Abstract

There are many aspects of water based muds (WBM) which must be improved before they can truly approach the performance of an oil based mud (OBM). OBMs provide increased drilling performance by combining shale hydration inhibition, drill string lubrication, reduced stuck pipe risk, low formation damage, corrosion avoidance and high temperature stability. The importance of these attributes is briefly discussed.

Historically, it has been common practice to focus on a single factor (such as shale inhibition) when developing ecofriendly WBM products. Insufficient attention has been given to the mutual compatibility of mud additives designed for different functions. Examples are provided on incompatibilities and side effects which can result from a blinkered component development process.

This paper describes the development of a new, integrated WBM system designed to provide much of the performance benefit of an OBM in all but high temperature well applications. The separate approach to high temperature WBM development is briefly discussed. Each component of the new system was developed to display not only high performance in its intended function, but also mutual compatibility and, where possible, synergy with the other additives. Other design criteria included the avoidance of detrimental side effects such as formation damage and/or adverse environmental impact.

Taking each criterion in order, results are presented on the performance of individual components, both alone and in combination. Synergism and product multifunctionality are highlighted. Finally, results are given for all the performance indicators of a fully formulated drilling fluid containing all the new components. Field trials have commenced on a partial system and this experience, together with subsequent case histories, is discussed.

Although OBM performance may not yet be replicated, an integrated approach does allow the target to be approached.

Introduction

In recent years it has become clear that the discharge of cuttings from drilling operations using low toxicity mineral oil-based muds (LTOBM) can cause significant environmental damage which is slow to recover. In offshore operations the degree of sea bed contamination has been proved to be unacceptable. Consequently environmental legislation is increasingly restricting the discharge of cuttings contaminated with LTOBM.

Much work has therefore been applied to the development of improved water based muds (WBM) of low eco-impact. By far the largest effort has been applied to the minimisation of wellbore instability caused by the hydration of sensitive shales. Although shale hydration inhibition is very important it is not the only fluid function requiring improvement before a WBM can approach the performance of a LTOBM. This has been recognised within the industry but the targets remain some way from achievement. OBMs allow high drilling performance by combining shale inhibition, drillstring lubrication, reduced risk of stuck pipe, low formation damage, corrosion avoidance and high temperature stability.

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