Representation of Steam Distillation and In-Situ Upgrading Processes in a Heavy Oil Simulation

Abstract

Abstract This paper investigates the role of compositional processes during the steamflooding of a typical California heavy oil. In the past, compositional effects have usually been ignored in heavy oil steamflood thermal simulation studies. Results indicate that the coupled, nonlinear steam distillation/in-situ solvent generation process is capable of significantly reducing oil saturations upstream of the advancing condensation zone, under conditions representative of typical steamfloods Very fine grid 1 -D and 2-D mechanistic simulation models are required to accurately resolve the relevant physics. An appropriate pseudo-component equation of state, tuned to laboratory data is also essential. A procedure for optimizing the steam distillation mechanism in mature steamfloods using a horizontal well is presented. Introduction This study will focus on the numerical and physical representation of the steam distillation and in-situ solvent generation processes during the steamflooding of a California heavy oil reservoir. Thermal simulation studies have generally ignored these mechanisms because of the low concentration of light, distillable components in the heavy oil. It has generally been assumed that the only benefit of steamflooding a heavy oil is the mobility enhancement through viscosity reduction. Further, the relatively low pressures, and hence low steam temperatures for shallow reservoirs has also discouraged consideration of steam distillation. However, experimental studies have shown that steam distillation, coupled with downstream in-situ solvent generation can be an important mechanism for reducing residual oil saturations below those expected from straight waterflood or steamflood fractional flow residuals. Also, indirect field evidence for these processes includes the production of an upgraded light oil from a heavy oil field the presence of hydrocarbon distillate in the produced steam, and very low residual oil saturations in cores taken from mature steamfloods near the top of the steam chest. While steam distillation is recognized as an important mechanism for light oils, there is also supporting evidence for its role as a recovery mechanism for heavy oil. Thermal simulation is well suited to the direct study of these processes provided that accurate physical and numerical models are constructed. It is the main objective of this study to construct such models, for the purpose of investigating the physical mechanisms at work. We will show that the distillation/upgrading process can be an important recovery mechanism in the steamflooding of heavy oil, even at relatively low pressures. We will also show that a proper study of the detailed mechanisms requires an extremely fine grid numerical model, as well as an accurate equation of state (EOS) to characterize the PVT behavior. Armed with these investigative tools, the local details of the recovery mechanism may be understood. From this understanding, we can ascertain the operating conditions under which the distillation/upgrading process can be optimized in the field. This optimization includes, not only modifying existing operating conditions, but also the use of steam additives tuned to the appropriate conditions, as well as the use of horizontal wells. P. 551