Methodology for Estimating Recovered Solvent in Solvent-Aided Process

Summary The solvent-aided process (SAP) is a solvent-based enhancement of steam-assisted gravity drainage (SAGD) in which small amounts of solvent, such as light alkanes or natural-gas liquids, are added to the injected steam to enhance reservoir performance and associated project economics. Expectedly, the economics with SAP are sensitive to the solvent recovered from the reservoir, making its measurement in a field test an important factor. When a single-component solvent such as butane, which is not generally present in the produced heavy oil or bitumen, is used in SAP, estimation of the recovered solvent can be achieved uniquely. But when the solvent also has heavier components, some of which overlap with the lighter components of the produced oil, the measurement is not straightforward. The problem is compounded by the fact that the interaction with the reservoir changes the composition of the produced solvent and makes it time variant on account of different resident times associated with different components. The issue is further complicated by the fact that produced oil also undergoes an in-situ solvent deasphalting process (SDA), which is also time and space variant in the reservoir. If there were no in-situ SDA, one potential method to measure the amount of produced solvent would be to measure the total asphaltene content as an oil "marker" in the produced blend. Use of a tracer with injected solvent, as well as regression-based analyses for solvent fraction (using compositional analyses of solvent, bitumen, and the blend) of the produced blend, is error prone for these same reasons. Because of the issues in these approaches, a new method is desirable for a more-robust and unique assessment of the solvent amount in the produced fluids. This paper elaborates on the current challenges and proposes a couple of workable methods, including use of maltenes/metals content as oil markers as well as the use of boiling-point curves of the produced blend compared with the boiling point of the base oil. Such techniques of estimating the recovered solvent can facilitate a more-objective assessment of SAP field tests and enable economic evaluation of SAP application to a commercial scale.