Abstract

The paper summarises the strategy and methodology employed to maximise well productivity and minimise formation damage during the well construction process for the Wanaea and Cossack projects. Actual performance against expectations and targets is reconciled. The outcome has been oil wells with lower than expected skins and as a direct consequence, aggregate well potential is in excess of nameplate process capacity. The incremental cost associated with achieving the better than expected well potentials has been small due to the methodology employed, but the incremental value is high.

The strategy adopted can be summarised as:

  • Where are we now?

  • What is possible?

  • How do we get there?

rather than the incremental methodology of 'how can we improve'.

This was found to be a very powerful strategy. The methodology used to implement this strategy is described, which included a life cycle approach to completion design and a laboratory based programme to assess various aspects of the drilling and completion processes on productivity. The application of 'fit for purpose' technology; including a novel horizontal completion design and use of long (100m) wireline conveyed perforating guns is also described.

Introduction

The Wanaea and Cossack oil fields are located in some 80m of water approximately 100km offshore North Western Australia. The two fields are being developed in parallel using subsea wells tied back via manifolds and flexible flowlines to an FPSO with a design life of 25 yrs; Reference Figure 1. Well construction activities occurred between July 1994 and February 1995, with production start-up in November 1995. Maximising Net Present Value (NPV) over the life-cycle of the well requires achieving the correct balance between:

  • Initial CAPEX (ie. well construction costs);

  • Tubing size vis a vis well productivity (initial production rate, forecast production profiles);

  • Well reliability and intervention costs (surveys, workovers etc.); and

  • Installing the appropriate degree of flexibility to cater for reservoir performance uncertainties.

The prime cost driver for the process of well construction was identified as rig time and therefore reducing rig time became the primary focus for cost reduction. However, well productivity was identified as potentially having the most significant impact on Net Present Value (NPV) over field life and therefore any proposed cost reduction was weighed against potential impact on well productivity.

This paper focuses on the application of the above strategy on well productivity and completion design and summarises the solutions adopted. The development and application of the philosophy and the necessary cultural climate is fully described in Reference 1.

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