The effect of CO2 on creep of wet calcite aggregates

Abstract

This paper reports uniaxial compaction creep experiments performed on porous calcite aggregates in the presence of CO2 at controlled conditions similar to those relevant for geological storage of CO2 in carbonate reservoirs. The experiments were conducted on pre-compacted calcite aggregates of various mean grain sizes in the range 1 to 250 μm, under dry and wet conditions, at temperatures of 28–100°C and applied effective stresses of 4–40 MPa. Carbon dioxide was added to wet samples at pressures up to 10 MPa. The results demonstrate that dry granular calcite shows virtually no creep, but that significant creep occurs when saturated aqueous solution is added. In wet samples, the strain rate increases with increasing grain size and applied stress. When CO2 is added from the outset, the strain rate decreases with increasing grain size up to 106 μm, and increases with grain size above 106 μm. Below 106 μm, the strain rate also increases with applied stress and strongly with CO2 (partial) pressure, but decreases with increasing temperature. The mechanical data together with microstructural evidence indicate that combined grain scale microcracking and diffusion controlled pressure solution best explain the behavior observed. Notably, in experiments where CO2 was added before loading, pressure solution dominated creep at fine grain size, giving way to subcritical cracking control at grain sizes above 106 μm. Our results point to pressure solution accelerating by up to 50 times at CO2 pressures increased from 6 to 10 MPa. Integrating our findings, we suggest that if a depleted carbonate reservoir exhibits measurable compaction creep due to diffusion-controlled pressure solution, then injection of CO2 has the potential to speed this up by amounts up to 50 times or more.