Drillstring-borehole contact may result in backward whirl: a common and catastrophic phenomenon. To increase BHA (bottom hole assembly) reliability, reduce non-productive time (NPT), and ultimately improve service delivery, an in-house backward whirl testing rig is developed to investigate dynamic loads. Analytical and numerical models elucidate dynamic responses and are validated against backward whirl test results. A 6 ¾-in. BHA is tested in an 8.5 in. borehole using a continuous and discontinuous borehole profile. At a rotary speed of 60 rpm, and using a continuous borehole, lateral acceleration amplitudes range between 20 to 30 g with an RMS value of 3 gRMS. In comparison, a discontinuous profile results in acceleration amplitudes between 40 to 50 g with an RMS value of 7 gRMS. In regard to the discontinuous profile, the response spectrum shows a broader range of frequency content up to 200 Hz while distinct frequency peaks are evident in the spectrum when the continuous profile is used. Results indicate that backward whirl may occur from friction induced contact with different borehole profiles. Further, discontinuous profile yields drastically higher dynamic loads with a broader frequency spectrum than a continuous profile. Furthermore, validated models may serve as useful predictions for the backward whirl phenomenon and the system response. Backward whirl testing of full scale drilling tools with realistic, discontinuous borehole contact, supplemented with validated modeling, is a new approach to understand downhole dynamic events.
This paper presents an investigation of true downhole dynamic conditions through improved testing methods and a new measurement-while-drilling (MWD) tool. Shock loads may cause severe damage to bottomhole assembly (BHA) components. Consequently, characterizing the shock loads and analyzing their distribution within drilling tools produces information for the design process that can improve tool reliability and efficiency. The difference between shock loads and traditional dynamics is rarely discussed within the drilling industry, so this paper first introduces a mathematical description of shocks. Experimental shock and vibration tests are used to investigate the shock and vibration loads experienced by full-scale downhole drilling tools. The test rigs for this effort enable testing of full-scale drilling tools as long as 10 m and a maximum weight of 2 tons. Testing of entire drilling tools, and the emulation of downhole conditions, enables realistic dynamic loading and an analysis of the interaction of tool internal components. These results are not possible when testing single components on a vibration table. During the tests, acceleration sensors positioned at several locations on the outside of the tool body measure the shock amplitude. Simulation models enable analysis of the load distribution within the tested tool. Depending on the simulation results, the reliability of all internal components can be ensured. If necessary, further design improvements are initiated. The simulation models are validated and improved with data from experimental modal analyses of the test rigs and the tested tools. Identifying uncertain parameters for complex dynamics simulation models is a particularly challenging task. Results from the shock tests are compared to the simulation results for additional validation because shock loads, in contrast to typical vibrational loads, are transient. Shock and vibration tests also serve to evaluate the capabilities and limits of current MWD tools in terms of sampling rate and sensor range. A newly developed MWD tool for dynamics measurement was tested, evaluated, and optimized using this testing process. The tests and simulations enable optimization of drilling tools for shock and vibration loads. In contrast with the limitations of historical measurements, enhanced dynamics-measurement capabilities with higher sampling rates and extended sensor ranges provide increased data quality and better characterization of downhole shock and vibration loads. This improved understanding of drilling dynamics loads leads to reliability enhancement and cost optimization by reducing shock and vibration-related tool failures.
Whipstock casing exits are a milling operation that enable operators to sidetrack from a primary wellbore. This paper describes a prescriptive data analytics workflow that was developed and applied to optimize casing exit applications. This workflow involves three major steps: Pre-processing data to cleanse, transform, and aggregate data from past casing exit jobs Building accurate machine learning models to predict milling performance Optimizing a weighted objective function into recommend best-case operational parameters. In collaboration with a North Sea operator, a downhole measurement-while-drilling (MWD) system was used in multiple wells during casing exit jobs to collect a rich data set of downhole measurements. Through a stringent data processing and modeling methodology, prescriptive models were developed and tested in an offset well in the North Sea. Success of the offset casing exit job resulted in greater than a 30-percent reduction in vibration, a 14-percent increase in rate-of-penetration (ROP), and a 23-percent reduction in average mill time.
Casing exits provide operators with an additional wellbore path in which subsequent well operations may be performed. In many instances, a casing exit operation provides an operator with an economic method for accessing a known reservoir zone from an existing wellbore. This is especially true in deepwater settings, which have higher baseline costs. However, milling casing in a deepwater application is often cumbersome and unreliable. The downhole environment is complex, the nature of the formation is often unknown, and directional steering capability is ineffective in casing. Therefore, optimizing casing exits is essential to minimize costs of rig operation, especially in high-cost and high-risk deepwater settings like the Gulf of Mexico. This paper presents a real-time optimization solution using data analytics to improve casing exit efficiency, quality, and consistency. In this respect, a prescribed milling schedule is developed using advanced analytics on historical job data. Further, the use of downhole data and telemetry tools enables the collection and transmission of real-time downhole measurements. Finally, a surface acquisition system provides real-time readouts of the downhole measurements to ensure BHA parameters are optimized in real-time. This optimization methodology was successfully employed in the Gulf of Mexico in collaboration with an operator. Using prescriptive analytics, a milling schedule was provided before a casing exit. During operation, the schedule for downhole weight-on-bit (WOB) and surface rotary speed (RPM) was followed and adjustments were made using real-time downhole measurements. The 13 5/8-in casing exit was successfully performed in a single run and resulted in a high-quality window. Vibration and window drag were minimal, and the milling time was reduced by 10% compared to the average for similar casing exits.
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