A Collaborative Approach to Risk Assessment and Mitigation ofPre-Production Cross-Flow for a Multi-Billion Dollar Sour Field DevelopmentProject in the Sultanate of Oman: A Case Study

摘要

The Yibal Khuff/Sudair reservoirs were discovered in 1977.The field contains both Non-Associated Gas inthe Sudair & Lower Khuff reservoirs and Associated Gas with oil rims in the Upper Khuff reservoirs.TheUpper and Lower Khuff hydrocarbons contain 2-3% H2S and 4-6% CO2,whereas the Sudair gas contain1-1.5% CO2 and less than 50 ppm H2S.The Field Development Plan (FDP),a multibillion dollar sourdevelopment project,was completed in 2011 proposing a total of 47 wells,34 dedicated horizontal/verticalwells for oil rim production and 13 commingled vertical/deviated gas wells,and the construction of newsour surface facilities with a gas production capacity of 6 MMm3/day.FDP execution started in 2016 while the details of field start-up,scheduled a few years later,were stillbeing planned.As part of this planning,it was noticed that a number of pre-drilled wells required perforationand clean-up before facility startup.Due to the time necessary to prepare all the pre-drilled wells,pre-production wellbore cross-flow was expected to occur in wells located in the West block of the field.Adedicated subsurface team was assigned in 2017 to evaluate and mitigate the potential risks associated withthis expected cross-flow through the wellbore resulting from the pressure difference between the LowerKhuff and Upper Khuff layers.This paper covers the integrated approach that the team followed to address the expected cross-flowissue,including: 1.Basis for pre-production cross-flow.2.The quantification of the cross-flow using analytical and numerical simulation methods.3.The assessment of the impact of cross-flow on process safety and the environment (i.e.drillingrisks with potential blow out of sour gas) and social responsibility (i.e.production capacity andultimate recovery losses resulting in lower benefits to the community).4.The identification and assessment of solutions to stop/reduce the cross-flow.5.The implementation of a robust and feasible mitigation plan.The conducted study demonstrated that the impact of cross-flow at well level would be severe.The cross-flow rate could reach up to 25-137 Km3/day/well,while the field level cross-flow rate could reach up to 400Km3/day.The oil rate capacity reduction in the West Block wells could reach 20-30% at start-up,resultingin a total only 1% oil ultimate recovery loss at field level since the West block contribution is small to totalproduction and West block wells are constrained.The study also showed that the casing design is adequateand drilling risks are manageable even in case of cross-flow.Out of several solutions identified to stop/reduce cross-flow,phasing perforation was considered the most robust and feasible option.This paper presents the novel approach of a collaborative study that resulted in improved safety andreduced environmental risks and potential ultimate recovery losses.It also presents the methodologies usedto allow the Assessment and Mitigation of Pre-Production Cross-flow and evaluation of the best option tomitigate the cross-flow in order to minimize the impact of cross-flow at minimum cost,well interventionsand impact on well deliverable.