Drilling Fluids for Highly Deviated Wells in North Sea Petroleum Drilling
Authors: Holder Barry J. 
Published: December 2007

Summary
This paper reviews drilling operations at eight highly deviated wells in three North Sea fields, with emphasis on the use and engineering of drilling fluids to drill the wells and to maintain a stable wellbore until protective casing is set and cemented. Wells with angles deviated more protective casing is set and cemented. Wells with angles deviated more than 500 from vertical are considered highly deviated. The eight wells have an average angle of 560 and an average course length of 3300 m (10,827 ft). In most cases, angle buildup occurs in the measured-depth range of 600 to 1400 m (1,969 to 4,593 ft) at a rate of 20/30 m (98 ft). Figures showing a cross section of the well geometry and pertinent mud and drilling information are presented with each well study.

Introduction
Drilling fluids play an integral part in any drilling operation. Their main functions are (1) to lubricate and to cool the bit and drillstring, (2) to maintain hole stability (i.e., provide a gauge-size hole), (3) to carry drilled cuttings to the surface, and (4) to prevent the entry of subsurface fluids into the annulus.When a deviated well is drilled, Functions 1 and 2 warrant special consideration because the drillstring tends to lie along the underside of the slope formed by the well in the formations. The increased area of contact between the drillstring and borehole wall, and the increased force of contact as the angle becomes more severe, present the drilling and mud engineers with a challenge. Carrying capacity of the drilling fluid also merits particular attention because the vertical lifting power of the mud decreases as the angle of deviation increases and the cuttings tend to be rolled out of the hole along the low side rather than be lifted out.Pipe rotating torque and drag are important considerations when a deviated well is drilled. Factors affecting the amount of torque and drag are (1) dog-leg severity (in both the horizontal and vertical directions), (2) angle of deviation from the vertical, (3) bottomhole-assembly makeup (number, type, and position of stabilizers), (4) mud lubricity coefficient, and (5) hole stability (torque and drag become more severe if the hole becomes undergauge).Of these factors, the last two can be enhanced by mud engineering. Muds formulated with oil as the continuous phase (oil muds) have a very low lubricity coefficient, as shown by field results (Table 1). At the same time, oil muds provide a very stable borehole wall (especially in claystone) to provide a firm base or area of contact between drillstring/stabilizers and the formation. Also, oil muds have an inherently low fluid loss, giving added assurance against sticking because of differential pressure in underpressured permeable formations (especially produced zones). Most operators do not approve the use of oil muds in the produced zones). Most operators do not approve the use of oil muds in the North Sea until they have proved that water-base muds are unfeasible (too costly in terms of mud cost and drilling time) for drilling development wells through the strata overlying and within the oil field. Specialized products available to treat water-base muds to overcome the problems of products available to treat water-base muds to overcome the problems of drilling highly deviated wells include lubricant surfactants and solid-bead lubricants. These products have been applied with success. Most water-base muds now used for development drilling in the North Sea are inhibited with an electrolyte (potassium chloride, gypsum, lime, or, in some cases, seawater alone, which may contain sufficient electrolyte to inhibit clay hydration) and various types of polymer. The advantages of using efficient and sufficient mud-solids control equipment (maintaining a low mud-solids content without excessive dilution) are recognized by all operators. This is essential to maintain an effective and economical mud system, either water- or oil-base, for development drilling.
JPT
p. 703