Application of an environmentally-friendly synthetic base drilling fluid to horizontal drilling of shale gas wells

The shales' instability and susceptibility to collapse is currently a crucial problem bottlenecking the large-scale horizontal drilling of shale gas wells. The synthetic base drilling fluid, due to its strong inhibition, has been widely applied abroad to horizontal drilling of shale gas wells. Learning experience from foreign countries, we prepared a new drilling fluid as follows: take the Shell GTL Saraline 185V as the base oil to obtain the basic formulation of synthetic drilling fluid through indoor tests; then adopt the potassium acetate as the water-phase inhibitor in substitution of calcium chloride so as to strengthen the system inhibition and simplify the preparation process. Finally, its basic performance was evaluated in experimental tests, the result of which demonstrated that this prepared drilling fluid with density increased to 2.40 g/cm3 has excellent rheological property and relatively high emulsion-breaking voltage. According to the comparison of performance before and after the hot-rolling aging experiment at 130°C, the rheological property and emulsion-breaking voltage show very little change. Pilot tests have been successfully conducted in horizontal drilling of 3 shale gas wells in the foreign-cooperative Fushun Block in the Sichuan Basin. In the horizontal well section with the length of 2000 m and high inclination, the returning shale cuttings had angular shapes; the pulverized cuttings were dry inside; the wellbore was smooth in pipe tripping; the average mechanical drilling speed of the deviated section reached 5.33 m/h, while that of compound drilling 8.8 m/h; and the electrical logging, casing running and cementing proceeded smoothly in the well completion. All these above indicate that this drilling fluid fully meets the requirements of shale gas drilling.