Influence CO2-induced geochemical reactions on the oolithic limestone: a lab-investigation

This pilot study aims at gaining insights into how the physical and petrophysical properties of brine-saturated carbonate rock systems change due to CO2 injection, with implications for places where carbonate rocks – depending on their porosity and permeability - form either the cap rock or the host rock of the storage site. In particular, we are interested in the influence of CO2 -induced geochemical reactions on the mechanical integrity of an oolitic limestone being subjected to typical reservoir conditions at the laboratory scale. To achieve this target, we perform CO2 high pressure - high temperature tests on brine saturated samples, to identify changes in the fluid chemistry and the limestone mineralogy upon the completion of each test. By preparing pre- and post-treatment thin sections from regions close to the sample edges we aim at identifying possible deformation micro-processes and mineral changes that could occur due to the thermo-chemo-mechanical loading of the samples.

SEM imaging on the oolitic limestone before the high pressure-high temperature tests indicated that this is a low porosity material. Calcite cement grew into the pore space from the ooids’ margin, as a fine-grained radially orientated cement phase, followed by a coarser pore occluding cement. However, calcite was not everywhere well-attached to the ooids. Moreover, micro-porosity and pressure solution were identified within some of the ooids and at their edges, respectively.

On the one hand, grain-scale investigations based on thin section observations (SEM imaging) on a brine-saturated limestone sample exposed to 230 bar CO2 pressure and to 37.5o C (i.e. supercritical conditions) for a total time of 2 weeks, revealed no obvious fractures linked with the thermo-chemo-mechanical loading of the sample. Micro-porosity in a few ooids together with occasional big voids in cement between ooids were identified locally in the post-deformation/treatment sample; however, similar observations were also made in the pre-treatment sample. Moreover, a few sporadic fractures along ooids were identified in both pre- and post-deformation samples. All micro-scale observations were made in regions 1-3 mm from the sample edges, indicating no actual grain-scale deformation. On the other hand, the increase in concentration of calcium ions in the post-treatment brine was linked to dissolution of the limestone, which possibly took place along the surface of the sample. Thus, we wish to argue that under the current test conditions (230 bar, 37.5o C, 2 weeks exposure) CO2 does not much damage this repository. We are currently performing further research on different conditions (particularly long exposure and higher brine -rock rations that can be linked with faster dissolution of calcite) in order to understand the dominant CO2 sequestration mechanism in oolitic limestone-containing reservoirs as well as the effects of CO2 injection on their mechanical integrity.

This pilot project has been funded by the Heriot-Watt Energy Academy 2015 Fledge Award.