Time-dependent deformation in unconsolidated reservoir sands

This dissertation discusses observations, implications, and micromechanical mechanisms of time-dependent deformation in dry reservoir sands from the Wilmington Field, Long Beach, California, and the South Eugene Island Field, Gulf of Mexico, with grain sizes between 10 to 300 {dollar}mu{dollar}m under elevated pressures. Experimental results obtained in a triaxial loading system at confining pressures of 20-30 MPa and axial stresses of 40 MPa provide evidence of viscoelasticity as indicated by time-dependent deformation and ultrasonic pulse transmission data. Creep strain data suggests that dry sand deformed under high pressures has a fundamental creep and relaxation time constant. These results may provide a mechanism that explains creep, dispersion, and low differential stress observed in unconsolidated reservoir rocks. The combination of creep and relaxation implies that the dry rock matrix is viscoelastic which may result in dispersion even in the absence of pore fluids. The viscoelasticity of the dry sand matrix adds an additional relaxation time constant besides those associated with pore fluid motion thus complicating previous descriptions of frequency dependent moduli. Control groups of Ottawa sand mixed with small amounts of Montmorillonite clay suggest that the phenomena is controlled by the deformation of intergranular clay and not the deformation of quartz grains. Creep tests on synthetic idealized "two grain" models could replicate the creep strain observed in the sands with equal volume fractions of Montmorillonite clay indicating clay's role in the overall matrix rheology. It appears that microporosity reduction in the clays allows the quartz sand matrix to viscously rearrange its grains hence leading to time-dependent deformation.