Coupled mechanical deformation and fluid flow in experimentally yielded granular reservoir materials

Unconsolidated sands and weakly cemented sandstones from five different hydrocarbon reservoirs have been studied experimentally in order to better understand the coupling between mechanical deformation and fluid flow response. Steady-state permeability has been measured continuously on core plugs deforming under complex nonhydrostatic stress paths. The degree of consolidation was observed to have first order control on the rheological behaviour and permeability response of such granular materials. All normally consolidated test reservoir horizons exhibited a strong single function relationship between permeability decline and differential stress, irrespective of stress path ratio or initial stress conditions. The exact nature of this functional relationship is material dependent and related to specific microstructural characteristics. For over consolidated material permeability reduction with increasing differential stress was observed to increase systematically as the magnitude of the stress path ratio increased, perhaps reflecting the decrease in mechanical strength with increasing stress path ratio associated with the elliptical shape of the yield cap.