Experimental compaction of quartz sand at low effective stress and temperature conditions

Abstract

Experiments have been carried out on dry and fluid-saturated quartz sands to investigate the behaviour during compaction under diagenetic 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°C, applied effective stresses (σ e ) up to 20.7 MPa and pore fluid pressures (P 1 ) of 12.5 and 15.5 MPa, using material with a grain size ( d ) in the range 20–100 μm 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, β on σ e , with both increasing volumetric strain (e v ) and increasing σ e and a decreasing dependence of β 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 °C, to compaction creep controlled by stress-induced intergranular solution transfer at T = 300–350 °C.