Freddy Gebhardt, Joe Dean Thompson
Wild Well Control Inc.
Spring, Tex.
Removing a tree under pressure can quickly prepare a well with an underground blowout for snubbing operations, rather than drilling an expensive relief well.
If a pressurized well cannot be killed by circulation or bull-heading, it may become necessary to remove the tree to allow a snubbing rig up. The procedure can be accomplished in a relatively short time and therefore may be more expedient than a relief well.
TREE REMOVAL
When a situation is encountered similar to the one shown in Fig. 1,(36979 bytes) snubbing equipment is often necessary. Before snubbing operations can proceed, the tree must be removed to allow full well bore access and installation of the blowout preventer (BOP) equipment.
In this situation, the well cannot be killed because fluid pumped at rates such that the pressures do not exceed the capabilities of the surface equipment will be lost to underground flow. Conventional circulation is not possible because of the tubing failure.
Experience has shown that the probability of reentering the lower tubing section with coiled tubing or small snubbing pipe is low, and there is a high probability of additional mechanical complications. A snubbing unit with a full complement of blowout preventers is needed to reenter the well and work with the tubing failure and underground flow.
In the example in Fig. 1,(36979 bytes), the upper section of tubing needs to be removed and the lower section tied back to surface by fishing operations. The tubing must be secured by a wrap around-type hanger that will maintain a seal while the tubing is moved through it. The following is the general procedure for the operation:
- Rig up firewater protection and have capping tools on hand.
- Run a wire line caliper survey from the tubing hanger to the end of the upper section.
- Place two wire line-set permanent bridge plugs in the bottom joint of tubing (or in the lowest competent joint as shown by the caliper survey).
- Place a back pressure valve in the tree.
- Remove all but the lower master valve of the tree (Fig. 2)(36564 bytes).
- Install crossover and pup joint to the master valve and secure it with elevators to a crane.
- Remove two bolts from the tubing hanger x casing hanger flange. Install and tighten cables, and then remove other hold-down bolts.
- Strip tubing up through the wrap-around seal 12-16 in. (Fig. 3)(38395 bytes).
- Install bar clamp, slack off cables, and remove tree (Fig. 4)(35866 bytes).
- Install pup joint (with safety valve in top) in tubing hanger coupling. Rig up a double BOP (with slip and pipe rams) on tubing. Install cables through the bottom flange of the BOP, tighten the cables, and then remove the bar clamp (Fig. 5)(39274 bytes).
- Strip the tubing and BOP down until the BOP flange mates to the wellhead (Fig. 6)(37696 bytes).
- Torque the flange bolts, remove the cables, and install and torque the remaining two bolts.
- Rig up snubbing BOPS.
- Install the snubbing unit, and test the BOP stack.
- Recover the tubing and wrap-around hanger. Snub in to make tie-back, and kill the well.
Preparations must be made to cap the well immediately in the event the wrap-around seal fails during the procedure and the well blows out. Calculations must be made to ensure that an adequate length of tubing (12-16 in.) can be safely raised (unsupported) above the tubing hanger with the given wellhead pressure.
HYDRAULIC METHOD
An alternative to the cable and clamp method is the hydraulic method of removing a tree under pressure. The process is basically the same, except an hydraulic jack-type snubbing unit is used to control thrust.
A substructure is positioned over the wellhead and secured with anchors to the ground. A snubbing unit is then mounted on top of the substructure. Thrust is transferred between the snubbing unit and tree via a landing joint supported by a pipe guide in the substructure. Once the equipment is in place, snub force is applied to test the stability of the substructure.
After a satisfactory test, force equal to calculated thrust is induced downward with the snubbing unit while four tubing hanger studs are replaced with 24-in. long studs. The remaining tubing hanger studs are then removed and the tree is raised approximately 14 in. The same type of bar clamp used in the conventional method is placed on the tubing and secured to the tubing hanger flange. Snub force is then transferred to the bar clamp, allowing the tree to be removed.
A pup joint is screwed into the top of the tubing hanger coupling. A BOP is stripped over the pup joint, and the slip and pipe rams are then energized. Force is transferred back to the snubbing unit, and the bar clamp is removed. The BOP is then lowered to the hanger flange and secured.
Snubbing operations can proceed once the substructure is removed and additional BOPs are installed.
In either the hydraulic or cable and clamp procedure, the lower outlets on the BOPs should have double valves installed. These valves should be open during the operation and should be directed toward a clear area away from equipment and personnel. With this configuration, it may be possible to complete the procedure in the event the tubing hanger wrap around fails while the BOPs are lowered.
Unless the well is producing abrasive material, the flow should be diverted through the casing valves to reduce wellhead pressure during the procedure. Both procedures allow the replacement of the ring gasket in the tubing hanger flange.
The selection of cable and clamp method or the hydraulic method is made more so on the basis of familiarity and availability of equipment than on any individual limitations. For both methods, a reasonable amount of room is needed to set up support equipment to facilitate an efficient work environment. This support equipment includes an hydraulic crane, BOPS, guy wire supports for the snubbing unit, substructure, large winch trucks, choke manifolds, flow lines, production equipment, and flare stacks, if necessary.
These procedures have been successfully applied in several situations with up to 6,000 psi wellhead pressure. The cable and clamp method and hydraulic method of removing a wellhead tree under pressure can be performed safely, providing the operations are planned properly and field-proven procedures are followed.
Note that these procedures rely wholly on the tubing hanger seal, which was not designed for stripping operations. There is a chance that this procedure could cause a serious leak and result in the necessity of a capping operation. Therefore, contingencies must be made for a possible capping operation.
