Hui, S.K., and Pillai, H., Esso Production Malaysia Inc. (Malaysia)

Abstract

Two oil fields which are characterized by weak natural pressure support and unfavorable rock properties were planned for waterflooding by Esso Production Malaysia, Incorporated. This paper documents the collection and analysis of data for selection of an optimum waterflood pattern. Included will be discussions on parameters that affect short and long term water injectivity, extensive use of numerical models to compare alternate waterflood patterns, on-site production/injectivity tests to validate predictions and injectivity enhancement methods. With a going-in development plan for an inverted 7-spot waterflood pattern, the final optimum plan was assessed to be a combination of 5-spot and peripheral injection.

Introduction

The Guntong and Tabu fields are located in the South China Sea, 210 kilometers off the east coast of Peninsular Malaysia in water depths of about 65 meters (Figure 1). Esso Production Malaysia Incorporated (EPMI) has explored and is currently developing the fields as a contractor to Petroleum Nasional Berhad (PETRONAS), the Malaysian national oil company.

Two major north-south trending faults divide the Guntong and Tabu structures into three areas (Figures 2 and 3). At Guntong the fault separating the west fault block from the remainder of the field is interpreted to be sealing. However, there is currently no conclusive evidence that the central fault block is separated from the east fault block. At Tabu there is no conclusive evidence that either of the two north-south trending faults are sealing. This paper will focus on the waterflood development program for the central and east fault blocks at Guntong and the east fault block at Tabu.

The majority of the oil-in-place at Guntong is contained in 6 main group I sandstone reservoirs. During the exploration phase large gas caps were encountered in the west fault block but only a small gas cap was intersected in the east in one of the lower I reservoirs. Consequently, gas caps are expected to be small or non-existent in the remaining east fault block group I reservoirs. Natural water-drive is expected to be weak. Oil-water relative permeability data from core samples show very low relative permeability to water at residual oil saturation. The oil is typified by a high API gravity, low viscosity and high gas-oil ratio. At Tabu, the east fault block group I reservoirs have similar characteristics. Waterflood was deemed to be most appropriate for development of the central and east fault blocks of Guntong and the east fault block of Tabu. Three 32-conductor platforms were planned for Guntong central and east fault blocks while one 32-conductor platform was planned for Tabu east fault block.

EPMI's previous field development experience had been with the groups J and K sandstone which typically have better reservoir qualities complemented by strong to moderate gas, water or combination drives. The task was to develop an efficient waterflood plan for the two fields.

RESERVOIR DESCRIPTION
Structure

The Guntong field is a east-west trending compressional anticline, approximately 11 kilometers long and 6.5 kilometers wide. The areal closure is about 50 square kilometers and the maximum structural relief is about 280 meters at the I-25 level in the east fault block. The maximum vertical relief in the west fault block is about 220 meters. The structure is asymmetrical, with dips of 5 deg. to 6 deg. on the north flank and 10 deg. to 14 deg. on the south flank. Figure 4 shows an east to west structural cross-section of the main group I sandstone reservoirs.

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