Field Measurement and Analysis of Circulating System Pressure Drops With Low-Toxicity Oil-Based Drilling Fluids

Abstract

The ability to predict accurately the circulating system pressure drop whilst drilling is a prerequisite of drilling optimisation. The quantification of the annular pressure drop is also necessary to calculate the equivalent circulating density of the drilling fluid. With invert emulsion drilling fluids, using 'low toxicity' paraffinic oils, these calculations are complicated by the viscosity changes under downhole conditions.

This paper presents data from three separate hole sections where annular and drill string pressures were continuously monitored using Measurement While Drilling (MWD) instrumentation. Readings were taken between 2625 and 14764 ft (800 and 4500m) with mud densities ranging from 9.33 to 11.67 ppg (1120 to 1400 kg/m3). The data are summarised and tabulated with respect to the hole geometry, circulating rates and drilling fluid properties.

The derivation of correction factors for the estimation of the drilling fluid rheology at elevated temperatures and pressures is detailed with results of laboratory studies using. HP/HT viscometry.

The theoretical pressure drops through the annular and drill string sections, with and without application of the rheology correction factors, are then presented. Two rheological models are employed, the power law with yield stress (Herschel-Bulkley) and the Bingham model.

The theoretical pressure drops are compared with the actual values. The application of the rheology correction factors is discussed and the comparative merits of the two models reviewed.

Finally the paper discusses the implications of the research to hydraulics planning and identifies critical areas of development required to enhance the predictions.

Introduction

The accurate prediction of the circulating system pressure drop, and the distribution of this within the various sections of the system, is required in the planning and monitoring of the drilling operation. Drilling optimisation demands an understanding of these pressures to maximise the energy transfer to the bit. The drilling fluid specification requires a knowledge of the systems pressure drop when considering annular flow rates and consequential hole cleaning performance. There is also a need to understand annular pressure drops, especially in weak formations, to minimise excessive overbalance within the wellbore.

Historically, the drilling industry has relied upon the application of non-Newtonian fluid flow models for these calculations using surface measured flow properties. However, particularly for invert emulsion systems, doubts have been raised as to the accuracy of these models, and of the distribution of the pressure drops within the system.

The use of the low toxicity, paraffinic, invert oil emulsion drilling fluids has developed within the North Sea operating area during the last five years. This is primarily due to the improved inhibition these systems impart to the soft Tertiary shales of the area. Additionally, enhanced rates of penetration have been documented when employing these invert emulsion fluids through the Cretaceous shales (Ref. 1). Therefore, despite the constraints imposed by environmental considerations, most development wells, many appraisal and some exploration wells now rely on the use of these fluids.

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