A new triaxial apparatus to study the mechanical and fluid flow aspects of carbon dioxide sequestration in geological formations
Highlights
► Triaxial apparatus for high confining and injection pressures for CO2 sequestration. ► To conduct mechanical and permeability tests on intact or fractured rock samples. ► Delivering fluids at up to 50 MPa injecting and 70 MPa confining pressures. ► Record injection and confining pressures, axial load and displacement and outflow rates. ► Leakage and sample tests have confirmed the effectiveness of the device.
Introduction
Triaxial tests are widely used in geomechanics to investigate the mechanical and fluid flow behaviour of soils and rocks under different confining stresses. Triaxial tests are of vital importance for rock characterisation and to provide the input parameters required for numerical modelling studies, and the development of suitable constitutive models [19]. To date much research have been conducted using triaxial experiments to study the coupled behaviour of fluid (e.g. CO2, CH4, N2, He, water, brine) migration during mechanical testing of various rock types (e.g. sandstone, mudstone, coal) and to examine their flow and strength behaviour (e.g. [20], [7], [15], [21], [9], [22]).
The advantage offered in triaxial testing lies in its ability to simulate in situ conditions (stress and pressure) at different depths. Therefore, triaxial testing is often used to investigate ground behaviour in deep geotechnical applications, such as construction of deep underground transport tunnels in mountainous areas, stability of deep underground mines and large open pit mines, deep underground disposal of nuclear wastes and sequestration of greenhouse gases in deep geological reservoirs [3]. To date most triaxial devices capable of simulating coupled fluid flow and mechanical behaviour of rocks in underground applications have been developed for low to medium confining pressure conditions. Among the various studies that have used triaxial experiments, Somerton [20], Durucan and Edwards [7], Viete and Ranjith [21] and Gentzis et al. [9] conducted triaxial experiments under maximum confining pressures of 18, 8, 10 and 14 MPa, respectively. Testing of various CO2 geological sequestration methods is important as the CO2 is the most dangerous green house gas in terms of its contribution to increase the radioactive forces [1]. Deep geological sequestration of CO2 involves injection and storage of CO2 in geological media at depths up to several kilometers below the ground surface. Thus, simulation of deep geological CO2 sequestration requires a triaxial device capable of simulating significantly higher in situ stresses and pressures. Barla et al. [3] introduced a new triaxial apparatus capable of investigating mechanical properties of rock at confining pressures up to 64 MPa. However, their setup was not designed to consider the mechanical influence of fluid flow within rock, and therefore is not suitable for investigation of deep geological CO2 sequestration. To achieve this, the triaxial device requires the ability to inject gas or liquid at high pressures relevant to deep geological CO2 sequestration. Additionally, the natural crustal geothermal gradient dictates that rock temperature increases with depth. Simulation of CO2 sequestration in deep geological formations requires the capability to apply testing temperatures that are elevated (above the supercritical temperature of CO2 of 31.8 °C) with respect to the normal temperatures of the laboratory [18]. An appropriate heating system is required to simulate ground conditions in deep geological CO2 sequestration.
This manuscript introduces a new triaxial apparatus developed and housed at the Department of Civil Engineering Laboratories, Monash University. The device was developed with the capacity to carry out mechanical and fluid flow (gas and/or liquid) testing at high confining pressures and injection pressures and under conditions of elevated temperature (from those of the laboratory). The design caters for experimental simulation studies of fluid flow and mechanical behaviour of geological reservoirs during injection and storage of supercritical CO2 and has the capacity to simulate the range of stresses, injection pressures and temperatures expected for deep geological CO2 sequestration.
Section snippets
Description of the high-pressure triaxial setup
The triaxial apparatus was designed to investigate the mechanical properties and permeability of rocks under in situ conditions. The setup allows continuous monitoring of lateral stress and displacement, axial stress and displacement, fluid injection and outlet flow rates and water saturation in fluids at the fluid injection and outflow points. The setup has been designed to simulate injection and flow of supercritical CO2 within deep sequestration reservoirs and thus incorporates a hydraulic
Type of testing that can be performed using the new triaxial apparatus
The new triaxial apparatus can be used to conduct permeability and strength tests for 38 mm diameter cylindrical samples consisting of rock (i.e. coal, sandstone, and mudstone) or synthetic materials analogous to rock (i.e. cement, reconstituted coal). Samples can be intact or fractured. Tests can be carried out at confining pressures up to 70 MPa to simulate ground pressures to depths in excess of 2.5 km. However, there is a temperature limit of 50 °C and temperature at such depth of 2.5 km could
Initial checks for the triaxial apparatus
To confirm that the triaxial setup was working properly, several initial checks were performed for the apparatus prior to testing. Leakage tests were carried out for the pressure cell and gas and water lines. Leakage tests for the pressure cell involved testing for stability in oil pressures for a period of 1 h using pressures at 5 MPa increments from 0 to 50 MPa. The results of the leakage test are shown in Fig. 3a, which confirms stability of pressure for the pressure cell at pressures up to 50
Example tests
A series of permeability tests and one strength test were conducted using the newly developed and tested triaxial set up. Tests were performed on New South Wales black coal samples obtained from the Sydney basin. The initial water content of the coal samples was approximately 3.2%.
Conclusions
This paper describes a new high-pressure triaxial apparatus developed at the Department of Civil Engineering, Monash University. The new setup consists of four main components: (1) the pressure cell; (2) the loading unit; (3) the plumbing system, and (4) the measurement system. The novel aspect of this new device is the nature of the complex plumbing system, which is capable of delivering single- or multi-phase fluids to the sample, at high injection pressures, during the loading process. The
Acknowledgments
This work was supported by the Australian Research Council Discovery Grant Scheme. The authors would like to thank Mr. Christopher Powell, Mr. Long Kim Goh and Mr. Mike Leach of the Department of Civil Engineering at Monash University for their continuous support during design, construction and modification of the triaxial set up and the various other technical staff from the Civil Engineering Laboratories at Monash University who also assisted in the setup.
References (22)
- et al.
Swelling of moist coal in carbon dioxide and methane
Int J Coal Geol
(2011) - et al.
Effects of stress and fracture on permeability of coal
Mining Sci Technol
(1986) - et al.
Evaluating geological sequestration of CO2 in bituminous coals: the southern Sydney Basin, Australia as a natural analogue
Int J Greenhouse Gas Control
(2007) - et al.
Geomechanical properties and permeability of coals from the Foothills and Mountain regions of western Canada
Int J Coal Geol
(2007) - et al.
Carbon dioxide gas permeability of coal core samples and estimation of fracture aperture width
Int J Coal Geol
(2010) - et al.
Coal swelling strain and permeability change with injecting liquid/supercritical CO2 and N2 at stress-constrained conditions
Int J Coal Geol
(2011) The effects of dissolved CO2 on coal structure and properties
Int J Coal Geol
(2004)- et al.
Numerical simulation of gas flow through porous sandstone and its experimental validation
Fuel
(2011) - et al.
The effect of CO2 on the geomechanical and permeability behaviour of brown coal: implications for coal seam CO2 sequestration
Int J Coal Geol
(2006) - Hoogwijk Monique, editor. WMO and UNEP Intergovernmental Panel on Climate Change. IPCC expert meeting on emission...