Compaction of granular calcite by pressure solution at room temperature and effects of pore fluid chemistry

Pressure solution is an efficient compaction mechanism leading to porosity/permeability reduction in both clastic and carbonate rocks. We performed compaction experiments on crushed limestone and granular calcite at room temperature and 1-4 MPa effective stress using various pore fluids. By applying a dry pre-compaction at 8 MPa, we could largely eliminate deformation mechanisms such as grain rearrangement, sliding and fracturing from runs employing a pore fluid. Compaction creep occurred only in samples filled with a saturated carbonate solution, in contrast to those filled with chemically inert fluids, at ε˙=10-6-10-9s-1. The measured strain rates increased with decreasing grain size and with increasing effective stress according to power laws with exponents of about 3 and 1-2, respectively. Samples made from naturally oil-contaminated limestone showed far less compaction than oil-free samples. Addition of dissolution and precipitation inhibitors (magnesium and phosphate ions) to the pore fluid significantly decreased compaction rates of wet samples, indicating that creep must have involved calcite dissolution/precipitation. Pore fluid with a salinity (NaCl) of 0.1-0.5 mole/l increased the compaction strain rate compared to non-saline solutions. The experimental results suggest the main deformation mechanism in calcite under our experimental conditions was pressure solution and that diffusion is likely to be the rate-controlling process in pure solution system.