Polymer Flooding for EOR in the Schiehallion Field - Porous Flow Rheological Studies of High Molecular Weight Polymers

Abstract

The BP operated Schiehallion Field, UKCS has been on production since 1998 through an FPSO which is in the latter stage of its original design life. In order to capture additional oil and gas that has been accessed by further exploration, field extensions and technology applications, in 2006 BP and its license partners embarked on a project to re-develop the greater Schiehallion area. This included the initiation of the Quad 204 Project to replace the existing FPSO, yielding an opportunity to implement an Enhanced Oil Recovery (EOR) scheme through incorporation of EOR equipment in the new project facilities. A polymer flood, utilising an acrylamide-based polymer (HPAM), is currently being evaluated as the leading EOR technology option. Polymer flooding seeks to improve oil displacement efficiency by increasing the viscosity of the injected water-flood. A more viscous flood gives a lower mobility ratio of injected brine to displaced oil, improving local, areal and vertical sweep efficiency and increasing oil recovery. The biggest challenge to the success of a polymer flood in Schiehallion is maintaining reservoir voidage. In maintaining voidage, down-hole pressures must increase when injecting a viscous phase. This is a concern on Schiehallion, where the injection pressure is limited. This restriction has led to the extensive program of laboratory work detailed in this paper, which shows, with associated reservoir modelling, that a relatively low viscosity polymer solution can materially improve water-flood recovery, while maintaining reservoir voidage. A further challenge is the adverse weather conditions west of Shetland which makes the deployment of a liquid, rather than a powder polymer the only practical choice. If the polymer flooding scheme is sanctioned, Schiehallion will be one of the first full-field offshore schemes utilising this technology. This paper details the laboratory test program covering both rheological and porous flow studies. Initial rheological work focused on optimum in situ flood viscosities and polymer dosages. This was followed by injectivity studies, measuring polymer Resistance Factors (RF) at different flow rates in aqueous-phase core floods, utilising field core. This paper then progresses to report results from a reservoir condition ‘live’ core flood. This measured polymer adsorption and Residual Resistance Factor (RRF) under field conditions. Finally a section on reservoir simulation identifies the importance of these two variables in reducing uncertainty in estimates of incremental oil in reservoir models.