Estimation and Improving Recovery Under Water Flood: A Case History From Ankleshwar Field
Authors: Mamgai D.C. , Rangaraj S. , Singh Lehmbar , Singh Daljit 
Published: December 2007

Abstract
Ankleshwar field in the Cambay Basin, Western India, is on production since 1961. It is producing from multilayered sandstone reservoir, wherein S3+4 is the major contributor. The Sand S3+4 is further having six sublayers. Water flooding in all sublayers of Sand S3+4 was started in the year 1966. From this sand, 85 per cent of expected 55 per cent oil recovery, calculated on the basis of core data and simulation studies, has already been realised.
As a normal reservoir phenomenon, in a water flooded heterogeneous reservoir, the advancing flood front generally leaves behind unswept pockets of oil, leading to low recovery. Mapping of flood front advancement, i.e. dynamic modeling, coupled with estimation of recovery factor have been used for Sand S3+4 in the present study as a tool to locate unswept oil. Inputs in the form of infill wells, transfer of horizon, selective perforation and regulated water injection, for exploitation, have been designed and executed for improving the recovery. Two different methods viz. (i) by use of initial and post flooding saturation values and (ii) calculation of ultimate oil production of each well/area, have been applied to estimate flushed zone recovery from different layers and flow regimes.
The study reveals that the ultimate recovery from the Sand S3+4 with water flood can be improved from 55 per cent to around 61 per cent with judicious inputs based upon this study.

Introduction
Ankleshwar Field in Cambay Basin of India is a multilayered reservoir with 9 oil producing sand units of Middle to Upper Eocene age. It has 133.62 MMT (1063 MMbbls) of initial in place oil reserves and 71.22 MMt (567 MMbbls) of ultimate oil reserves. The field was put on production on 15th August 1961 and as on 31st Dec. 1994 the cumulative oil production from the field is 61.34 MMt (488 MMbbls) with 9.88 MMt (79 MMbbls) balance of oil reserves.
Sand S 3+4 is the major oil bearing reservoir of Ankleshwar Field. It has an initial in place oil reserves of 69.33 MMt (552 MMbbls). The sand 53+4 was initially developed with a well spacing of 500 m X 600 m, that is, 74 acre spacing. 63 wells were initially planned and drilled for development of sand S 3+4.
Sand S 3+4 is under exploitation since October 1961. As on December 1994, cumulative oil production from this Sand is 34.31 MMt (273 MMbbls) amounting to 49.5 % of initial in place reserves and 89.9 % of expected ultimate reserves. The plateau oil production rate of 4400 to 4500 TPD (35024 BOPD to 35820 BOPD) was attained for 10 years during 1968-1978 through 65 to 73 flowing wells. The oil production rate as well as productivity of wells started declining from 1979 onwards as shown in Fig. 1. In 1968 the peripheral wells started producing water and since then there is a steady increase in water cut. Presently the Sand is producing oil at the rate of 694 TPD (5524 BOPD) with 66.9 % water cut through 64 wells.
The initial reservoir pressure of the sand was 1696 psi at mid of oil column (3652 ft below MSL). The saturation pressure of the oil is 1487 psi. Though the water drive is the prominent drive mechanism, for higher recovery and good production potential, the Sand was water flooded from early stage of production. Water injection was going on simultaneously for all the six sublayers, 54 (1+2+3+4) and 53 (1+2), in-spite of their varying permeabilities.
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