Coupled Fluid Flow And Stress Analysis Of Oil Sands Subject To Heating

Abstract

Abstract Theoretical expressions, in terms of elastic and thermal properties ofmulti-phase soil systems, have been derived for the change in pore fluidpressure under undrained and the change in soil stress under drainedconditions. A methodology is proposed for linking these equations to coupledflow and deformation finite-element formulations that are applicable to gassysoils. The developed finite-element program satisfies the complex temperature, fluid flow and stress interactions under any specified displacement and fluidpressure boundary conditions. These facets are demonstrated by comparing thecomputational results with closed-form solutions and analyzing a practicalproblem that is typical of an enhanced recovery process often employed inextracting highly viscous bitumen from oil sanddeposits. Introduction Temperature variations can significantly influence the behaviour of soils, suchas oil sands. These effects are of practical concern in the geotechnicalaspects of numerous engineering projects, such as power plants, undergroundstorage of nuclear waste, underground power cables, liquefied natural gasstorage and pipelines, geothermal energy development, and recovery of bitumenfrom heavy-oil reservoirs by the process of steam injection. In general, variations in temperature affect pore fluid pressure, inducechanges in volume, and modify some of the engineering properties of soil. Thechanges in volume include the expansion of solid particles, of the soilskeleton, and of the pore fluid. Invariably, the volume increase of the porefluid is greater than that of the voids in the soil skeleton, producing anincrease in pore fluid pressure and a consequent reduction in effective stress.If the thermal diffusivity of the soil is relatively more dominant than thehydraulic diffusivity, excess pore pressurewill develop as a result of anytemperature increase. The increase in pore pressure can potentially cause soilliquefaction in relatively impermeable soils subjected to a rapid increase intemperature. During subsequent consolidation, excess pore pressure dissipates, effective stresses increase, and displacements reduce to eventually becomeequal to those corresponding to the expansion of the solid grains alone. A theoretical analysis of volume change in saturated soil subjected totemperature variations, and pore pressure changes due to undrained heating hasbeen proposed by Campanella and Mitchell(1). The formulations arevalid for pressures above the gas/liquid saturation pressure (i.e. the porepressure sufficient to prevent gas evolution), with the assumption that thesolid particles are incompressible. Although in their analysis, Campanella and Mitchell made provision for solid particle compressibility, it is not treatedin a consistent manner and their formulation is in fact strictly true only ifthe compressibility of the solids is zero. The approach proposed herein takesinto account the compressibility of solid grains, as well as thecompressibility of occluded gases in the fluid phase. In this paper, expressions are derived for the change in pore pressure underundrained conditions, and the change in soil stress under fully restrainedconditions, due to changes in temperature. The expressions provide the basisfor the development of a finite-element code linking temperature changes tofully coupled stress I deformation and fluid-flow phenomena.