Overcoming Flow Assurance Issues in Deepwater Gas Well Testing and Clean-Up Operations: Best Practices and Lessons Learned

Abstract

Today many exploration, appraisal, and development well test operations are performed in new frontiers, remote locations, deepwater offshore environments, and complex gas-bearing reservoirs. Deepwater welltest and well cleanup operations are presenting unique challenges related to flow assurance aspects that are beyond the standard welltest playbook. Several failures and operational issues related to flow assurance such as the plugging of pipelines and/or tubing by wax or hydrates have hindered the interpretability of data, compromised well test objectives, significantly increased the total costs, or led to serious health safety and environmental (HSE) incidents. Several past experiences have shown that it is nearly impossible to perform a proper deepwater well test with achieving well test objectives without an appropriate test design, equipment selection and operations planning. The assessment and mitigation actions to reduce the risk of hydrate formation are of paramount importance in such welltest design activities. Hydrate formation is one of the most common flow assurance issues during the well test and well cleanup operations in gas and gas-condensate reservoirs, particularly in deep-water offshore environments. Low water column temperature in deep water along the marine riser can cool down the produced fluids considerably. This in turn can trigger hydrate formation introducing significant challenges to the operations while increasing the risk of HSE incidents and therefore compromising the welltest objectives. Well tests and well clean-ups are complex operations performed offshore today that involve multiple expertise across different domains and services including temporary surface well test package, landing string, and drill stem test (DST) string with specialized downhole tools and components or permanent completion. Therefore, the risk assessment of hydrate formation and plan of mitigation actions must be performed in line with the detailed welltest operation sequences and technical capabilities of the equipment used offshore. Today there are no definitive practical workflows available that would guide operators through the process of planning deepwater well test operations effectively with a detailed assessment of the risk of hydrate formation and mitigation plan. During the course of past three years, over twenty-five gas well tests were successfully completed in deepwater and ultra-deepwater environments around the globe. This success is mainly driven by the application of hydrate studies performed with new-generation transient multiphase wellbore flow simulation software. Hydrate studies coupled with fine-scale reservoir models for well test design, enable the evaluation of hydrate formation risks and help operators develop mitigation actions. The selection of an appropriate hydrate inhibitor and optimization of the chemical injection system is essential to achieve the required chemical injection capabilities to prevent hydrate formation. Despite all the mitigation actions against hydrate formation, our experience has shown that rigorous monitoring and quick reaction to early detection of hydrate formation was instrumental in preventing several substantial problems. This paper describes an innovative approach and provides practical recommendations that can be used to assess the risk of hydrate formation and develop mitigation actions during the design stage of well tests. Besides, this paper highlights the results and lessons learned from the multiple welltest operations performed in recent years.