Design of a Pilot Polymer Flood in the Marmul Field, Oman

Abstract

Abstract The Marmul heavy-oil field is located in South Oman in the province of Dhofar. The main sandstone reservoir consists of glacial deposits of Permo-Carboniferous age and contains about 2.5 billion Permo-Carboniferous age and contains about 2.5 billion barrels STOIIP of 21 degrees API crude. The field is in the stage of primary development with a current production of 45000 BPD. production of 45000 BPD. Oil production by depletion is expected to be low, while a water drive will be adversely affected by the high oil viscosity and high permeability. Thus the Marmul field offers ample scope for EOR techniques, for which two methods have been selected, viz. steam injection and polymer injection. In view of the long lead times involved early testing of these techniques is planned. This paper deals with the design of the relevant polymer flood and pilot tests. A suitable mobility polymer flood and pilot tests. A suitable mobility ratio is determined from calculated drive efficiencies and related polymer requirements', allowing for polymer retention and for viscosity grading of the polymer flood. In the selection of a candidate polymer, attention is paid to viscosifying power, retention characteristics, plugging tendency, shear stability, and last but not least maximum operational flexibility in the field. As for the latter, a liquid polymer seems preferable to a powder polymer. Results of laboratory experiments indicate that polyacrylamide emulsion polymers provide attractive polyacrylamide emulsion polymers provide attractive properties for application in Marmul. Retention in the properties for application in Marmul. Retention in the highly permeable sands is low and so is the plugging tendency. However, viscoelastic effects cause very high pressure gradients at high flow rates, which may adversely affect polymer injectivity. This can be resolved by subjecting the solution to controlled shear treatment prior to injection. Numerical simulations predict that the oil recovery is substantially enhanced by polymer injection. To have a qualitative indication of the sweep efficiency improvement in the first (inverted five-spot) pilot test, it is proposed to have the polymer drive preceded by injection of a limited polymer drive preceded by injection of a limited volume of water. The reversal in oil cut should then provide an indication of the better sweep. provide an indication of the better sweep Introduction The Marmul field, discovered in 1956 by Cities Service P.C., is located in South Oman (Fig. 1). In 1969 Petroleum Development Oman (PDO) acquired the concession and started appraisal drilling. Since 1978 the field has been developed full-scale and the first crude was produced in October 1980. The field contains a STOIIP of more than 400 million m3 of heavy crude oil, divided between three clastic units: Upper Haushi (Permo-Carboniferous) - 'stacked'reservoirs of fluvial origin, present only on the flanks of the structure, and not yet appraised indetail. Lower Haushi (Permo-Carboniferous) - complex and laterally variable glacial deposits which containmost of the STOIIP, with oil viscosities ranging from 50 to 120 mPa.s. Haima (Cambro-Ordovician) - fluvial sands generally of poor reservoir quality. A high proportion of the primary production to date has been obtained from Lower Haushi reservoirs, which also are the present target for enhanced recovery projects. projects. Production forecast studies indicate that the primary production of the viscous crude will be rather primary production of the viscous crude will be rather limited, the more so since no bottom-water drive is indicated as yet. There is evidence suggesting influx of edge water. The low recovery can be increased by pressure maintenance, but because of the unfavourable pressure maintenance, but because of the unfavourable mobility ratio the injection of water will result in early water breakthrough and long tail production at high water-cuts. For economic analysis an average recovery factor for waterflood is estimated at 20% of STOIIP. Thus the Marmul field provides ample scope for enhanced oil recovery methods which can improve the sweep efficiency by reducing the water/oil mobility ratio. Both polymer and steam drives are considered highly promising in this respect. P. 513