In today's high cost drilling environment, vibrations are recognized as the primary contributor to drilling inefficiency. As such, accurate prediction, identification, and quantification of vibration along the drill string has become increasingly important in order to reduce drilling costs. Near-bit measurements are also particularly important, as the drill bit is often predicted to be an important cause of vibration. This paper reviews a project within Ecuador, where high vibration and mechanical damage to drill bits has been observed, clearly affecting performance. This application is drilled with Fixed Cutter (FC) bits on a Rotary Steerable System (RSS) through a challenging sequence of formations including sandstone, shale, limestone, and conglomerates. Initial steps had been taken to optimize the drill bit design in terms of cutting structure, gauge geometry, and cutter technology. However, in order to truly understand the vibration issues, a unique downhole dynamic logging tool was utilized. This tool records drilling dynamics data at a high frequency sample rate, enabling lateral and torsional events to be accurately identified and recorded. Due to its size, and thus flexibility in string placement, the tool was located in two different points in the BHA.Evaluation and interpretation of the vibration data led to the recommendation of new drilling parameters, very different to traditional practices in this application, in a critical section for the next well in the same pad. This resulted from detection of torsional vibration, not only at the bit, but also at points along the BHA. Implementation of the recommended drilling parameters resulted in significant mitigation of torsional vibration at the bit and in the BHA. This resulted in reduction of wear to components within the BHA, as well as both time and cost savings in excess of $200K.
Many developments in the deepwater Gulf of Mexico target reservoirs are below salt sections. As well plans increasingly call for drilling through long salt sections; a common strategy in the Gulf of Mexico is to drill while simultaneously opening the hole with an under-reaming tool. This configuration presents unique dynamic behaviors to the drilling system. Understanding the drilling system is important in order to reduce vibrations and to meet desired run lengths. In the Gulf of Mexico dynamics have limited the effectiveness and life of hole opening tools. Therefore, the identification of drillstring dynamic behavior is critical to efficiently executing well plans that call for under-reaming while drilling. To study the vibration phenomenon unique to under-reaming while drilling bottomhole assembly dynamics through salt sections the operator employed a unique distributed downhole measurement system which included vibration sensors. The goals of this study were to evaluate the accountability of the real-time MWD and surface data, to illuminate complex dynamics and provide further stimulus for the development and adoption of real-time strategies. The distributed measurement system generated insights in all three areas. This paper presents examples of specific dynamic behaviors from several deepwater Gulf of Mexico wells, focusing on the drilling phenomenon occurring in salt. Key observations about the development of vibrations while opening the reamer, drilling, drilling transitions and back reaming are given. Attenuation of vibration to the surface is demonstrated. Predrill modeling of bottomhole assemblies was also cross-validated. The impact of cutting structure effectiveness and bottomhole assembly "neutral point management" is also highlighted with examples. Introduction Deepwater Gulf of Mexico wells often require drilling through the salt canopy. (see Figure 1.) In some plays, subsalt targets equate to the majority of the well being placed in salt. Similarly subsalt targets mean the wells may be very deep and therefore benefit from a large hole size to provide adequate clearances for deepening casing strings. Maintaining hole verticality is also important to limit casing wear due to hang down weight effects. Casing running in salt presents the added challenge of salt creep, often requiring oversized hole. (Zhang et al. 2008) A successful method to address the challenges of drilling deep wells through salt is simultaneous underreaming while drilling. The use of logging while drilling and rotary steering tools introduces constraints to underreamer placement and over all bottomhole assembly (BHA) design that may limit drilling performance. Chevron's (the operator) current developments in the deepwater Gulf of Mexico are successfully using rotary steerable tools and simultaneous underreaming. Distributed measurement in drilling assemblies is an established method of investigation for the operator; however this study is the first of its kind known for the deepwater Gulf of Mexico. (Marland et al. 2004) Many directional measurement schemes employ measurement at several locations. Distributed vibration measurement is often achievable in a BHA using multiple LWD tools with vibration logging capability. This ability is often marginalized or ignored as realtime telemetry and memory data prioritize formation evaluation data. Vibration measurement complements avoidance of destructive vibration modes by supplying symptomatic information about the downhole dynamic conditions. When vibrations are high, anecdotal evidence is used to diagnose a dynamic condition. Based on that diagnosis one may attempt to mitigate the phenomenon.
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.
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