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Valves at the surface, in the drill string and on the wellhead are used during drilling for well control, specifically to prevent the potentially disastrous consequences of a kick, which is the sudden influx of large amounts of gas into the well bore. The valves are designed to close the well exactly when required and under any circumstances although how it is done depends on what is in the well at the time.
If there is a string through the wellhead then, besides the protection afforded externally, intemal protection is also required. For this a non-retum valve (NRV) positioned close to the base of the string allows circulation downwards but not upwards. Other back-up valves are located on the drill floor (to be stabbed into the drill pipe) or on the kelly. The choke manifold in the mud flowing system also controls the back-pressure on the mud column.
However, the most important control system is the BOP either set at the surface or subsea. At the top of the BOP is a hard rubber sleeve that fits around any pipe and will seal the annulus of the well when sufficiently compressed by a hydraulically operated piston. The sleeve allows the drill string to move up or down (called stripping) adapting itself to irregularities as the string is stripped through.
Below the sleeves pipe rams are metal rings that fit specific pipe sizes. They can be hydraulically employed only when matched drill pipe is in the BOP but can hold back very high pressures. Also present are blind rams, which have plain ends that can be used to close the well when there is no drill pipe or obstruction in the wellhead. The rams are made with steel lined with rubber. Finally shear rams are sharp carbon steel mechanisms that can rapidly cut pipe and close off a hole completely.
For BOP operation a supply of fluid under pressure from a pressure vessel called an accumulator is always made available for instantaneous power. BOPs are rated according to the pressure they can handle with most offshore wells requiring 10,000 or 15,000 pounds per square inch capability. The wellhead equipment forms part of the general equipment rental but blowouts themselves are extremely rare and are not included as a drilling cost.
To extend the water depth capabilities of older rigs some have been reconfigured to use surface BOPs instead of subsea BOPs. Surface BOPs require that casing is cemented into the sea floor, which can be a potential source of fluid leaks from the high-pressure riser. To provide protection Cameron has developed a linking mechanism called a subsea Environmental Safe Guard (ESG), which not only seals the connection but also allows disconnection from the sea floor and increases the weather window for drilling. It is a sea floor positioned shear ram that allows a traditional BOP stack to be suspended below the rig floor to handle the well control functions.
The ESG has a basic control system which is tied back to the surface BOP via a high pressure riser, a telescoping joint and a motion compensation system. It is considerably smaller and lighter than normal systems so that lower rated rigs can use it even in deep waters. A similar surface BOP (SBOP) system has been used in Southeast Asia since the 1960s but is being adapted by Shell for use on dynamically positioned deep waterrigs.
In a normal open system the formation fluid pressures increase steadily as a function of the hydrostatic pressure (exerted by the weight of the column of water above the formation), however, abnormal pressures may be encountered in all types of geological settings. Overpressures arise most commonly either where hydrocarbons are present or where under-compaction has occurred. In the former the force which water exerts at the base of a hydrocarbon column is a function of the difference in density between the water and the hydrocarbons. A pressure anomaly thus emerges which is at a maximum at the top of the reservoir. In the latter buried sediments sometimes become sealed by surrounding clays and are unable to expel trapped water. As burial proceeds over geological time the water begins to support the weight of the overburden and overpressures are generated.
Safe drilling relies on the ability to detect increases in formation fluid pressure at theearliest possible stage. Predictions based on nearby wells and geology will be made before drilling commences but the Mud Logging Company is normally responsible for detection during drilling. Increases in ROP, drilling torque and mud pit levels indicate potential over-pressured zones whilst LWD tools can detect under-compacted shales. An engineer will analyse these data in real time to determine whether overpressures are a hazard. If a well begins to flow then a well kick is taking place downhole and remedial measures are required usually involving increasing the mud weight. The drilling of high pressure/high temperature (HP/HT) wells is increasing as shallow opportunities are exhausted and so these problems are also becoming more common.
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