For back-off operations to be successful on the first try, plans must be devised and then carefully followed; this should also keep the risk of injury low for rig floor personnel. Of course, the Contractor Driller/Tool pusher and the Fishing Tool Supervisor need to oversee the process since it’s considered a non-routine operation.
Steps for Safety
1) Firstly, a safety meeting, coordinated by the Drilling Supervisor, should take place before the procedure itself. With all rig personnel in attendance, the meeting will explain the no-go areas during torque application (and when torque is held on the drill string), the hazards of the operation, and the proper use of equipment in order to prevent injury.
For personnel not essential to the task at hand, they should stay well away from the rig floor until completion.
2) To hold right or left-hand torque in the string, sometimes rotary slips and rig tongs will be used. If this is the case, the slip insert dies need to be sharp (while also fitting into the slots themselves). If any dies have signs of wear during an inspection, they should be replaced.
3) While raising and lowering the pipe to work the torque downhole, the surface torque needs to be held in the string and this may require manual drill pipe tongs. Before operation, it’s important to check the snub line on the tong to make sure it has sufficient length.
It’s dangerous to operate with a short snub line; the tong may lose bite on the pipe when raised and this leads to an uncontrolled and potentially harmful backlash. To be extra safe, it’s possible to install extra-long snub lines before then attaching them to a suitable snub point (such as a derrick leg). When working torque downhole, this will ensure sufficient bite.
4) Using rotary slips, torque will work into the pipe; at this time, the positioning of the latched elevators is essential. Located just below the tool joint, this will allow rotation for the pipe through the elevators. While the torque is trapped in the string, elevators can be used to adjust and control the pipe. Depending on whether the blocks are secured to a guide rail system, the hook swivel will need to be Locked (if secured) or Unlocked (if not).
5) Normal procedure will see right-hand torque applied and worked downhole (to reach the planned back-off point) before applying any left-hand torque. If you had to free the drilling assembly by working the pipe hard to the right, this step isn’t a necessity.
6) Using a length of wire rope, slip handles need to be tied together when applying left-hand torque (with rotary slips). If the pipe were to break high, the slips won’t be thrown from the rotary. In turn, the loss of strain won’t cause the pipe to jump.
7) In some cases, a stuffing box and wireline lubricator will need to be used to run the string shot. Two common examples include;
When it’s believed that formation fluids have entered the drill string annulus
When the balance between inside and outside wellbore fluids has been thrown off
8) While running and making up the string shot, users must follow wireline safety procedures for perforating operations.
Identifying the Free Point (If Necessary)
With a stuck tubing assembly, how do we determine the free point?
There are a couple of options, and the first uses the length of free pipe and a measurement of pipe stretch for any given over-pull. This being said, the actual free point normally falls within 500’ either way which makes it useful in planning fishing operations and any subsequent operations. The second option is to use free point indicator tool which will give the precise stuck depth. This option may not available on every location. In this article, it will demonstrate
Calculate Free Point by Using Pipe Stretch Data
If the impact of hole drag is small in vertical holes, the use of pipe stretch data will lead to the most accurate free point calculations. Although this method can also be applied in directional wells, the effects of hole drag creates a potential of underestimating the free pipe length.
There are five main steps to follow with this particular technique;
- Pick up to the normal pick up weight and mark the pipe on the rotary table as a reference point.
- The next step is to set the brake after taking an over-pull of 50,000lb (+/-). Note: it’s vital not to go past 80% of the pipe’s minimum yield strength.
- After the over-pull, what was the stretch? Write this down in inches.
- From here, there’s a special equation for calculating the length of free pipe.
This can be seen below;
Lf = (L×Ap×E) ÷ (12×P)
Lf = length of free pipe, ft
L = length change due to over pull, lbs
Ap = pipe cross sectional area, square inch
E = steel Young Modulus, psi ( 30,000,000 psi for steel pipe)
P = overpull applied, lbs
- Finally, we should note that the calculated length can be checked by increasing the amount of over-pull. If this is done, remember step four (use the reference point and measure the amount of stretch).
Determining String Tension
If ideal conditions are present, zero is the perfect pipe tension at the back-off point. However, achieving this in practice is almost impossible. In our experience, it’s better to have a little tension at the back-off point rather than compression.
To successfully back off at the intended depth, it’s important to calculate the required surface tension before then considering the application of this tension before blind back-off. Whenever the pipe is in compression or there’s too much tension, back off is actually unlikely. In the rare event that it does happen, it will occur much higher up the string than desired. Sadly, this means making up the pipe for a second time and repeating the procedure.
When it comes to determining and optimizing tension at the back-off point, there’s some essential information that needs to be compiled. This includes;
- Drilling fluid density in the well.
- Length of individual components (and weight per foot) in the drilling assembly.
- In the section the drilling assembly is stuck, the average hole inclination.
- Before getting stuck, the off bottom rotating weights (fishing BHA string), pick-up, and slack-off; all measurements should be with the pumps off. Of course, these figures are hard to obtain if the pipe got stuck after engaged. In this case, we recommend estimating using what had been recorded prior to that point.
- After determining the hook load that’s needed for the back-off point to reach zero tension, you need to ensure that the pipe doesn’t go into compression which is why +/- 5,000lb of over-pull should be added. There’s a general equation when calculating the correct weight indicator reading after firing the string shot.
Weight indicator reading should be followed the equation below;
Weight indicator = Pick-up weight – Buoyed weight of back-off point – 5,000lb over-pull
In this equation, it assumes that;
- Buoyed weight is the combination of pipes to be free and the Fishing BHA (hole inclination is also considered).
- Pick-up weight includes both pipes to be free and the whole Fishing BHA.
Working Right-Hand Torque Downhole
First things first, the appropriate make-up torque needs to be applied to the pipe before even considering a downhole back-off. When this isn’t done, there’s a risk of a deep open hole back-off or a shallow back-off at the wrong depth.
What’s the perfect amount of right-hand torque when working down the string? There’s no universal answer, and it all depends on the wellbore profile, well depth, and degree of hole drag (torsion and tension). While locating the free point, the correct string tension also needs to be determined so it can be applied at the surface; this ensures torque is worked down to the back-off point.
With this in mind, it’s not necessarily the case that the pipe is worked between the slack-off weight and calculated pick-up weight at the back-off point. When determining the free point, the torque measurements taken provide a guide to correct surface tension not only for applying right-hand torque but left-hand torque too.
There are some important practices when working right-hand torque downhole;
- When following the procedure, a full-scale torque reading needs to be generated at the planned back-off depth plus 30%. When attempting to back-off, this reading will allow for maximum left-hand torque application to the pipe.
- When working the torque down the pipe, it should travel from surface to the free point depth. Between the zero surface tension and calculated free pipe pick-up weight (at the back-off point), the pipe should be lowered and raised because tool joints will only torque correctly with minimum axial tension. At the same time, right-hand torque should progressively increase.
- The goal is to work maximum make-up torque into the string and this can be achieved with three or four applications. It’s important to remember how many total turns were required to reach the maximum make-up torque; additionally, note the estimated number of turns when applying left-hand torque.
- Eventually, as the pipe cycles between the range of slack-off weight and pick-up weight being used, it will be the case that there’s no loss of trapped torque. This indicates a complete make up of all connections.
Working Left-Hand Torque Downhole
When it comes to left-hand torque, the amount applied should the highest possible while still comfortably avoiding a shallow or premature back-off. Since this is a hazardous operation, the safety precautions need to be understood and followed as closely as possible. For example, there are four main guidelines when working left-hand torque downhole.
1) Generally speaking, 70% of the right-hand make-up torque is the maximum for applying left-hand torque at surface; it should never exceed this amount.
2) When there’s a zero axial tension on the connection, there’s a real possibility of the tool joint breaking. Therefore, we recommend starting with maximum surface tension to work the left-hand torque down the hole before progressively working down to the planned back-off tension.
Starting with around half of the suggested left-hand torque, this is something that should happen gradually. At this point, we should note that the torque should only ever be increased to the next step once a given amount of torque has worked into the whole string.
3) As the torque works towards the planned back-off point, keep a record of cumulative left-hand turns going into the pipe.
4) What happens if the string is still not free and you’ve hit 70% of the left-hand torque? If this happens, add 5% of the torque until the pipe is eventually free.