Abstract

Some of the older oil fields in Malaysia can be characterized as mature water floods. Schemes for enhancing and prolonging the already declining production and diminishing reserves strategically call for timely implementation of Enhanced Oil Recovery, EOR applications.

Among various Enhanced Oil Recovery (EOR) processes and techniques, Malaysia's reservoirs appear especially amenable to gas injection. Feasibility studies showed that immiscible Water-Alternating-Gas (WAG) injection is potentially the most practical and viable option. Laboratory studies and numerical simulation study conducted on the applicability of immiscible WAG injection using high CO2 content gas indicated that an additional 5–7% of OOIP can be recovered [1]. Such a gain will result in reserves growth and extended producing life of these reservoirs. WAG application is being field tested by implementing a pilot in a sub-block, South-3 of the Dulang field, which is isolated from other sub-blocks, by major faults. One in-fill well was drilled specifically for the current pilot. Upon drilling this infill well, several geological complexities were uncovered, that called for a thorough revisit of modelling and pilot conceptual plans. The pilot area contains 6 wells, three of which are being used as producers during WAG operations.

This paper discusses detail of our first ever EOR pilot implementation (Immiscible WAG in Dulang oil field). It describes progression from conceptual design to various challenges, results and the lessons learnt during early periods of implementation.

Introduction

Dulang Field (Figure 1) is located 130 km offshore north east of Kerteh, Terengganu (Eastern Peninsular Malaysia; water depth of 76m) with an area 11km by 3.5 km. It is an East-West trending symmetrical anticline in the South China Sea. The field plunging to the west and is dissected by a number of NW-SE and E-W faults which sub-divide the field into several fault blocks. The WAG pilot covers the South-3 block. It has been developed through a total of 6 wells and production in this block commenced in March 1991. It was initially exploited by natural depletion. Declining reservoir pressure and production rates led to the implementation of a peripheral water injection scheme through down-dip wells A31 & A29 in 1996. Water was injected mainly for pressure maintenance.

Enhanced Oil Recovery (EOR) is being considered for increasing reserves of the Dulang field. Feasibility studies identified re-injection of the produced gas (containing almost 50% CO2) as a cost-effective EOR option. Immiscible gas injection in the WAG mode was considered for mobility control and improving sweep-efficiency.

Field (pilot) testing was initiated in November 2002 in an attempt to improve recovery from the E12/13 and E14 reservoirs within this fault block and also, to evaluate its suitability as future EOR scheme for the rest of Dulang field and potentially for other Malaysian fields. This is the first EOR scheme to be implemented in Malaysia. In September 2002 the in-fill well, B16, was brought on-stream as part of the overall exploitation plan for the block.

Reservoir Description

The E12/14 reservoirs in Dulang field were deposited in a tidal environment. E12/13 reservoirs are better represented by stacked sand flats with tidal and sub tidal channels traversing through them. E12/13 sediments are capped by coal throughout Dulang field. Both cross ripples and laminar beds are fairly common. These are separated from the underlying E14 reservoir by extensive shale and coal beds.

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