Profile modification using in situ gelation technology — a review

Abstract

Oil production during the primary stage is achieved due to the natural energy stored in the reservoir. Upon depletion of this energy, the production ceases or the oil production rate becomes so small that it will not be economical to operate. At this stage, a large fraction of the initial oil in place is still trapped under the ground. The oil recovery efficiency during the primary stage is within 10% to 30% depending on the nature of the reservoir. This means that more than70% of the initial oil in place is the target for the secondary and/or improved oil recovery techniques. During the secondary recovery stage, some kind of fluid is injected to push the oil from the injection well toward the producer. Water and gases are the most commonly used displacing fluids in this process. Waterflood is the most common practice of secondary oil recovery techniques. Injection of carbon dioxide or other gases is also a common practice to improve oil recovery efficiency. Regardless of the type of the fluid used to displace the oil, the displacing fluid could bypass the oil and early breakthrough could occur. In the case of waterflood, the water/oil ratio could become so high that the process ceases to be economical any more. For injection of CO 2 or other gases, the high gas/oil ratio renders the process uneconomical. This is more dramatic for heterogeneous and layered reservoirs with contrasting permeability variation among the layers. To remedy the above problem, some kind of polymer solution is injected into the reservoir and is allowed to gel under certain conditions. The gel viscosity being much higher than the displacing fluid could impede the flow of displacing fluid through the already flooded regions; therefore, the displacing fluid is bound to find new paths which means additional oil can be displaced. Profile modification based on in situ gelation technology is an already proven economical process for improving oil recovery. There is a variety of gelation systems available in the market for the treatment of the reservoir. Most of the gelation systems in the market are based on cross-linking of polyacrylamide-type polymers by some kind of heavy metal ions such as chromium to produce a three-dimensional gel structure in situ in the reservoir. Recent research efforts at the University of Kansas have produced a new type of bio-polymer which gels without cross-linker. Gelation occurs by reducing the pH of the alkaline solution and the gelation process is reversible. This paper will discuss the in situ gelation techniques based on the commercially available systems and the newly discovered bio-polymer as mentioned above.