Abstract

This paper describes the use of an under balanced drilling (UBD) system in a remote desert exploration well to directly evaluate the hydrocarbon reservoir inflow potential. The integrated sub-surface and well design team, planning, activity execution and results are discussed, with an assessment of the added value realized. The lessons learnt will be used to optimize the subsequent exploration and appraisal drilling activities, and in the longer term, optimization of subsequent field development activities.

The vision is effectively to inflow test all available hydrocarbon reservoirs while drilling, thereby generating a pseudo reservoir inflow profile as a function of depth without the uncertainty of drilling induced formation damage. To achieve this objective, the available under balance drilling operating parameters and drilling fluids, were proactively controlled. This allowed the rig crew to manage the inflow while drilling, and near well bore skin damage, reducing the uncertainty to optimize the reservoir information acquired. The activity was executed in Q3 2007.

Inflow testing in a known 'non damaged state' while drilling exploration and appraisal wells in desert areas remote from hydrocarbon processing and export infrastructure helps to reduce overall project risk. In low permeability reservoirs, over balance drilling induced near wellbore reservoir inflow impairment, has historically delivered inconclusive inflow data, resulting in higher project uncertainty and higher costs. UBD reservoir characterization information may be used to reduce subsequent well flow testing activities, which in turn, lowers project costs, speeds up the field appraisal process and reduces waste.

Introduction

A venture in South West Algeria aims to explore and develop low matrix permeability gas reservoirs as part of its stakeholder's corporate strategy. Tight (tight=<1millidarcy) gas reservoirs are usually complex, with high stratigraphic and/or reservoir quality uncertainty. Tight gas developments require in-depth understanding of all reservoir units reserves and gas mobility, to reduce the project risk. This is especially true in new country entries for exploration, where well sequencing decisions are critical to effective appraisal of discovered gas bearing structures, aiming to optimize appraisal value from ideally only one appraisal well.

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