Abstract

Wandoo is a shallow, very permeable, unconsolidated reservoir located 80 kmoff the coast of north-west Australia. The oil has a viscosity of 14.4 cp at the original reservoir conditions of 50 C and 926.5 psi. There is a smallgas-cap and the reservoir is underlain by a moderately large aquifer. The reservoir is highly productive with permeabilities in excess of 30 Darcies inlarge sections of the field. Production tests on the discovery and appraisal wells produced significant quantities of sand. The reservoir characteristics have resulted in development of the field with horizontal wells equipped withpre-packed sand screens.

This paper describes the evolution of the horizontal wells from the initial500 m completion run with a wash-down string, to the latest 1100 m completions run without wash-down strings, the effects of different clean-up mechanisms on well productivity, what we believe are key parameters in the well construction and installation, and the findings from coiled tubing production logs run to date.

Introduction

The Wandoo oilfield is situated 80 km North West of Karratha, Western Australia in Permit WA-14-L. The field lies in 55 m of water and has an arealextent of approximately 2 km × 6.5 km (Figs 1 and 2 show the location and a top structure map).

The field has recoverable reserves of 75 mmbbls. The 22 m oil column is located in very high permeability, unconsolidated, glauconitic sands at approximately 580 m 55 TVD. This thin oil column has a small gas cap above and an active aquifer below. The oil is moderately heavy (190 API), has a high live oil viscosity (14.4 cp) at reservoir conditions and a GOR of 99 scf/stb. Key reservoir and fluid properties are given in Table 1.

The M. Australis formation is of Cretaceous age with two main reservoir units, the 'A' and 'B' sand. The 'A' sand has a higher glauconite content, lower permeability and is slightly more consolidated than the underlying 'B'sand. During production testing of the early appraisal wells Wandoo 1 and Wandoo 2, significant quantities of formation sand were produced (up to 15lb/bbl). These early DSTs also proved that gas and water coning were easily established.

In summary, the early discovery and appraisal well data highlighted the following:

  • Sand production from the shallow, unconsolidated reservoir sands was likely without some form of sand control

  • The thin hydrocarbon column leads to uncertainty in long term well productivity due to potential gas and water coning

  • Processing Wandoo crude would be difficult because it is heavy, viscous, has a low GOR and forms stable emulsions.

The combination of these factors prompted Ampolex to undertake a stageddevelopment of Wandoo, rather than commit to an immediate full field development (FFD) option. The first stage was a 120 day Extended Production Test (EPT), with process facilities installed on a converted jack-up rig located next to the 5 slot Wandoo 'A' monopod.

Phase I

The ability to drill deviated or horizontal wells in the unconsolidated M.Australis formation was uncertain due to concerns regarding hole stability and the ability to steer the well in such a soft formation. The well designs initially incorporated a cautious approach. In order to effectively compare different completion techniques for potential application during FFD, two completion designs were selected for the initial EPT wells. P. 57

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