Effects of pore fluid flow and chemistry on compaction creep of calcite by pressure solution at 150°C

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Uni-axial compaction creep experiments were performed on crushed limestone and analytical grade calcite powders at 150°C, a pore fluid pressure of 20 MPa, and effective axial stresses of 30 and 40 MPa. Previous experiments have shown that compaction under these conditions is dominated by intergranular pressure solution (IPS). The aim of the present tests was to determine the inter-relationship between pore fluid chemistry, compaction rate and the rate-controlling process of IPS. Intermittent flow-through runs conducted using CaCO3 solution showed no effect on creep rate at low strains (<4–5%) but a major acceleration at high strains (5–10%). Measurements of the Ca concentration present in fluid samples revealed the build-up of a high super-saturation of CaCO3 during compaction under zero flow conditions, especially at high strains. Active flow-through led to a drop in Ca concentration, which corresponded with creep acceleration. Addition of NaCl to the pore fluid, at a concentration of 0.5 m, increased the creep rate of the analytical grade calcite samples roughly in proportion to the enhancement of calcite solubility. Addition of Mg2+ and to the pore fluid, in concentrations of 0.05 and 0.001 m, respectively, caused major retardation of compaction creep. Integrating our mechanical, flow-through and chemical data points strongly to diffusion-controlled IPS being the dominant deformation mechanism in the calcite-water system under closed-system (zero flow) conditions at low strains (<4–5%), giving way to precipitation control at higher strains. Our fluid composition data suggest that this transition is because of accumulation of impurities in the pore fluid. As Mg2+ and phosphate ions are common in natural pore fluids, it is likely that retarded precipitation will be the rate-limiting step of IPS in carbonates in nature. To quantify diagenetic compaction and porosity-permeability reduction rates by IPS in carbonates needs to account for this.