Cesium formate brine is a heavy low-viscosity clear fluid, purpose-designed to provide the oil industry with a better and safer well control fluid for HPHT well interventions. Its unique hydrothermal chemistry ensures its stability and durability over long periods in HPHT wells. Furthermore, cesium formate brine protects common viscosifiers and fluid loss control additives against thermal degradation – allowing the formulation of very effective well kill pills that can control fluid loss for several days under HPHT conditions. Some operators routinely have cesium formate-based kill pills ready on a stand-by basis at the rig site. Cesium formate brine is a powerful suppressant of gas hydrate formation and can be used in pills to melt hydrate plugs (e.g. as used in Elgin/Franklin wells). To date cesium formate brines have been used as well intervention and suspension

Cesium formate is ideal for challenging well interventions

fluids in around 20 challenging HPHT wells and have demonstrated the following advantages over traditional high-density intervention fluids:

• Reduced formation damage – cesium formate is essentially incapable of causing permanent formation damage. Most users report that after a few weeks of production their well productivity indices exceed expectations after exposure to cesium formate brine during well interventions.
  
  
• New improved HSE standards and reduced liability – no other high-density brines come close to matching the high HSE standards set by cesium formate brine. Using cesium formate brine avoids compromising safety and greatly reduces the risk of creating long-term liability.
  
  
• Improved well integrity – the use of buffered cesium formate brine reduces the risk of catastrophic localised corrosion and stress corrosion cracking in well tubulars exposed to HPHT acid gas influxes.
  
  
• Extreme durability – cesium formate brine can be safely deployed for long periods in extreme HPHT wells where traditional well intervention and well suspension fluids would cause major problems. Field experience in the Mako-6 well (BHST: 235˚C/455˚F) in Hungary has shown that suspension fluids based on buffered cesium formate brine will tolerate months of exposure to a combination of extreme temperatures/pressures and acid gas influxes.

The worth of cesium formate brines as HPHT well intervention fluids has been clearly shown in the Elgin, Franklin, Rhum, Mako and Kvitebjørn fields. This has included HPHT well suspensions for up to two years. Cesium formate brines quickly destroy OBM filter cakes and have a niche application as pipe-free pills.