Performance Of Polymer Flood In Sanand Field, India - A Case Study

Abstract

Abstract Twenty two years of history of polymer flood in KS-III sand in Sanand field, Gujarat State, India, proves the efficacy of the process in a reservoir of 850 C temperature. KS-III sand of Middle Eocene age, is one of the few successful cases of polymer flooding in such high reservoir temperature. Reservoir is acting under partial edge water drive. Oil viscosity of 20 centi poise led to adverse mobility ratio which resulted in cusping of water in structurally higher wells. Hence polymer flood was considered the best option in this reservoir for improving mobility ratio of oil and overall areal and volumetric sweep efficiency which was also supported by the laboratory studies on native cores of the field. Polyacrylamide polymer was selected for injection. Polymer flood process in Sanand field has undergone many stages of implementation. After initiation of polymer pilot in an inverted five spot pattern in 1985 with one injector, 4 producers and one monitoring well, it was extended to four inverted five spot patterns in 1993 with nine producers and four injectors. Effect of the injection was visible in terms of reduction in water cut, improvement in oil production and increase in reservoir pressure. Success of pilot projects encouraged to full scale field implementation in 1996–97. Polymer injection was done in peripheral pattern in the main part of the field. Subsequent reviews have been carried out from time to time and mid course corrections have been taken accordingly. Current paper discusses different stages of development in polymer flood, the steps taken from time to time, lessons learnt and conclusions drawn on the basis of practical experience and reservoir simulation. Introduction KS-III sand in Sanand field in Ahmedabad-Mehsana tectonic block of Cambay Basin is a sandstone reservoir in Kalol formation of Middle Eocene age. The field was discovered in 1962 and was put on commercial production in May 1969. KS-III sand is the major hydrocarbon bearing sand in the field with 64% of proved oil-in-place and 95% of total oil production. Till date, the sand has produced about 23% of proved oil-in place. Geological, Reservoir & Fluid Properties Sanand field is located at the western margin in the southern part of the Ahmedabad-Mehsana tectonic block of Cambay basin which is a north-south trending intracratonic rift basin. Structure is NNW-SSE trending elongated doubly plunging anticline(Fig.1). Sanand is a multi-layered reservoir in Kalol sands but KS-III is the main reservoir. Three NNWSSE trending longitudinal faults separate the oil bearing area of main block from western gas bearing blocks and the nonreservoir part in the west. One transverse fault, in the southern part, makes the southern boundary of main block, KS-III. Another transverse fault separates block of S-17 from the main block based on fluid contacts. OWC in the main block is at 1327 mts MSL and in the block of S-17, it is at 1347 mts MSL on the eastern part. A shale layer of varying thickness of 0.5 to 2.5 mts separates the KS-III reservoir into 2 units. This reservoir is operating under partial aquifer support. KS-III sands are a combination of shale, silt, sand and coal deposited under a combination of marine, coastal marsh and deltaic plain environment. Reservoir sand appears consistent laterally with the main portion of the field but vertical heterogeniety is present which can be seen on logs as well as cores visible in the correlation profile (Fig 2). Maps made on the basis of well data, indicate the deposition of the reservoir facies in few depocentres (Fig. 3).