Successful Drilling Of Permafrost With A Bentonite-Xc Polymer-Kci Mud System
Authors: Kljucec N.M. , Yurkowski K.J. , Yurkowski L.R. 
Published: March 2010

The Arctic, with its permafrost, oilers drilling operators a new challenge. To cope with problems such as hole enlargement, fill on bottom and stuck pipe, operators have tried air, foam, and refrigerated and conventional water-base drilling fluids with varying degrees of success.

A Bentonite – XC Polymer – KCI system is a water-base fluid being successfully used in Imperial Oil's arctic Operations. Laboratory and subsequent field tests demonstrated that excellent rheological properties, a depressed freezing point and low corrosion rates with proper pH control can be obtained with this system.

Introduction

ARCTIC PERMAFROST presents drilling operators with a new challenge. A perennially frozen subsurface formation found in the arctic and subarctic regions of North America and Asia, its thickness can vary from a few feet to 2000 feet or more. Topography, climate, vegetation and type of formation all influence its depth and temperature regime. Equilibrium temperatures of permafrost can range from 5 to 32 °F; seasonal temperature changes affect only the upper 50 to 100 feet. Above the permafrost is a thin active surface layer, 1 to 4 feet thick, which thaws in summer and re-freezes each winter.

Drilling through permafrost poses some unique difficulties. Hole enlargement, fill on the bottom, stuck pipe and poor primary-cement jobs on surface casing when attempting to cement to surface – all resulting in loss of productive rig time – are problems regularly reported by arctic operators. To cope with these problems, operators generally try to drill through permafrost as quickly as possible. Drilling fluids such as air(l,:z,3l, foam (4,5), chilled diesel fuel&lt;6l and refrigerated water-base mud(2,7,8) have been used with varying degrees of success. As permafrost is not homogeneous, but varies in thickness lithology and thermal characteristics, it often happens that drilling methods successful in one area cannot be used in another.

Imperial's Early Experience

Prior to the 1971–72 arctic drilling season, Imperial Oil experienced various difficulties in drilling surface holes through the permafrost zone in the Mackenzie Delta area. Among the more common were hole fill, stuck pipe, washed-out sections and mud contamination with subsequent mechanical failure of mud-handling equipment. Such problems were more serious when drilling in poorly consolidated gravel sections. Initially, conventional water-based bentonite drilling fluid was used. For adequate hole-cleaning, the yield point of the mud had to be kept high, and this was achieved by maintaining a very high bentonite concentration (50 lb/bbl). Solids control in this system was difficult, particularly when drilling through sand sections. Typical mud properties of such a system are shown below:

Plastic Viscosity...... 40–60 cp

Yield Point..........50–150 lb/100 sq.ft

Density.........9.4–9.8 lb/gal.

As valuable rig time was often lost while making up mud and repairing eroded parts of surface mud-handling equipment, average drilling time spent on a surface hole was 17 days.

To-ensure adequate hole cleaning at relatively low temperatures and thus reduce drilling time, a research project was initiated to design a water-base drilling fluid with a high yield point to plastic viscosity ratio.