This paper presents parts of the work performed to develop and qualify a polymer-assisted surfactant flooding (PASF) system for economical use in the Gullfaks Field, Norway. The paper addresses experimental work done in the laboratory, numerical simulation of PASF, and the evaluation of the potential for PASF in full-field scale.The experimental part comprises core-flooding experiments at different temperatures, pressures, and gas/oil ratios (GOR) to optimize the PASF system for the Gullfaks Brent formation conditions. The surfactant in the PASF system is a branched sulphonate (5,000 ppm) and xanthan (500 ppm). The surfactant-polymer slug is followed by a slug of xanthan (500 ppm) for mobility control. No cosolvent is used. In coreflood experiments more than 70% of the waterflood residual oil was recovered.Using reservoir simulation, a suitable pilot area was found in the Brent reservoir. Additional results from simulations were the amount of chemicals, the time needed for the pilot test, and additional oil recovery.Much effort was put into estimating the full field PASF potential. First, the areas of the field where PASF possibly could be used were selected. Key factors were existing and planned well locations, production data, and long-term production forecasts. Then, the amount of chemicals needed and the expected technical efficiency for each area were calculated. To verify these calculations, an area of the field containing two possible injection wells and three producers was selected for a simulation study. This area was considered the most promising area for PASF.The main conclusion from this work is that, with the present crude-oil price and chemical costs, the PASF process is not economically attractive for use in the Gullfaks field, mainly because the residual oil was considerably lower than believed at project start.

The surfactant test iriitiated in a Gu!Ifaks Well in January 1991 1 . was nterrupted before surfactant injection due to operational problems iri the well. Based on this expenence, a new well was identified, and a single well surfactant injection test was planned and conducted dunng the first haif year of 1992. The results, and a discussion of borh the results and the experiences, are presented for the two well operatons.The main objective of the test was to evaluate the efficiency of the surfactant to mobilize oil at reservoir conditions, by measuring the residual oil saturation after vater flooding, and after ihe surfactant was injected.The residual oil saturation was measurcd by the SWCT method. applyirig ethyl forrnate as the reactive tracer. A ota1 of six SWCT tests were performed during the operations. In both welis, extensive data collection programs were executed. This included measuremenr.s of fluid saturations (GST Iogs), fluid inflow and outflow performances (PLT 1025,) as well as pH, ionic composition, and gas content in the back produced fluid. These data were integrated in the evaluaiion of the SWCT tests, and proved to be crucial in understanding arid interpreting reservoir processes dunng testing. A chemical flood simulator was used to model the tracer profiles from the SWCT tests.Special attention was paid to the reduction ofpH in back produced water due to hydrolysis of ethyl formate, and the reduced gas content in back produced water due to stripping of light components of the residual oil by the injected water.The residual oil saturations interpreted from these tests.showed that between 40 and 70% of the remaining oil after water flooding was mobilized by the surfactant.

No abstract

Durch Anwendung von 20 Äquiv.