Monobore Completions For Slimhole Wells
Authors: Robison C.E. 
Published: October 2008

A growing number of operators are looking at monobore and slimhole completion techniques as possible solutions to the problems in the oilfield today that demand greater cost efficiency in well completions. Each technique is capable of providing enhanced completion capabilities that can substantially impact completion costs, and since these benefits affect different phases of well technology, attempts have been made to unify the techniques into a single completion approach that will attain the combined benefits. Each philosophy requires specialized equipment to provide its operational benefits, however, and attempts to unite the two strategies have created unique challenges for equipment designers. In many cases, these challenges are not even fully understood.

This paper will present several equipment enhancements that are ideally suited for monobore completions for slimhole wells. Also discussed will be new servicing methods available and how use of these options and the newly-designed equipment can address the difficulties currently challenging proper design of completion equipment.

INTRODUCTION

The volatile price of oil has forced the oil and gas industry to focus more heavily than ever before on drilling and completion costs. Any techniques, therefore, that can offer a method for effecting reduction in the cost to drill and complete a well are worthy of investigation. This explains the recent resurgence of interest in the monobore and slimhole concepts.

Monobore completions offer a highly-desirable degree of flexibility for maintenance operations to the producer, By design, the tubing imposes no restriction to any tool that may be deployed in the liner. Since use of special tools is eliminated for these wells, lower costs result. Unfortunately, these operational savings are typically not realized until later in the life of the well. Because cost savings from use of the monobore concept are anticipated rather than immediate, it is difficult to justify additional expense during the completion phase, which plays an important role in decision making for the complexity of the downhole components.

In contrast to the cost savings generated by monobore completions, the cost savings from the slimhole concept is normally realized during the drilling phase, as use of slimhole drilling techniques reduces the cost of the cement, fluids, tubulars, etc. The timing of the cost savings, therefore, explains why many operators are more inclined to adopt the slimhole concept and attempt these procedures in their wells.

BACKGROUND

In the mid 1950's, slimhole drilling re-surfaced as a viable means of drilling in soft rock formations.l At that time, technology had advanced to the point that a 4¾-inch [12.07 cm] bit could achieve penetration rates similar to that of an 8 ¾inch [22.23 cm] bit, and the cost savings for slimhole drilling came in the form of reduced horsepower requirements for the rig. However, the hazard from the reduced mud volume and the limitations on the other downhole services resulting from the small hole size imposed an obstacle that was insurmountable with the technology available at that time.