Pushing the Envelope of Polymer Injectivity in Low Permeability Sandstones

Abstract

Summary Chemical EOR is one of the more attractive methods to improve oil recovery. Numerous successful projects including injectivity tests, pilots and full-field developments have been executed without major injectivity issues or decline. Nevertheless, this topic remains a concern among operators. Polymer Flooding has seen more interest from the industry, and more challenging reservoirs (low permeability formations) are considered—thus raising concerns about injectivity. Filter ratio is routinely used as an injectivity screening criteria, but does it correlate with polymer injectivity and propagation during coreflood experiments, especially in the presence of crude oil? This paper provides new insights on polymer injectivity in cores considering polymer molecular weight, chemistry, rock permeability and mineralogy. The results are obtained from dedicated experiments and examination of several extensive data bases (including the literature). State of the art commercial polymers of varying chemistry with molecular weight ranging from 5 to 27+ MDa were injected into different sandstone cores having permeabilities between 10 to 200 mD with a range of clay content. Filter ratio was also determined and compared to injectivity in cores. All the data comes from field project case studies using reservoir cores and representative outcrop cores. For HPAM, injectivity was not a concern. It was possible to propagate up to 27+ MDa HPAM in a 100–200 mD core without significant pressure build-up. Concerning ATBS polymers, injectivity initially appeared to be constrained by the ATBS content; a 15 MDa polymer with a medium-high ATBS content poorly propagated below 200 mD. However, optimization based on molecular weight for similar ATBS content showed stable propagation in representative porous media. Finally, the filter ratio test did not always correlate to injectivity. Indeed, it was observed that several 1.2 µm FR tests (performed on high Mw polymers) failed despite successful transport in cores having permeability below 200 mD. In addition, acrylamide-based terpolymers allowed improvement in transport of ATBS polymers - a 20 MDa polymer containing medium level of ATBS was able to propagate in less than 100-mD cores. These observations are applicable to cores having clay content below 5%. For higher clay content, injectivity should be assessed case by case using reservoir core and crude oil. This paper establishes new references in terms of polymer transport behavior in porous media and highlights the importance of appropriate selection of polymer, polymer quality and experimental protocols to properly assess polymer injectivity in cores. Significance of the proposed paper: 1. Extensively examines the lower limits of permeability for injection of synthetic polymers, especially as a function of polymer molecular weight, polymer composition, rock mineralogy, and the presence of residual oil. 2. Better characterizes the relations between filter ratio, permeability and polymer injectivity in low-permeability rock.