A Drilling Fluid with Water Block Preventive Capacity and Strong Plugging Capacity for Offshore Low Permeability Reservoir Drilling

The formation rocks of the reservoir in Block Wushi in Beibu Gulf, west of South China Sea, have low permeability and are developed with hard and brittle shales. Shales, plus small-sized pore throats and severe water sensitivity resulted in serious formation damage by water block, and high skin factors. Borehole wall instability and washout during drilling resulted in frequent drilling accidents. To resolve these problems, a new drilling fluid has been formulated based on the composition of the drilling fluid previously used, through selection of a fluorocarbon water block preventing agent, reduction of oil/water interfacial tension, using flming plugging agents and optimizing the size distribution of CaCO₃ particles. This new drilling fluid not only helped stabilize borehole wall, it also had low fltration rate and was able to prevent drilling fluid particles from entering the reservoir formations, thereby protect the reservoir from being damaged. Laboratory evaluation results showed that the fltration rate of the drilling fluid and the invasion depth of solid particles and fltrate were all remarkably reduced. HTHP experiment and solid particle distribution in mud fltrate showed that the invasion depth of solid particles was reduced. SEM experiment showed that HTHP mud cakes were dense and smooth. The percent damage by water block was reduced to 17.5%, and the percent recovery of core permeability was greater than 80%. Field application of the new drilling fluid in Block Wushi showed that it has the ability of better stabilizing borehole wall; the rate of hole enlargement was only 5.4%. Skin factor was only 1.2, indicating that the drilling fluid is capable of protecting the reservoir. The drilling fluid technology used on this well is of reference signifcance in designing drill-in fluids for low permeability reservoir drilling.