fax 01-972-952-9435. AbstractConoco has had a sustained, multi-rig, development program in the Lobo field of South Texas since the mid-1990's.
Casing Drilling is an emerging technology for simultaneously drilling and casing a well where the casing is used to transmit mechanical and hydraulic energy to the bit, instead of using a conventional drill-string. A drilling assembly, positioned in the lower end of the casing, replaces the tools normally located on the lower end of the conventional drill-string. This assembly is retrieved with a wire line to access bits, motors, under reamers, MWD/LWD, and other components while leaving the casing in place. The Casing Drilling process has been used on portions of ten wells during a field trial phase. The system is still undergoing development but these wells have demonstrated the functionality of the casing drilling system. Introduction The conventional drilling process for oil and gas utilizes a drill-string made up of drill collars and drill pipe to apply mechanical energy (rotary power and axial load) to the bit, as well as to provide a hydraulic conduit for the drilling fluid. The drill-string is pulled out of the hole each time the bit or bottom hole assembly needs to be changed or the final casing depth is reached. Casing is then run into the hole to furnish permanent access to the wellbore. The Casing Drilling System (CDS) provides an alternative to the conventional drilling system by using ordinary casing for the drill-string. Thus the well is cased as it is drilled which may reduce well costs or enable problematic hole sections to be drilled. The CDS may eliminate costs related to purchasing, handling, inspecting, transporting, and tripping the drill-string, reduce hole problems that are associated with tripping, and save on rig equipment capital costs and operating costs. While the potential savings from reducing drill-string tripping and handling times are important, the savings from reducing hole problems may be more significant. There are many situations where problems such as lost circulation, well control incidents, and borehole stability problems are directly attributed to tripping the drill-string and other situations where these problems prevent the drill-string from being tripped. Since the CDS process provides a continuous ability to circulate the well, it is inherently safer than leaving the well static without a means of circulating it while a conventional drill-string is tripped. Reduced pipe tripping with the CDS should also reduce surge and swab pressure fluctuations. Sometimes it is difficult to run the casing after the drill-string is tripped out because of poor borehole quality. A portion of these problems may be directly attributed to drill-string vibrations causing borehole enlargement.1 The casing drilling system may reduce these incidents by eliminating the tripping and providing a drill-string that is less prone to vibrations. The CDS has been under development for three years as described by Tessari, et al.2 In addition to drilling two directional test wells, it has recently been used to drill portions of ten wells as part of the first field trials. These wells were chosen to prove and refine the technical aspects of the CDS. In the following sections the system is described, engineering considerations for key technical areas are discussed, and key learnings from the field trials are presented. CDS Equipment. The casing drilling process eliminates the conventional drill-string by using the casing itself as the hydraulic conduit and means of transmitting mechanical energy to the bit.3 A short wire line retrievable bottom hole assembly (BHA) consisting of at least a bit and expandable under reamer (Fig. 1) are used to drill a hole of adequate size to allow the casing to pass freely.
Retrievable Casing Drilling * tools have been used to drill more than 600,000 ft of hole in over 120 wells encompassing six casing sizes ranging from 4-1/2" to 13-3/8" and reaching inclinations of 90 o . This drilling system is composed of downhole and surface components that provide the ability to use normal oil field casing as the drill string so that the well is simultaneously drilled and cased. The casing is rotated from the surface for all operations except slide drilling with a motor and bent housing assembly for oriented directional work.A retrievable drilling assembly is attached to the casing inside a proprietary profile nipple located near the casing shoe. The drilling assembly extending below the casing usually includes an underreamer and a PDC or roller cone pilot bit. Other conventional drill-string components may be included so that almost any normal drilling activity can be conducted. These assemblies are retrieved with a wireline at the casing point or at any point in the drilling process when there is a need to change drilling tools.
TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractEngineers and managers make decisions related to drilling operations every day that must balance the risk of various alternatives. These decisions may be based on a mental, experienced based risk model that has been developed over years of decision making. However, many companies are moving to explicit risk models where risk and its economic impact are quantified. Risk is composed of two components: the probability of an event occurring and the economic consequences if it occurs. This modeling uses a logical progression to predict economic impact of trouble events such as well control, loss of circulation and sticking of the drill string or casing string. Applying a risk model to new technology is difficult because there is no experience base for quantifying either the probability of an event occurring or the consequences of the event. This is particularly true when the technology has operational components that go against normal practices. Such is true with Casing while Drilling. Casing is used as the drill string to solve problems that one would normally expect to be made worse by drilling with a smaller wellbore clearance, higher annular velocity, and little ability to make conditioning trips. This paper will show the results of more than 280 wells and over two million feet of hole drilled with casing over the last eight years. Comparisons will be made to conventional drilling methods in several of the areas where the technology has been applied. Elements of study include casing fatigue, casing wear, tool reliability, underreamer durability and reliability, differential sticking, lost circulation induced by increased ECD, acceptable penetration rates, etc. Results of these wells in difficult drilling areas show positively that drilling with casing is more efficient and encounters less trouble time than conventional drilling, saving operators both time and money. These savings can be seen in daily operating costs, personnel safety and in insurance premiums for the operators and drilling contractors.
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