fax 01-972-952-9435. AbstractThe effects of downhole vibration on drill bit performance have been discussed a great deal in the past several years. Data acquired on near-bit vibration has proved useful for assessing bit selections, design features and running parameters. Typically, engineers have had only limited access to this type of data because measurement-while-drilling (MWD) tools capable of acquiring vibration measurements are generally placed well above the bit in the bottom hole assembly (BHA), where the dynamics can be significantly different than at the bit. Also, these conventional MWD tools are expensive to operate.The recent development of an easy-to-use, memory-mode, vibration-logging tool has made obtaining relevant " at-bit" data much more feasible. The tool is very small and can be used in collar sizes as short as 8 in. from shoulder-to-shoulder. The tool can be placed directly above the bit without disturbing the BHA. This paper highlights the development of this tool and presents data from recent field tests that show how it has been used to optimize and evaluate drill bit performance in various applications.

Motor Steerable (MS) assemblies are used globally in the majority of directional drilling applications. However, the limitations of MS systems are often highlighted as well paths, become more complicated and efficient drilling performance becomes more difficult to obtain. Tools that impart cyclic, axial oscillations into the drill string have been shown to extend the operating range of MS assemblies, particularly in slide-drilling mode, including coiled tubing applications (Robertson-2004). Adding the correct Axial-oscillation Generator Tool (AGT) to a drill string can dramatically improve slide drilling (Rasheed-2001); and significantly improve work over operations with coiled tubing as well (Tongs-2007). When evaluating the overall performance of a drilling system, it is necessary to determine the interaction between the AGT, Bottom Hole Assembly (BHA) and drill string as a whole. This paper will document the magnitude of the forces generated by the AGT and compare them to the specifications of other common drilling components. It will confirm that the correct selection and configuration of an AGT will impart benign forces into the drilling assembly, improving the effectiveness of the drilling system, and maintaining full compatibility with the entire drill string. Historically, measuring the actual forces transmitted by the AGT has been a challenge, as most downhole measuring devices record at sample rates significantly lower than the operating frequency of the oscillation tool. These recording devices often lack the necessary number of data channels required to fully describe the downhole dynamic environment. Furthermore, variations in bit type, BHA configuration, drilling parameters and formations make evaluating results from different wells difficult. To accurately measure downhole vibration and other drilling parameters, a Drilling Research Tool (DRT) was used. The DRT is a high-speed, sixteen-channel, downhole dynamics recorder. By ensuring identical BHA's, drilling parameters and formations, the differences in downhole activity can be attributed to the axial oscillation system. This paper will compare a number of field runs with and without an AGT and will quantify the actual downhole accelerations caused by adding the oscillation system to the drill string. The results will show that the addition of an effective axial oscillation tool positively influences drilling performance while maintaining compatibility with all drill string components. Introduction The effects of adding pressure pulses to drilling assemblies have been studied and is well documented. The practical benefit of adding the appropriate axial oscillation device to the drill string has often meant the difference between being able to slide drill and having to stop short of finishing a desired hole section. Recently, there has been research into how hydraulic vibrations (pressure pulses) can influence static friction of pipe against borehole walls (Barakat-2007). External, axial oscillation has also been shown to reduce friction more effectively than other oscillation types (Newman-2007). The results clearly show that hydraulic vibrations and axial oscillation can be helpful in reducing static friction. This paper expands on the laboratory tests that were performed and uses actual field data to quantify the performance benefits associated with an AGT system. In addition to evaluating the performance improvement, this paper also analyzes the compatibility of the AGT system with other elements of the drilling assembly; particularly Measurement While Drilling (MWD) tools and the dynamic influence the AGT could have on these tools.