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.

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Erosion is a form of wear that has been found on the drilling tools used in the oil and gas industry that can, in some cases, severely shorten the life of the tools. In spite of its importance, there has been virtually no attention paid to it in drilling engineering research. This paper focuses on the modelling of erosion and the application of the developed models to improve the design of drilling tools. The mechanism of erosion, which is controlled primarily by the impact velocity and angle, has been formulated based on experimental and theoretical work done in material science. These algorithms were developed for transient simulations of the erosion of any surface in 2D geometry. Based on these, an "Erosion Simulator(1)" has been written, which is able, in combination with any computational fluid dynamics (CFD) software, to simulate erosion in downhole tools. Actual data from TESCO Corporation casing drilling tools(2) has been used to calibrate the physics of the process and validate the software. The simulator has been used to modify the geometry of the under-reamer casing-drilling tool, which resulted in a substantial decrease in erosion rate and therefore an increase in the tool's life. Introduction Erosion, as a form of material wear, has been reported in many areas of the oil and gas industry. An example of erosion in drilling tools is the TESCO Corporation (hereinafter TESCO) "underreamer" tool used in casing drilling(1) (see Figures 1 and 2). Erosion in the drilling industry is important because it can lead to an increase in time and cost of operation. In the case of drilling tool failure from erosion, eroded parts must be replaced. This will cause unexpected extra rig time in order to pull out the drill string and run it with new parts into the wellbore. Besides the cost of operation, erosion may be dangerous. Tool failure can cause a blowoutand be a potential for loss of lives. This paper focuses on an erosion simulation in order to understand and predict erosion phenomena in drilling tools, with the ultimate goal of improving tool design and extending its life. History of Erosion Research Erosion (i.e., a form of wear) occurs when fluid containing solid particles impacts a solid surface. The intensity of erosion is commonly measured as a specific weight loss (rate of material removal from the surface) and expressed as Er (the weight of material removed by unit weight of impacting particles). During the 1960s and 1970s, a number of important experiments were done in the area of metal wear that laid the foundation for the current understanding of the phenomena. Erosion experiments during that period covered impact velocities up to 550 m/s and particle sizes of up to 1,000 μm [Tilly(3)]. Different velocities and particles can cause different types of damage. During this time, scientists and researchers determined the relationship between erosion rate, the type of material, size of particles, velocity, and angle of impact.

Erosion is a form of wear that will cause loss of material. It has been reported in the drilling tools used in the oil and gas industry, and it can in some cases severely shorten the life of the tools. In spite of its importance, there has been virtually no attention paid to it in drilling engineering research. This paper focuses on the modeling of erosion and the application of the developed models to improve the design of drilling tools. Mechanism of erosion, which is controlled primarily by the impact velocity and angle, has been formulated based on experimental and theoretical work done in material science. Algorithms were developed for transient simulation of the erosion of any surface in 2-D geometry. Based on this, an "Erosion Simulator 1 " has been written, which is able, in combination with any CFD software, to simulate erosion in down-hole tools. Actual data from TESCO Corporation casing drilling tools 2 has been used to calibrate the physics of the process and validate the software. The simulator has been used to modify the geometry of the under-reamer casing-drilling tool, which resulted in a substantial decrease in erosion rate and therefore increase in the life of the tool.