Chpater 12 - Underbalanced Drilling

Abstract

This chapter discusses underbalanced drilling operations, which require that the bottomhole annulus pressure during circulation of the drilling fluid be maintained at a magnitude that is less that the static bottomhole pore pressure of the reservoir producing pressure. When underbalanced conditions are maintained during drilling or completions, reservoir fluids enter the well bore and are carried out as the drilling fluids are circulated through the well. Underbalanced drilling and completions operations are restricted to mature sedimentary basins. 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. Low head drilling is the most utilized conventional drilling technique. In underbalanced drilling operations the primary control barriers that restrain the reservoir fluid from flowing to the surface are the RCH, the blooey line, and so on. These primary control barriers restrict the flow of the reservoir fluids that are entrained in the annulus flow of the circulating drilling fluid. The secondary control barrier in these operations is also the mechanical BOP. Underbalanced drilling allows the bottom hole pressure to always remain below the pore pressure of the producing reservoir. A number of drilling fluid types that can be used in underbalanced drilling operations are: conventional drilling muds, gasified drilling fluids, stable foam drilling fluids, unstable foam fluids, and air and gas drilling fluids.