Sloughing Drilling Fluid Sealing Closure Mechanism And Evaluation Methods

Wellbore instability of the clay shale formation is one of the most important problems during oil&gas well drilling, which may solved by adding sealing additives into drilling fluids. The wellbore instability of the clay shale occurs in different types of clay mineral and the formation of different clay content. The type of wellbore instability contains borehole diameter enlargement, reduction of well diameter, fracturing formation. The reasons can be summarized as the mechanical factor, physical and chemical factor and the engineering technical factor. The initial stress state change, lithological differences lead to different degrees of clay hydration both impact wellbore instability. Clay hydration swelling, diffusing double layer theory from the point of view of chemical analyses the instability phenomenon. Fluid column pressure surge and rheological properties of drilling fluids both influence the wellbore instability.According to principle analysis of sealing, the instrument was applied to test the sealing performance of the drilling fluid at high pressure and high temperature. It simulates stratigraphic the microns (20～100μm) fractures, the observable from in micro fracture the sealing material internal and external state of being, and it has good reliability with good reproducibility and stability, thus the method for evaluating the drilling fluid sealing capacity has been established. Several other typical traditional sealing materials experiment obtain plugging phenomenon of plugging material.This paper also analysis the sealing effect in the micro and macro scale and probes into the sealing mechanism of sealing materials. Tiny cracks Caisi asana microscope to the microscpic observation the sealing condition reach a result. Emulsified bitumen in the certain temperature under pressure to soften the deformation, then embedded blocking tiny cracks. It compacts sealing layer throughout the internal tiny cracks. Polyethylene glycol and emulsification paraffin wax seal tiny cracks through "cloud point effect" and "Jia Min effect".