Underbalanced Drilling (UBD) technology has been applied around the world to drill wells where it is imperative to obtain benefits, as it minimizes the formation damage that is common result in conventional overbalanced drilling operations. In addition to minimizing formation damage, other benefits related to Underbalanced Drilling include increase of penetration rate, elimination of lost circulation problems, elimination of differential pipe sticking and reservoir evaluation while the well is being drilled.

The reservoir screening analysis and the engineering feasibility study performed during the planning stage of this project determined that the Hamra reservoir is a good candidate for underbalanced drilling technology application.

Based on the reservoir pressure and the Lower Hamra formation depth in the Sabria field, a weighted drilling fluid system was required to achieve a controlled drawdown and therefore underbalanced drilling conditions. Comprehensive hydraulic simulations were carried out to determine the required liquid injection rate and back pressure on the choke manifold to ensure safe and manageable drilling conditions. The simulations were also used to predict the anticipated hole cleaning and the impact of reservoir production due to downhole hydraulic conditions while drilling.

Drilling the Multilateral Well A in underbalanced condition allowed Winstar Resources Ltd to evaluate the reservoir and successfully acquire real time reservoir characterization data and production capability of the well while drilling.

A multidisciplinary team contributed to the engineering, planning and execution of the Underbalanced Drilling operation. This paper discusses the planning phase, operational results, lessons learned and recommendations that will permit to improve the UBD benefits for future wells to be drilled in the area.

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