Abstract

In prolific gas reservoirs with strong aquifer drive, water coning is often the determining factor in well productivity and ultimately, gas recovery factor. One of the main drivers in optimising the field development in these cases is to reduce the drawdown of the wells and consequently, a horizontal well design is often used. It is the case, however, that in such highly permeable gas fields extending the horizontal length beyond 1000–2000 ft often does not help to improve well productivity any further, as well bore friction becomes the constraining factor. This paper presents a cost effective and simple solution to increase well productivity by 60% compared to single horizontals at a marginal cost increase. A simple level-1 dual lateral with large bore casing-flow well design has shown to be able to deliver the additional productivity at only a 5–10% incremental increase in cost. Furthermore, we present the results of the numerical simulation that helped to justify the well design and to deal with interference between the branches.

Introduction

Horizontal wells have become common place in oil and gas fields to increase well productivity. In the Central Luconia region 250 km offshore Sarawak, Malaysia, horizontal wells are a key element of the field development approach for a large number of gas bearing carbonate build-ups that have prolific reservoir quality (200 to >1000 mD) and a strong aquifer drive. As a consequence of the field characteristics, (fast) water encroachment and water coning play a dominant role in the field behaviour. The horizontal wells aim to reduce the drawdown and thus the effect of coning, whilst at the same time maximise the offtake per well, and the ultimate recovery.

However, in the prolific gas fields, extending the horizontal well length beyond 1000–2000 ft does not reduce drawdown any further as the well bore friction becomes the constraining factor, and the toe of the well does not contribute to the well's production. To further enhance the productivity of our horizontal wells, a cost-effective dual lateral well design was identified to potentially increase the well productivity by some 60% compared to a single horizontal.

This paper will present the well design, the details of a numerical simulation study to justify and optimise the design concept, and finally a discussion of the simulated versus the actual well results.

Field summary

The field is a prolific carbonate build-up that has been in production since Dec 1996 at an average field offtake rate of several hundred MMscf/d. The average field permeability is in the range of several hundred mD, and the original gas column height was some 300 ft. To date the field has produced 74% of the ultimate recovery and has been experiencing water breakthrough in its deviated producer wells since mid-1998. The aquifer has risen on average some 150 to 200 ft, leaving only a relatively small gas column unswept. To maintain the field capacity and to further increase the field ultimate recovery it was decided to drill two horizontal infill wells, one according to the newly-identified well design, and one conventional 7.5/8-in tubing-flow single lateral horizontal well. In this field, coning and cusping would be a major constraint in maximising ultimate recovery as well as reducing the well capacity upon water breakthrough.

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