An Integrated Methodology to Locate the Remaining Oil Opportunities in Mature Reservoirs in offshore Sarawak, Malaysia
Authors: Zulhaimi Muhammad Jazlan , S. Wan Ibrahim Wan Mohd , Sanyal Satyashis , Zamri Khairul Azhar , Mohamad Muhammad Naim , M Zaini Khairul Nizam , Tewari Raj Deo , Pradhan Akshaya Kumar 
Published: May 2016

The matured field is located in the Sarawak offshore, Malaysia. Discovered in 1968, the field is an anticlinal structure with vertically stacked reservoirs deposited in a shoreface environment. The brown field has been producing under active aquifer drive with a very high recovery factor. The field is currently producing with more than 85% WC and has significant levels of uncertainties with respect to oil-water contacts, flank structure, depth of spill points, production allocation, SCAL and residual oil saturation.

Major challenges were observed in building fine static models for further dynamic simulation work. A coarse mega model were built covering all reservoirs to capture the STOIIP uncertainties in the field as well as to support an integrated conventional approach to locate the remaining oil and estimating the potential infill reserves. A robust workflow were developed to identify the infill opportunities by integrating material balance study, case hole logs information, water diagnostic plots, decline curve analysis and many more. The comprehensive LTRO methodologies applied has been successful in identifying the location of the remaining oil as well as estimating the potential reserve. This approach has also managed to accelerate field development plan and afforded the team efficient resource optimization by avoiding the more time consuming re-cycling the conventional static-dynamic modeling approach.

Introduction

The field is located 20 km offshore Sarawak in water depths of 100 ft. The field, which was discovered in May 1968, fell under the BDO PSC, which was to expire on 31stMarch 2018. A new EOR PSC was signed on 16thJanuary 2012, which extends the expiry date to 2040.

The field structure consists of a 30,000 by 9,000 ft elongated anticline running NE-SW and comprises a series of stacked reservoirs at a depth of 4000 to 9500 ft. The field is considered structurally simple, with a normal growth fault affecting only the deep reservoirs and no other faults visible on 2D seismic or in the wells.

The field reservoirs were deposited some 20–23 million years ago during Cycle V of the late Miocene in a lower coastal plain to coastal environment. The depositional environment of this field is dominated by a sequence of shore-face deposits, which formed 6–11 million years ago during Cycle V of the late Miocene.

Sands are loosely consolidated, fine to very fine grained and interbedded with layers of silts and clays. Porosity ranges from 14 to 26 % with a field-wide mean of 20%. The permeabilities are in the order of 50 to 3000mD. Net sand thicknesses are less than 30 ft, with most individual sands around 10 ft. The reservoirs are subdivided into three categories (Figure 1):

Shallow Reservoirs at depths of 4000 to 5300 ft, which are characterized by large gas caps and mostly thin oil rims, and are currently considered unfaulted.