HEAVY AND THERMAL OIL RECOVERY PRODUCTION MECHANISMS

Abstract

The Stanford University Petroleum Research Institute (SUPRI-A) studies oil recovery mechanisms relevant to thermal and heavy-oil production. The scope of work is relevant across near-, mid-, and long-term time frames. In August of 2000 we received funding from the U. S. DOE under Award No. DE-FC26-00BC15311 that completed December 1, 2003. The project was cost shared with industry. Heavy oil (10 to 20{sup o} API) is an underutilized energy resource of tremendous potential. Heavy oils are much more viscous than conventional oils. As a result, they are difficult to produce with conventional recovery methods. Heating reduces oil viscosity dramatically. Hence, thermal recovery is especially important because adding heat, usually via steam injection generally improves displacement efficiency. The objectives of this work were to improve our understanding of the production mechanisms of heavy oil under both primary and enhanced modes of operation. The research described spanned a spectrum of topics related to heavy and thermal oil recovery and is categorized into: (1) multiphase flow and rock properties, (2) hot fluid injection, (3) improved primary heavy-oil recovery, (4) in-situ combustion, and (5) reservoir definition. Technology transfer efforts and industrial outreach were also important to project effort. The research tools and techniques used were quite varied. In the area of experiments, we developed a novel apparatus that improved imaging with X-ray computed tomography (CT) and high-pressure micromodels etched with realistic sandstone roughness and pore networks that improved visualization of oil-recovery mechanisms. The CT-compatible apparatus was invaluable for investigating primary heavy-oil production, multiphase flow in fractured and unfractured media, as well as imbibition. Imbibition and the flow of condensed steam are important parts of the thermal recovery process. The high-pressure micromodels were used to develop a conceptual and mechanistic picture of primary heavy-oil production by solution gas drive. They allowed for direct visualization of gas bubble formation, bubble growth, and oil displacement. Companion experiments in representative sands and sandstones were also conducted to understand the mechanisms of cold production. The evolution of in-situ gas and oil saturation was monitored with CT scanning and pressure drop data. These experiments highlighted the importance of depletion rate, overburden pressure, and oil-phase chemistry on the cold production process. From the information provided by the experiments, a conceptual and numerical model was formulated and validated for the heavy-oil solution gas drive recovery process. Also in the area of mechanisms, steamdrive for fractured, low permeability porous media was studied. Field tests have shown that heat injected in the form of steam is effective at unlocking oil from such reservoir media. The research reported here elucidated how the basic mechanisms differ from conventional steamdrive and how these differences are used to an advantage. Using simulations of single and multiple matrix blocks that account for details of heat transfer, capillarity, and fluid exchange between matrix and fracture, the importance of factors such as permeability contrast between matrix and fracture and oil composition were quantified. Experimentally, we examined the speed and extent to which steam injection alters the permeability and wettability of low permeability, siliceous rocks during thermal recovery. Rock dissolution tends to increase permeability moderately aiding in heat delivery, whereas downstream the cooled fluid deposits silica reducing permeability. Permeability reduction is not catastrophic. With respect to wettability, heat shifts rock wettability toward more water wet conditions. This effect is beneficial for the production of heavy and medium gravity oils as it improves displacement efficiency. A combination of analytical and numerical studies was used to examine the efficiency of reservoir heating using nonconventional wells such as horizontal and multilateral wells. These types of wells contact much more reservoir volume than conventional vertical wells and provide great opportunity for improved distribution of heat. Through simulation and analytical modeling of the early-time response of a reservoir to heating with a horizontal well showed that cyclic steam injection is an effective technique to heat a reservoir prior to continuous injection during a gravity drainage process.