While backreaming out of hole, pumping at 1150 gpm with 120 RPM and max 30K torque, experienced total losses. Regained flow by varying rates to 200 gpm but could not backream out. String could still go down but not up. RPM was then set at 160 rpm and torque limit at 40K, string stalled and could not go up, taking a 100 k overpull. After about 5 mins, Observed string backout from saver sub and dropped into the well.
Fished the string successfully, but in two. One part was string to Jar stand crossover the other part from the HWDP to remaining of BHA. The boxend of the HWDP was seen to have some thread of the jar stand pin indicating a twist off.
What must have gone wrong?
Who has experienced this before?
Nice discussion. All responses have been helpful. I have some additional information. The well was a directional well, inclination builing from around 5700 ft to 82 deg at 11000 ft, creating a bucket between 7200 ft and 8500 ft. The picture as attached shows the trend of events just before it happened.
Also from the pictures of the box end that back out from the TDS saversub, no noticeable damage. The picture from the box end downhole that parted also indicates some thread coatings of a pin on it, making us to believe it was twisted off.
Backreaming at full drilling rate has been the normal practice in this field for these kinds of wells. Should this continue?
Thanks for the added information.
How much of the 5000ft long 17 ½” OH was distributed between 20-40° / 40-65° inclination and above?
What was the BHA design (STABs amount and space out / DC size and amount), and how tortuous was the wellbore drilled (average dog legs through build & slant intervals)?
Indeed, one can understand the need to ream at 120rpm above 65° to manage / clean any cutting bed through a long slant 80° section, ERD type well design.
About Torsional inertia: several 17 ½” STABS and 9 ½” DCs/NMDCs/MWD size collars likely present in the BHA and these are heavy, when spinning them at 160rpm, they build up lots of centrifugal energy. Your say indicates string stall-out at 160rpm. So the energy built up in the high speed rotating string needs to dissipate all of a sudden, when stalling, explaining what you experienced: TDS SS backed off and string twist off.
Furthermore, think of the linear speed at the edge of the STAB blades, the side force and associated torque. Your DrillString above winds up, as a spring, storing further torsional energy (Stribeck type stick & slip vibration can develop along the string).
In terms of BHA design, for enhanced string integrity, your Jar OD size should not cross-over to a different tubular sizes or connection types. Ideally JAR & ACCELERATOR should form an homogeneous built, with same tubular size and connection, above, between and below these critical string components. This should be assured upfront with your down hole tool provider. You can cross-over a stand or more below JAR and Above ACCELERATOR, but not in between as this would likely be a weak link. Tensile strength and Make Up Torque design shall be checked through advance TnDnB analysis, involving stiff string style models, and investigating load cases such as back-reaming, accounting for wellbore tortuosity (effect of cutting bed would be more challenging).
Your case worth post analyzing thoroughly to identify limiters that put you in that situation: ROP vs hole cleaning, ie: Mud pumps capability? Bit-BHA design, i.e. drill string components compromises ?
Hope this helps
The reaming rate was guided by hole cleaning procedures... it was needed to properly clean the hole - 17-1/2" hole section, 5000 ft long moving from 20 degrees to 82 degrees
If you can, please explain torsional inertia and how it affect design window of drill string (Tensile strength of drill string, Make up torque) Pipe at saver sub, 6-5/8 FH. Pipe twisted at bottom 5-1/2 FH. Pipe strength S135