With the advent of horizontal wells in Improved Oil Recovery (IOR) of heavy oils, a distinct change is tacitly taking place in our approach to improved recovery of oil: from displacing mobilized oil in a flood pattern from injector to producers over long distances of the order of hundreds of meters to short-distance oil displacement (SDOD) processes (typically over a few meters). Due to the high viscosity of oil, its displacement to producers located long distances away may not always be profitable. In these cases, the required pressure drop may be too high, and/or mobility ratio between injectant and oil could be exceedingly large. Either gravity override or very intensive channeling may take place, resulting in extremely low volumetric sweep efficiency, low oil rate and recovery, and hence, marginal or poor economics.
Horizontal wells provide a means of contacting a large part of the reservoir. Therefore, travel distances for the mobilized oil to reach the producer can be shortened. The SDOD processes are aimed at mobilizing oil and producing it immediately, via the shortest path, into a horizontal producer. Thereafter, mobilized oil flows through the horizontal well rather than through the porous media. The SDOD processes utilize these concepts and use 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:
Processes in which the swept zone surrounds the horizontal producer, forming an ever increasing chamber containing the injected fluid (displacement front surface quasi-parallel to the horizontal producer).
Processes in which the displacement front is quasi-perpendicular to the horizontal producer; the swept zone starts from its toe and progressively moves towards the heel.
While the first type of SDOD processes use 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 producer located in the proximity of the shoe of injector. In the first type of processes, streamlines are roughly perpendicular to the horizontal section of the producer, and the well produces through the whole horizontal section during the entire producing life. In the second type of processes, the streamlines bend towards the producer, due to a distribution of flow, which results from a combined effect of drive induced by flow inside horizontal producer and injectant/oil gravity segregation. Successively reduced sections of the horizontal well are thus utilized for production; only a portion of the horizontal section close to the heel is used for production during the entire life of the well.
Steam Assisted Gravity Drainage (SAGD) and Vapor Extraction (VAPEX) processes belong to the first type; cyclic steam stimulation also belongs to this type but is not analyzed here because it is essentially a method for stimulation of a well. Toe-To-Heel Displacement Processes (TTHDP) are of the second type. These processes can be applied in a non-thermal mode, such as Toe-To-Heel Waterflooding or a thermal mode. Thermal TTHDP comprise of THSF (Toe-To-Heel Steamfloding) and THAI (Toe-To-Heel Air Injection) along with its variant CAPRI, aimed at in-situ upgrading of the oil.
The status of SAGD field applications is reviewed, with a focus on the most important aspects to be clarified before the method can be used on a field scale. The main considerations testing of the VAPEX process in the field are also presented.
Underlying concepts behind different TTHDP are examined in light of the available laboratory and simulation results. The main thermal and non-thermal TTHDP and the prospects of their applications are reviewed, including ways to test the process in the field.
Finally, an in-depth analysis of all SDOD processes (SAGD, VAPEX and TTHDP), is made, based on their relative merits in terms of the following factors:
Override/underride due to gravity segregation
Injectant channeling due to reservoir heterogeneity
Injectant/oil mobility ratio causing frontal instabilities.
The key strengths and weaknesses for each of these processes are analyzed and conditions for their applications are identified. The main message of this presentation is: these factors are very important in defining the efficiency of the long-distance oil recovery techniques (patterned floods or line drives), but they are substantially less important in the SDOD processes. This is the reason that in many situations, the efficiency of heavy oil exploitation via SDOD IOR/EOR processes may improve, leading to the foundation of a new technology for oil displacement.