Underbalanced Drilling (UBD) as a Tool to Test Tight Gas Plays – An Example from the Empty Quarter, Saudi Arabia: Drilling and Reservoir Characterisation Lessons

Abstract

Executive Summary The South Rub Al-Khali Company Limited (SRAK) is an incorporated Joint Venture formed by Shell Saudi Ventures Limited (50% share) and Saudi Arabian Oil Company (50% share) to explore for non-associated gas and associated liquids in the South Rub’Al-Khali Basin (Empty Quarter) in the Kingdom of Saudi Arabia. This paper discusses specific data acquisition strategies that have been developed to address the challenges of deep wells in tight reservoirs in remote and harsh environments. While drilling the first three wells SRAK has gone through a very steep learning curve with respect to well data acquisition and integration. It also allowed optimising the data acquisition strategy for future wells that SRAK was planning to drill. As a direct result of this initial work, and the experience gained by other wells drilled in the Rub’ Al-Khali, SRAK realised that the traditional approach and formation evaluation using static well data gathering was not entirely satisfactory. Industry experience also showed that tight gas exploration and appraisal wells historically involved lengthy multi-year multi-well programs due to inconclusive results from static logging, coring and well testing operations. In line with Shell's global initiative to use underbalanced drilling (UBD) for tight gas exploration and appraisal, UBD was implemented in SRAK's fifth exploration well in the Rub’Al-Khali. The deeper Palaeozoic reservoirs in the Rub’Al-Khali are known to be commonly tight and susceptible to near-wellbore formation impairment during conventional overbalanced drilling. UBD offered a possible solution for better quality reservoir evaluation and demonstrating any un-impaired potential flow. In other prospects, previous conventional drilling, stimulation and well clean-up had typically failed to unambiguously characterise pay and produce hydrocarbons to surface. UBD drilling operations were designed and implemented with the support and expertise of the shareholders. Both internal and external subject matter experts supported operations. Whilst numerous UBD specific drilling related aspects were understood pre-drill, significant uncertainty remained around data acquisition and interpretation during UBD drilling operations. The SRAK UBD project was safe and successfully executed as per the drilling program. Due to the UBD operations there was no ambiguity on presence and mobility of hydrocarbons, thereby avoiding SRAK the completion, fraccing and testing cost which are far higher than the UBD expenditure. The well SRAK-F offered firsthand experience to the SRAK team to assess the advantage and disadvantage of UBD operations on reservoir characterisation. This paper aims to capture SRAK's learning in UBD drilling and reservoir characterisation.