Shear Dilatancy in Unconsolidated Laboratory Samples Improves Relative Permeability Performance

摘要

Steam Assisted Gravity Drainage (SAGD) can cause dilatant shear failure in unconsolidated heavy oil reservoirs. Ourexperimental work documents changes induced by such shear failure in absolute permeability, relative permeability to oil andwater, and residual saturations. Tests were performed on 2 inch diameter synthetic specimens made from lower fine tomedium grain size Ottawa sand formed with wet vibration to an initial porosity of about 32%. Loading paths includedtriaxial compression and radial extension. Axial and volumetric strains were measured directly during deformation, andsingle and two phase permeability tests were executed at axial strains of 1 to 10%. Triaxial compression tests followed apath of increasing mean stress (under constant confining stress) while radial extension tests provided a decreasing meanstress loading path (reducing confining stress under constant axial stress). The relative permeability was determined by theunsteady state method for relative permeability endpoint assessment. Results show similar absolute permeability trends tothose published by others, but our multiphase measurements appear unique. Relative permeabilities are more stronglyinfluenced by shearing, with water relative permeabilities being increase by as much as a factor of 2. The effect seems to beslightly greater for finer grain sand than for the medium grain size. Absolute permeability changes were greatest for thesamples with the lowest initial porosity, which would include those that were poorer sorted. Poorer sorting of the sand packseems to reduce the impact on relative permeability changes, however. Residual oil saturations were reduced from 0.28 to0.13 in the lower fine Ottawa sample, suggesting the possibility for significant improvement in oil recovery when the sand issheared. During triaxial testing, it was interesting to note that at 50 psi effective confining stress, absolute permeabilityincreased with shearing, but at 200 psi effective confining stress, absolute permeability was reduced, even though thevolumetric strain was still dilatant. In addition to providing new data on the magnitude of absolute permeability and multi-phase flow properties for unconsolidated sands, this paper also demonstrates that dilatant shear failure does not guaranteeincreased flow capacity for such rocks.