Influence of pore fluid salt content on compaction creep of calcite aggregates in the presence of supercritical CO2

We have performed uniaxial compaction experiments on simulated porous carbonate rocks (crushed and sieved Carrara Marble), using pore fluids of varying salinity saturated with CO2, in order to investigate the long term, chemically coupled effects of CO2 on creep of such rock materials. Experiments were performed at reservoir conditions equivalent to 3 km depth, which is up to 30 MPa effective pressure and a temperature of 80 °C, on samples with mean grain sizes of 28, 37, 50, and 106 μm. The tests were carried out using saline pore fluid solutions containing 0, 0.6, 1, 2 and 3 M NaCl or 0, 1 and 2 M MgCl2, with and without supercritical CO2 at a pressure of 10 MPa.Addition of either salt (NaCl or MgCl2) to the pore fluid in samples compacted with CO2 caused a reduction in compaction creep rate at low salt concentrations (0.6–1 M). As salt concentration was increased to values of 2–3 M, creep rates increased to approach or exceed values obtained in salt-free experiments. By comparison of our data on the compaction creep of carbonate aggregates in the presence of added salts and supercritical CO2 with microphysical models and previous work on fluid-enhanced deformation mechanisms, we suggest that the effect of increasing salt concentration in samples tested with supercritical CO2 is the net result of progressive inhibition of pressure solution and enhancement of the role of subcritical crack growth.The present results imply that storage of CO2 in highly porous carbonate formations increases creep rates by up to ten times, depending on the salt concentration. However, it remains difficult to quantify the rate of compaction creep of specific carbonate reservoirs in the context of CO2 storage, as compaction behaviour is strongly influenced by porosity and other microstructural and compositional factors, as well as by salinity.