Drilling Fluid with Superior Plugging Performance Used in Deep Well Drilling in Manshen Block

The deep formations drilled in the Manshen block are broken and full of microfractures. They have high clay contents with large differences in clay composition. Hard and brittle shales and water-sensitive shales are coexisting in the formations drilled. These formations easily lose their stability during drilling because of hydraulic splitting and difference in the speed of hydration. A technical measure named “physical support plus chemical inhibition and plugging” was presented and an anti-collapse drilling fluid with superior plugging capacity formulated to deal with the borehole wall instability problem. Laboratory study showed that the drilling fluid functioned normally at 180 ℃ and was able to resist contamination by 10% saltwater. Shale samples taken from the T layer and S layer were tested in the drilling fluid formulated. Hot rolling test gave percent cuttings recovery of 89.36% and 91.33% respectively, and expansion test gave rate of expansion of 7.3% and 4.2% respectively. This drilling fluid can plug the pores in cores made with quartz particles from 20 mesh to 120 mesh. In drilling the well ManS5-H4, the drilling fluid showed stable properties and had low filtration rate. It had good inhibitive capacity and collapse prevention performance. Tripping was smooth with no overpulls and resistance, the second and the third interval found average hole enlargement of 4.28% and 6.75% respectively. The cuttings returned to the shake shakers kept their original shakes and no downhole troubles were ever encountered. This drilling fluid has satisfied the needs of drilling the complex formations and saved the total cost of drilling.