Underbalanced drilling has generally been recognized as providing drilling penetration rates exceeding those of conventional overbalanced drilling operations. The use of coil tubing, with its absence of connections and resulting continuous drilling, can further influence rates of penetration. Under the right reservoir and drilling conditions, these rates can far exceed conventional rates, and it begs the question as to how fast wells can be drilled.

Four wells were drilled in 1996 for Mobil Oil Canada in the Glauconitic Formation in the Taber area of Alberta. The horizontal sections of the wells were drilled underbalanced with coiled tubing and one of the objectives was to maximize the rate of penetration of the drilling operation. Penetration rates in the order of 60 metres per hour were observed for sustained periods. The fastest of the four wells was a 164 m horizontal leg drilled in under 6 hours elapsed time, with another well having a 486m horizontal leg drilled in an elapsed drilling time of approximately 25 hours.

High rate drilling has its obvious economic advantages by virtue of minimizing rig time resulting in lower associated costs. It also adds some complexities, however, as high rate drilling can affect the ability to directionally steer in the formation, react to any changes in lithology or underbalanced conditions, as well as increases the potential for hole cleaning problems due to loading the wellbore with solids.

This paper will present the drilling results of these four wells and address the issues described above.


As with a conventional drilling operation, the economics of a coiled tubing drilling operation are subject to both fixed and variable costs. The most significant variable costs are the day rate for the rig and nitrogen consumption, both of which are highly dependent on the rate of penetration (ROP) and the time taken to drill the well. Figure 1 shows the sensitivity of coiled tubing drilling costs to rate of penetration for a typical Southeast Alberta Glauconite oil well. From this figure it is clear that optimization of penetration rates can have significant impacts on the economics. Note that these costs have been normalized to reflect undiscounted costs of the drilling contractor only, and are exclusive of bits, production testers, trucking, and other third party costs and is included primarily for illustrative purposes.

Penetration rates can be optimized by selection of bits and motors, the degree of underbalanced pressure, and maintaining continuous weight on bit. The selection of drilling fluids and the ability to drill continuous hole by ensuring hole cleaning also has a significant effect on ROP. A common strategy in coiled tubing drilling is to conduct regular wiper trips, which are short or partial trips out of the hole while circulating for the purpose of "wiping" the hole clean and removing any cutting beds that may have developed in the horizontal or build sections of the well. Wiper trips may range in duration from 30 to 90 minutes, and can have a significant influence on the drilling time and the resulting day costs.

It should be noted that the term "ROP" can have different definitions or applications depending on the intended use of the data. For example, evaluation of bit performance would consider the ROP as the metres drilled divided by the number of hours in which the bit was actually on bottom and rotating, such that the performance of the bit could be evaluated independent of external influences (eg. survey time, wiper trips, downtime). For the purpose of this paper, this definition of ROP is referred to as "true ROP". From an economic perspective, however, the term "ROP" can have a significantly different connotation as it is intended to pertain to the overall time interval over which drilling occurs inclusive of wiper trips, surveys, equipment adjustments, etc. such that it reflects "billed" time rather than "drilled" time. For the purpose of this paper, ROP for economic purposes is referred to as "effective ROP".

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