Abstract

The Moomba field was first discovered in 1966 in the Cooper Basin, South Australia with first production commencing in 1969. From inception, the gas wells were drilled conventionally overbalanced with the majority of wells fracture stimulated to optimise production from the tight inter-bedded gas sands. Today the reservoir is highly depleted and conventional methods of accessing reserves are no longer viable over most areas of the field. Santos Ltd. investigated the application of new technology to accelerate the remaining reserves while avoiding formation damage and optimising potential production rates. Coiled Tubing Underbalanced Drilling was identified as a potential enabling technology for this application and 3 sidetracks from existing depleted wells and 2 new wells were drilled in the field in early 2007.

This paper will discuss the planning, implementation and post drilling analysis of the campaign. The paper will also discuss the technical challenges encountered due to the high pressure depletion, complex geology of the reservoir, hostile drilling environment and remote location.

Of the five wells drilled, four produced with increased rates when compared to modelling with a physically recorded zero skin on all wells. One of the two new wells achieved production rates not seen in the field for over 20 years. Overall, the campaign showed it was technically viable to drill wells in a geologically complex, depleted, tight gas field without damaging the reservoir. The paper concludes with a detailed discussion of how the degree of success achieved is reliant upon the selection of the right technology with a comprehensive understanding of the reservoir and its physical properties. In addition candidate selection is illustrated to be a critical element in achieving projects goals.

Introduction

The Moomba field has a history of high productivity since first production in 1969, Many of these wells are now into their production 'tails' and producing at low rates or have ceased production. The field is highly depleted with reservoir pressures in the range of 600 psia from an initial pressure of 3400psia. Field wide static gradient and Pressure Build-Up surveys (PBUs) were performed and revealed that the pressure depletion was widely varied and many wells were subject to high near wellbore skin damage. Typically, wells were drilled conventionally and fracture stimulated in earlier field development phases. However, with the field now severely depleted and the reservoir showing characteristic interbedded sands, shales and coals, the ability to effectively apply fracture stimulation technology is particularly challenging. Coiled Tubing Underbalanced Drilling (CTUBD) was identified as the technology with the best potential for accelerating reserves capture.

The aim of the project was to achieve a zero skin wellbore through the two gas bearing intervals in the Moomba area and therefore, lower the abandonment pressure. This will accelerate reserves from long production 'tails' and potentially add incremental reserves by accessing previously undrained sands.

Well Candidate Selection

Candidate Selection is a key element when applying new technology such as CTUBD (M. Brown et al1). The following criteria were applied in order to rank possible candidates within the existing wells in the Moomba field as re-entry candidates.

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