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Underbalanced drilling limitations
There are limitations, as well as advantages, to underbalanced drilling (UBD). Before embarking on a UBD program, the limitations of the process must be reviewed.
Factors that negatively affect underbalanced drilling
There are technical limitations as well as safety and economic limitations to the UBD process. The following are conditions that can adversely affect any underbalanced operation:
- Insufficient formation strength to withstand mechanical stress without collapse.
- Spontaneous imbibitions because of incompatibility between the base fluid used in the UBD fluid and the rock or reservoir fluid. Use of a nonwetting fluid can prevent or reduce this situation.
- Deep, high-pressure, highly permeable wells presently represent a technical boundary because of well control and safety issues.
- Noncontinuous underbalanced conditions.
- Excessive formation water.
- High-producing zones close to the beginning of the well trajectory will adversely affect the underbalanced conditions along the borehole.
- Wells that require hydrostatic fluid or pressure to kill the well during certain drilling or completion operations.
- Slimhole or drilling conditions that result in a small annulus create high backpressures because of frictional forces.
- Wells that contain targets with significant pressure or lithology variations throughout.
Wellbore stability is one of the main limitations of UBD. Borehole collapse as the result of rock stresses is one issue to consider. The other issue is chemical stability, which is a problem seen in shale and claystone formations. Both these issues can have serious implications in UBD. Defining maximum drawdown and reviewing chemical compatibility with the proposed drilling fluids is a key issue in the feasibility of UBD.
Water inflow in a depleted reservoir can cause severe problems in an underbalanced drilled well. If the flow rate is high enough, the well will be killed as a result of the water influx. Gas lifting a well that produces water at a high rate is almost impossible. Care must be taken that the water leg in a depleted reservoir is not penetrated when drilling underbalanced.
Directional drilling equipment
Directional drilling equipment can have limitations on UBD. Hydraulic operated tools cannot be used in underbalanced wells, and if a gasified system is used, the measurement while drilling (MWD) pulse systems may not work. Certain motors and other directional equipment may be prone to failure as a result of the rubber components becoming impregnated with the gas used. Explosive decompression of rubber components is a consideration when selecting equipment.
The higher torque and drag seen in underbalanced wells (as much as 20 to 100%) may prevent certain trajectories from being drilled underbalanced. The higher torque is caused by the reduced buoyancy combined with the lack of filter cake on the borehole wall.
The reservoir may not be suitable for UBD. A highly porous, high-permeability reservoir can provide too much inflow at low drawdown. It is important that the perceived benefits of UBD are kept in mind when planning for underbalanced operations.
Safety and environment
The health, safety, and environment issues of a UBD operation may prove to be too complicated to allow UBD to proceed.
The placement of the surface equipment may prove to be impossible on some offshore locations. There can be problems with rig-floor height and with deck space or deck loading. Both the wellhead equipment and the surface separation equipment must be carefully designed to fit the platform or rig.