Optimising Big Bore Gas Well Completion Design in a Carbonate Field
Authors: Yeow Ming-Lei 
Published: March 2021

A large gas field with carbonate formations was discovered about 200 km (kilometers) offshore, in water depths of 108 m (meters) below mean sea level. Flow analysis and reservoir depletion studies by the Operator show 7" tubing is required to provide high production rates of about 80 to 120 million Standard Cubic Feet per day (mmscfd) per well as the optimum production and depletion strategy for the field. The field presents significant challenges to well construction and completion. These challenges were considered in the design stage to optimise well completion design and operations. This paper describes the approach and methods used to overcome the challenges in the design and execution stage to optimise the completion design and to manage challenges during construction of the well including: –Due to loss of circulation in the fractured gas carbonate reservoirs, the well will be drilled with Pressurised Mud Cap Drilling (PMCD), a form of Managed Pressure Drilling (MPD). Thus, the design needs to provide for installing the well completion in this condition–The wells required heavy mud weight for drilling and thus, heavy brine weights for well completion. Challenges to avoid or minimise the loss of high cost heavy brine were considered–High reservoir temperatures of about 325 deg. F (degrees Fahrenheit) which lead to challenges of ensuring equipment will continue to work in high temperature environments, high loads / stresses on completion & casing during production, and Wellhead & Christmas Tree & Flowline movement / growth.–Presence of sour gas and CO2 (Carbon Dioxide) which require optimising metallurgy of tubulars and downhole equipment, considering corrosion and cost requirements–Understanding loads / stresses on the tubing and casings with high flowrates and high flowing temperatures–Concerns with formation collapse during production–Corrosion studies to optimise material selection and ensuring well integrity for at least twenty (20) years’ field life–Manage hydrates and scaling while carrying out well completion, well clean and well test operations and during the field life–Acceptable seals, barriers and completion equipment to manage high pressure gas–Wellhead and Christmas Tree that can take the high temperature and sour gas content–Well clean-up and testing after completion in conjunction with drilling operations–Cost and time optimisation to maximize returns on investment for the project.

Well completion engineering studies were carried out for each challenge or consideration. Experiences and lessons from similar gas fields were also considered. Corrosion laboratory analysis was carried out to optimise the corrosion resistant alloy for the completion tubular and completion equipment. For each solution to the challenge, cost and time considerations were reviewed and studied to optimise the design, cost, integrity and safety of the wells and operations. This paper describes the approaches and methods taken by the Operator to optimise the Big Bore Gas Well Completion Design including some lessons for improvement after successfully drilling, completing, clean-up and testing of the first well with this completion design.