Summary Ultralarge-diameter polycrystalline-diamond-compact (PDC)-bit drilling is a fast-growing cost-effective solution in high-tier deepwater drilling operations in the US Gulf of Mexico (GOM) where salt is encountered in the shallow part of the wellbore. Conventional design called for roller-cone (RC) (IADC Code 111-115) drill bits on positive-displacement motors (PDMs) in these ultralarge-diameter intervals. Cost savings on drilling fluid alone, in the form of rate-of-penetration (ROP) gains through the salt interval, has the industry trending to drill these riserless sections with the use of PDC drill bits on rotary-steerable-system (RSS) drilling assemblies. New robust high-torque-capacity topdrives, stronger drillpipe (DP) connections, larger-diameter RSS tools, and improved mud programs have all largely contributed to this step change in drilling performance. In addition, evolved bit and bottomhole-assembly (BHA) design, efficient operating parameters, improved hydraulics, and vibration-prediction modeling have all aided in the success of these runs. Although this emerging new trend reduces drilling times and associated cost, experience has shown there are multiple challenges that must be overcome to complete a successful run in a single trip. These challenges vary from well to well and include, but are not limited to, BHA steerability, rig-equipment limitations, efficient operating parameters, identification of both sediment and salt formations, hole cleaning and hydraulics, salt creep, drilling-fluid displacement, DP torque limitations, stabilization placement, lateral/ torsional BHA vibrations, and others. This paper will concentrate on the multiple aspects of ultralarge-diameter riserless PDC-bit drilling applications and the considerations that have been used to optimize them. Prior SPE papers and data from previous deepwater GOM case histories were heavily researched and scrutinized to support the conclusions provided within the body of this paper. Together with industry experience available, these findings have resulted in a set of defined recommendations, providing operators with a guide to justify a lower-cost-per-foot approach through the potential reduction of drilling time in these challenging applications.

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Electron field emission from diamond films was calculated using a model consisting of the projection of the energy-band surfaces in the <111>, <110> and <100> emission directions. It is found that the tunneling from bulk conduction and valance bands is negligible in p-type diamond, while emission from n-type doped diamond, surface states located near the conduction band edge, and hypothesized defect bands in the energy gap all have sufficient transmission probabilities to produce the low power-high current observed in experiments. To understand the mechanism for supplying electrons to these tunneling states, we analyze charge transport in diamond with internal fields. In particular, a Monte Carlo simulation was performed for electrons in the conduction band as a function of field and film thickness. The results predict hot electron behavior at lower fields with a transition to ballistic-like behavior for high fields (≥ 10 V/μm) and thin samples (≤0.1 μm). It is suggested that if a viable electron injection mechanism into the conduction band of a diamond-metal or diamond-semiconductor interface could be found for those faces of diamond exhibiting NEA, then a copious cold cathode electron emitter with field tunable energies is feasible.

The advent of wired drill pipe has the ability to allow a variety of measurements to be distributed throughout the whole drilling assembly. One such measurement is acceleration to better determine the impact of how vibration events are distributed through from the bottom hole assembly to the upper drillstring or vice versa. In turn we can now investigate the potential use of distributed dynamics by utilizing a set of designed for purpose independent Downhole Dynamic Data Recorders (DDDR), for real-time decision making. A test project was executed to acquire vibration data along the drill string on a horizontal well in Oklahoma's Woodford Shale. This project allowed the evaluation of data acquired from the bit and the bottom hole assembly (BHA), in the horizontal section, as well as the sensors located in the upper assembly showing the dynamics throughout the vertical section, curve, and landing point of the horizontal. This paper focuses on the analysis of the measurements gathered during the project and it will provide detailed descriptions of the obtained results. Several concepts as well as common known misconceptions related to drilling dynamics will be discussed, among them the decoupling effect of the mud motor to drilling vibrations, the value of downhole torque, weight and bending moment for the understanding of distributed dynamics along the drill string. The importance of the vibration sensors' placement and data recording frequency in order to diagnose and mitigate drilling dysfunctions will also be discussed.

Ultra-large diameter Polycrystalline Diamond Compact (PDC) bit drilling is a fast growing cost-effective solution in high-tier deepwater drilling operations in the U.S. Gulf of Mexico (GOM) where salt is encountered in the shallow part of the wellbore. Conventional design called for roller cone (RC) (IADC Code 111-115) drill bits on positive displacement motors (PDM) in these ultra-large diameter intervals. Cost savings on drilling fluid alone, in the form of Rate of Penetration (ROP) gains through the salt interval, has the industry trending to drill these riserless sections with the use of PDC drill bits on Rotary Steerable System (RSS) drilling assemblies. New robust high torque capacity top drives, stronger drillpipe connections, larger diameter RSS tools and improved mud programs have all largely contributed to this step change in drilling performance. Additionally, evolved bit and BHA design, efficient operating parameters, improved hydraulics and vibration prediction modeling have all aided in the success of these runs. Although this emerging new trend reduces drilling times and associated cost, experience has shown there are multiple challenges that must be overcome to complete a successful run in a single trip. These challenges vary from well to well and include, but are not limited to: BHA steerability, rig equipment limitations, efficient operating parameters, identification of both sediment and salt formations, hole cleaning and hydraulics, salt creep, drilling fluid displacement, drillpipe torque limitations, stabilization placement, lateral/ torsional BHA vibrations, and others. This paper will concentrate on the multiple aspects of ultra-large diameter riserless PDC bit drilling applications and the considerations that have been used to optimize them. Prior SPE papers and data from previous deepwater GOM case histories were heavily researched and scrutinized to support the conclusions provided within the body of this paper. Together with industry experience available, these findings have resulted in a set of defined recommendations, providing operators with a guide to justify a lower cost per foot approach through the potential reduction of drilling time in these challenging applications.