Some fundamental mechanisms of hydraulic fracturing in unconsolidated formations have not been well understood although they have been studied quite extensively for hard rock formations. Laboratory and field evidences demonstrated that during the hydraulic fracturing of the unconsolidated porous materials such as oil sands reservoirs in Alberta, Canada, a single planar fracture is unlikely to occur, and the outcome is a high-porosity zone permeated by a network of micro-cracks. Thus, the numerical approaches based on the conventional fracture mechanics have limitations in modeling such complex processes. The purpose of this paper is to present numerical simulation techniques in modeling the progressive mechanical breakdown of, unconsolidated oil sands. We will follow the continuum mechanics and use elasto-plastic model with strain softening for simulating the sands matrix deformation. The intimate coupling between the deformation and fluid flow is considered via a porosity-saturation-dependent isotropic permeability model. All such theoretical approaches are tested by history matching of mini-frac tests in the oilsands reservoirs.
Fractures in the earth's crust are desired for a variety of reasons, including enhanced oil and gas recovery, reinjection of drilling or other environmentally sensitive wastes, measurement of in situ stresses, geothermal energy recovery, and enhanced well water production [1]. Fracturing unconsolidated formations is an increasingly important application of hydraulic fracturing in petroleum industry. In these unconsolidated formations, hydraulic fracturing has been mostly used for creating high conductivity channels for steam injection in cyclic steam stimulation operation, determination of the in-situ stress in mini-fracture tests, disposal of petroleum drilling waste into deep unconsolidated formation and frac-pack completion in deepwater wells. Although hydraulic fracturing in hard rock has been comprehensively studied both experimentally and numerically, some fundamental mechanisms of hydraulic fracturing in unconsolidated formation have not been well understood. Both experimental data and field testing data clearly show that fracturing in unconsolidated formations is significantly different than those encountered in hard rock. Field observations regarding the occurrence of fractured zone as opposed to one single dominant fracture have been reported in many publications. By mining back into the hydraulic fractures created in unconsolidated formations, Schmidt [2] reported a very complex fracture patterns. Abnormal fracturing pressure in massive hydraulic fracturing treatments that was observed which was attributed to the presence of zone fracturing [3]. Beattie et al. [4] reported the measurement of surface heave as large as 45 cm during steam injection in Cold Lake oil sand. This is far larger than that attributed to thermal expansion or a single tensile fracturing of the formation. Also, steam injectivity was much greater than what might be expected based on the native reservoir properties. Multiple shear fractures and the resulting dilation zone have been identified as the main cause in both cases. Based on many field observations about hydraulic fracturing in oil sands which contradicted the conventional assumptions, Settari et al [5] concluded that very high fluid leak-off in the porous materials, such as oil sand, and the creation of fractured (dilated) zone is the main reason for the observed discrepancies.
