A 2.145 g/cm3 (17.88 lb/gal) cesium formate brine was used as well-control fluid during the appraisal of a basin-centred gas accumulation in Hungary. The appraisal activities involved fracturing various zones in an extreme HPHT well, Mako-6, with a total depth of 5,000 metres (16,400 ft.), a BHST of 235°C (455°F) and pressures in excess of 96 MPa (14,000 psi).
After drilling operations started in December 2005, the well was completed in July the following year and temporarily abandoned with 1.33 g/cm3 (11.1 lb/gal) calcium chloride brine in the hole. Log analyses revealed significant gas from numerous intervals and an extensive fraccing operation was started in spring 2007. As part of this process, a two-metre interval was perforated. Subsequent fraccing saw success, but after discovery of H2S gas a well-kill operation was initiated with cesium formate brine. A total of 57 m3 (359 bbl) cesium formate brine at 2.147 g/cm3 (17.92 lb/gal) was bullheaded down the 5½ casing to the bottom of the perforated zone to minimise well-head pressure and enable snubbing of a packer and test string into the hole. The bottom of the column of cesium formate brine was at a depth of 5,300 metres with local temperature of 225°C (437°F). High-density cesium formate was chosen because alternative brines with lower densities develop much higher surface pressures, placing more wear and tear on the snubbing equipment and increasing operational risk.
This case history and all others are downloadable here.
Howard, S. and Downs, J. D. (2008, April 8–9). Formate brines in extreme HPHT well construction operations – Do they have limitations? Proc., AADE Fluids Conference and Exhibition, Houston, Texas, USA. AADE-08-DF-HO-11.
Howard, S. K. and Downs, J. D. (2008, May 27). The Hydrothermal Chemistry of Formate Brines and Its Impact on Corrosion in HPHT Wells. Proc., SPE International Oilfield Corrosion Conference, Aberdeen, UK. SPE-114111-MS. doi.org/10.2118/114111-MS.