Bitumen mobility at reservoir (cold) conditions is a major problem in the exploitation of the Athabasca oil sands. A thermal drive can mobilize the in-place bitumen, but the low mobility of bitumen in the vicinity of the producer precludes its production under realistic pressure gradients.
This experimental and theoretical investigation addresses the problem of bitumen mobilization under reservoir conditions, including the case of an underlying water layer. Two situations were considered: One, where the in-place bitumen is mobilized by a solventand surfactant, and the second, where a bottom water path is employed for bitumen mobilization and production, In some areas of the Athabasca oil sands, a communicating bottom water layer of varying thickness is present, that could utilize the techniques developed in his study.
It was found that the bitumen recovery depends on the thickness and permeability of the water zone, the injection-production strategy, well completion and injection rates. Depending on the process used, bitumen recovery varied from 4 to 46% of the bitumen in place, under reservoir conditions, Results of experimental as well as numerical simulation studies re presented, and their significance with regard to field application is discussed.
Economical exploitation of Alberta's vast Athabasca resource remains one of the foremost challenges. A viscosity of several million mPas and limited reservoir energy makes the bitumen virtually immobile under reservoir conditions1,2. A complex reservoir combined with an overburden thickness that ranges from zero to 700 meters, places limitations on the choice of recovery methods in many areas of the deposit. Another factor ffecting bitumen recovery is an underlying high water saturation zone ("bottom water") of varying thickness and extent.
Most of the previously unsuccessful attempts and current promising recovery methods utilize heat to mobilize bitumen, Problems that hindered commercial exploitation include: poor injectivity which may makethe injection pressures unacceptably high; maintenance of interwell communication between the producer and injector; tar blocking which occurs when bitumen moves to the cooler regions and seals the path, creating a zone of zero permeability3,4; and last but not least, the presence of bottom water.
The presence of bottom water in any recovery process has two conflicting effects. It provides the needed injectivity for injected fluid and serves as atransporting medium for the mobilized bitumen5,6,7. On the other hand, bottom water can act as a solvent (if used) or heat sink. This work builds on the work done by Kaleli5 to address the problem of bitumen obilization under bottom water conditions and deals specifically with the mobilization of bitumen under cold conditions. Two situations are considered in thispaper. One, where the in-place bitumen is mobilized by means of a solvent in the absence of an initial channel. In the second instance, an existing bottom water path is utilized for bitumen mobilization by means of solvent and solvent/surfactant combinations. The effect of variables such as bottom water Permeability, injection strategies etc, on bitumen recovery is investigated.
One of the primary problems in the exploitation ofthe Athabasca oil sands has been the difficulty in maintaining interwell communication at injection pressures below fracture pressure3,4,8.