Geotechnical Centrifuge Experiments to Improve Understanding of Sand Production from Heavy Oil Reservoirs

Abstract

Since the 1990’s, Cold Heavy Oil Production with Sand (CHOPS) has been a common practice in Western Canada where sand can be easily mobilized from weakly cemented sandstones. The more sand produced, the greater the oil recovery, with recovery factors of 10% being typical. In recent years, operating companies have enhanced recovery in reservoirs previously operated using CHOPS by applying thermal and/or chemical methods; however, effective implementation of these methods requires an improved understanding of sand production mechanisms. Numerous field-based, numerical, and laboratory modelling studies have been completed to better understand the shape of the voids that form within heavy oil reservoirs during CHOPS, sand transport mechanisms, and CHOPS impact on production rate. The objective of this research was to design and implement a laboratory testing system to investigate these subjects. A purpose-built geotechnical centrifuge model was used for this research, with a sandpack of dense, uncemented sand representing the reservoir. The results demonstrate that the predominant sanding mechanism is the development of a cone-shaped cavity at the top of the sandpack, both with stiff (steel) and compliant (clay) caprock. The results also demonstrate that erosion channels may develop around the cavity, in cases where flow rates near the top of the sandpack are greater than flow rates at depth; however, with the presence of a flexible caprock and uniform radial flow, this effect can be nullified. In addition, the results for one of the experiments, in conjunction with numerical modelling, suggest that a wormhole may form during seepage in the upper part of the reservoir where in-situ stresses are relatively low, though it might collapse once seepage is ceased. Such wormholes may have been a major cause of the high flowrates observed during the tests after sand production, enhanced to some extent by dilation-induced increase in hydraulic conductivity in the near-well and near-cavity areas. Moreover, results obtained using two-phase fluid saturations (water and oil) suggest that capillary pressures increase the effective strength of the sandpack, which reduces the tendency of sand failure as was observed in deep perforated zones in experiments conducted using single-phase oil saturation. The practical implication of this work is the suggestion that it may be possible to improve the effectiveness of enhanced recovery operations if heated fluids/solvents are injected into the deeper, less disturbed part of the reservoir in order to avoid premature breakthrough (production) of injected fluids.