Applicability of the VAPEX Process to Iranian Heavy Oil Reservoirs

Abstract

Abstract The objective of this paper is to evaluate the viability of the VAPEX process for an Iranian heavy oil reservoir according to the reservoir characteristics. Also, based on the reservoir fluid conditions, the optimum solvent system to meet the requirements of the VAPEX process is determined. In addition, a mechanistic model was developed for the study of the VAPEX process by considering the mass transfer and fluid flow mechanisms characteristics of the VAPEX process. The model is capable of predicting the drainage rates of heavy oil in all stages of the process, including the pseudo-steady state. Also, various initial and boundary conditions can be specified to the model. The results obtained by this model agree well with the experimental data. The model was further applied to a fractured system and effects of solvent injection rate, fracture and matrix permeability, matrix to fracture permeability ratio, and initial viscosity of the reservoir heavy oil on the performance of the VAPEX process were studied. Introduction Development of heavy oil resources in the Middle East as supplementary energy resources requires the application of promising EOR processes that are suitable to the nature of these reservoirs, e.g. highly fractured, low-permeability carbonate rocks, high reservoir pressure, etc. While the recovery of heavy oil from reservoirs with low porosity, low thermal conductivity, and high fractures and/or fissures using thermal methods is problematic and uneconomical, the Vapor Extraction (VAPEX) process needs to be studied as an alternative to the thermal EOR processes. The Vapour Extraction (hereafter called the VAPEX) process was originally proposed by Butler and Mokrys [1–2] as an alternative EOR method to the Steam- Assisted Gravity Drainage (SAGD) process. The process involves the application of a pure hydrocarbon vapor or a vaporized hydrocarbon mixture as solvent to diffuse and dissolve in heavy oil to reduce its viscosity and make it mobile. The process was to be applied in reservoirs where the SAGD process is problematic. These reservoirs include thin reservoirs, low-permeability carbonate reservoirs where the heat capacity per unit volume of contained oil is high, and reservoirs underlain by aquifers and/ or gas cap, where application of the SAGD process leads to excessive heat losses to the under burden and overburden, aquifer, and /or gas cap, and make the SAGD process questionable from economical point of view. The main hydrocarbon solvents applied in the VAPEX process include ethane, propane, and butane. The use of pure solvents in VAPEX experiments makes the process relatively simple in that the diffusion in the gas phase and the competitive diffusion of solvent components in heavy oil are absent. However, from practical point of view, the use of pure hydrocarbon solvents in the VAPEX process is feasible in a limited number of reservoirs, where the relatively low reservoir pressure allows the pure solvent to be injected at its dew point (saturated vapour) conditions. On the other hand, in most reservoirs, the use of pure solvent may not meet the saturation conditions at reservoir temperature and pressure. To overcome this problem, a mixture of hydrocarbons mixed with a carrier gas is used as the solvent. This method is some times regarded as an alternative to the VAPEX process [3]. Existing studies on the VAPEX process as a new method for heavy oil recovery have been limited to low-pressure, low-temperature sandstone reservoirs, where a pure light hydrocarbon serves as the solvent. However, for reservoirs with relatively high pressure, like those found in Middle East heavy oil reservoirs, hydrocarbon solvent systems, rather than pure solvent components, should be designed for a specific reservoir according to its temperature and pressure.