Laboratory study on drilling fluid of thick salt bed in Lower Fars of Middle East

Drilled wells reveal that the drilling fluid for Lower Fars salt-gypsum bed in Middle East has such problems as poor rheological property, lost circulation and overflow, etc. A composite saturated brine fluid with a density of 2.30 g/cm3 was developed, whose PV is 53 mPa·s, YP is 15 Pa and API filtration loss is less than 2 mL; its HTHP filtration loss is 8 mL under 120 ℃/3.5 MPa; its filter cake thickness is 1 mm; the recovery rates of salt-gypsum sample and mudstone sample are 88% and 79% respectively. When contaminated by 10% NaCl, 1% CaCl2 and 8% bad soil, its rheological property changed little, with filtration was less than 3.5 mL, lubricity was less than 0.1, fracture width was 200 μm , and the cumulative loss under 5 MPa was 2.4 mL/3 min. It can not only meet the drilling of Lower Fars salt-gypsum bed in the Middle East, but also provide experiences for drilling of salt-gypsum bed in similar oilfield in other countries.