Abstract

In view of the high permeability, fairly high oil viscosity, moderate reservoir temperature, low salinity formation water and availability of low salinity injection water, the Marmul Al Khlata formation was considered a good candidate for polymer flooding. A pilot consisting of an polymer flooding. A pilot consisting of an unconfined 5-spot on a 200 m spacing was initiated in May 1986. After the injection of a water preflush of 0.23 PV, a polymer slug of 0.63 PV and preflush of 0.23 PV, a polymer slug of 0.63 PV and a water postflush of 0.34 PV, the total oil production was 59% of STOIIP. The injection of production was 59% of STOIIP. The injection of polymer resulted in a sharp increase in oilcut in polymer resulted in a sharp increase in oilcut in all four wells. From the viscosity/concentration relationship of the produced polymer and from a detailed rheological characterisation it was shown that no significant degradation of the polymer has occurred. A satisfactory history match of the oilcut response and of the polymer production was obtained with a numerical simulator. From the simulation results and from analytical calculations it was deduced that the reduction of oil in place in the pilot area since the beginning of the preflush was some 50% of STOIIP. This indicates that a pattern polymer flood in the pilot region offers scope for an increase In pilot region offers scope for an increase In ultimate recovery of some 25% to 35% of STOIIP compared to primary production with aquifer support. The corresponding amount of polymer is about 2.3 to 3.2 kg of active matter per additional m of oil (0.81–1.1 lb/bbl). In a forthcoming pilot extension project polymer flooding will be carried out in an unconfined 5-spot on the original 600 m Marmul well-spacing.

Introduction

The Marmul field, situated in the southern part of Oman was discovered in 1956 by Dhofar City part of Oman was discovered in 1956 by Dhofar City Services but relinquished as non-commercial soon afterwards. Appraisal drilling was initiated by Petroleum Development Oman (PDO) in 1976 and the Petroleum Development Oman (PDO) in 1976 and the field was brought on stream in 1980.

The stock tank oil initially in place (STOIIP) is estimated at 390 × 10 m of which 60% is contained in the Al Khlata formation, a complex sequence of very heterogeneous glacial deposits. The oil in this high permeability formation (1 -20D) has a viscosity of 80 mPa. s. The Al Khlata is currently producing around 5000 m/d of oil from some 90 wells at an average watercut of 50%.

There is a natural flank waterdrive which in some areas is quite strong. It was recognised that in view of the unfavourable water-oil mobility ratio (M=45), there is considerable scope for EOR by polymer flooding and steam-flooding. It was decided to pilot test both processes.)

A small pilot was recommended as a first stage in the testing of polymer flooding.) Its results would serve as guidelines for a possible larger scale pilot.

This paper is concerned with the performance and analysis of this first pilot which is also the first polymer flooding project undertaken in the Middle East.

DESIGN
Objectives

The objectives of the pilot were defined as follows:

  1. To obtain information on improved sweep efficiency by polymer flooding.

  2. To obtain experience with the make-up of a polymer solution in the harsh Marmul desert environment.

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