Underbalance

Key Takeaways

• Underbalanced drilling (UBD) technique requires specialized rotating control devices (RCDs, also called rotating head or rotating blowout preventers) that seal around the drill string at the surface while allowing it to rotate and reciprocate, containing the produced formation fluid and directing it to a dedicated multiphase flow handling system (separators, choke manifold, and produced fluid storage) while drilling continues: the UBD surface system receives the produced formation fluid mixed with the drilling fluid returns and separates them in a dedicated UBD separator (a horizontal or vertical three-phase separator that separates gas, liquid hydrocarbon, and water from the returns), with the gas directed to a flare or sales pipeline, the liquid hydrocarbons directed to storage tanks, and the water directed to disposal; the underbalance pressure is controlled by adjusting the choke opening on the choke manifold at the surface, which changes the backpressure on the returns and therefore the downhole wellbore pressure (higher choke restriction increases backpressure and reduces underbalance, lower choke restriction decreases backpressure and increases underbalance); the continuous monitoring of the return flow rate, pressure, and composition (oil, gas, water fractions) provides real-time information about the formation's production response to drilling that is used both to control the underbalance and to generate a continuous reservoir evaluation dataset (similar in value to a drill stem test but acquired during drilling rather than in a separate test).
• Formation damage prevention by underbalanced drilling is the primary economic justification for UBD in the reservoirs where it is most commonly applied, including naturally fractured carbonate reservoirs (where overbalanced drilling plugs the fractures with mud solids that reduce permeability by orders of magnitude), tight gas sandstones with reactive clay minerals (where drilling fluid invasion causes clay swelling and fines migration that permanently reduce near-wellbore permeability), and coalbed methane wells (where aqueous drilling fluid filtrate reduces gas desorption from coal matrix pores by increasing water saturation in the near-wellbore cleats and matrix): the formation damage caused by overbalanced drilling is quantified by the skin factor (a dimensionless measure of the additional pressure drop in the near-wellbore region beyond what would be expected from formation permeability alone), with skin values of +5 to +50 being common in overbalanced-drilled wells in formation-damage-prone reservoirs; UBD eliminates the positive overbalance pressure differential that drives fluid filtrate and mud solids into the formation, and may actually clean the formation by drawing formation fluids into the wellbore during drilling (particularly beneficial in naturally fractured reservoirs where fluid influx during drilling flushes solids back from the fractures before they can cause permanent plugging); the production benefit of UBD is quantified by comparing production from UBD wells against matched control wells drilled with conventional overbalanced techniques in the same reservoir, with documented improvements of 50 to 300 percent in initial production rate being reported in field applications in naturally fractured and clay-sensitive reservoirs.
• Underbalance in perforating (underbalanced perforating) uses a wellbore pressure below the formation pressure at the time of gun firing to cause formation fluids to flow immediately into the wellbore after perforating, flushing the crushed zone and charge debris from the newly created perforation tunnels before they can compact and plug the perforation entry: in conventional overbalanced perforating, the wellbore pressure is above the formation pressure at the time of firing, causing the drilling or completion fluid to flow into the formation in the initial moments after perforating, potentially plugging the perforation tunnels with fluid filtrate and suspended solids before the well can be cleaned up by swabbing or nitrogen lifting; underbalanced perforating (also called surge perforating) uses a wellbore pressure below formation pressure to drive the initial fluid flow outward from the formation and through the perforation tunnels at high velocity, mechanically cleaning the crushed zone and carrying the charge-generated debris into the wellbore where it can be circulated to surface; the degree of underbalance required for effective perforation cleanup depends on the formation permeability, the compressive strength of the crushed zone, and the size and density of the charge debris particles, with higher permeability formations requiring less underbalance than low-permeability tight formations where higher differential pressure is needed to achieve the minimum surge velocity for crushed zone cleanup.
• Underbalance detection in conventional overbalanced drilling operations requires continuous monitoring of the pit volume, return flow rate, and wellbore pressure to identify the onset of formation fluid influx (a kick) before the influx volume grows to a level that creates a well control emergency: the primary indicators of the onset of underbalance and kick initiation include an increase in pit volume (formation fluid entering the wellbore displaces an equivalent volume of drilling fluid at the surface, increasing the total fluid volume in the active system), an increase in return flow rate above the pump rate (formation fluid entering adds to the return flow), a decrease in standpipe pressure (gas entering the wellbore reduces the hydrostatic head of the fluid column, reducing the circulating pressure required to maintain circulation), and a change in the density and gas content of the return mud (gas-cut mud returns indicate gas influx, heavy brine returns indicate water influx, oily returns indicate oil influx); early detection of a kick is essential because the volume of formation fluid that has entered the wellbore at the time of detection determines the subsequent well control procedure complexity, with small kicks (less than 5-10 barrels) being manageable with standard well control procedures and large kicks (greater than 30-50 barrels) requiring more complex or time-consuming remediation and potentially posing a risk of wellbore communication to other formations or to the surface if the gas migrates before the well is killed.
• Managed pressure drilling (MPD) is an intermediate approach between conventional overbalanced drilling and full underbalanced drilling that uses a rotating control device and a choke manifold to maintain precise control of the wellbore pressure profile throughout the drill string and annulus, allowing the wellbore to be operated at a pressure window that is narrower than the pressure window required by conventional drilling and that may include brief periods of underbalance (to minimize formation invasion) without allowing sustained uncontrolled influx: MPD is particularly valuable in wells with narrow drilling margins (where the gap between the pore pressure gradient and the fracture pressure gradient is so small that even minor mud weight adjustments cause either underbalance and kick or overbalance and lost circulation), in depleted reservoirs where the pore pressure and fracture gradient have converged to near-equal values that make any drilling without pressure control challenging, and in high-angle and horizontal wells in reactive shale formations where the wellbore stability pressure window and the pore pressure margin are both narrow; the distinction between MPD and UBD is that MPD is designed to maintain the wellbore at or slightly above pore pressure (minimizing both formation damage and kick risk) while UBD intentionally operates below pore pressure with full management of the resulting formation fluid influx.