Underbalanced Drilling With Coiled Tubing: A Safe, Economical Method For Drilling And Completing Gas Wells
Authors: Graham R. 
Published: March 2010

Abstract
With the recent resurgence of the gas market and the continued commitment of the oil and gas industry to pursue safe, efficient and economical technology to stay competitive and viable, Underbalanced drilling with coiled Tubing is a natural progression for the 1990s and beyond.
The successful marriage of conventional drilling technology with the obvious advantages of coiled tubing have brought about a new wave of economical gas wells which can be drilled and completed for a fraction of the cost of cased, cemented, perforated and stimulated wells.
This study looks at coiled tubing drilling technology and its advantages. The methodology is discussed and case studies are examined. A peek into the development of this technology is offered and future developments are postulated.

Why Drill Underbalanced?
There have been many papers written over the years discussing formation damage and resultant loss of production and reserves. The debate over the subsequent ability to remove this formation damage via various stimulation and hydraulic fracturing techniques is a broad topic. However, all debate aside, if formation damage is avoided, or at least minimized from the outset, the technique used to eradicate the formation damage becomes a moot point.
Hydraulic fracturing goes well beyond the scope of merely removing formation damage, but serves to increase effective drainage area and potentially tap into other permeability channels such as natural fracture conduits. However, in many areas where there is underlying water, the potential also exists to create a conduit into these zones and effectively negate the ability to produce the well.

Barefoot Completions
The increasing proliferation of barefoot completions is testament to the industry's push for more economical means of producing marginal gas reservoirs.
One of the downsides to barefoot completions is the potential costs incurred by running and cementing casing and then finding that the reservoir is not economical to tie-in. This represents an increase in expenditure over "drill and abandon" costs. However, it has been this author's observation that when such a well is drilled, often considerable expense is incurred in running extra electric logs and testing. Casing is frequently run to attempt stimulation that will reduce potential formation damage.
Setting casing above the zone of interest and then drilling into the zone underbalanced serves two main goals: First, formation damage is minimized and second, the well is production tested while being drilled. This serves to minimize the need for extensive formation evaluation and provides the means to cost effectively determine the merits of tieing in the well.
Often high permeability reservoirs are subject to losses of circulation. Incremental costs in lost circulation material and additional rig time can be incurred and cementing difficulties are also probable. Setting casing above these zones can improve the probability of achieving a good cement job and can eliminate the costs and problems associated with a loss of circulation.

Why Coiled Tubing?
The next consideration involves the technique employed to drill the reservoir underbalanced. There are essentially three options to consider to complete a well underbalanced: drilling rig, service rig and coiled tubing rig.