Summary Heavy oil and bitumen are expected to become increasingly important sources of fuel in the coming decades. Steam assisted gravity drainage (SAGD) is a commercially viable and widely used recovery technique for heavy oil and bitumen. However, it remains an expensive technique and requires large energy input in the form of steam. Energy intensity of SAGD, as well as environmental concerns such as fresh water usage and CO2 emission, make it imperative to find new oil extraction technologies. Coinjecting a hydrocarbon additive with steam offers the potential of higher oil rates and recoveries with lower energy and water consumption. A reservoir simulation study using a 20X12X15 3D Cartesian model and Athabasca fluid and reservoir properties was conducted to evaluate this process. The role of hydrocarbon additive in the steam chamber and its effect on the performance of SAGD was investigated. Simulation results revealed the parameters that will have the greatest impact on the process performance and indicated the effectiveness of each hydrocarbon additive in improving the performance of SAGD. The results also showed that selecting the most suitable hydrocarbon additive depends on the operating conditions as well as the original reservoir fluid composition.

Summary The Grosmont formation, a carbonate reservoir in Alberta, Canada, has 400 billion bbl of bitumen resource, which is currently not commercially exploited. The carbonate reservoir is karstified by groundwater and tectonically fractured, resulting in three classes of porosity: matrix, vugs, and fractures. The viscosity of bitumen is lowered by four to six orders of magnitude when heated by steam. Since December 2010, the Saleski pilot project evaluated steam-injection-recovery processes by use of four well pairs, two each in the Grosmont C and Grosmont D units. For the first year of the pilot, two well pairs were operated with continuous injection and production similar to successful steam-assisted-gravity drainage(SAGD) projects in Alberta oil sands. Reservoir observations of steam/oil ratio (SOR) and calendar-day oil rate (CDOR) indicate recovery by gravity drainage is viable, although operating practices from conventional SAGD must be modified for the Grosmont formation. The decision to evaluate cyclic injection and production from single wells was made in early 2012, although it was recognized that cyclic operations created new challenges for the facility (which was built for SAGD operations) and artificial lift. The pilot data indicate that the drilling conditions (balanced vs. overbalanced), completions (openhole vs. slotted liner), and acid treatments of the wells have a significant impact on the individual-well performance. Injectivity into the Grosmont reservoir is high, even into a cold reservoir, because of the existing fracture system. Injection pressures stayed less than 40% of the estimated pore pressure required to lift the overburden. 4D-seismic results indicate that the injection conformance along the well axis is close to 100% and that the heated area is laterally contained around the well. Productivity is comparable to oil-sands project performance. The decline of oil rate is not only dependent on pressure but also on temperature. For cyclic operations, a CDOR of 43 m3/d (for a 450-m-long well) and an SOR of 3.4 were achieved, demonstrating that with sufficient scale, a commercial project can be established successfully. The pilot has satisfactorily derisked the Grosmont reservoir at Saleski. While cyclic operations have demonstrated economic performance, continuous injection and production similar to SAGD remains an alternative recovery strategy beyond startup in the later depletion stage. Successful future developments will advance the optimization of drilling, completion, artificial-lift, and plant capacity issues, while the reservoir itself has demonstrated its production capacity.

Bitumen production from the Grosmont formation is enabled by bitumen-viscosity reduction caused by heating with steam, and is driven by three processes: thermal expansion, gravity drainage, and spontaneous imbibition. Gravity drainage is the dominant recovery mechanism. Maintaining a balance of injected and produced fluid is indicative of good performance. The projected steam/oil ratio (SOR) for the carbonate Grosmont formation is comparable to that of the clastic Clearwater formation; the impact of lower porosity is compensated by lower water saturation.On the basis of the experience from the pilot project, a followup development of the Grosmont formation relies on cyclic operation of injection and production. Saleski Phase 1, approved by the Alberta Energy Regulator, is designed for 1700-m 3 /d oil capacity from the Grosmont formation. For the first time, probable undeveloped reserves have been assigned to a fractured-carbonate bitumen reservoir. The cyclic-to-continuous steam-assisted-gravity-drainage drainage (C2C-SAGD) concept, where initial cyclic operation of individual wells is converted into continuous injection and production with well pairs as the reservoir depletion matures, intends to maximize recovery in future exploitation projects.Spontaneous Imbibition. In general, fractured carbonate reservoirs are initially oil-wet (Al-Hadhrami and Blunt 2001). Specific to the Saleski Grosmont, laboratory experiments are currently being executed to confirm the wettability. Assuming bitumen is the wetting phase (Fig. 2, left), capillary pressure in the matrix is low at high bitumen saturations. Fluid movements into or from

Heavy oil and bitumen are expected to become increasingly important sources of fuel in the coming decades. There are extensive deposits in Alberta that could be a principal source of fuel in the coming century. The Athabasca Oil Sands, the largest petroleum accumulation in the world, the Cold Lake oil deposit, and the Lloydminster reservoir are all major Canadian oil sands deposits. SAGD, which has shown considerable promise in all three of these major deposits, remains an expensive technique and requires large energy input. Energy intensity of SAGD and the environmental concerns make it imperative to find new oil extraction technologies.Co-injecting hydrocarbon additives with steam offers the potential of lower energy and water consumption and reduced greenhouse gas emission by improving the oil rates and recoveries. In a previous paper by the same authors (Hosseininejad Mohebati, Maini et al. 2009), it was shown that the selection of a suitable hydrocarbon additive and the effectiveness of this hybrid process are strongly dependent on the operating conditions, reservoir fluid composition, the heavy oil viscosity, and the petrophysical properties of the reservoir. Among these factors, the heavy oil viscosity which is the most prominent difference between these three reservoirs could be a very important parameter in the performance of this hybrid process. Therefore, it is important to evaluate the effect of oil viscosity on solvent assisted SAGD.Extensive numerical studies in a 3D model by means of a fully implicit thermal simulator were conducted to evaluate the efficiency of each hydrocarbon additive in Athabasca, Cold Lake and Lloydminster reservoirs. Varying mole percents of hexane, butane and methane were co-injected with steam in with different values of heavy oil viscosity. The effect of oil viscosity on the performance of each solvent was compared in terms of oil production rate and cumulative steam oil ratio.

Solvent SAGD hybrid processes have attracted considerable attention in recent years. The perceived benefits of solvent addition to steam in SAGD are higher oil rate, lower energy and water consumption, higher recovery by lowering residual oil saturation (Sor) and higher return on investment. Despite numerous investigations that have been published regarding different aspects of solvent SAGD processes, this hybrid process is poorly understood and the solvent effects are difficult to predict. In fact, there is no available theory to model to the transport phenomena and the role of solvent within the steam chamber. Numerical simulation studies typically model the viscosity reduction of bitumen by solvent dissolution but do not capture other plausible mechanisms that yield higher oil rate and recovery, for example, lowering of Sor or partial in-situ upgrading. Laboratory experiments at realistic reservoir conditions are needed to gain more insight into these hybrid processes. This paper presents the results of a series of laboratory experiments for evaluation of solvent addition to SAGD. These experiments were conducted at different representative reservoir pressure in a 3-D scaled physical model. Hexane, which has shown the best performance in many studies, was co-injected as solvent with steam in these experiments. Oil rate, recovery, and steam oil ratio were compared and the hybrid solvent/SAGD process performance was evaluated at different operating conditions. Additionally post-test sand samples were extracted from the model to examine residual oil saturation in different parts of the model after each experiment. Experimental results showed improved performance of SAGD with addition of hexane, both at high and low operating pressure. However, the impact of hexane on the shape of the steam chamber and distribution of residual oil was significantly affected by operating pressure. This behavior of hexane, which appears to be related to its phase behavior, shows that solvent SAGD processes are considerably more complex than first thought.