Oil Recovery and Technology Sequencing

Introduction Steam-Assisted Gravity Drainage (SAGD) has attracted much attention since its inception in 1990 to 2000. However, developments have also occurred in other production technologies. Table 1 lists commercialized production technologies for viscous oils (μ> 100 cP in situ), along with suggested screening criteria for use as the first and major extraction method in a reservoir. These guidelines are approximate only; there are other important criteria, and each reservoir must be evaluated before a production approach is chosen. For example, IGI and VAPEX are gravity drainage methods that can give high RF; however, in viscous oils, production rates may be a small fraction (10 – 30%) of those for a thermal process. Nevertheless, these methods will become more widely used, particularly for the range μ< 1,000 cP, once reservoir engineers acknowledge the advantages of no heat costs and high RF. This article recommends deliberate technology sequencing planning at the beginning of a project(1). Of course, everyone wants low costs, high RF and high rates. More realistically, technology sequencing could give high early profitability followed by a long production life that eventually achieves a high life-cycle RF because low production rates can be tolerated if OPEX is low (i.e. non-thermal). Choosing a single exploitation technology now seems simplistic in view of possibilities for technology sequencing over the productive life span of a reservoir. Flow Instabilities To serve as an introduction to sequencing, the three classes of instabilities are reviewed: gravitational (vertical phase segregation), viscous (mobility-ratio issues) and capillary (multiphasic surface tension effects) instabilities. Gravitational instabilities, such as steam or gas override in steam drive and water underride in WAG methods, lead to impairment of recovery efficiency. However, gravity drainage methods using long horizontal wells (IGI, SAGD, VAPEX, etc.) now exploit vertical phase segregation to achieve high RF values. Viscous instabilities associated with pressure gradients and viscosity differences include coning, fingering, channeling and hydraulic fracturing. To reduce or avoid them, viscosity differences can be reduced (solvents, steam) or production can be undertaken at gravity drainage conditions with no significant pressure gradients. Capillary instabilities arise because of interfacial tensions between two fluids, restricting entry of the displacing fluid into a small pore throat. However, this does not occur if the continuityTable 1: Viscous oil production technologies. Available in Full PaperFIGURE 1: Maintaining oil films in IGI to achieve high RF. Available in Full Paper of the oil phase through the pore throat is maintained. In gravity drainage, high pressure gradients are avoided, thereby eliminating pinch-off and ganglia formation. Exploiting these instabilities means high RF values become possible; ignoring them leads to low RF, although not necessarily low rates. For example, CP is a solution gas drive approach: in viscous oil, as pressure is dropped, gas exsolves and eventually severely degrades permeability to oil because gas bubbles are immobilized in pores by interfacial forces. This leads to the low projected RF values for this technology (RF = 0.10 – 0.15) when applied to thick, high permeability, 1,000 – 3,500 cP reservoirs in the Venezuelan Faja del Orinoco(2).

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