Guidelines for Polymer Flooding Evaluation and Development

Abstract

Abstract Field experience has shown that polymer flooding can be an effective means to improve oil recovery. Evaluating whether a polymer flood is suitable for a given field and developing the optimal design requires considerable analysis and testing prior to full-scale implementation. To help manage this process, guidelines for polymer flooding evaluation and development were developed that are described in this paper. The guidelines are a specific case of a more general staged process, which is also described in this paper, for evaluating enhanced oil recovery methods. The polymer flooding guidelines cover initial screening, laboratory measurements, reservoir simulation, and field activities that are considered best practices. Descriptions of specific activities have been compiled into a matrix that serves as a valuable guide to managing the various aspects of polymer flood evaluation and development. These activities cover a range of topics including reservoir simulation, evaluation of polymer solution properties, polymer solution preparation, injectivity, facilities, quality assurance, and economics. Introduction Use of a polymer-augmented waterflood (i.e., a polymer flood) is a technique to enhance oil recovery from a reservoir by improving reservoir sweep and reducing the amount of injection fluid needed to recover a given amount of oil. Polymer floods work by adding low concentrations of water-soluble polymers to injection water to increase the injectant viscosity. This is done to more closely match the injectant viscosity to that of the in situ oil and thus achieve a more favorable mobility ratio. A number of reviews on the application and benefits of polymer flooding exist.1–4 Over the past thirty years, polymer flooding has been applied on modest scales in a number of areas and in large-scale applications in China, but its application has not been widespread.5–9 One reason for the lack of widespread use may be the technical challenges associated with designing an economically attractive polymer flood. Although the basic concept of polymer flooding is straightforward, the evaluation and design of polymer floods is significantly more complex than primary depletion or waterflooding. Evaluating whether a polymer flood is applicable for a given field depends on a number of factors, which include: oil viscosity, mobile oil saturation, ability for the polymer to propagate through the reservoir, compatibility of the polymer with reservoir rock and fluids at in situ conditions, reservoir heterogeneity, well spacing and flow rates, polymer costs, preparation and quality control of injected polymer solutions, and the ability to sustain injectivity. As such, a proper polymer flood evaluation and design requires a combination of reservoir characterization, laboratory testing, reservoir simulation, facilities design, and field testing. If a polymer flood is found to be suitable for a given reservoir, design variables such as polymer type, polymer slug size, and polymer concentrations need to be optimized. Optimization is complicated by the additional physical phenomena that are not present in conventional waterfloods. Simulation of full polymer-flood physics requires modeling polymer concentration-dependent viscosities, shear-thinning rheology of the polymer solution, extensional-thickening rheology near the wellbore, in situ mixing (dilution) of the polymer solution and native brine, thermal degradation, shear degradation, polymer adsorption onto the reservoir rock, inaccessible pore volume (physical exclusion of macro-molecules by narrow pore throats), and relative permeability changes due to adsorption (e.g., residual resistance factors).1,2 To help manage the complexity and challenges associated with polymer flooding, guidelines were developed for a staged process to evaluate and develop a polymer flood. The guidelines, which are summarized in this paper, represent recommended procedures that are generally applicable to any field (e.g., offshore, onshore, large, small). The guidelines cover initial screening, laboratory work, reservoir simulation, field testing, field piloting, and finally commercial application.