Abstract

Cyclic steam stimulation (CSS) and steam-assisted gravity drainage (SAGD) are successful commercial methods for enhanced oil recovery (EOR) used in Canada. Although SAGD exhibits higher oil recovery than CSS, it is more sensitive to reservoir heterogeneities such as top water and lean zones. In most cases, these baffles cause channeling in steam injection operations, thus leading to a high steam-oil ratio (SOR) and low injectivity. Therefore, recovering bitumen by using SAGD in a pilot well pad with such baffles in Long Lake has been a considerable challenge. To overcome the aforementioned issues, a hybrid process that combines CSS and SAGD (hybrid CSS/SAGD) is investigated in this study. Hybrid CSS/SAGD takes advantage of the benefits of both methods. During this process, all SAGD wells operate as cyclic injection/production wells. When the wells are shut in for a soak period, the injectivity of SAGD is highly improved. Simultaneously, the producer can also function as a heat source. After good conductivity is achieved between the injector and the producer, oil recovery is improved significantly.

This study aims to construct a hybrid CSS/SAGD simulation model for the Long Lake pilot well pad. The simulation is based on a well-defined 3D geostatistical model. By strictly controlling detailed lithological facies, lean zones and top water zones are accurately described in the geological model. These baffles are mainly located in the upper-middle portion of the McMurray Formation. By adjusting relative permeability curves, a SAGD model with a ten-year production history (beginning in April 2003) is history matched well. Based on this model, we then run hybrid CSS/SAGD simulations by changing well constraints from April 2003 with a submodel of well pair 1. Both wells in the well pair 1 operate as producers and injectors. The simulation results show that CSS-SAGD exhibits higher oil recovery and lower SOR than SAGD alone. Moreover, sensitivity analysis on preheating pressure is performed. Based on the net present value, we optimize the steam injection pressure and steam injection rate to establish criteria for each parameter for the pilot well pad.

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