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Modeling Slick and
Stabilized BHAs
Katie Mills
DrillScan
++
BHA modelRock-Bit model
SPE 74459, PA-82412, 79795, PA-87837, 110432
Bit-Rock-BHA Model
BHA Requirements
▪ What do we want out of our BHA?
– Build in the curve ~ 10 deg/100ft DLS in the plan
– Neutral tendency in the lateral ~ 90 deg
– Fast but with minimal vibration
– Stay in the hole for the entire section
– Easy to control and easy to predict
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Slick vs Stab
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Stabilized BHA
Motor
- 7 ¾” Sleeve
- 7 1/8” Kick Pad
- 2deg bend 6ft from bit
Stabilizer
- 8 1/8” gauge
Slick BHA
Motor
- Slick Sleeve
- 7 1/8” Kick Pad
- 2deg bend 6ft from bit
Rotating Tendency in the Lateral
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Rotating at 90deg
½ in overgauge
2in gauge bit or 6% steerability
Varied WOB from 5klbs to 35klbs
Sliding Build Rate Potential
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Slide Drilling at 100% Highside
0 in overgauge
2in gauge bit or 8% steerability
Constant 10klbs WOB
Inclination increasing
Slick vs Stab?
▪ Stabilized BHA helps us rotating in the lateral
– Less Sliding necessary to maintain 90deg
– But can it build the curve?
▪ Slick assembly has a great building potential
– More sliding necessary to maintain 90deg in the lateral
– Is it worth the trade off?
How do I know which BHA will achieve my objectives?
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Toolface Chart
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High Side
90°270°
Low Side
Toolface Chart
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High Side
90°270°
Low Side
0° GTF
100% Toolface Efficiency
Build Yield = 14 °/100ft
Toolface Chart
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High Side
90°270°
Low Side
30° GTF
100% Toolface Efficiency
Build Yield = 12.2 °/100ft
Toolface Chart
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90°
30° GTF
70% Toolface Efficiency
Build Yield = ?? °/100ft
High Side
270°
Low Side
Toolface Chart
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High Side
90°270°
Low Side
Assume 70% Efficiency yields:
70% of TF in the green
20% of TF in the yellow
10% of TF in the red
Toolface Chart
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High Side
90°270°
Low Side
• 0°GTF = 14 (°/100’) BR
• 30°GTF = 12.2 (°/100’) BR
• 50°GTF = 9.2 (°/100’) BR
• 70°GTF = 5.2 (°/100’) BR
• 90°GTF = 0.6 (°/100’) BR
Which would give an overall build rate of:
BR = 10.9 (°/100’) or 89% of 12.2 (°/100’)
or 78% of 14 (°/100’)
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Example: Trajectory
TFO: Sliding Sheet vs EDR
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Average TFO = 324 deg
TFO = 340 deg
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Results: Trajectory prediction
Average BUR = 14.7 deg/ 100 ft
Average BUR = 16.5 deg/ 100 ft
Resulting BUR About 90% of the predicted BUR
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Results: Trajectory prediction
Resulting trajectory is more erratic
Importance of TFO
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▪ A small variation of TFO can generate significant variations of
BUR/TR
▪ TFO control/efficiency is key to drill a smooth wellbore
▪ Variation of WOB (and TOB) will generate variation of TFO (twist of
the BHA) and thus tortuosity or unexpected BUR/TR
BHA Design Bottom Line
▪ Curves are designed with 10 (°/100’) DLS
▪ 100% slide in the curve is optimal, but adding in a safety factor, 80% slide will be the design goal
▪ To achieve 80% slide in a 10 (°/100’) DLS curve, we need a BHA with actual build rates of 12 (°/100’)
▪ Knowing that the actual build rate comes out to be about 90% of the potential build rate, the designed BHA needs to be able to achieve potential build rates of about 13.2 (°/100’).
Revisit - Sliding Build Rate Potential
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Slide Drilling at 100% Highside
0 in overgauge
2in gauge bit or 8% steerability
Constant 10klbs WOB
Inclination increasing
Redesigned Stabilized BHA
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Stabilized BHA
Motor
- 7 7/8” Sleeve
- 7 1/8” Kick Pad
- 2deg bend 6ft from bit
Stabilizer
- 8 1/8” gauge
Sliding Build Rate – Redesigned BHA
▪ Keep WOB over 5klbs
▪ Build at more than 13.2
deg/100ft for all
inclinations
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Contact Points
▪ Stabilized BHA shifts near-bit contact point to the sleeve
▪ Slick BHA keeps that point on the kick pad behind the bend
▪ Contact on the kick-pad could contribute to TF fluctuations
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Stabilized 7 7/8in Sleeve Slick with Slick Sleeve
Conclusion
▪ Design requirements for BHAs are many
– Depend on application
▪ Slick or Stabilized can work for either
– Different applications for each
– More important to design the whole system to meet objectives
▪ Important to account for TF fluctuations when we
consider overall build rates
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Thank You
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Context: BHA
