Experimental and theoretical modeling of creep compaction of quartz sand: Rate laws and evolution of porosity and fluid chemistry

Experiments were conducted in flow-through reactors to investigate the creep compaction and quartz-water interactions under hydrostatic conditions at 150°C and 34.5MPa of effective pressure (pore pressure P_p = 0, 0.14 MPa, and 11.7 MPa). The starting materials are St. Peter quartz sand (90–124μm, 124–180μm, and 250–350μm) and disaggregated novaculite (35μm). Theoretical models were developed to investigate (1) the relationships between grain convergence rate and pore-fluid flow, and (2) the evolution of porosity and fluid chemistry, during creep compaction of quartz sand by intergranular pressure solution.; Experiments indicate that the presence of water increases the rate of compaction. Creep compaction rates decay exponentially with strain most rapidly under dry conditions, less rapidly under vapor-dominated conditions, and slowest for water-dominated conditions. Strain rate varies inversely in power law with grain size raised to power 1.5–2.5 in water-dominated conditions.; Experiments indicate that the creep compaction rate of quartz aggregates depends on fluid flow rate. Theoretical modeling shows that an increase in flow rate leads to higher removal rate of dissolved materials from the system, lower saturation state, and more rapid grain convergence. Theoretical modeling also indicates that grain convergence rate is nonlinearly related to flow velocity, and also varies with intergranular dissolution rate, strain, grain size and grain boundary properties.; Theoretical models indicate that porosity loss is nonlinearly related to strain. Porosity loss also depends on grain packing arrangements, stress states, and the saturation states of solution. At early stages of compaction porosity loss is dominated by intergranular pressure solution, but with increasing compaction, cementation becomes increasingly important. Isotropic compaction leads to more porosity loss by pressure solution, but less porosity loss by cementation, than uniaxial compaction.; Both experiments and numerical models indicate that pore fluids are supersaturated at early stages of creep compaction and gradually become saturated with increasing compaction. Accordingly, pressure solution should be rate-limited by precipitation kinetics at the early stages of compaction but switch to rate-limited by grain-boundary diffusion or dissolution at grain contacts with increasing compaction. The decrease in silica concentration probably results from the lessening in pressure solution and in creation and dissolution of ultrafines and high-energy surfaces.