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Completing underbalanced wells
Underbalanced drilling (UBD) can create special challenges for well completion.
Damage caused by overbalanced conditions during completions
The majority of wells previously drilled underbalanced could not be completed underbalanced - the wells were displaced to an overbalanced condition with kill fluid prior to running the liner or completion. Depending on the completion fluid type, some formation damage would take place. The damage is not as severe for completion brine as with drilling mud because there are no drilled cuttings and fines in the brine. However, reductions in productivity of 20 to 50% have been encountered in underbalanced drilled wells that were killed for the installation of the completion.
Completion methods for underbalanced drilled (UBD) wells
If the purpose of UBD is reservoir improvement, it is important that the reservoir is never exposed to overbalanced pressure with a nonreservoir fluid. If the well has been drilled underbalanced for drilling problems and productivity improvement is not impaired, then the well can be killed, and a conventional completion approach can be taken.
A number of completion methods are available for underbalanced drilled wells:
- Liner and perforation
- Slotted liner
All of these options can be deployed in UBD wells. The use of cemented liners in an underbalanced drilled well is not recommended if the gains in reservoir productivity are to be maintained.
Regardless of the liner type run, the installation process for the completion is exactly the same. It is assumed that a packer-type completion is installed. The production packer and tailpipe are normally run and set on drillpipe, with an isolation plug installed in the tailpipe. If the well is maintained in an underbalanced condition, well pressure will normally require the production packer and tailpipe to be snubbed into the well against well pressure. The use of pressure-operated setting equipment in underbalanced drilled wells is not recommended. A mechanically set production packer should be used.
Installation of a solid liner
Using solid pipe for the liner is no different from snubbing in drillpipe or tubing. The shoe track of the liner must be equipped with nonreturn valves to prevent flow up the inside of the pipe. The liner is normally run with a liner packer, and the liner can be snubbed into the live well. Once on bottom, the liner hanger and packer are set and the reservoir is now sealed. If zonal isolation is required, ECPs must be run at predetermined intervals. Once the liner is set, the pipe must be perforated to obtain flow. This can be achieved using the normal procedures, but it should be remembered that any fluid used must maintain the underbalanced status.
Installation of a perforated liner or a sandscreen
The main disadvantage of running a slotted liner or sandscreen in an underbalanced drilled well is that isolation is not possible across the slotted section of the liner or screen with the blowout preventors (BOPs). The use of plugged slots that dissolve once the liner is installed downhole is not deemed safe for offshore operations. The pressure integrity of each slot would need to be tested prior to running each joint, and this is not feasible.
The use of special blanking pipe in sandscreen adds further complications to the installation procedures. Running a slotted pipe or screen into a live well cannot be done safely, because, even if all the holes are plugged, the potential for a leak is too great. The only way to install a slotted liner in a live well is by using the well as a long lubricator and by isolating the reservoir downhole.
There are mechanical methods of downhole isolation that are available, if it is necessary to run a slotted liner. The underbalanced liner bridge plug system is one of the systems currently on the market. This system allows a retrievable plug to be set in the last casing. This isolation plug is released by a retrieving tool that is attached to the bottom of the slotted liner. This retrieving tool unseats the isolation plug and then swallows the isolation plug or packer. The swallowing action of the retrieval tool ensures that the plug and retrieving tool are rigid, and can be run to Total Depth (TD) without hanging up in the open hole. Both the packer and retrieval tool are specifically designed to be released by the liner. If necessary, the well can be lubricated to kill fluid on top of the plug, and displaced via the slotted liner when the drillstring is sealed by the rotating diverter.(This sentence doesn’t make sense to me. Are they killing fluid? I have never heard that phrase used before.) The procedure for running a slotted liner and the completion in an underbalanced drilled well is outlined in the following diagrams (Fig. 1).
The main problem with running the completion in a live well is the installation of the subsurface safety valve control line. Once the control line is connected, the BOPs no longer seal around the pipe. The simplest method is to isolate the reservoir prior to running the completion.
In the case of the completion, the production packer with a plug installed in the tailpipe is snubbed into the live well, and the production packer is set on drillpipe. The packer assembly would be lubricated into the well by utilizing the snubbing well-control system.
Once the production packer is set, the drillpipe can be used to pump completion fluid to provide an additional barrier that can be monitored if required. The completion is now run conventionally. The isolation plug in the tailpipe is retrieved during the well commissioning. Before pulling this plug, the fluid must be displaced out of the completion string. This can be achieved with coiled tubing or with a sliding sleeve. Once the completion has been installed, the well is ready for production. No cleanup or stimulation is required in the case of underbalanced drilled wells.