Cementing Practices to Solve Well Integrity Challenges of Ultra Deep High Temperature Wells in Western China

摘要

Ultra-deep wells (maximum measured depth of 8000 m) located in the Tarim Kuqa foreland oil field in western China pose significant challenges to well integrity because of the high temperature (maximum bottom hole static temperature of 180°C), high pressure (maximum 145 MPa), and a deeply buried, thick, salt-gypsum formation (maximum measured depth of 7800 m). An engineering cementing approach with continuous improvements has been applied in the cementing operations in over 200 wells from 2008 to 2015. The ultra-deep casing program was designed to meet the complex drilling and high-pressure completion requirements. Meanwhile, fit-for-purpose cement systems have been developed to seal the long, high- pressure interval of the salt-gypsum formation. In addition, a procedure on spacer optimization and evaluation has been established to optimize target zone cementing in oil-base mud. Tailored cementing procedures from well preparation to post-job remedial work were developed to meet the severe lost circulation challenge caused by the narrow pore-fracture pressure window in the reservoir. Since 2008, various problems related to well integrity have been encountered in the field, including casing leaks, incomplete zonal isolation post cementing and sustained casing pressure (SCP) during the exploration and development activities. Substantial local operational practices have been accumulated to solve these intractable problems through the 1200 primary and remedial cementing operations undertaken to the present. A comprehensive pool of cement slurry formulations has been established with optimized properties including extreme high density with good pump-ability, saturated salinity resistance, and cement setting under an extended temperature difference to meet the various cementing requirements resulting from complex and extreme downhole conditions. A design-execution-evaluation (DEE) cycle of cementing operation has been implemented to achieve continuous improvements and shorten the learning curve in the face of the new challenges continuously encountered during the extended exploration activity of this region. The annual average percentage of qualified cementing jobs has been enhanced steadily from an initial 55% in 2008 to the present 73%. The nature of this ultra-deep reservoir limited the application of lessons learned from other oil fields. A series of cementing practices developed empirically to address the challenges of this field have been documented and optimized over the years. They have proven to be successful in the field and form the foundation of designs going forward.