The following chapter presents the results from the simulations done in WellPlan and includes discussion and evaluation of the results. N.B. The simulations involve comparing various hole/wellbore sizes ranging from 12¼” to 15” (i.e. changing the size of the under-reamer).
The following hole sizes are used:
• 12¼”
Hole size 12¼” and 15” are “the most critical” (the extreme) ones in conjunction with the base case, 13½”. The minimum pump rate used in the simulations is 600 gpm and the maximum rate used is 1100 gpm (increment pump rate 100 gpm). The surface equipment working pressure is 7000 psi in each simulation.
The following Table 12, Table 13 and Table 14 show the different simulations performed in WellPlan and their objective. The overall objective for all the simulations is to study the effect of varying hole size in the 12¼” x 13½” hole section.
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Table 12: Drilling the 12¼” x 13½” hole section.
Simulation Objective
WOB to induce sinusoidal buckling
Observe how the minimum WOB to induce sinusoidal buckling at any point in the drillstring for a range of bit depths changes with varying hole size
WOB to induce helical buckling
Observe how the minimum WOB to induce helical buckling at any point in the drillstring for a range of bit depths changes with varying hole size
Surface torque Observe the maximum torque found at the surface/a point in the drillstring vs. the make-up torque limit and how it changes with varying hole size
Hook load Observe how varying hole size affects the string load, also including rig capacity and yield limits
ECD Observe how the ECD progresses with depth
and varying holes size
Bed height (annular velocity) Observe how the bed height of the cuttings in annulus changes with both increasing wellbore inclination and flowrate
Annular velocity Observe how the velocity of the fluid in the annulus changes along the string and for a range of flowrates and hole sizes
Minimum flowrate
Observe how the minimum (critical) flowrate required to remove cuttings beds during drilling evolves with both increasing wellbore inclination and flowrate Suspended volume Observe how the suspended volume of
cuttings evolves with both increasing wellbore inclination and flowrate
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Table 13: Running the 10¾” production liner.
Simulation Objective
WOB to induce sinusoidal buckling
Observe how the minimum WOB to induce sinusoidal buckling at any point in the liner string for a range of bit depths changes with varying hole size
WOB to induce helical buckling
Observe how the minimum WOB to induce helical buckling at any point in the liner string for a range of bit depths changes with varying hole size
Surface torque
Observe the maximum torque found at the surface/a point in the liner string vs. the make-up torque limit and how it changes with varying hole size
Hook load Observe how varying hole size affects the string load, also including rig capacity and yield limits
Side forces Observe how the side forces acting on the liner string changes with varying hole sizes
ECD Observe how the ECD progresses with depth
and varying holes size
Pressure loss Observe how the total pressure loss is affected by a change in hole size and pump rate
Table 14: Change in section depth for the 12¼” x 13½” section.
Simulation Objective
Minimum flowrate Observe if a change in section depth affects the minimum flowrate for hole cleaning during drilling
ECD Observe how the ECD at shoe (6693 ft. MD)
and at bit for the liner string is affected by a change in section depth
Pressure loss
Observe how the total pressure loss for the liner string is affected by a change in section depth, hole size and pump rate
171 6.1 Drilling the 12¼” x 13½” hole
The most important ERD focuses while drilling the 12¼” x 13½” hole are, as presented in section 5.3, hole cleaning, ROP, ECD management, mud consumption tracking, torque &
drag monitoring, evaluate real-time pore pressure prediction, mud-logging service, avoid high DLS (dog leg severity) and to avoid pack-offs.
6.1.1 Minimum WOB
This simulation shows the minimum weight-on-bit to induce/initiate sinusoidal or helical buckling at any point in the drillstring for a range of bit depths [WellPlan user manual]. These forces (WOB) are compressional forces; parts of the drillstring will be in compression while drilling, other parts will be in tension (and will not affect the WOB to either sinusoidal or helical buckling).
Figure 91 and Figure 92 show that the WOB to induce buckling increases with decreasing hole sizes for both sinusoidal and helical buckling. As presented in section 2.4.4, larger annular clearances will result in less buckling tolerance since the tubular is less constrained in the wellbore. This implies that big holes are more prone to buckling compared to small holes since the pipe has more room to move in big holes (which again increase the risk of buckling) as expected from theory presented in section 2.4.4.5. The WOB to induce sinusoidal buckling decreases with depth. The buckling tendency thus increases with increasing depth; for this specific case.
Higher weight of drillstring will lead to increased drag and increased compressional forces, which again may increase the risk of buckling and deformed pipe. According to theory presented section 2.4.4.6 the best way to limit pipe buckling is to preserve the ability to rotate in order to release and get rid of a lot of friction. The simulation shows that small clearance between the wellbore and drillstring reduces the risk of buckling as expected, implying that the buckling tendency is lower in a 12¼” compared to a 15” hole thereby allowing a more effective transfer of weight on bit.
172 6.1.1.1 Sinusoidal buckling
The WOB to induce sinusoidal buckling increases with decreasing hole sizes and decreases with depth (overall). The curve could also be seen from surface, but the WOB values are the same down to the 13⅝” casing shoe; represented by the horizontal line in Figure 91. The figure shows that from around 12.300 ft. MD the WOB to induce sinusoidal buckling is more or less constant until the string reaches TD (the WOB to induce buckling appears to drop just before TD).
Figure 91: Sinusoidal buckling 12¼” x 13½” hole.
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