Sweet Success in Sour Coring

Abstract

Abstract The South Rub Al-Khali Company Limited (SRAK) is a Joint Venture between Shell and Saudi Aramco and is engaged in exploring in the Rub Al-Khali Basin of the Kingdom of Saudi Arabia. The licence area contains significant quantities of ultrasour gas in the Jurassic Arab Formation reservoir. Acquiring accurate subsurface data forms a key objective for SRAK in its ongoing exploration drilling campaign in this remote and challenging desert environment. Health, Safety and Environment (HSE) is at the forefront of SRAK's operations. This paper presents how extensive pre-job planning and proper execution of new procedures led to the safe and successful coring of the hydrogen sulphide (H2S) rich Arab Formation in the eastern Rub Al-Khali of the Kingdom of Saudi Arabia. In SRAK's most recently completed well, five continuous cores, with a total length exceeding 350 ft were successfully cut, with a high recovery factor of 99%, covering the Mesozoic Arab C and D members of the Arab Formation. These gas-bearing reservoirs were known to be very rich in H2S and carbon dioxide (CO2) posing environmental and technical challenges for data gathering. Through strict adherence to extensive procedures and training, the operations were successful and were conducted safely and efficiently. The operations were planned well in advance with a dedicated workshop incorporating the well engineering group, subsurface teams and the coring and core analysis companies. Detailed step-by-step discussions of the coring workflow identified gaps in existing core acquisition procedures. The coring contractor, based on the expected high levels of H2S, updated existing HSE coring procedures. Specific HSE procedures that covered the interface between the different parties were set-up and communicated. Mandatory H2S-specific training was extended to cover all staff, from logistics, operations team to the core analysis personnel. In addition to the HSE aspects, the mud system and core barrel vibrations were closely monitored to guarantee good core recovery with a maximum of H2S scavenged. Special attention was also given to the cores after recovery to ensure no poisonous gas is released in an uncontrolled manner. The data obtained from the Arab Formation cores has been a vital component in establishing SRAK's understanding of the geology, hydrocarbon-fill history and in-place volumes of the company portfolio. The intention of this paper is to share the proven guidelines for coring, core handling and core processing, of rocks from H2S-bearing formations, for the benefit of other operators of sour gas fields.