porous rocks of deep saline reservoirs is considered a practical choice for reducing CO2 emissions into the atmosphere. A fundamental necessity for long-term storage of CO2 in saline reservoirs is the integrity of non-permeable rock called the caprock. The caprock overlies the porous saline reservoir and prevents migration of CO2 upwards out of the reservoir. As such, the primary focus of this research is to test the effectiveness of caprock as a seal or barrier to fluid migration under representative conditions of a geological storage site. Additionally, a secondary focus is the assessment of the rock thermal properties and geomechanical modeling of CO2 injection. This is because thermally induced stress changes resulting from the injection of relatively cold CO2 can lead to the creation of leakage paths (e.g., formation fracturing or reactivation of existing discontinuities). The study area of this research is the proposed CO2 injection location of Project Pioneer (TransAlta) in Alberta. The caprock and storage reservoir in the study area are the Calmar and Nisku Formations, respectively.

An experimental setup was developed to measure the permeability of intact and fractured caprock samples exposed to CO2-rich brine under representative temperature and pressure conditions of the injection site. The objective of determining the permeability of fractured caprock samples exposed to CO2-rich brine was to examine how caprock seal effectiveness may evolve over time in the presence of fractures which either preexisted or are generated during CO2 injection. The outcomes were used to assess the collective effects of chemical and physical processes that could lead to caprock leakage.

Geomechanical modeling was conducted in this work to investigate injection-induced stress changes and to see whether their effects (deformation) within the storage reservoir are observed on the ground surface. The models incorporate the thermal, geomechanical and geometrical parameters of the saline reservoir and surrounding rocks.

The permeability of intact samples of the Calmar Formation (primary seal/caprock) were measured as 0.3 nd (0.3·10-21 m2), and measured permeability of fractured caprock samples ranged between 10 to 40 µd (10·10-18 to 40·10-18 m2). The intact rock permeability is very low, hence, the rate of leakage would be very low (7.4·10-7 m3s-1). Potential leakage rates could be up to four orders of magnitude higher if the caprock is fractured from base to top. The geomechanical deformation model predicted that CO2 injection in the Nisku zone is not likely to cause any significant surface heave (< 2 mm), and it likely too small to be measured effectively using standard surface deformation monitoring techniques. Numerical modeling conducted by a research collaborator using results generated in this research suggests that thermally induced fracturing may occur at the study site if the injection rate is not carefully chosen.