The Application of an Oil-Based Drilling Fluid with Strong Plugging Capacity in Pilot Test Shale Oil Wells in Qibei Area

The proved undeveloped shale oil reserves in block Qibei of Dagang oilfield are still very high and their burial depths can be 4,500 m or deeper. This oil and gas enrichment area is located in the Shahejie reservoir which has high content of clays as well as developed interlayer pores and microfractures. Past experiences show that borehole wall instability and difficulties in tripping of drill strings have been encountered. To deal with these problems, three pilot test wells were drilled first in this area with oil-based drilling fluids. In laboratory studies an oil-based drilling fluid was formulated with 2% nanolatex, 2% sized calcium carbonate and 2% oxidized asphalt. Sizing of the particles rendered the drilling fluid high plugging capacity. In field application, the filtration rate of this drilling fluid through 750 mD sand-disks was reduced to 0.8 mL because the particles in the drilling fluid plugged the formation, minimizing the volume of the filtrates into the formations. In this way the wellbore stability was improved. The well QY6-31-2 was drilled in 40.98 d, refreshing PetroChina’s shortest drilling time record of wells with depths raging in 6501-7000 m, and setting 5 records in Dagang oilfield, such as the deepest completed well (6558 m) of horizontal shale oil wells. The use of oil-based drilling fluid has ensured the smooth construction of the pilot test wells.