Abstract
Conformance issues have plagued the SACROC miscible CO2 flood for many years resulting in poor sweep efficiency and in some cases no sweep with CO2 channeling between wells in major conduits within hours. These conformance problems have led to very high producing GOR's and CO2 gross utilization ratios as well as significant amounts of by-passed pay and unprocessed reservoir volume.
Conformance problems can be addressed in several ways which can be characterized as mechanical (in the wellbore), near wellbore, and/or in-depth treatments in the reservoir. All of these methods have been utilized with varying degrees of success at SACROC. Recently the most long-term success is with large volume gel polymer treatments which plug high permeability channels and conduits and divert injected CO2 to unswept portions of the reservoir.
These treatments generally involve injecting 20,000 barrels or more of polymer gel consisting of Chromium crosslinked medium and high molecular weight polyacrylate polymer. Gel concentrations typically increase from 5000 ppm at the start to as much as 12,000 ppm in the later stages. One of the keys to the success of these treatments is a final, tail-in stage of very high concentration polymer (> 30,000 ppm) or even cement in certain cases to prevent breakdown of the near wellbore gel when the well is returned to injection or production.
These treatments have been implemented in project areas as a result of CO2 early breakthrough and in project areas prior to CO2 injection startup which has resulted in different degrees of success. This paper will discuss 1) candidate selection, 2) treatment design, 3) treatment timing, 4) discussion of results and 5) conclusions with future recommendations. In summary we have concluded that improving CO2 utilization is most effective with large volume treatments. Additionally, maximum benefit is realized when the treatments are implemented prior to CO2 injection in a new set of patterns.
