Field Implementation of Polymer EOR Technique - A Successful Experiment in India
Authors: Pratap Mahendra , Roy R.P. , Gupta R.K. , Singh Daljeet 
Published: December 2007

Abstract
ONGC has implemented a large scale polymer flood project in Sanand oil field which lies in Ahmedabad-Mehsana block of Cambay Basin in India. The project is well under way and is designed to improve the mobility and sweep efficiency in highly heterogeneous sandstone reservoir. Adverse water flood characteristics were identified and extensive laboratory work was undertaken to demonstrate the benefits of reducing water mobility by increasing viscosity with PHPA polymers. Displacement experiments on long cores and sand packs were used to design the polymer flood slug size and the graded polymer concentrations.
On the basis of laboratory work, a pilot was designed and implemented which was followed by an expanded pilot. Encouraging results in both the phases led to the fieldwide commercial application of the process. Facilities have been developed to ensure proper testing of chemicals before use, low- shear polymer injection, optimum use of biocides, oxygen scavengers and monitoring of injection fluid quality.
This paper aims to present main features of laboratory work, pilot, expanded pilot and fieldwide commercial application of polymer flood in Sanand field. The data generated and presented in this paper shows that a properly designed polymer flood is economically attractive as it significantly improves sweep efficiency thereby improving ultimate recovery.

Introduction
ONGC has been exploring for and producing oil and natural gas from Cambay Basin in India since 1950's. The 1039 - acres (4,204,701 - m2) Sanand Horizon-III unit is the main pool in Sanand oil field which lies in Ahmedabad-Mehsana block of Cambay Basin. The field was discovered in 1962 and was put on production in 1969. In April, 1985, an experimental polymer pilot was commissioned in a limited portion (141 - acres; 5,70,609 m2) of Sanand horizon-III (Fig. 1). The polymer flooding process was selected because extensive laboratory tests had indicated that a considerable increase in oil recovery would result over plain waterflooding in the field. It was believed that increasing the displacing fluid viscosity would improve the pore-to-pore, areal, and vertical sweep efficiencies, The pilot was expanded to 338-acres (1,367,843 m2) in Feb. 1993 (Fig. 2). On the basis of encouraging results of laboratory studies, pilot and the expanded pilot phase, fieldwide commercial application of the polymer flooding process started in Dec, 1995 (Fig. 3). Realising the high degree of heterogeneity and high mobility ratios, which may allow large amount of oil to be bypassed, infill drilling has been planned to further improve recovery efficiencies (Fig. 3).

Reservoir Description
The presence of oil and gas in Sanand field was established in Kalol equivalent pay zones III, IV and IX+X. Pay zone III is the main producing horizon whereas pay zones IV and IX+X have proved to be oil bearing only over a small area in the southern part of the field, Horizon-III, which is overlained by a gas cap on the western side and is underlained by a weak aquifer on the eastern side, is an elongated doubly plunging anticline trending NNW-SSE. Lithologically, the rock is composed of sandstone, calcareous silty shales and siltstones which was despoited under alternating regressive and transgressive marine environments.
Average depth is 1300mts. and the pay thickness ranges from 6 to 8 mts. Porosity determined by log analysis ranges from 24 to 32% and permeability determined by pressure transient tests averaged 1000 md, varying from 3.4 md. to approx. 7 d.
The gravity of the oil averaged 22.3 degree API and the viscosity at reservoir conditions of 140 kg/cm2 and 82 degree C was 18-20 cp.
The reservoir is operating under mixed drive mechanism with a gas-cap support from western flank and a weak aquifer support from eastern flank (Table. 1).

Field History
Sanand field was discovered in 1962 through seismic survey. P. 303^