Keith RappoldDrilling Editor
The use of vibration measurements to detect bit and drillstring dysfunctions in real time can improve drilling efficiency. Elf Aquitaine Production has developed a surface sub to detect vibrations in the drillstring in the well. This drilling dynamics sub, or dynameter, measures resonance phenomena (axial, torsional, and lateral vibrations) from the bit, bottom hole assembly, and drillstring.
A description of how this sub is used to identify and help eliminate stick-slip will appear in the concluding article in this series. During stick-slip, the bit stalls and accelerates cyclically as the rotary speed at the surface remains constant. During stick-slip drilling, penetration rates are low and the bit wears faster.
In addition to identifying stick-slip, the measurement of these vibrations can also help identify downhole drilling problems such as bit bouncing, bit wear, blocked cones, bit or stabilizer balling, stabilizer hang up, and backward whirl. Surface measurements of weight-on-bit and torque alone cannot measure these phenomena.
For example, conventional surface torque measurements may show constant torque. During stick-slip, however, the torque period is low; too low for detection on ordinary gauges. With bit bouncing, the small up-and-down movements are dampened by the elasticity of the drillstring. Previously, drillers could only suspect bit bouncing after analyzing a worn bit at the surface-too late to alter parameters to improve drilling or prevent damage. With drilling dynamics measurements, the driller can detect bit bouncing as it occurs and can modify drilling conditions to eliminate the dysfunction.
When vibration measurements indicate a specific problem, or for that matter an unknown source of inefficient drilling, the driller can modify the drilling parameters (weight-on-bit, rotational speed, flow rate) to attempt to eliminate the dysfunction before severe problems develop. Drillstring dysfunctions can reduce the energy transfer between the surface and bit, increase metal fatigue, increase premature bit wear, decrease the rate of penetration, and cause well bore damage.
After the drillstring and drilling parameters are optimized, further signal changes may be an indication of changing geology.
Traditional rate of penetration, torque, and pressure logs can provide only a simple picture of the drillstring, bit, and well behavior. A driller must use this information, a few rules of thumb, and experience to manage the rig operations efficiently. The drilling dynamics measurements are not a full replacement for these logs, however. Rather, the intent is to have the drilling dynamics measurements complement conventional data (ordinary measurements, measurement-while-drilling, mud logging, etc.).
Drilling dynamics measurements have several aims:
- Optimization of the rock destruction process
- Elimination of premature pull out of the bit
- Reduction of the number of drillstring fatigue failures
- Tracking of the rock formations drilled without a time lag
- Optimization of mud circulation (cleaning and lubrication)
- Assessment of the overall state of the well during drilling.
The use of real-time drilling dynamics data helps a driller achieve these goals through adjusting the drilling parameters in real time, optimizing bottom hole assembly configuration, continuous monitoring of the bit (teeth and bearings), and identifying and remedying drilling dysfunctions.
DYNAMETER
The dynameter is a centralized measuring sub placed either beneath the power swivel or above the kelly. The use of the dynameter below the power swivel offers the most information because the drillstring torque is input above the tool. Above the kelly, the dynameter cannot directly access the torque. The changing position of the kelly with respect to the rotary kelly bushing also influences data interpretation. Careful processing of the measured data helps eliminate the effects of the type of drive.
The dynameter, about 1 m tall and 44 cm in diameter, can be installed easily in the drillstring and will not interfere with normal drilling operations (Fig. 1). The tool is placed at the top of the drillstring like any other standard sub. Once in place, the tool needs no other attention from the driller; drilling and tripping continue normally.
The tool can operate continuously for about 200 hr on an integrated battery pack. During smooth drilling, the sub can be turned off to save the battery life. Before a trip out of the hole, the sub is removed from the drillstring. During the trip, the battery is recharged.
Various strain gauges and accelerometers inside the tool measure the vibrations which travel from the bit up the drillstring (Fig. 2). The tool measures axial, torsional, and transversal accelerations, tension, torque, temperature, and rotational speed. This information is digitized and then sent by radio transmitter to an antenna located nearby on the rig. The data then travel by cable to a control cabin for computer processing and interpretation by a signal-processing specialist.
The data can be analyzed as a frequency spectrum on a computer screen in the drilling dynamics cabin on the rig.
Traditional surface measurements can be graphically displayed alongside these measurements for trend analysis.
The drilling dynamics specialist analyzes the data, then confers with the driller about which drilling parameters, if any, need adjusting.
Elf Aquitaine Production's Dynafor team began development of the dynameter in 1986 as part of a project to study the dynamic behavior of the drillstring. Drillstring behavior was considered as fundamental as the rock in optimizing the drilling process.
The Dynafor project sought to optimize the rock destruction process by improved control of the dynamic drilling system behavior, to improve real time information about the nature of the formation being drilled, and to reduce drilling costs by approximately 5-10%.
Elf Aquitaine Production has licensed the drilling dynamics technology to Exlog Inc.
DYNAMICS MEASUREMENT
The dynamics measurements use the bit/rock boundary conditions as the source for the main excitations (Fig. 3). The drill bit converts the rotational energy into destructive vertical translation energy by successive percussions on the rock. The length, elasticity, and composition of the drillstring, the complex well geometry, and the different types of formations drilled affect the vibrations generated. The drillstring transmits the signals from the drill bit and transforms them according to its own dynamic behavior. The drilling rig is the topside boundary.
The vibration data from the drillstring can be broken into two intervals: a low frequency interval from 0 to 25 hz and a high frequency interval from 25 hz to 100 hz (Fig. 4).
The low frequency interval corresponds to the first modes of the drill pipe and bottom hole assembly, the fundamental harmonics of rotational speed, and to the tertiary harmonics of triplex mud pumps. This frequency range also represents the drillstring failures and the complex bottom hole assembly behaviors (bit whirl and stick-slip).
The high frequency interval corresponds primarily to the bit signature. These data are useful for optimizing rate of penetration. The plot of amplitude of signals in this range during drilling with a tricone bit produces the Snap log. Snap logs permit identification of the respective involvement of bit wear and rock formation in drilling performance.
A roller cone bit destroys rock by a series of percussions. Some of the energy from these percussions destroys the rock, some of the energy is reflected into the drillstring and goes to the surface, and the remainder of the energy is dissipated by damping and friction.
At constant bit and cone rotation speeds, the bit produces a characteristic frequency spectrum. This spectrum has various peaks which correspond closely to the number of teeth in the rows, the diameters of the cones, and the bit speed.
To differentiate these signals, the drilling dynamics operator runs a series of drilling tests with the various drilling parameters briefly modified. These tests establish reference bases for later comparison during normal drilling operations. These reference spectra are chosen when drilling conditions are considered ideal or satisfactory.
Several computer algorithms were developed to analyze the data: stick-slip detector, washout detector, drilling break detector, and torque and vibration analyzers. These algorithms continuously compare the vibration data against the reference spectra determined earlier during the bit run.
The algorithms interpret the data and identify their respective drilling operations as green (acceptable drilling), orange (transition), or red (problem drilling).
Ultimately, the three-color display may be placed on the floor for the driller. Drillers typically have enough problems to worry about; they don't need to watch complex graphs and charts. A simple, three-light display can indicate the existence of a problem for quick, minor adjustments.
AUDIODRILL
The dynamics measurements can be converted for an audible interpretation of the bit's drilling performance. The audiodrill measurements are not intended for constant listening by the well-site engineer. Rather, the audiodrill has ideal applications during the first 5-10 min when the bit starts drilling, during occasional drilling anomalies, and during the last 3-10 min when the bit reaches its life expectancy.
At the beginning of a bit run, any drilling peculiarities will be clearly audible. Near the end of the bit run, if the bit makes the sounds of satisfactory drilling, the bit can be allowed to drill a little longer.
On a recent Elf Aquitaine Norge well in the North Sea, the drilling supervisor wanted to pull a 17 1/2-in. tricone bit after 80 hr of drilling. The drilling dynamics measurements and audiodrill indicated the bit, still drilling at an acceptable penetration rate, had a few more hours of useful life. The bit was pulled 23 hr later, after it began to sound odd. The bit drilled from 2,551 m to 2,793 m in 103 hr.
The drilling sounds of a bit change characteristically over time as the teeth and bearings wear. For example, efficient drilling conditions produce a smooth sound. Stick-slip rotational instability produces an intermittent sound of the bit stopping and accelerating. During bit bouncing, the sound resembles that of a pile driver.
The audiodrill function is useful because the ear is an excellent time/frequency analyzer-it can easily detect slight changes. Additionally, any person, not just a signal processing specialist, can detect changes in drilling with the audiodrill. After a brief period of adaptation to the sounds generated, a listener can learn to detect the various drilling dysfunctions.
APPLICATIONS
Coherent dynamic signals at the surface and a stable torque typically indicate satisfactory drilling conditions and well conditions. Drilling dynamics optimization techniques help detect several dysfunctions:
- Absence of bit noise (Snap log): bit balling, locked cones, stabilizer hang up, mud motor failure, worn bit
- Intermittent bit noise: heave compensator problems offshore, axial sticking, bit bouncing, stick-slip
- Dynamic torque: stick-slip, bit whirl, drillstring bending
- Axial and torsional signals: drillstring, casing, or well bore contacts.
A regular series of drill-off tests helps establish guidelines for optimum weight and rotary speed for optimum penetration rate with minimum vibration levels. These tests can help determine the critical rotary speed/bit weight combinations that should be avoided. Once these guidelines are established for a given formation, only minor changes are necessary to minimize wasted energy. Drillstring resonance, stick-slip, and bit whirl can reduce penetration rate and damage downhole tools.
Bit bearing life can be extended about 20% by reducing the downhole levels of axial and torsional stresses. Reducing these stresses can also increase the cutting structure life of polycrystalline diamond compact bits.
Bit bounce is a relatively common damaging phenomenon involving axial vibrations and can damage a bit rapidly. Bit bounce can be a problem if shock subs or compliant tools are in the bottom hole assembly or if a new bit is being broken in improperly.
Adjusting operating parameters to reduce resonances in the low frequency range can reduce fatigue in the drillstring and bottom hole assembly. Downhole lateral bending of the bottom hole assembly is detectable in the axial mode at the surface. Avoiding this cyclic bending motion can minimize fatigue that contributes to washouts and twist offs.
Although drillstring vibrations do not normally cause fatigue cracks, the vibrations can accelerate propagation of microscopic cracks already present in the pipe. Washouts typically occur from incorrect makeup torque or damaged shoulders and from pipe body failure in the internal upset area, where stresses are high and scarring from slips aggravates the problem. Thus, detecting and minimizing vibrations can slow the onset of washout problems.
Twist offs typically result from a washed out area or from fatigue failure in a component subject to periods of lateral bending.
Detection of lateral bending of the bottom hole assembly through its coupling into the axial mode is an important part of avoiding this type of failure.
The dynamic torque, or rotational acceleration, response is a good indicator of impending bit failure (cones and cutting structure).
Detection of the dynamic torque can prevent bit damage from bit whirl and stick-slip. Axial acceleration and dynamic force are indicators of bit bounce.
Geology
As each tooth of a tricone bit contacts the rock, the interaction creates a frequency of excitation (Snap log). This frequency is broadened by both the cone skew angle and the stick-slip dysfunction, which cyclically alters the bit rotational speed. During good drilling conditions, the amplitude of the Snap frequency is a function of the mechanical properties of the rock.
The boundary changes between formations with differing mechanical properties show up clearly at the surface, without the time delay common to waiting for drill cuttings or from sensor distance of measurement-while-drilling tools. Rock fractures can be detected through sharp changes in observed axial energy.
Other applications include the monitoring of vibration levels during coring. Detecting and then reducing downhole vibrations can help minimize coring problems such as core loss, jammed core barrels, and irregular cores.
Editor's note:
This article was prepared from a series of interviews with Elf Aquitaine Production's Dynafor project engineers Denis Becq, Jean Magendie, Elyes Draoui, Luc Fambon, and Henri Henneuse and with Exlog Inc.'s drilling dynamics manager Matthew A. Kirkman. Also interviewed were research engineers Michel Lalanne, Alain Berlioz, Regis Dufour, Guy Ferraris, and Christophe Ulrich with the Laboratoire de Mechanique des Structures-URA CNRS 862, Institut National des Sciences Appliquees de Lyon.
BIBLIOGRAPHY
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Copyright 1993 Oil & Gas Journal. All Rights Reserved.
