Experimental study of nano-drilling fluid based on nano temporary plugging technology and its application mechanism in shale drilling

Abstract

During drilling operation in gas shales, the high capillary force leads to drilling fluid filtration continuously, which eventually results in severe wellbore instability, reservoir damage, and other problems. To solve this urgent problem, in this study, CaCO3 nanoparticles (NPs) enhanced nano-drilling fluid is developed to reduce the drilling fluid permeation into the shale matrix by sealing. In addition, the sealing nanoparticles can be removed by acid washing to restore reservoir permeability. The size distribution of CaCO3 nanoparticles is determined based on the pore size distribution of the shale matrix. The dispersion evaluation experiments are conducted to select and optimize the type of CaCO3 nanoparticles and dispersion agents. A specially designed pressure transmission experiment is used to evaluate the effects of CaCO3 nanoparticle concentration and rock/fluid contact time on the degree of shale matrix sealing by measuring the matrix shale liquid permeability before and after sealing, and the shale liquid permeability as a function of contact time, respectively. Finally, scanning electron microscope technique is used to explore the sealing mechanism of the CaCO3 nanoparticles enhanced nano-drilling fluid on the shale matrix surface. Results show that 10–40 nm hydrophobic CaCO3 nanoparticles and cetyl trimethyl ammonium chloride are the optimal nano-additive and dispersion agent, respectively. When the optimal nanoparticle concentration is 1%, the reduction rate of the shale matrix permeability reaches up to 95.5%. Moreover, the mechanisms of the nanoparticles seal the matrix pores are discovered: (i) A single nanoparticle can seal the shale matrix pore which has a size similar to the nanoparticle size by plugging method; (ii) Several nanoparticles come together and can seal the shale matrix pore which has a diameter greater than nanoparticle size by bridging method. Finally, after washing by 15% HCl for 30 min, the nano-drilling fluid sealing effect can be removed effectively. This research shows that the NPs could be used as a temporary filtrate reducer in drilling operation of gas shales. In particular, the detailed application mechanism of nano-drilling fluids in shale drilling is presented. The experimental research methods and fundamental data provide significant practical guidelines for the development of new NP enhanced drilling fluids.