It is a well-accepted rule in high-temperature electronics that, at best, every 10°C increase in operating temperature shortens operating life by an additional 50%. Just as designers of downhole systems for use in wells above 175°C face new challenges in making their designs work to reliability and operating time requirements, so are they finding newly-available opportunities to help them meet those requirements. This paper summarizes a recent cross-disciplinary effort to identify and address the key challenges involved with development of a new 200°C MWD System.
In 2012 we completed a feasibility study, commissioned by a major North American operator, for a 200°C directional & natural gamma MWD system with a mud pulser for real time telemetry and a turbine alternator for power. The primary users of this system would be MWD service providers servicing those operators drilling hot shales in North America. This study included a comprehensive survey of the state-of-the-art in components, materials, system architectures, fabrication & assembly techniques, and statistical reliability & quality techniques required to assure a successful implementation. Sufficient downhole non-volatile memory for directional, gamma and diagnostic recording, along with a "time-stamping" facility for data records, were essential parts of this project. Due to the potentially drastic life-shortening environmental effects of drilling these hot shales, we also specified recording of temperature, shock and vibration data to enable Condition-Based Maintenance (CBM) based on an Accumulated Damage Modeling (ADM) "scoring matrix."
Our completed feasibility study showed that several key technological developments, both inside and outside the oilfield, made the 200°C MWD system technically achievable. The system was also shown to be operationally viable for all three parties in the application chain: our organization, our independent drilling service provider customers and their operator customers. These factors were expected to have a positive effect on service availability for hot hole applications.
This paper provides insights into the drilling technology development process in general and the key technologies available for implementation, specifically, of a 200°C MWD directional & natural gamma system. The diagram below illustrates the key design considerations investigated and the general work flow for the technical portion of the study, and serves as the focal point for the detail developed in the rest of the paper.
A novel, real time drilling optimization system is shown to improve drilling performance by increasing the rate of penetration (ROP) while extending the length of the bit run. Additionally the optimization system reduces the wear and tear on the drill string realizing additional cost savings by minimizing down-hole equipment failures. The result is lower total cost to drill the well.
The optimization system's new features include combining surface data with down-hole vibration data to get the available results in realtime. Additionally, the method uses mechanical specific energy to determine the drillability through stringers and fractures. While the average mechanical specific energy maybe minimized for a given set of surface parameters, the mechanical specific energy spikes whenever stringers, fractures or any other changes in the formation are encountered. The optimization program combines all of these features to determine the optimal parameter set points.
To date, seven wells have been drilled utilizing this optimization system. Significant cost reductions are achieved whenever the optimization recommendations are followed. The system combines analytical drill string modeling with real time drilling data. The real time drilling data includes surface data from the rig EDR system and down hole MWD data. The optimization system analyzes the data and then determines the optimized drilling set points based on the rate of penetration, mechanical specific energy, down hole vibration and data scatter. The system has the ability to identify and resolve drilling dysfunctions in real time, mitigating damage and avoiding unplanned trips.
In the planning stage, drill string modeling analyzes the proposed drill string to assess the structural integrity of the BHA, predicts the drilling tendencies to ensure the planned BHA is capable of developing the directional plan and identifies critical drilling speeds associated with excitation of the motor, bit and or drill string RPM. The modeling application also analyzes the drill string for torque and drag, stick slip and the planned hydraulics. During the drilling process the model is continuously updated accounting for differences from plan. The system optimizes drilling performance in real time and offers solutions for mitigating drilling dysfunctions that could result in unplanned trips if not addressed in a timely manner.
Several examples are discussed in depth, which illustrate the efficacy and benefits of this new drilling optimization system.
Automated downhole rotary steering devices that were developed in the 1990's for directional control were initially deployed on extended reach wells. They are currently more widely applied to improve both drilling performance and wellbore quality. Automated surface control systems have also been developed for safer pipe-handling, downhole pressure control and for mitigating stick-slip when induced by cumulative friction along the drill string. A novel self-adapting downhole drilling tool can now autonomously detect and suppress excessive downhole shock and vibration levels by adjusting the stiffness of its damping fluid which in turn instantaneously changes the vibration characteristics of the bottom-hole-assembly.
The downhole drilling environment is continuously and unpredictably changing as the drill string gets longer and as stabilizers navigate around micro-doglegs or past geological strata where the borehole has enlarged and no longer provides stabilization. Whenever the driller (or an automated control system) makes changes to the surface drilling parameters this also changes the downhole vibration characteristics. Each blade of the bit can additionally induce torsional shocks as it alternately cuts softer and then harder rock at a lithological transition in an inclined borehole when the formation on the upper side of the wellbore circumference is harder or softer than that on its lower side.
In order to respond to a frequently changing environment, vibration sensors in a shock-sub send data to its autonomous control system with an intelligent algorithm that iteratively adjusts the stiffness of a magneto-rheological damping fluid in response to the severity of the various modes of downhole vibrations. In this way the novel self-adapting shock-sub seeks out the optimal dynamic stiffness of the drilling assembly for each change in the downhole drilling environment.
This paper presents insights as to the nature of downhole vibrations encountered when drilling in the Middle East, describes the intelligent self-adapting shock-absorber, examines some of the characteristics of a magneto-rheological damping fluid and explains how the intelligent shock-absorber can be configured for various drilling scenarios.
The state-of-the-art approach to downhole shock and vibration mitigation is to insert into the bottom-hole-assembly (BHA) a shock-sub or torque-limiter with response characteristics pre-configured prior to tripping into the borehole and then continuously monitor downhole vibrations with the driller making adjustments to surface control parameters whenever the downhole shock levels are excessive. Some top-drive control systems are also capable of mitigating the torsional stick-slip dysfunction. A novel approach is to use downhole shock and vibration measurements to actively adjust the damping characteristics of a downhole shock-absorber, thereby changing the axial (and optionally torsional) stiffness of the BHA itself, in response to transient downhole dysfunctions and without the inherent delay of transmitting data from downhole to the driller at the surface.
At the heart of this new adaptive shock-absorber is a magneto-rheological damping fluid whose viscosity is modified whenever excessive downhole vibrations are detected by adjusting the electro-magnetic field through which the damping fluid passes. This is similar technology to that used for protecting buildings during earthquakes and in high performance vehicles that have intelligent shock-absorbers that let the driver select a preferred stiffness for the vehicle's suspension.
This paper describes this innovative downhole self-adapting vibration damper and its autonomous control system that detects drilling dysfunctions and then iteratively adjusts the device's damping characteristics until the destructive downhole forces and motions are mitigated – potentially before the driller at the surface is even aware that there was a problem. Modeling of the magneto-rheological fluid response is presented together with some initial downhole test results demonstrating the tool's ability to control the stiffness and vibration characteristics of a bottom-hole-assembly. Comparisons of downhole shock levels and the amplitude of various types of vibrations at different frequencies illustrate how the self-adapting damper can suppress different modes of BHA dysfunction and thus also demonstrating the device's ability to extend bit life and drilling tool life and to improve overall drilling performance.
Automatically controlling drilling dysfunctions from within the BHA at their source is timelier and potentially more effective than the current approach of telemetering downhole vibration information to the driller at the surface after the bit and drill string components have already been damaged over some incremental time period. The more instantaneous response of a self-adapting tool has the ability to prevent drilling tool failures, reduce bit wear, sustain higher penetration rates with a sharper bit and extend the interval drilled during a single bit run. For certain drilling applications this can significantly reduce the number of bit runs per hole section as well as the related drilling time and cost.
Drillstring vibration is a serious problem, particularly in deep and hard rock drilling; it can reduce the rate of penetration (ROP), shorten bit life, and damage expensive downhole components. Testing of an active drilling vibration damper (AVD) system at TerraTek Laboratory, under conditions designed to induce vibration, demonstrated that the use of the AVD reduced vibration, maintained more consistent weight-on-bit (WOB) and increased ROP.
The AVD has a structure similar to that of a shock sub with the shock absorber filled with magnetorheological fluid (MRF), rather than hydraulic oil. Under the influence of a magnetic field, MRF instantaneously increases its viscosity. Using a series of coils to induce intense electromagnetic fields across the fluid gap, the damping coefficient can be changed in milliseconds by a factor of 7-10. A linear motion detector provides feedback to control the AVD in response to bit motion.
In these tests, the AVD was used behind a tricone bit to drill through blocks of hard concrete, each of which had a 12" granite slab mounted within it at a 10° angle. By inducing an asymmetric load on the bit, the interfaces produced severe vibration during drilling the control holes. A total of 28 holes were drilled, including 11 control holes, at varying WOB and rotation rates.
Analysis of the data confirmed the anecdotal observations made during drilling. The vibrations at the bit were reduced significantly; the variation of the measured WOB was significantly curtailed, and the ROP was increased. These tests demonstrated that the AVD is likely to provide significant time and cost savings, particularly in deep wells. These will arise, not only from the increased instantaneous ROP, but also from fewer trips for bit or equipment changes, and lower costs for replacing damaged MWD tools, motors or other expensive components.
The deep, hard rock drilling environment induces severe vibrations, which can cause reduced rates of penetration and premature failure of the equipment. Conventional shock subs are useful in some situations, but often exacerbate the problems.
APS Technology is developing a unique system to monitor and control drilling. This system has two primary elements: (1) An active vibration damper (AVD) to minimize harmful vibrations, whose hardness is continuously adjusted. And (2) A real-time system to monitor drillstring vibration and related parameters. This monitor adjusts the damper according to local conditions.
APS Technology is pleased to announce the successful deployment of 6-3/4" Drilling Isolation Subs to the North Sea and Canada. A major international service company has integrated the novel design into a Multi-lateral completion system. The sub is used in the initial BHA to both cut a casing window and drill 30 feet of new formation successfully protecting setting tools instrumentation, enabling the program to proceed.
Since the following paper was written, the design has been successfully tested both in the lab and on drilling machines at all applicable levels of WOB and drilling torque. The concept has proven both rugged and strong enough to stand up to the rigors of the drilling environment, and APS is currently preparing to deliver other sizes. The first 4-3/4" tools should be delivered in Q4 2004.
The deep hard rock drilling environment induces severe vibrations into the drillstring, which can cause reduced rates of penetration (ROP) and premature failure of the equipment. The only current means of controlling vibration under varying conditions is to change either the rotary speed or the weight-on-bit (WOB). These changes often reduce drilling efficiency. Conventional shock subs are useful in some situations, but often exacerbate the problems.
APS Technology is developing a unique system to monitor and control drilling vibrations in a 'smart' drilling system. This system has two primary elements: