Abstract:
Steam assisted gravity drainage (SAGD) adopts a stimulation process through water injection to a pair of horizontal wells. Two deformation mechanisms named shear dilation and tensile parting occur in this process. The linear Drucker-Prager (DP) model had been used to model the injection of the marine-facies Alberta oil sand in Canada, whose structure consists of densely packed, interpenetrative sand grains. However, for the land-facies Karamay oil sand, the grains were loosely packed and isolated by a mixed matrix of bitumen, sand and clay. Triaxial tests confirmed that the sand exhibited substantial volumetric expansion upon hydrostatic unloading that demonstrated apparent plastic behavior. In consequence, the modified DP model with cap plasticity was selected as the constitutive framework, whose yielding surface takes into account not only shear dilation (plastic softening) but also hydrostatic expansion (inelastic strain softening). Numerical analysis was carried out on a finite element platform to evaluate and compare the capacities of the isotropic elasticity, linear DP and modified DP model with cap plasticity. Surprisingly, the analysis disclosed that all of the three constitutive models gave rise to exactly identical results in terms of cumulative injection volume, pore pressure distribution and Mises stress distribution.
Introduction
Steam assisted gravity drainage (SAGD) has become the mainstream technology to develop the heavy and ultra-heavy oil sand reservoirs in Karamay city of Xinjiang province, northwest China. The stimulation before preheating the reservoirs in terms of water injection has been frequently adopted to save both time and cost spent in preheating. The stimulation involves controlled water injection to a SAGD wellpair such that a dilative zone is created in the inter-well domain. Here water injection instead of hydraulic fracturing is a more appropriate term due to the absence of stress shadow effects in weak rocks (Lin et al. 2015). Two mechanical mechanisms named shear dilation and tensile parting-induced dilation control this process. The former originates from the rollover motion of sand grains relative to each other upon shearing, while the latter is in fact a hydrostatic unloading of the oil sand matrix (Dusseault 1977, Dusseault and Morgenstern 1978, Agar and Morgenstern 1983, Oldakowski 1994, Samieh 1995, Yuan et al. 2011a&b, Lin et al. 2015). Both mechanisms result in a generation of flow channels in terms of microcracks and expanded pore space at regions in close proximity to the wellbores (Yuan et al. 2011b). The injection process and mechanisms are depicted in Fig.1.
