With the advent of horizontal wells, a distinct change is tacitly taking place in our approach to improved recovery of heavy oil: from displacing mobilized oil in a flood pattern from injector to producer over long distances of the order of hundreds of metres, to short-distance oil displacement (SDOD) processes (typically over a few metres). Due to the high viscosity of oil, its displacement to a producer site located a long distance away is usually inefficient. In these cases, the required pressure drop may be very high, and/or the mobility ratio between injectant and oil exceedingly large. Either gravity override or very intensive channeling may take place, resulting in extremely low volumetric sweep efficiency and hence, generally marginal or poor economics.
Using horizontal wells, travel distances for the mobilized oil to reach the producer can be shortened. SDOD processes are aimed at mobilizing oil and producing it immediately, via the shortest path, into a horizontal producer. SDOD processes can utilize horizontal producers and injectors, or combinations of horizontal producers and vertical injectors. Based on the displacement front's position relative to the horizontal section of a producer, SDOD processes could be divided into two categories:
Those with a displacement front quasi-parallel to the horizontal producer.
Those with a displacement front quasi-perpendicular to the horizontal producer.
While the first type of SDOD process uses two parallel horizontal wells (one for injection and the other one for production), the second type uses a vertical injector and a horizontal producer with the toe of the producer located in the proximity of the shoe of the injector. In the first type of process, production occurs throughout the entire horizontal section. In the second type, the swept zone extends and moves from the toe towards the heel, utilizing reduced sections of the horizontal well for production.
Steam Assisted Gravity Drainage (SAGD) and Vapour Extraction (VAPEX) processes belong to the first type. Toe-To-Heel (TTH) displacement processes are of the second type. These TTH displacement processes can be applied in a non-thermal mode, such as Toe-To-Heel Waterflooding or a thermal mode such as THAI (Toe-To-Heel Air Injection) along with its variant, CAPRI, aimed at in situ oil upgrading. An analysis of all SDOD processes (SAGD, VAPEX, and TTH displacement processes) was performed, focusing on their relative merits in terms of override or underride due to gravity segregation, injectant channeling due to reservoir heterogeneity, and injectant/oil mobility ratio causing instabilities. These negative factors are very important in defining the efficiency of the long-distance oil recovery techniques, but they are substantially less important during the SDOD processes. Therefore, in many situations, the efficiency of heavy oil exploitation via SDOD IOR/EOR processes is usually better than alternate recovery schemes.
Finally, basic concepts behind different TTH displacement processes are examined in light of available laboratory and simulation results. Also, various aspects of SDOD process implementation in the field along with practical considerations are discussed.