Rheology Control and Application of Ultra-High-Density Compound Brine Drilling Fluid

Well Lu-206 is an appraisal well drilled in the oil/gas and shale gas exploration block in Changning, Lu County, south of Sichuan Basin. The target zone of this well is the Baota Formation. The vertical pilot hole of this well is 4082 m in depth. During drilling operation, high pressure fracture gas was encountered, caused the hole to overflow, and the well was finally killed with a high density (2.42 g/cm³) water base mud. To control the formations wit active gas, ultra-high density had to be used, this in turn resulted in difficulties in solids control of the drilling fluids. Furthermore, the top formations are very water sensitive, making control of mud rheology is very difficult. Based on the past drilling experiences gained and comprehensive laboratory experiment, the sulfonate polymer drilling fluid previously used to drill the top section of the well was replaced with an ultra-high-density compound brine drilling fluid with excellent contamination resistance and inhibitive capacity. A lignite resin KJ-4, a high-density dispersant and a sulfonated tannin were mixed together to control the rheology of the compound brine drilling fluid, which was also a problem needed to be resolved. Application of this ultra-high-density compound brine drilling fluid in drilling the fourth interval of the vertical pilot hole of the well Lu-206 showed that the rheology of the drilling fluid was under control during the whole drilling process. The success of the field practice has provided a clue for controlling the rheology of high-density drilling fluids.