Horizontal Underbalanced Drilling of Gas Wells with Coiled Tubing

Abstract Coiled tubing drilling technology is gaining popularity and momentum as a significant and reliable method of drilling horizontal underbalanced wells. It is quickly moving into new frontiers. To this point, most efforts in the Western Canadian Basin have been focused towards sweet oil reservoirs in the 900–1300 m true vertical depth (TVD) range, however there is an ever-increasing interest in deeper and gas-producing formations. Fig. 1 shows a composite vertical section of the horizontal or directional under-balanced wells drilled with coiled tubing in Canada, and also highlights the aforementioned trend towards oil over gas. Significant design challenges on both conventional and coiled tubing drilling operations are imposed when attempting to drill these formations underbalanced. Coiled tubing is an ideal technology for underbalanced drilling due to its absence of drillstring connections resulting in continuous underbalanced capabilities. This also makes it suitable for sour well drilling and live well intervention without the risk of surface releases of reservoir gas. Through the use of pressure deployment procedures it is possible to complete the drilling operation without need to kill the well, thereby maintaining under-balanced conditions right through to the production phase. The use of coiled tubing also provides a means for continuous wireline communication with downhole steering, logging and pressure recording devices. Coiled tubing drilling rigs currently exist in several different forms and setups, but generally consist of a drilling coil, injector to feed the tubing into or out of the well, (blowout prevention) (BOP) system for well control and return fluid path, and a control cab and power unit. The coiled tubing drilling equipment used to drill the case studies presented later in this paper consisted of either 60.3 or 73.0 mm coiled tubing and injector with a hydraulic power unit with control cabin and accumulator. A coiled tubing hybrid mast unit and substructure complete with V-door and catwalk was used. Pumping equipment included a fluid pumper capable of liquid rates of 0.10–1.5 m3/min, nitrogen pumper capable of 10–120 m3/min, and a nitrogen bulk storage unit of 50,000 m3 capacity. When drilling sour gas formations, a chemical injection pump for corrosion inhibition is typically used.