The Buda formation in Texas presents extreme challenges to prevent formation damage during drilling operations. After using many fluid selection variations, several operators have determined that underbalanced drilling with native crude is the most optimum method. The initial startup project for coiled tubing drilling (CTD) operations in this formation has been completed, with more activity to come. However, as the preferred recirculation of the native crude drilling fluid presents two major challenges. First, the formation is up to 2% sour, and the native crude is relatively volatile with a low flash point and high vapor pressure.Recirculation of sour crude will lead to double-sided exposure of the coiled tubing, which historically has resulted in extremely short fatigue life. The use of active fluid property testing, in addition to scavengers and inhibitors with defined mitigation plans, reduces the risk to an acceptable level. Significant laboratory testing of the fluid properties as the material degrades with time, temperature, and mixing have been completed. Additional mitigation actions are utilized in conjunction with the laboratory results to further reduce the volatility properties. Internal technical and operational reviews, along with additional sourced subject matter expertise, have challenged existing safety, operational, and technical limits. Operational procedures have been continuously monitored and adjusted to compensate for the adverse dynamic wellbore conditions encountered during the campaign.The discussions within this paper detail the background challenges, laboratory testing, operational/HSE planning and mitigation practices to allow operations to commence. Additionally, the paper covers operational results as the wells are drilled. These results may provide a basis for future operations utilizing extreme fluid conditions in other applications within the industry due to the economic benefits from native crude.

With the large amount of tight gas reserves remaining in Oman, new innovative techniques and methods to unlock these reserves have become imperative for the future economic success and stability of the country. Among the various technologies considered, the concept of underbalanced coiled tubing drilling (UBCTD) was introduced. In order to address the harsh downhole challenges such as high temperatures, deep burial depths, under pressured reservoirs, abrasive and hard sands and logistical constraints; a fully integrated well delivery solution was developed jointly by the operator and energy service company. In accordance with this strategy, best in class downhole drilling tools, a state-of-the-art fully automated coiled tubing drilling unit together with technical and project management experts were deployed. Application specific solutions to the challenging subsurface conditions included utilization of underbalanced drilling (UBD) techniques, deployment of high temperature drilling assemblies, fit-for-purpose bit drive mechanism and a robust integrated management system. All of the above was achieved whilst ensuring the safety of all personnel during the project and reducing carbon emissions through a flare minimization strategy and diesel consumption reduction initiatives. Over the course of the pilot campaign, reservoir exposure per well was doubled, average penetration rate compared to conventional rotary drilling was more than tripled and incremental production improvements of up to 230% were observed. This paper discusses the challenges faced and the solutions implemented during this three well pilot campaign.