An Environmentally-Friendly Drilling Fluid for Stabilizing Shaximiao Formation in Block Zhongjiang

The Shaximiao formation during drilling has found serious borehole wall collapse and the fallen debris were difficult to be brought out of the hole. To deal with this problem, an environmentally friendly drilling fluid suitable for drilling the Shaximiao formation in block Zhongjiang was selected and optimized for hole stabilization. The Shaximiao formation is developed with stack-structured fractures, and the bonding strengths between the clay minerals and the other minerals are low, thus, in the long-term exposure of the borehole walls into a water-based drilling fluid, it is easy to induce the propagation of the fractures in the formation to produce spalling and fallen debris, causing the borehole wall to collapse. The drilling fluid selected is an environmentally friendly biomass synthetic-based drilling fluid having good compatibility with the formation. Laboratory evaluation results showed that this drilling fluid has good inhibitive capacity, the 16 h linear expansion rate of rock cores tested with this drilling fluid is 0.62%, the permeability of the drilling fluid is low, and the wetting angle is 28.9°. This drilling fluid has the property of low viscosity and high gel strength, at 80℃-100℃ and 40 MPa, the YP/PV ratio is higher than 0.3 Pa/(mPa·s). The drilling fluid has stable properties at low temperature and high density. The PPA filtration rate of the drilling fluid tested on sand disks of different pore sizes is zero, and the penetration depths of the mud filtrates into visual sand-beds of different meshes are between 7 mm and 20 mm, indicating that the drilling fluid performs very well in plugging micropores and microfractures. Using this drilling fluid, borehole wall instability problem was solved, and all wells were successfully drilled to the designed depths with no downhole problems. The use of this drilling fluid provides an environmentally friendly technical guarantee for efficiently drilling the Shaximiao formation in the block Zhongjiang.