Abstract
Using Calcium Carbonate (CaCO3)to improve waterflood conformance is a new concept that was borrowed from drilling operations, in which sized CaCO3 particles are routinely used to prevent lost circulation. The application for water shutoff is to inject a predesigned volume of CaCO3 particles to reduce the permeability contrast between the fractures and the matrix, thereby improving the injection conformance to achieve better waterflood sweep efficiency.
The journey of developing this new conformance technology started in early 2021 by working with a vendor to redesign a bridging agent used for drilling fluid losses into a simpler, less expensive recipe comprising CaCO3 particles with different sizes and a friction reducer to suspend and carry the particles deep into the formation. This technology features several key advantages: low-cost rigless operation, a flexible recipe for materials of different sizes, the volumes and pumping schedule can be adjusted, and the treatment is reversible in case of a blockage.
The first campaign of implementing this technology was successfully completed in January 2022. Three water injectors were treated in the Safah field in Oman, a mature waterflood with an area with high water cuts and low recovery factors. A drop in injectivity was seen after pumping a total of 300 bbl of the two sizes of CaCO3 particles (600 micron and 150 micron), indicating that the treatment achieved the objective. After logging one of the injectors and comparing the injection profile before and after the treatment, we found a clear improvement at the heel section of the openhole horizontal lateral. Injectivity in the thief zone was reduced from 99% to 62% at the heel section, and injectivity at the middle of the lateral improved by 38%.
To date, four separate campaigns of this new injection conformance treatment have been executed targeting a total of 16 water injectors. Excellent operational success of 93% has been achieved, with a significant drop in injectivity noted in most of the treated wells. Six of the treated injectors were logged before and after the conformance treatment to measure the improvement in the conformance profiles. Remarkable improvement was achieved across the injection profile after the treatment. At the heel section of the openhole lateral (the thief zone) injectivity was reduced by up to 60%. Injectivity of the middle section and deeper in the openhole laterals improved significantly for better conformance and sweep efficiency in this mature waterflood.
As a result of improving injection well conformance, a 20% oil gain was observed from several of the treated patterns, which confirms the improvement in waterflood sweep. To our knowledge, this is the first application of injecting CaCO3 particles in water injection wells for conformance purposes. Based on the encouraging results obtained, there are plans to expand its application to other parts of the Safah field and to other fields in northern Oman.
