How To Design a Bottomhole-Assembly Rotary-Speed Sweet Spot

Topics: Drilling
Fig. 1—The three BHA designs are progressively more compact, with BHA-3C being the most compact.

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Correct placement of the rotary-speed sweet spot of a bottomhole assembly (BHA) provides multiple benefits toward achieving the ultimate goal of drilling to section total depth in a single trip. The process of BHA redesign and tuning for the desired operating parameters provides greater control over the drilling process. Vibration minimization through proper design and operation of the BHA facilitates drilling objectives that include reaching section depth in fewer trips and providing acceptable borehole quality to run casing to depth.

Introduction

A musician understands the basic principle of a stringed instrument: The frequency of the note made by the instrument varies with the length of the string span. Other parameters play a role, including the string properties and tension. However, once the instrument is tuned and the concert has begun, the musical notes are played principally by changing the length of the vibrating string.

As the finger position is changed on the fingerboard, the pitch of the note changes. Although this is a basic concept to a musician, it is not common today in the oil and gas industry for engineers to apply this concept to BHA design selection.

A BHA is a beam that may be represented by a fourth-order differential equation. Model calculations proceed by subdividing the BHA into elements comprising short sections of pipe in a lumped-parameter model. A 2D model uses a state vector at each mass node comprising the lateral displacement in a plane, tilt angle relative to the centerline, bending moment, and beam shear load. In the lateral bending flex mode, a dynamic side force is applied at the bit at integral multiples of the rotary speed. In the rotary twirl mode, offset mass elements are used to investigate the stability of the BHA to eccentric mass and centrifugal forces.

The dynamic response is considered to be a perturbation from the static solution. The bending strain energy is related to the square of the bending moment in the beam, and calculating the length-averaged BHA bending energy provides a BHA bending-strain-energy vibration index.

Lower values of vibration indices are sought because the dynamic response to a reference input is then minimized (i.e., lower vibrations are then predicted by the model). All such modeled designs are perturbed by identical inputs, and the model results may then be compared on an apples-to-apples basis.

BHA lateral dynamic modeling is now applied before drilling in nearly every well the operator drills. In most applications, a proposed BHA design is solicited from the directional-service provider with the required measuring and logging tools for the hole section. This baseline configuration is entered into the model. Alternative configurations are then obtained by moving contact points through stabilizer movement, tool rearrangement, or both.

The model results for a suite of such designs are then generated and compared on a single chart, facilitating selection of the BHA that has the lowest vibration indices over the targeted ­rotary-speed range, usually determined by hole size and drillstring modeling. In most instances, BHA designs that have been evaluated and run may be reused successfully, provided that the governing assumptions are still correct.

Redesign of the planned BHAs for a well is considered to be an important part of the well planning because early redesign provides the most flexibility to modify the tool configurations. Changes made to BHAs late in the process are constrained by tool availability and logistical considerations.

Case Study

BHA designs for 16.5-in. intervals of three offset wells in a deepwater application provide a good example of BHA redesign to optimize the rotary-speed sweet spot. The designs shown in Fig. 1 above include a rotary-steerable tool, two measurement-while-drilling (MWD) tools, two roller reamers, drill collars, and subs. Each of these designs drilled salt sections exceeding 3000 m. The contact-point nodal positions are indicated by black diamonds on the right edge of the BHA profiles in Fig. 1.

In each successive well, the opportunity to increase rate of penetration (ROP) through higher rotation speeds or weight on bit (WOB) was pursued, and BHA-3B and BHA-3C were modified to have shorter spacings to increase the sweet-spot rotary speed and increase the stiffness to carry more WOB. In BHA-3C, the MWD tools were rearranged to avoid magnetic interference with the steel roller reamer.

Model results show that twirl values decrease for increasingly compact assemblies.

The BHA 3B run was complicated by increased reaming requirements in the lower section, which negatively affected the interval average ROP. However, the instantaneous ROP values of BHA‑3B demonstrated improvement over BHA‑3A. Moreover, the BHA-3C run showed a substantial increase in drilling performance over both previous runs. The interval ROP was 44% higher than the average for the BHA-3A and BHA‑3B runs.

The higher ROP of BHA-3C can be ­explained by an increase in the rotary speed (13% on average) with an increase in the depth of cut (DOC) (27% on average). Combining these values, the overall improvement is calculated to be 44%, in agreement with the data.

BHA-3A and BHA-3B both ran an average of 36,000-lbm WOB, and BHA‑3C was operated with a WOB increase of approximately 2,000–3,000 lbm on average. However, there was a significant portion of the WOB distribution between 45,000 and 60,000 lbm, unlike the first two runs. The rotary speed in this run was approximately 13% higher, typically 160 rev/min compared with 140 rev/min in the first runs.

DOC was, on average, 27% higher for the third run. Increased DOC and higher rotary speeds together yield a faster ROP. An increased average DOC of 27% with a small change in average WOB is indicative of increased BHA stability.

BHA-3B shows increasing ROP with increasing rotary speed and WOB. However, both BHA-3A and BHA-3C show relatively constant ROP at the dominant rotary speed. Plotted against WOB, ROP is relatively constant at a range of WOB parameters for BHA-3A, and this certainly is seen in the BHA-3C response, where 150 ft/hr is achieved at WOB values ranging from 30,000 to 55,000 lbm. This again suggests that adjustments to the bit DOC elements might be appropriate in future drilling-optimization efforts. Note that the three runs discussed in this case study used the same bit design.

The BHA-3C assembly performed well at higher rotary-speed and WOB conditions, suggesting improved stability. To characterize the stability of these assemblies, the BHA lateral vibrations can be evaluated and trends with rotary speed and WOB identified. Data show that there is a tendency for the lateral vibrations of BHA-3A to increase from 1 g (g-force) or less below 35,000-lbm to between 1 and 1.5 g above 35,000‑lbm WOB. On the other hand, BHA-3B shows flat or decreasing lateral vibrations with WOB greater than the range from 30,000 to 45,000 lbm. For this reason, BHA-3B is considered to be more laterally stable at higher bit weight than BHA-3A.

BHA-3B has the lowest average torsional vibrations, and BHA-3C the highest, with a 24% increase relative to the average of the first two runs.

Summary and Conclusions

In many field applications, the authors have found that the lateral-vibration sweet spot of a BHA can be controlled through improved design practices. In particular, the analogy to musical instruments shows that some controlling span between contact points needs to be shortened to increase the frequency of a lateral-vibration sweet spot. Conversely, a controlling span needs to be lengthened to reduce the rotary-speed sweet spot. It may not be obvious which span is the controlling factor, but a proper design procedure can quickly assess the different options and provide interpretable results that concur with a substantial amount of field evidence.

In the practical application of this methodology, the initial BHA design is entered into a lateral-vibration model and the simulation is calculated for a range of rotary-speed and WOB conditions. The observed rotary-speed sweet spot is compared with the desired characteristic, and BHA design adjustments can be modeled accordingly until the ­desired characteristic is obtained or until some design limit is encountered.

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18836, “How To Design a BHA Rotary-Speed Sweet Spot,” by Jeffrey R. Bailey, SPE, and Andrius Minkevicius, SPE, ExxonMobil; Alexander Bolzan and Victoria R. Wilkins, Hibernia Management and Development; and Joshua S. Pokluda, Esso Exploration and Production Nigeria, prepared for the 2016 International Petroleum Technology Conference, Bangkok, Thailand, 14–16 November. The paper has not been peer reviewed. Copyright 2016 International Petroleum Technology Conference. Reproduced by permission.

How To Design a Bottomhole-Assembly Rotary-Speed Sweet Spot

01 December 2017

Volume: 69 | Issue: 12

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