Abstract: Plugging material of nanometer and micron in size were introduced into the formulation of reverse osmosis low free water drilling fluid to solve the borehole wall instability problem in Block Nanpu. A semipermeable membrane near the borehole formed by the plugging material, can, when in combination with bonding water, reduce the water activity of the drilling fluid, which in turn, resulted in reverse osmotic pressure, balancing the pressure of the drilling fluid column and capillary pressure existing in shale formations. Shale formation stability can be improved by this process. HSM, a micelle agent, and HGW, a borehole wall strengthening agent, were used to formulate the reverse osmosis low free water drilling fluid. HSM, when dissolved in water, assumes spheroidal, laminar, or rod shapes with sizes ranging between 1 nm and 100 nm. HSM, with its hydrophilic groups adsorbed onto the surface of borehole wall, has its hydrophobic groups entangled with each other to form a protection membrane against invasion of free water from drilling fluid to enter into the formation. HGW, a hydrophobic submicron emulsion, protect borehole wall by forming a hydrophobic layer on the surface of the borehole wall. By adding 3% HGW and 2% HSM, the membrane efficiency can be increased from 0.11 to 0.3, and to 0.50 if 20% bonding agent was further added. Laboratory test showed that this drilling fluid was similar to oil base drilling fluid in terms of inhibitive capacity, lubricity, and reservoir protection. Two wells, Nanpu2-35 and Nanpu4-65, drilled with this drilling fluid had no borehole instability problems, with gauge holes obtained. Compared with offset wells, drilling fluid density was reduced from 1.26 g/cm³ to 1.20 g/cm³, and ROP was increased by 49.26%.