Formulations for the magnitude of excess pore pressure and change in effective stress developed In gassy soils due to temperature change, and the finite element methodology adopted to incorporate these effects, are described. Application of the model to simulate the s team injection recovery process demonstrates a mechanism which can inhibit the development of fracture at injection pressures above the estimated in-situ stresses.
Temperature variations have a significant influence on the behavior of soils. In general, temperature affects pore pressures and interparticle forces, and induces changes in volume. Temperature can also significantly alter some of the engineering properties of the soil, for instance it may change the hydraulic conductivity or result in gas exsolution which will increase the compressibility. Some of these effects have been considered by Campanella and Mitchell(1), Lambe(2) Henkel and Sowa(3), Scott(4), Mithcell and Campanella(5), and Mitchell et al.(6). A knowledge of changes in volume, pore pressure, and stress as a result of variation in temperature is of particular importance in problems dealing with the extraction of bitumen from oil sands by the process of heat injection.
Although oil sands have been found in abundance, they have hitherto been relatively unexploited as the extremely high viscosity of the crude bitumen makes conventional recovery by pumping impractical. In-situ extraction methods generally involve heating the oil sand with pressurized Bream or by in-situ combustion. Such a process decreases the viscosity of the bitumen and thus enhances the rate of flow. But the temperature increase also tends to increase the pore fluid pressure which, under undrained conditions, result in a reduction in effective stresses and hence a loss in shear strength. This In turn will cause some shear deformations that can adversely affect well casings or inhibit fracture formation. Moreover, volume changes in the affected strata will occur as a consequence of temperature increase. With the propagation of hest through the oil sand, the thermal expansion will produce ground movements towards underground utilities or the surface. Thermal expansion may further cause differential displacement which could result in the propagation of fractures in cemented reservoirs, threatening the stability of well casings and shafts, Scott and Kosar(7) Agar et al. (8). The severity of the problems can be further compounded if the change in temperature causes the dissolved gases to come out of solution thus increasing the expansion of the sand matrix. Determination of the thermal expansion of the soil under investigation, the pore fluid pressure generation, and the fluid flow is therefore of paramount importance to the full understanding of the processes taking place during in-situ heat injection of the oil sand reservoirs.
In this paper, formulation for the magnitude of the excess pore pressure developed due to temperature change for the case of s three phase soil system is derived. To increase the general applicability of the method, the solid particles and the liquid component of the sand matrix are considered to have finite compressibilities.