Surveillance and Initial Results of an Existing Polymer Flood: A Case History From The Rayoso Formation

Abstract

A semi-regional 3D model of the Rayoso Formation in the Neuquén basin, Argentina, has been used to predict that several millions of barrels of additional oil can be recovered by polymer flooding in six fields of YPF S.A. As a result of static and dynamic modelling, two polymer flood pilots in separate fields have been approved. The first pilot started polymer injection in July 2016, while the second one is expected to commence in late 2017. This paper describes the approaches currently used and proposed for surveillance to ensure that the pilot objectives are met and to demonstrate the feasibility of field-scale application of polymer flooding across the Rayoso formation. A multidisciplinary integrated surveillance plan has been implemented to prevent the pilot from failing due to avoidable causes. Data on polymer quality monitoring, tracer surveys, interwell tracers, fall-off tests, and production and injection review are regularly fed into simulation models. We describe how this is achieved while limiting additional staffing and equipment costs. The pilot test discussed here lies in a multi-reservoir field where oil production is commingled with two deeper reservoirs. In order to make the oil production response to polymer injection directly detectable, the central producing wells in the pilot were recompleted to produce only from the Rayoso formation. Since there was no practical possibility to do the same for the second-line (offset) production wells, geochemistry techniques were used to identify the oil produced from each formation and thus estimate incremental produced outside of the pilot area. Although the Rayoso formation has been exploited for more than 30 years, only recently has it been given priority as a production target. At the beginning of the study, several major uncertainties exisited that could adveesely affect the potential success of polymer flooding in this reservoir, especially those regarding the initial and remaining oil saturation in the formation. As a result of the appraisal and surveillance plan applied, key project uncertainties were addressed or assessed by monitoring potential deviations of a set of variables from the project prognosis in real time. Interwell tracers and pressure fall off tests during the baseline waterflood provided data to reduce the uncertainty in the presence of possible flow barriers in the pilot area. Frequent production tests and continuous well bottomhole pressure data enabled calibration of the simulation model and to predict the behavior of the declining phase of the baseline waterflood with resasonable confidence. A single well polymer injectivity test with back flow, performed in one of the pilot injectors, showed that the selected polymer at the target concentration was not significantly degraded in the reservoir.