Elsevier

Fuel

Volume 90, Issue 8, August 2011, Pages 2751-2759
Fuel

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

https://doi.org/10.1016/j.fuel.2011.04.004Get rights and content

Abstract

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 (CO2) in deep geological formations (Perera et al., in press [18]). For CO2 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 CO2 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 CO2 injection and storage process for deep geological CO2 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 CO2 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 °C 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 N2 and CO2 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 N2 gas is much higher than CO2. Moreover, test results are consistent with matrix swelling due to the adsorption of CO2 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.

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)

  • S. Arafin et al.

    Equation of state of crude oil through temperature dependent ultrasonic measurements

    J Geophys Eng

    (2006)
  • Cited by (0)

    View full text