The Successful Application of Underbalanced Drilling Technology for Reservoir Evaluation and Drilling Performance Improvement in Kuwait
Authors: Vieira P. , Larroque F. , Al-Saleh A.M. , Ismael H. , Qutob H.H. , Chopty J.R. 
Published: October 2008

Abstract
The Shuaiba formation is a highly fractured limestone reservoir. The use of conventional overbalanced drilling technology in this formation usually results in the complete loss of drilling fluids to the formation. These losses cause drilling problems and results in the masking of the potential for hydrocarbon production. Underbalanced Drilling Technology was proposed as an alternative drilling method in order to evaluate the Shuaiba formation for producible hydrocarbons while simultaneously eliminating fluid loss to the formation and improving drilling performance.
The Shuaiba formation was successfully drilled underbalanced without any fluid loss during the drilling phase and a successful evaluation of the formation for hydrocarbons was completed for the first time. A multidisciplinary team contributed to the engineering, planning and execution of the first underbalanced drilling operation in Kuwait. This paper highlights the key factors in the underbalanced drilling design phase and the operational results of the first well drilled in Kuwait using underbalanced drilling technology.

Introduction
Controlled Pressure Drilling (CPD) technology has enabled the commercial development of numerous oil and gas reservoirs around the world that would have otherwise not been exploited due to technical and/or economic limitations. Controlled pressure drilling has only become possible as a result of the integration of many developing technologies that facilitate directional control of the well trajectory, control of the bottom hole circulating pressure and the safe handling of drilling fluids and drill cuttings on surface.
Underbalanced Drilling (UBD) is a subset of CPD where the annular pressure profile is maintained below the expected reservoir pore pressure and is particularly beneficial in evaluating the productivity of prospective formations in their virgin state. This means that the formation can be evaluated without first subjecting it to drilling damage and requisite clean-up operations. The Shuaiba formation is a dolomitic limestone with significant fractures and massive losses of circulation are normally experienced in this zone. KOC believes that these massive losses have masked the potential evaluation in the Shuaiba by flushing any hydrocarbons beyond the reach of log evaluation. Similarly, drill stem tests may have also failed to reach the hydrocarbons due to the massive losses of drilling fluid to the formation and subsequent of plugging of the pore throats with drilled cuttings and drilling mud.
The Shuaiba is also naturally sub-normally pressured formation due to its depositional environment, which further lends itself to losses of circulation when drilling with conventional drilling fluids. It was anticipated that the formation pressure in the Shuaiba is in the range of 7.1 ppg - 8.9 ppg. Due to the exploratory nature of this well and the depositional layers, these fractures could contain oil, gas or water, or any combination and in no particular orde1.
Therefore, it was proposed to drill this formation in an underbalanced state with the intention of identifying the potential for hydrocarbon production from the fracture permeability in the Shuaiba dolomite.