What does underbalanced drilling mean

Underbalanced drilling refers to drilling deliberately when the formation pressure or pore pressure is greater than the gas column pressure generated by the annular drilling fluid column or gas column. In this respect, balanced drilling belongs to the category of underbalanced drilling, because during balanced pressure drilling, the movement of the pipe string may cause the annulus pressure to be lower than the formation pressure. Underbalanced drilling can prevent or reduce annulus leakage to protect the reservoir, prevent sticking, and increase the ROP.

Drilling below the formation pressure will produce a situation similar to a well kick or blowout. Controlling these pressures and maintaining a safe working environment requires special surface pressure control equipment and trained and qualified employees. The required equipment is mainly determined according to the rock properties, permeability, formation pressure and other conditions of the formation to be drilled.

In addition to special equipment, underbalanced drilling also requires some surface treatment equipment similar to overbalanced drilling. Regardless of the relationship between the shortage of liquid column and formation pressure, the drilling fluid must be able to clean the bottom hole cuttings and carry them to the surface. Before the drilling fluid enters the drill string in the next cycle, the ground must use corresponding equipment to remove the solid phase. When the drilling fluid is not recycled, such as aerated drilling or foam drilling, corresponding measures must be taken to deal with the solid and liquid phases in the drilling fluid.

Compared with the solid phase in overbalanced drilling, the emphasis on solids control in underbalanced drilling is not enough. There are many reasons for this. Some people think that the main reason may be that the formation pressure is higher than the pressure of the drilling fluid column. Perform solids control. But this is not consistent with the actual situation, because the annulus is not always in an underbalanced state, so too high solid content may cause reservoir damage. Moreover, as with overbalanced drilling, too high solid content will also reduce drilling efficiency. In addition to the special equipment in the system that should be redesigned, various existing solids control equipment can be used in underbalanced drilling.

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