High Density Clay-free Oil Based Drilling Fluid

Barite in oil based drilling fluids at elevated temperatures is easy to sag, resulting in poor stability of the muds. Conventionally this is solved by adding more organophilic clay. High clay content in the mud results in high mud viscosity and high ECD, causing mud losses to happen. To deal with this problem, a small particle size emulsifier DEMUL with high electric stability and viscosifying property has been developed to formulate a high density clay-free oil based drilling fluid. The drilling fluid was treated with DEMUL, SEBS (a styrene-butene/butadiene-styrene block copolymer) and a gellant, the synergy of these additives helps improve the stability of the oil based drilling fluid. This drilling fluid has simple formulation, and each additive is added at a low concentration. Laboratory experiment showed that the oil based drilling fluid still retained good rheology after aging at 200 ℃ for 160 h. After standing still at 160 °C for 336 h, the settling factor of the drilling fluid was 0.5074, indicating that the drilling fluid had good stability. Furthermore, this drilling fluid also has good inhibitive capacity and extreme-pressure lubricity. This drilling fluid has been used in drilling the horizontal section of a high pressure shale gas well in Chuanyu area. During the whole drilling operation, the drilling fluid showed stable rheology, good sand carrying capacity and high inhibitive capacity, and no downhole trouble has ever been encountered.