Case Study of Underbalanced Drilling Operations in Fractured Carbonate Reservoirs in OML 11, Rivers State

Underbalanced drilling techniques have demonstrated significant benefits in fractured carbonate reservoirs by minimising formation damage from drilling fluid invasion and enabling real-time productivity testing during drilling. This study presents a case study of underbalanced drilling operations applied to fractured carbonate reservoir sections in OML 11, Rivers State, South South Nigeria. Operational data including borehole pressure profiles, real-time formation pressure while drilling measurements, influx volumes, and rate of penetration records were analysed for three wells where underbalanced conditions were intentionally maintained using nitrogen aeration of the drilling fluid. Formation pressure gradients estimated from pressure while drilling data were compared with planned equivalent circulating density windows. Surface equipment including rotating control devices, degassers, and multiphase flow meters were evaluated for operational performance against their design specifications. The underbalanced wells were completed and tested for productivity, with results compared against two adjacent conventionally drilled wells in the same carbonate sequence. Skin factors calculated from post-drilling production tests indicate values of minus 2.1 to minus 3.6 for underbalanced wells versus plus 4.7 to plus 8.2 for the conventionally drilled reference wells, confirming substantial damage avoidance benefits. The study concludes that underbalanced drilling delivers measurable productivity improvements in the OML 11 fractured carbonate context but requires enhanced casing design to manage well control risk. Keywords: underbalanced drilling, fractured carbonate, OML 11, formation damage, skin factor.