Horizontal Slim-Hole Drilling With Coiled Tubing: An Operator's Experience

Summary What is believed to be the first horizontal well drilled with directionally controlled coiled tubing recently was completed in the Austin Chalk formation. An existing well was sidetracked out of 4 ½ -in. casing with a conventional whip stock. An average build rate of 15°/ 100 ft was achieved in the curve, and a 1,458-ft vertical section was drilled with 2-in. coiled tubing, downhole mud motors, wireline steering tools, a mechanical downhole orienting tool, and 3 7/8-in. bits. This paper discusses the orienting and directional tools and techniques developed during this operation. It also describes improvements made for the second well. Introduction The Austin Chalk formation within the Pearsall field in Texas consists of a Cretaceous limestone through which production occurs from natural fractures. Drilling through these fractures requires careful preparation because the rig may encounter prolific flow of oil or complete lost circulation. During the discovery of the Pearsall field, wells were drilled vertically to the Austin Chalk. Horizontal drilling currently is the preferred technique for development of the Austin Chalk. Through horizontal drilling, a single wellbore can intersect several fractures, potentially increasing initial flow and ultimate oil recovery. Fresh water usually is used as a drilling fluid because it will control formation pressures when circulation is lost. Surface pressure containment equipment allows the flow of well fluids and gases to the surface under controlled circumstances where they are processed during drilling. In the field, this technique is commonly called "flow drilling." This method minimizes loss of drilling fluid to the formation and increases rate of penetration (ROP). The severity of either lost circulation or prolific flow can pose operational and safety problems. Loss of circulation can prevent adequate removal of drill cuttings, increasing the potential for sticking the drillstring. In addition, the logistics for providing an ample water supply in this semiarid area are complex because the large volumes required to power the downhole mud motors are lost to the natural fractures. Prolific flow from the fractures necessitates the handling of large volumes of oil and gas at pressures that can approach the limits of the surface equipment. Safely tripping into or out of the hole with drillpipe requires killing the well, which sometimes calls for the use of expensive and environmentally undesirable heavy brines. Therefore, a method of tripping while under pressure without killing the well would greatly enhance flow drilling operations. This problem was addressed by replacing the conventional drillstring with coiled tubing.