Dissolution and compaction of natural quartz sand as functions of temperature, pore-fluid pressure, and strain

Abstract

Experimental studies were conducted using quartz sand and distilled water in a hydrotherinal flow-through system at conditions which simulate diagenesis. The flow-through system can monitor fluid chemistry and time-dependent compaction data simultaneously, and has been shown to provide accurate and precise data on mineral solubility at elevated temperatures and pressures. Experiments were conducted to evaluate the effects of temperature, pore-fluid pressure (Pp), and strain on fluid chemistry and compaction. At all temperatures, silica in the pore fluid at 34.5 MPa effective pressure (Pe) was supersaturated with respect to quartz solubility. Dissolved silica increased slightly as a function of Pp, but was consistent with the amount of increase in quartz solubility with Pp. This indicates that the "law of effective pressure" appears to hold for fluid chemistry. Time-dependent compaction was observed in all experiments. Strain rate increased exponentially with temperature from 2.14xlO-10 s-I at IOOOC to 4.43xlO8 S-1 at 225'C. Strain rate at constant temperature decreased with increasing strain, probably due to enlargement of grain contacts of the solid. For example, at 225C the strain rate dropped from 4.43xlo-8 S-1 to 2.90xlo-8 s-1 as the strain increased from 10% to 24.7%. Strain rate also decreased slightly with increasing pore-fluid pressure at constant Pe. No significant change in dissolution kinetics were found at high strains 25%) during or after loading. High Ea values (-73 kJ/mol) indicate that the mechanism is surface controlled, and is identical for both loaded and unloaded conditions. Under load, no variation was shown among kinetics studies at various pore-fluid pressures. Scanning electron microscopy and thin section analysis reveal large amounts of fines, fractured grains, and fluid inclusion trails, along with "pressure solution" features such as indented contacts. Although both chemical and mechanical processes were at work during this experiment, it is concluded that stressdependent mechanisms similar to water-film diffusion (Weyl, 1959) dominate at grain contacts at high strains.