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Drill String
The drill string design must be as simple as possible to avoid damage to the sealing elements of the rotating BOP or rotating control head. In gasified liquid drilling, the string is usually tripped while the well is pressurized. A float at bottom enables the string to be pulled with pressure underneath. A large diameter is desirable for high rate wells to reduce the frictional pressure drops in the string.
Typical gasified liquid drilling applications in the United States are conducted with water or mud to apply back pressure on a formation. Rotating heads are installed on top of conventional BOP stacks.
For higher back pressure applications, a rotating blowout preventer is more suitable than a rotating head. If installed, blind rams can allow irregularly shaped BHA components to be lowered into the well under pressure. Typically, BOP stacks used in this process are larger than the ones used in conventional drilling applications. Therefore, the clearance beneath the rig floor must be properly designed to allow more room.
The surface system configuration for gasified liquid drilling depends on the hydrocarbon production potential of the well. If aerated muds are used, additional gas separators are needed. A typical surface system configuration is shown in Figure 1.
Returns are directed to the rig’s shale shaker or a mud/gas separator as shown in Figure 2 when brine is used as the liquid phase.
Closed systems should be used if H2S returns are expected.
However, care must be paid not to collect combustible mixtures in the separators.
An MWD unit may be necessary for monitoring the bottomhole pressure to avoid formation damage in sensitive formations. If the well is filled with a compressible fluid, electromagnetic MWD systems can be used to ensure the data transmission to the surface. Real time monitoring of the annular pressure can be used to adjust the injection rates for maintaining underbalanced conditions.
Water producing zones can be identified based on estimated downhole pressures.
