A new triaxial apparatus to study the mechanical and fluid flow aspects of carbon dioxide sequestration in geological formations

摘要

Climate scientists are practically unanimous in the belief that anthropogenic greenhouse gas contributions have added to the thickness and thus the effectiveness of the greenhouse gas layer, leading to a warming of the planet (IPCC, 2005 [1]). Engineers and scientists around the globe are researching and developing measures to reduce greenhouse gas emissions. These measures have included proposals to sequester carbon dioxide (CO_2) in deep geological formations (Perera et al., in press [18]). For CO_2 sequestration in deep geological reservoirs to become a feasible strategy to reduce greenhouse gas emissions, a sound understanding of the manner by which mechanical properties and permeability changes with the introduction of CO_2 to the geological reservoir will influence the stability of that reservoir is required. Thus there is a need to develop laboratory equipment capable of simulating the CO_2 injection and storage process for deep geological CO_2 sequestration under the expected in situ pressure (confinement and fluid) and temperature conditions. Triaxial experiment has been identified as the best method for this purpose (Perera et al., 2011b [19]). Therefore, we present a new high-pressure triaxial apparatus which can provide the high confining and fluid injection pressures and elevated temperatures expected for deep geological CO_2 sequestration. The new setup can be used to conduct mechanical and permeability testing on intact or fractured natural rock samples or synthetic rock samples subjected to high-pressure injection of up to three fluid phases (gas and/or liquid) at high pressures and temperatures corresponding to field conditions. The equipment is capable of delivering fluids to the sample at injection pressures of up to 50 MPa, confining pressures of up to 70 MPa and temperature up to 50 ℃ and will continuously record fluid injection and confining pressures, axial load and displacement, radial displacement and independent outflow rates for liquid and gas fluid phases (under drained conditions). Leakage tests have confirmed the effectiveness of the device at pressures up to its maximum capacities. Additionally the temperature-pressure relationship for the hydraulic oil used to apply confining pressure to the sample has been calibrated to account for the influence of changes in temperature on confining pressure. Several permeability tests (using N_2 and CO_2 as the injection fluid and 10 MPa confining pressure) and one strength test are reported for black coal samples from the Sydney Basin, New South Wales. According to the results of the permeability tests, coal mass permeability decreases with increasing effective stress for both gases. However, the permeability for N_2 gas is much higher than CO_2. Moreover, test results are consistent with matrix swelling due to the adsorption of CO_2 in coal. The strength testing results are in agreement with the results of testing carried on similar black coal samples from literature, certifying the ability for the new device to accurately measure strength and deformation properties of rock under deep ground conditions.