Challenges in Design of Wellfluid H2S Mitigation on an Offshore Wellhead Platform - A Case Study

Abstract

Abstract Hydrogen Sulfide (H2S) occurs naturally in well fluid and imposes variety of challenges in oil and gas industry. Although there are many methods that may be employed to remove H2S from gas and liquid streams, selection of technology, design, installation & operation on a remote unmanned wellhead platform offers unique challenges. This paper presents NPCC’s experiences in implementation of H2S mitigation unit on a recent offshore well head platform project. Offshore wellhead platforms are designed to minimize manning frequency and duration of stay. Further, climatic conditions - heavy rains during monsoon impose additional challenges to logistics, restricting movement of crew members, service boat requirements to replenish consumables on unmanned wellhead platform. Therefore, it’s preferred to select wellfluid H2S mitigation unit requiring minimal operational intervention. Sweetening of multi-phase wellfluids (mixture of oil, gas & produced water) can be performed by direct injection of scavenger chemicals but pose problems in downstream facilities. Alternately, it was proposed to separate the wellfluid into gas & liquid streams and treat them independently for H2S removal. Experience demonstrates that proper selection and application of proven technology are critical for wellfluid H2S mitigation. Based on assessment of various technologies, the most optimized approach for this case is use of a solid based scavenger media packed in an Adsorber vessel to treat the associated gases and a triazine based liquid scavenger to treat the separated liquids. Adsorber vessel needs to be designed considering replacement of spent media with fresh solid scavenger based on worst possible monsoon period in the region. The important design challenges include unloading of spent media and loading of fresh media into the Adsorber vessel. This involves establishing logistics in transporting fresh media to offshore and sending spent media to onshore for safe disposal. Safe land disposal practices are recommended to meet the local environmental regulations. H2S removal provides distinct advantages and focus is now shifting towards removal at wellheads due to potential high returns on investment in current oil market. Adding new sour wells to existing production may pose limitations in downstream facilities such as existing pipeline and process complex. It is worth to consider sweetening of wellfluid at wellhead platform thus mitigating H2S at the source itself instead of augmenting existing sweetening system at downstream facility. This paper discusses a case study based on an actual project, highlighting the design challenges and experience gained during execution of wellfluid sweetening system on a remote wellhead platform, thus providing economic benefit to all stakeholders.