Effect of Initial Gas Saturation on Steam Assisted Gravity Drainage Process

Abstract

Abstract Cold Heavy Oil Production with Sand, CHOPS, is a primary recovery method commonly used in many Canadian heavy oil reservoirs. In spite of its commercial success, the oil recovery factors achievable with this process range between 5 to 20 percent. In order to recover more oil from primarily depleted reservoirs, Steam Assisted Gravity Drainage (SAGD) process can be considered. However, the performance of SAGD process is likely to be negatively affected by the initial gas saturation present in such depleted reservoirs. Unfortunately, no systematic study of the effect of initial gas saturation on SAGD performance has been reported. This paper presents an experimental evaluation of the effect of initial gas saturation on SAGD performance using physical models. Different gas saturation levels were established in the model by modifying the model packing and saturating techniques. High pressure physical model experiments were carried out by injecting steam at controlled rate and producing the oil from the production well at constant pressure. The oil production behavior was analyzed to evaluate the effect of initial gas saturation on thermal efficiency of the process. Based on the results, it is possible to conclude that initial gas saturation of about 9 percent, or higher, accelerated heat dissipation and retarded the formation and growth of the steam chamber. Due to the higher relative permeability of steam in the gas saturated zone, it becomes difficult to confine the steam in a chamber as seen in the conventional SAGD. The temperature in the steam zone was noticeably below the steam saturation temperature and the oil production rate was substantially lower compared to similar test without the gas saturation. The results strongly suggest that SAGD would not be a viable option in Heavy oil reservoirs containing high free gas saturations.