Stress Analysis on BHAs Katie Mills

DrillScan Expert Services, Innovative Software Solutions, Trainings for the drilling industry Directional Drilling, Torque & Drag & Buckling, Survey, Casing Wear, Fatigue, Drilling Bit Performance, Drilling Dynamics Advanced Modeling Solutions Strong research collaboration Laboratory Validation & Permanent improvement Strong collaboration with Operators Field Validation First off, I’d like to give you a little background on Drillscan. We do software, training and engineering services for the drilling industry. We focus on all the sub categories you see here. Our software is based on advanced modeling solutions developed over years of research in collaboration with both the paris school of mines and insa in lyon and everything we implement in the software is both highly scientific and has been through extensive field validation in collaboration with our various partners.

+ + = Bit-Rock-BHA Model Rock-Bit model RSS/Motor model Build/Turn Rate, Bending Moment, Contact Forces, etc = One such model is our bit-rock-bha model. We’re going to use this quite a bit today so I want to give you a little bit of background on it. Basically we use the bit and the rock in combination to come up with a steerability and a walk of the bit, couple that with the BHA and are able to get out a predicted build/turn rate of a particular system. Something we talk less about is the fact that we can also get the bending moment along the BHA from this model, so today we’re going to play with that a little and see what effects the bending moment.

Validation Case Reference: SPE-184074-MS If you’d like more information about the validation around our bending stress model, you can check out this paper referenced here at the bottom. The bending stress was a side effect of the rest of the case study, but it’s helpful. Basically we see in red the measured bending stress from an MWD tool and in blue we have the modeled bending stress while the tool is going through a series of micro-doglegs. We can see that we’re pretty close, maybe even underestimating the total bending stress at the MWD. Reference: SPE-184074-MS

Drill Collar M-N Curve M-N or S-N curves Gives bending stress or moment vs number of cycles Used to estimate fatigue limits for different stress levels or bending moments Specific to an individual element Reference: US Patent US20120016589 A1 Starting out, I want to introduce an M-N curve, courtesy of the US patent office, google patents and an unnamed big blue service company who filed a patent on fatigue tracking. What is an M-N curve? Most of you might be more familiar with the idea of a S-N curve, which gives the bending stress as a function of the number of cycles to failure. An M-N curve is the same idea, just using the bending moment which doesn’t takes into account the moment of inertia, or the structure of the element. Because we’re using the bending moment, these curves are each specific to a particular element or collar in this case.

So what gives a 50,000ft-lb bending moment? Drill Collar M-N Curve Translate into: Ft-lbs for bending moment Fatigue life in hours rotating at 100rpm What do we see? High bending moment (50,000 ft-lbs) 7 hours for collar 1 12 hours for collar 2 35 hours for collar 3 When rotating at 100rpm So what gives a 50,000ft-lb bending moment? I took this curve and did a little data manipulation to come up with the bending moment in ft-lbs so we can understand it, and with the fatigue life in hours rotating at 100rpm. So what do we get out of this? Well with a high bending moment, 50,000 ft-lbs, for collar 1 we can rotate for about 7 hours at 100rpm until the expected fatigue life or an expected failure. For collar 2, that’s about 12 hours and for collar 3 that’s about 35 hours. So, then what does a 50,000 ft-lbs bending moment look like?

Slick BHA 8.5in Bit 6 ¾” motor w/ 2.12 bend Slick assembly 40 klbs WOB, 60 deg inclination Build up rate of 15° sliding / 1.87° rotating Max bending moment @ 59,800ft-lbs This is a pretty typical BHA for shale curve and lateral. We’ve got an 8.5in bit, a 6 ¾” motor with a 2.12 bend and a 7in kickpad. Above that, we have a pony collar. Call that a flex, a non-mag, whatever you want. And then the MWD. At 40klbs WOB, and that’s WOB actually at the bit, not from surface btw, and 60 deg of inclination, we’ll predict a build rate sliding 100% highside at about 15deg and 1.87 rotating build. We get a max bending moment at 59,800 ft-lbs. Way over our 50,000 ft-lbs in the previous example. We’ll talk about the life expectancy in a minute. First take note of the purple stars.

Slick BHA Bend Power Section Start Power Section End Top Sub DC/MWD Connection Now we’re looking at that same BHA, with a few different weights on bit, still at 60deg of inclination. The purple stars here are the same points we saw before. I’ve labeled them so you can have an idea of where they sit. One thing to notice about this is that the different weights on bit have a fairly minimal effect on the bending moment. Reducing the WOB by half does not reduce the bending moment by half but by only about 15%. The other thing to note is the position of the bending moment and the way it gets shifted forward with increased WOB. Increasing the WOB from 30 to 60 we basically push that max bending moment directly on to the top sub connection. That should set off a few warning bells.

Stabilized BHA 8.5in Bit 6 ¾” motor w/ 1.75 bend Stabilized motor w/ 8 1/4” sleeve Upper 8 1/4” stabilizer at 31ft from bit 40 WOB, 60 deg inclination Build up rate of 13.4° sliding / 0.36° rotating Max bending moment @ 51,900 ft-lbs So now lets look at a stabilized BHA. Basically the same set up. 8 ½ in bit, 6 3/4in motor, we’ve lowered the bend a little since it’s stabilized, now it’s at 1.75. We’ve added a motor sleeve at 8 ¼” and an upper stab right above the motor at about 31 ft, also 8 1/4in gauge. Then the same pony collar and MWD, all the same lengths, all the critical points are at the same distance. 40 klbs WOB, 60 deg inclination, we get a build rate of 13.4 deg sliding, a little less than before and a rotating tendency of 0.36, which is probably better for you once you hit the lateral. Now our bending moment has changed to only 51,900 ft-lbs instead of the 59, 800 we had before. Maybe not a monumental shift, until you look at the positioning.

Stabilized BHA NB Stab 8 ½” Bend Power Section Start Power Section End Top Sub Upper Stab 8 ½” DC/MWD Same purple stars, same distances from the bit. Now we see the distribution of the bending moment has shifted. The upper stabilizer breaks up that moment so it’s no longer concentrating at the top of the motor. The overall bending moment still doesn’t change very much with WOB but it’s even more stable than before. The overall bending moment is lower than the slick BHA, but also the stress is moved back in to the MWD/DC. Which is great if you provide motors, maybe not so great if you’re an MWD provider. But, of course you can always swap out your pony collar for a full DC and then that moment is in the middle of the collar anyway. Ok so before I move on to the next slide, a word of caution. I’m using moment-cycle curves that aren’t meant for motors, they’re meant for collars, probably big blue MWD/LWD collars, so the rotating time shouldn’t be taken as a direct time, but instead just an idea. I tried to use percentages, but because the M-N curves are logarithmic it skewed the data in weird ways.

40 klbs @60deg Inclination ∞ 10 DLS Point Slick BHA Stabilized BHA % Change in Bending Life Collar 1 % Change in Life Moment ft-lbs Δ Slick - Stab Hrs @100 rpm 10 DLS Bend -22,000 -506 -98% 2,000 ∞ +++ Motor Top Sub 37,000 27,300 -26% 43 400 +830% DC/MWD 37,400 40,100 7% 38 24 -37% So what we’re looking at here is a fixed dogleg scenario. Instead of seeing what the BHA can achieve, I’m setting the curvature at 10 deg/100ft and putting the BHA in that curvature. This way, we can compare the slick and stabilized BHAs in equal conditions. What do we see out of this. Let’s start in the middle with the motor top sub. Comparing the same point on the slick and the stabilized bha we see that the stabilized BHA was able to reduce the bending moment by about 26%. That’s great, however the really interesting thing is that when we correlate that to hours rotating at 100rpm, we’re not increasing our time by 26% but by 830%. We’re able to go from 43 hours to 400hours. The compromise on that is of course, that actually the moment on the MWD/DC connection goes up and we lose a little of our rotating life. In this case, a 7% increase results in a 37% decrease in rotating hours going from the slick to the stabilized BHA. This is of course where my method of using the same bending moment curve for different elements doesn’t work. There’s a very real possibility that the rotating hours on the MWD are greater than the motor.

Rotating at 60 rpm vs 100 rpm Reducing rpm in the curve results in a dramatic increase in life of top sub Effect is more pronounced with stabilized BHA 100 RPM 60 RPM Now imagine we lower the rotating speed from 100 rpm to 60 rpm, well that’s a great idea! Now we can rotating a lot longer with our motor top sub for all the BHAs but especially for our stabilized BHA.

40 klbs @90deg Inclination ∞ 3 DLS Point Slick BHA Stabilized BHA % Change in Bending Life Collar 1 % Change in Life Moment ft-lbs Δ Slick - Stab Hrs @100 rpm 3 DLS Bend -44,500 -25,700 -42% 11 625 5,582% Motor Top Sub 9,700 5,150 -47% ∞ - DC/MWD 16,600 16,100 -3% 60,000 One more scenario for you. Here we’re looking in the lateral, at 90deg of inclination, still with the same 40klbs at the bit and a fixed curvature of 3Deg, again assuming that we’re putting the BHA into that curvature exactly. If we look, now the highest bending moment has shifted down into the bend, for both the slick and the stabilized BHA. In fact for the slick bha, the rotating life is pretty limited, in part an effect of that large 2.12 deg bend.

Conclusions Lowering motor bend positively impacts motor life Adding stabilization Decreases overall bending moment Moves high bending moments back away from critical connections Small decrease in bending moment has a large impact on fatigue life Conclusions. Thank you IADD

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