document

Abstract
Large heavy oil resource presents in reservoirs containing overlying gas cap. However, production from such reservoir is challenging. At the moment there is no proven recovery technique that can be successfully applied to these viscose oil reserves. Recently, vapour extraction process (Vapex) is
introduced as an attractive alternative method for such problematic reservoirs.

The objective of this study is to investigate the applicability of Vapex process in reservoirs containing gas cap. This paper is grouped into two parts (i) developing a validated numerical model via experimental data to carry out sensitivity analysis of different parameters which are expensive and difficult to
perform on experimental setup (ii) screening study of the Vapex process to real reservoir by using simulation model. The results show that small gas cap can even be advantageous in this process and improves the recovery by expending the interfacial mass transfer rate and the inter-well communication. In addition, there is a threshold value for the ratio of initial gas to oil volume which beyond that the process loses its efficiency. Moreover, increasing the lateral spacing between the injector and producer is not beneficial in the presence of a small gas cap.

Introduction
Heavy oil and bitumen reserves represent a considerable portion of worldwide energy resources. It is estimated that these reservoirs contains six trillion barrels of OOIP which is much further than the total conventional oil reservoirs. Great consumption of light oil reserves and hence their dramatic
decline encourages more interest in exploitation of highly viscous oil and bitumens for future energy demands. For the optimum conditions, the primary recovery of these reservoirs would not exceed 10 percent of the OOIP(1).

Since in the waterflooding operation, the water tends to flow at the lower portion of the reservoir and touches a small part of it only a small overall incremental recovery of about 1-2 percent can be achieved. High sensitivity of oil viscosity to temperature motivates the application of thermal recovery
techniques. Variety of methods including Cyclic Steam Stimulation (CCS), In-Situ Combustion (ISC), and Steam Assisted Gravity Drainage (SAGD) are currently being applied for extraction of these crudes. As far as the reservoir’s oil is heated, main fraction of the thermal energy is used up in heating the formation itself. For the reservoirs with very thin payzone, bottom water zone and/or overlying gas zone, low thermal conductivity of rock matrix, high water saturation etc. the economical applicability of the thermal methods is doubtful. With respect to current available recovery technologies, we categorize these reservoirs as problematic reservoirs(2,3).

Addition of vaporized hydrocarbon solvents (HCS) can also drastically reduce oil viscosity. This concept is the fundamental of vapor extraction process, Vapex. In a typical Vapex approach, solvent is injected into a horizontal well located directly above a horizontal producer. As the solvent diffuses and dissolves in the bulk of the reservoir, a chamber of solvent vapor grows around the injection well and helps the gravity drainage of diluted oil toward the production well, from where it is pumped out to the surface. Several other configurations of the wells are also possible. The main affecting parameter in choosing the separation between injector and producer is the oil mobility at reservoir condition. As the mobility increases, the separation can also be increased as well.

Less energy consumption, less environmental pollution, and lower capital cost are the main advantageous of Vapex process over that of thermal methods. Since the HCS are soluble only in the oil then the possibility of solvent loss to the formation and its confining environment is vanished. In addition, the Vapex process appears to be the only IOR technology that can be economically justifiable for problematic reservoirs(4).