What is Underbalanced Drilling?
The International Association of Drilling Contractors defines Underbalanced Drilling (UBD) as, “a drilling activity employing appropriate equipment and controls where the pressure exerted in the wellbore is intentionally less than the fluid pressure in any part of the exposed formation.” To achieve an underbalanced condition, a gas-liquid mixture is often used in place of conventional drilling mud. The result is that multiphase flow can be present in every aspect of a UBD operation. This presented new challenges for drilling engineers who had historically dealt strictly with conventional drilling mud (i.e. an effectively incompressible, single-phase fluid) for which the hydraulics focused on mud weight and the corresponding hydrostatic head. In UBD they were dealing with complex behaviour associated with mixtures of gases and liquids (both drilling fluids and produced fluids). The early 1990s saw a dramatic growth in UBD as it became popular for drilling horizontal wells. Maintaining a
The International Association of Drilling Contractors defines Underbalanced Drilling (UBD) as, “a drilling activity employing appropriate equipment and controls where the pressure exerted in the wellbore is intentionally less than the fluid pressure in any part of the exposed formation.” To achieve an underbalanced condition, a gas-liquid mixture is often used in place of conventional drilling mud. The result is that multiphase flow can be present in every aspect of a UBD operation. This presented new challenges for drilling engineers who had historically dealt strictly with conventional drilling mud (i.e. an effectively incompressible, single-phase fluid) for which the hydraulics focused on mud weight and the corresponding hydrostatic head. In UBD they were dealing with complex behaviour associated with mixtures of gases and liquids (both drilling fluids and produced fluids). The early 1990s saw a dramatic growth in UBD as it became popular for drilling horizontal wells. Maintaining a
