|Fig. 1: Typical Heavy-oil viscosity vs temperature plot.|
About 70% of all the liquid oil of all types estimated today falls into the category of heavy-oils (<20 API gravity). Among these resources, Canada and Venezuela together account for about 55-65%.  Thermal Enhanced Oil Recovery (EOR) remains the most frequently used method for extraction of heavy oils.  Without any surprise, thermal EOR projects have been concentrated mostly in Canada, Venezuela and the US. The main idea behind thermal EOR methods is to introduce heat into the reservoir in order to reduce viscosity of the heavy-oil. Fig. 1 demonstrates oil viscosity as a function of temperature for a typical heavy-oil sample. As can be seen in Fig. 1, oil viscosity decreases rapidly as temperature increases, especially at lower temperatures. This means that a relatively small addition of heat in a cold formation can significantly change mobility of the oil. Heavy-oil properties are discussed by Alnoaimi.  The present paper describes the most popular thermal EOR methods applied nowadays. These methods are i) cyclic steam injection (sometimes referred to as Huff & Puff), ii) steamflooding, and iii) Steam- Assisted Gravity Drainage (SAGD).
Cyclic steam injection (CSI) is usually done is 3 stages: injection, soaking, and production. First, a predetermined amount of steam is injected into the well to heat the oil in the surrounding reservoir (injection stage). Once the desired amount of steam is injected, the well is shut down to allow the steam to heat reservoir around the well (soaking stage). In the last stage, the injection well is converted to a production mode until the heat is dissipated with the produced fluids (production stage). This cycle is repeated until the response becomes insignificant and economical limits are reached. Obviously, most of the oil is produced in the first few cycles.
CSI is most commonly applied to the reservoirs with thickness greater than 30 ft and depth less than 3000 ft; high reservoir porosity and oil saturation are desirable. Typical recovery factors for this method are in the range of 10-30%.  It is very common for wells to produce for a few Huff&Puff cycles before switching to a different thermal EOR method, namely steamflooding.
For steamflood operations, some wells are used for injection and others are used for production. During the steamflood, high-quality steam is injected into the heavy-oil reservoir; steam heats the oil and pushes it toward a producing well. As opposed to the CSI, during the steamflood, two mechanism are involved in the recovery process. As in the case of any other thermal method, the first mechanism is viscosity reduction due to increase of temperature; the second mechanism is a physical displacement of oil by steam and hot water. The recovery factors for the steamflood operations are usually higher than one for the CSI and are in the range of 40-60%. 
As steamflood matures, a large amount of heat is retained in the reservoir rock and it may become uneconomical to inject steam into the reservoir. In this case the steamflood project is usually stopped or field is converted into a waterflood mode.
Stea-assisted gravity drainage (SAGD) was first proposed and developed by R. Butler and his coworkers in Imperial Oil in the late 1970s. The Foster Creek plant in Alberta Canada (1996), was the first commercial SAGD project. Since Foster Creek plant, SAGD has been used increasingly for the recovery of bitumen and heavy oil in Canada. SAGD is considered to be an advanced form of steam injection: here, a two horizontal wells are placed, one a few meters above another. The upper well is used as a steam injector; whereas, another well is used as a producer. The steam from the injection well flows towards the perimeter of a steam chamber and condenses. The steam chamber grows both vertically and horizontally heating the surrounding oil with subsequent reduction of oil viscosity. Heated oil and steam condensate flow toward the production well due to gravity.
|Table 1: SOR for different thermal methods.|
Generally, SAGD leads to a higher recovery factors (about 40-60%) compared to the CSI.  This high recoveries are obtained because of the systematic nature of the drainage process. Several different reservoir cutoffs for an economical SAGD operations are described by McCorkack.  Among the most restrictive cutoffs are: pay thickness greater than 40 ft, permeability greater than 3 Darcy, and absence of top/bottom water. Currently, there is an attempt to improve SAGD technology by introducing noncondensible gas to the steam stream as in SAGP process; or by injecting solvent vapor along with noncondensible gas as in Vapex process. A common feature of the three processes (SAGD, SAGP, and VAPEX) is the use of a pair of long horizontal wells.
All of the methods described above require steam to be generated on the surface and injected into the petroleum reservoir. Most of the energy consumed during thermal EOR is due to steam generation. The amount of oil produced per amount of steam injected determines the method energy efficiency. For the steam based methods, the commonly used metrics are the volumetric steam-to-oil ratio (SOR) and oil-to-steam ratio (OSR). Since the amount of oil produced and the amount of steam injected vary in time, sometimes it is more meaningful to use cumulative volumes instead of instantaneous. The SOR's are strongly dependent on the quality (density, viscosity, etc) of the produced hydrocarbons, reservoir properties (porosity, permeability), and surface facilities being used. The steam-to-oil ratios for CSI, steamflooding, and SAGD at different locations are presented in Table 1. 
One of the ways to reduce SOR for a given reservoir is to optimize the design of the surface facilities. For instance, it is beneficial to minimize transportation of hot water to avoid heat losses; or to maximize heat integration between hot and cold process streams to minimize external cooling. On the reservoir side, several optimizations can be done: well placement optimization, injection strategy/production strategy optimization, etc. Commonly, both reservoir and surface facilities optimization are performed during the conceptual stage of the project.
© Rustem Zaydullin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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