EXPERIMENTAL COMPACTION OF QUARTZ SAND AT LOW EFFECTIVE STRESS AND TEMPERATURE CONDITIONS

Experiments have been carried out on dry and fluid-saturated quartz sands to investigate the behaviour during compaction under diagnetic conditions and to evaluate the importance of stress-induced solution transfer ('pressure solution') as a compaction mechanism. The experiments were performed at temperatures (T) in the range 150-350-degrees-C, applied effective stresses (sigma(e)) up to 20.7 MPa and pore fluid pressures (P(f)) of 12.5 and 15.5 MPa, using material with a grain size (d) in the range 20-100-mu-m and an initial porosity of 45-52%. Dry quartz sands underwent significant compaction during the loading stage, but showed very little compaction creep once full load was achieved (i.e. essentially time-independent compaction). In contrast, fluid-saturated material at constant applied effective stress showed substantial time-dependent compaction (i.e. creep). With increasing temperature, there is a decreasing number of grains in the wet-compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains. In addition, there is a decreasing dependence of the volumetric strain rate, beta, on sigma(e) with both increasing volumetric strain (e(v)) and increasing sigma(e), and a decreasing dependence of beta on e(v) with increasing temperature. These observations suggest that, for wet quartz sand, a gradual change might occur from compaction creep controlled by time-dependent microcracking, at T = 250-300-degrees-C, to compaction creep controlled by stress-induced intergranular solution transfer at T = 300-350-degrees-C.