TABLE OF CONTENTS Underbalanced drilling is an oil field term. These drilling operations require that the bottom hole annulus pressure during circulation of the drilling fluid be maintained at a magnitude that is less that the static bottom hole pore pressure of the reservoir producing pressure. These underbalanced operational requirements extend to the follow-on completions. Oil and gas drilling operations are generally the only drilling operations that require the drill bit to advance into rock formations that contain high pore pressure reservoir fluids. When underbalanced conditions are maintained during drilling or completions, reservoir fluids will enter the well bore and be carried out as the drilling fluids are circulated through the well. Note that the only other operations that drill into high pore pressure rock formations are geothermal drilling operations and artesian water well drilling. Geothermal drilling and completion operations are usually carried out with the oil industry.
Conventional drilling operations are either balanced or overbalanced. These operations require that the circulating fluid exert bottom hole pressures that are equal to or greater than the fluid pore pressure of the reservoir. In these operations the drilling fluid is the primary control barrier that keeps the high pressure reservoir fluids in the reservoir. Thus, the circulating drilling fluid is not cut with reservoir fluids. The secondary control barrier in these operations is the mechanical BOP. In underbalanced drilling operations the primary control barriers that restrain the reservoir fluid from flowing to the surface in an uncontrolled manner are the RCH,...
