Horizontal Re-entry Drilling With Coiled Tubing: A Viable Technology
Authors: Graham R.A. , Cox R.J. , Stadlwieser J.A. , Stinn R. 
Published: March 2010

Abstract
Horizontal drilling technology has been the fastest growing segment of the oil and gas industry over the last few years. With the diversity of reservoirs in the Western Canadian Basin, Canada is widely recognized as a leader in this technology. Due to economic, environmental, and surface logistics concerns, reentry drilling from existing wellbores is often an extremely viable solution to horizontal development in existing reservoirs. By utilizing an existing wellbore, many of the costs can be avoided and often troublesome formations are already secured behind casing.
Coiled tubing drilling is also a fast growing technology in Canada, and the obvious advantages of coiled tubing for a safe, efficient drilling operation which lends itself easily to "underbalanced" or "live well" operations have propelled it into the horizontal re-entry market.
This paper examines the rationale behind the selection of coiled tubing as a viable drilling technology for horizontal reentry drilling. A summary of the field trials is also included along with a discussion on the current focus of technology development.

Objectives
In 1995, Morrison Petroleums initiated a program of re-entry drilling in several Mannville pools of Southern Alberta. The objectives of the program were as follows:Accelerate production of Mannville pools in Southern Alberta.Improve total recovery within economic limits.Determine most cost effective technology for reservoir management.Take advantage of royalty relief schemes available for reentries under current regulatory structure.

Technology Evaluation

Why Horizontal Re-entries?
The primary technology options for reservoir management available were as follows: The selection of coiled tubing underbalanced drilling as a reservoir management tool to enhance economics of pool development was arrived at after consideration of the above objectives. The primary technology options available were as follows:Vertical infill drillingHorizontal new well infill drillingHorizontal re-entry drilling
The above options could further be divided into overbalanced and underbalanced drilling techniques and could be conducted with conventional jointed pipe drilling technology or coiled tubing drilling technology.
The overall economics of re-entering an existing well and drilling a horizontal wellbore versus drilling a new vertical or new horizontal well were projected to favour the horizontal re-entry. The elimination of requirements for lease acquisition allows for a more cost effective and timely reservoir development scheme. The royalty schedule for re-entry drilling also offered additional incentive over new drills in this area.
Old marginal wells do not recover reserves in place before economic limits are reached. Horizontal drilling was expected to provide better reservoir access, improved drainage efficiency and an improved recovery factor.
Three wells were selected as candidates for coiled tubing underbalanced drilling after consideration of the above objectives.

Why Underbalanced Drilling?
The reservoir in question consists of tight sandstone with low permeability and porosity and depleted pressure. As a result, these wellbores are difficult to clean up with post-drilling stimulations. Formation damage problems can be circumvented by maintaining underbalanced conditions while drilling.