Historically, invert-emulsion drilling fluids (IEFs) require organophilic clays to provide viscosity and suspension characteristics. While effective, these chemicals are prone to stratification in certain conditions, slow chemical-reaction times, high pressure spikes, and high equivalent circulating densities (ECDs). This paper describes the first application of clay-free IEFs in the Norwegian continental shelf (NCS), with an emphasis on an impressively low and consistent ECD contribution. Further, a treatment was developed to allow the IEFs to be used to drill into a section exhibiting temperatures greater than 160°C.

Introduction

IEFs have been used in the NCS for almost 50 years. These systems commonly use organophilic clays and lignites as their primary viscosifiying and ­filtration-control agents. IEFs containing these compounds have been found to exhibit a higher propensity for fluid stratification than IEFs that use polymers for viscosity and filtration control. Until recently, organophilic clays and lignites have been the primary means to provide viscosity and filtration control in IEFs. While these chemicals are effective, there are several inherent limitations, including high pressures spikes when applying force following a static period and stratification of the fluid. The latter can lead to settlement of weighting agents in a wellbore. In addition, organophilic-clay additives are known to react much slower than polymer-based additives, reducing their effectiveness to maintain fluid properties. This can lead to destabilization of the fluid such that stratification and high filtrate loss can occur, leading to instances of stuck pipe and poor hole cleaning and to well-control concerns. Polymer-based viscosifiers generate suspension characteristics by seemingly unconventional and contradictory means; high gel strengths are generated extremely rapidly but are easily broken. Because of such behavior, they are often referred to as strong yet fragile gels. In addition, at low fluid densities, these systems are less likely to induce losses through decreased solids loading, which can often be attributed to clay and lignite concentration.

Before new chemicals can be introduced into the Norwegian market, stringent environmental requirements must be met. The pre-existing viscosifying polymer used globally in clay-free systems is classified as “black” in Norway because of its inability to biodegrade, meaning it cannot be used. To address this, a polymer-based viscosifying agent has been developed to replace organophilic clay and serve as a much more environmentally acceptable substitute.