West Africa Salt-Zone Cementing Best Practices:Laboratory Evaluation and5-Year Field Application Review

摘要

To determine which salt-based cement system [potassium chloride(KCl)or sodium chloride(NaCl)] wassuitable for cementing across halite and anhydrite salt sections in West Africa,eight slurry recipes weretested to assess how formation salt contamination would affect slurry properties.The formation salt used fortesting was sampled from a deepwater,presalt well in Angola.The recommendations developed from thelaboratory study were implemented in 10 projects across West Africa over 5 years with 100% operationaland well integrity success.A candidate deepwater well was selected in which the surface and intermediate strings penetrated saltformations.A total of four slurry designs(a lead and tail slurry used on each casing string)was programmed.Each slurry was designed and tested as two distinct systems using KCl and NaCl salt respectively,yieldinga total of eight slurry designs.Using the methodology and data presented by Martins et al.at the 2002IADC/SPE Drilling Conference(SPE-74500-MS),the mass of dissolved formation salt that each slurrymay receive during placement was estimated and duly incorporated into each slurry design.Subsequently,the salt-contaminated slurries were tested and compared with the properties of the initial uncontaminatedslurries.Based on these results,conclusions were then made on which salt slurry system(KCl or NaCl)exhibited better liquid and set properties after contamination with formation salt.Subsequently,thisknowledge was applied to 10 projects across three countries in West Africa.This study showed that when the contact time of liquid cement slurry to salt formation was low-typicallywhen the salt formation interval across which the cement slurry flowed was less than 100 m thick-the levelof formation salt dissolution entering the slurry during placement was limited.In this case,a KCl salt-basedslurry delivered improved liquid and set properties as compared with a NaCl salt-based slurry.In the field,this knowledge was applied in all oilfield projects cemented by an oilfield service company between 2015and 2020.This included deepwater,shallow offshore,and onshore wells.All related salt-zone cement jobs,including sidetrack plugs,placed across the salt formations were successful on the first attempt.In an absence of industry consensus around salt-formation cement slurry design,this paper validatesa guideline for West Africa,based on results from laboratory testing and 5 years of field application.Incontrast to current literature that recommends only NaCl salt-based slurry designs across halite or anhydrite salt intervals,this work demonstrates that KCl salt-based slurry systems can effectively be used to achievewell integrity where a halite or anhydrite salt interval is less than 100 m [328.1 ft] thick.