UNDERBALANCED DRILLING WITH CONVENTIONAL DRILLING FLUIDS OR WEIGHTED DRILLING FLUIDS

UBD or ‘‘balanced’’ drilling with conventional drilling fluids is no different than conventional overbalanced drilling as far as basic solids control is concerned. Special pressure-control equipment is required to control well pressures as they occur. A rotating head, choke, and separator are basic to UBD systems (Figure 1).

Use of this equipment modifies the methods of feeding the shale shaker. The assumption should be made that the well will have to be drilled under pressure and fluid returned through the choke and separator.Nevertheless, the solids-control system has to be capable of handling 50–100% of the variations in flow volume.

An open, or ‘‘atmospheric,’’ separator typically is set high enough to feed the shaker through a U-tube seal. If the underbalanced or near balanced bottom-hole pressure is strictly controlled, the output of the separator to the shaker screen will be relatively constant. Any upset to the system from gas will tend to send a surge to the separator, followed by a flow reduction. The separator will act as a buffer, but it may not be large enough to minimize surges to the shaker screen. Some fluid may have to be bypassed to the settling tank or to a surge tank, where it will be stored temporarily as extra volume. The surge tank has been discussed under ‘‘gaseated systems.’’