The mechanical behaviour of weak artificial rocks with bond degradation with application to CO2 geosequestration

Abstract

Sedimentary rock deposits, comprised of mudstone, siltstone, sandstone and shale, at depths of 1000 m or more have been attracting interest because of their potential to store and control the movement of sequestered CO2. From a geo-mechanical perspective, injection of CO2 into these rocks can lead to changes in stress and hence deformation, and to chemical effects that can change the nature of the rocks. This study is primarily concerned with siltstones that are often considered to be suitable as cap rocks to contain the CO2. To overcome the difficulties associated with obtaining uniform, repeatable specimens at reasonable cost the approach used in this study has been to create artificial siltstone specimens with a range of strengths and densities that are typical of natural soft rocks. These specimens have been manufactured from quartz sand, feldspar silt and gypsum. Tests have been performed to characterise the artificial siltstones and to explore the influence of chemical degradation to the bonding.

Unconfined compression tests have been performed at different loading rates to investigate the rate-dependent behaviour for specimens with different strengths, produced by varying the gypsum content. The UCS of specimens with gypsum content from 30% to 60% vary from 4.67 MPa to 10.77 MPa. The compression behaviour of the specimens has been studied by isotropic consolidation up to 45 MPa. A series of drained triaxial tests has been carried out on the artificial siltstones with effective confining pressures up to 13 MPa. These tests have enabled the effects of gypsum content and effective confining pressure on the stress-strain response of the specimens to be obtained. The results have shown elastic behaviour inside a well-defined yield locus that expands with the gypsum content, which is also associated with density increase. The failure mode changes from brittle at low confining stress to ductile at stress levels relevant to CO2 geo-sequestration. It is also shown that the specimens appear to be approaching critical states at large strains.

The chemistry of the gypsum is sensitive to temperature and pH. This has been used to investigate the effects of bond degradation on the mechanical response. Specimens of the artificial siltstone have been immersed in hydrochloric acid solutions and then subjected to standard triaxial tests and water saturated specimens have been injected with CO2, followed by shearing. Results have shown that immersion in acid can lead to significant loss of bonds and changes in void ratio reducing stiffness, strength and changing the critical state. CO2 injection caused minor changes to strength and stiffness and resulted in more brittle failure modes and large amounts of cracks.

Finally, a constitutive model based on the framework for weak rocks proposed by Gens and Nova has been developed. The ability of the model to reproduce the mechanical behaviour and the effects of chemical degradation is explored. The effect of degradation in the model is captured through a reduction in the cement content whereas experiments have shown that this only partly captures the degradation effects and more sophisticated models will be required.