Horizontal Underbalanced Drilling of Gas Wells with Coiled Tubing Challenges and Experiences

Abstract

Introduction/Background Coiled tubing drilling technology is gaining popularity and momentum as a significant and reliable method of drilling horizontal underbalanced wells, and 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 metre (3000 – 4300 feet) true vertical depth range, however there is an ever-increasing interest in gas-producing formations. Figure 1 shows a composite vertical section of the horizontal or directional underbalanced 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, as well as its suitability for sour well drilling and live well intervention without 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 underbalanced conditions right through to the production phase. The use of coiled tubing also provides the means for continuous wireline communication with downhole steering, logging and pressure recording devices. Coiled Tubing Equipment 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, 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 60.3mm or 73.0mm (2.375 or 2.875 inch) 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, catwalk and pipe racks was used. Pumping equipment included a fluid pumper capable of liquid rates of 0.10–1.5 m3/min (0.6 – 10 barrels per minute), nitrogen pumper capable of 10–120 m3/min (350 – 4250 standard cubic feet per minute), and a nitrogen bulk storage unit of 50,000 m3 (1.8 million standard cubic feet) capacity. When drilling sour gas formations, a chemical injection pump for corrosion inhibition may be used. Of the two coils commonly available, 73.0 mm is the preferred size due to its added weight on bit, stiffness, increased annular velocity and decreased pump pressure compared to the 60.3mm coil. However, to remain transportable under local highway size and weight restrictions, length of 73.0mm coil is limited to approximately 3000m of useable coil. Wells deeper than 3000 mMD, or alternatively those requiring drilling through 114.3mm (4.5 inch) casings would require use of the 60.3mm size, with a depth restriction more in the order of 4200 mMD.