Expanding Solvent SAGD in Heavy Oil Reservoirs

Abstract

Abstract In recent years, several Steam Assisted Gravity Drainage (SAGD) projects have proven effective for the recovery of heavy oil and bitumen and Expanding Solvent (ES) SAGD pilot projects have shown positive indications of improved performance. This paper presents the results of a simulation study performed to investigate important aspects of the ES-SAGD process. In an ES-SAGD process, a solvent is added to the injected steam that remains in the vapor phase in the SAGD steam chamber and condenses along the walls of the steam chamber. Thus the solvent will have enough time to dissolve/disperse in the bitumen in the mobile zone before steam condensation occurs. Because the solvent blends with the bitumen, it significantly lowers (up to 5 fold) the oil viscosity. This process has the potential to accelerate recovery with less steam requirement per barrel of oil produced. The important factors that control the performance of the ES-SAGD process are the solvent type, concentration, operating pressure and the injection strategy. Results of sensitivity studies performed on each of these aspects are presented with conclusions and recommendations for operating strategy. Frequently, in heavy oil recovery processes, shear dilation has been reported as a mechanism that enhances the fluid conductivity of the reservoir medium. Even though dilation is typically adjusted as a history matching variable, one of the main problems encountered with that procedure is the huge disparity in production rates that result depending on whether the process is carried out at high or low operating pressures. The capability and limitations of the geomechanical constitutive relations used to model permeability variations with reservoir pressure, built in to the thermal simulator used for the study were explored. It is concluded that dilation is an important factor for SAGD performance at high operating pressure. In order to history match the performance of such projects, it is necessary to increase the porosity and/or permeability within a heterogeneous model and dynamic dilation factor was found to play a crucial role in matching the early time data. I. Introduction The SAGD process injects steam through an upper horizontal well (placed 5 m above the producer well: 5 meters of separation is targeted as an optimal case for establishing circulation and subcool control in SAGD operations) and produces heated bitumen that flows to a lower parallel horizontal well (target is the base of the continuous bitumen section). With time, the injected steam forms a hot heart-shaped "steam chamber" above the steam injector. Bitumen at the edge of the chamber is heated to about 175–270°C by the 3500 kPa steam and flows by gravity drainage to the producing well. The SAGD process enables continuous injection and production once initial reservoir heating has been achieved. The key economic indicator in steam assisted production processes is the amount of steam required to produce a barrel of oil, also referred to as SOR. The ES-SAGD process injects solvent with the steam, and when the solvent mixes with the bitumen in the reservoir (solvent sits on the edges of the steam chamber) a greater reduction in viscosity is achieved than with steam alone. A typical SAGD (or an ES-SAGD) configuration is shown in Figure 1 along with a heterogeneous permeability distribution (heterogeneity), well placement and pay thickness along a vertical cross-section of a reservoir.