Investigations into Heavy Oil Recovery by Vapour Extraction (VAPEX)
Creators: name: Al-Hadhrami, Munira
Publisher: name: Imperial College London
Year: 2014

description: It is anticipated that resources from extra-heavy oils and bitumen may resolve the expected future escalation in oil demand. Such oils are usually recovered by thermal methods, however these can be energy intensive, especially for reservoirs with thin net-pay or those bounded with large aquifers or gas caps. This is primarily due to excessive heat losses. On the other hand, VAPour EXtraction of heavy oil (VAPEX) is a more energy-efficient, economically attractive and pollution-free alternative, especially for these problematic scenarios. Despite all the potential benefits of this process, there are many uncertainties associated with the actual physics of the process. The question as to whether the oil drainage rates are sufficient for the mechanism to be economically feasible for field scale application remains unanswered. Prediction of field scale recovery factors by numerical simulation is challenging since a very fine grid is needed to ensure that the physical diffusion dominates the numerical diffusion and then to model the subsequent gravity drainage. Thus, there is a tendency to rely upon the Butler-Mokrys (1989) analytical equation to estimate oil rates. A further uncertainty in field scale application, which has only been investigated in a few studies, is the impact of geological heterogeneity on the process, since this can influence the solvent-oil dispersive mixing as well as the shape of the solvent chamber. This research first investigated the oil drainage rates with VAPEX by performing a series of laboratory experiments in both homogenous and heterogeneous systems (including layered and single discontinuous shales). All experiments were performed in well-characterized glass bead packs using glycerol and ethanol as analogues of heavy oil and solvent, respectively. The porous medium and fluid properties were measured independently. The experimentally measured rates were compared to the estimates derived from the Butler-Mokrys (1989) analytical model. In addition, numerical simulations were performed to validate whether the physical diffusion boundaries were captured correctly. Our experiments revealed that the Butler-Mokrys analytical model substantially underestimated the drainage rates in all cases, even when the effects of convective dispersion and end-point density difference were factored in. Results from the heterogeneous models further suggested that layering may not reduce VAPEX oil drainage rates significantly. The performance in systems with layers and discontinuous shale barriers, however, was less than in homogenous models with higher or equivalent permeabilities. The numerical simulations, therefore, under-predicted the physical oil drainage rates, although the pattern of solvent-oil distribution was correctly captured. The research was then extended from lab-scale experiments to field-scale numerical investigations, using a fine grid, high resolution model with realistic petro-physical properties. The solvent–oil PVT were based on real field properties. Three key criteria were examined: the oil production rates and the recovery factors that it is possible to achieve; the full range of static parameters influencing VAPEX, and; identification of the most sensitive parameters (i.e. reservoir thickness (h), vertical permeability (kv/kh) and average arithmetic permeability). In addition, we compared the performance of VAPEX against Steam Assisted Gravity Drainage (SAGD). These, field scale numerical simulations revealed that VAPEX oil extraction rates incorporating diffusional mixing alone were insufficient for the mechanism to be feasible. Although incorporating single-well tracer test (SWTT) dispersivities into the numerical simulations significantly improved the recovery rates, they still remained unacceptably low.