Although key shales vary considerably in terms of reservoir pressure, temperature, mineralogy and local stresses, the principal drilling-related issues are wellbore stability, shale inhibition, hole cleaning and rate of penetration. Because most of the shale reservoirs are in environmentally sensitive and/or densely populated areas, stricter environmental regulations require new types of enviroment-friendly water-based drilling fluids. The traditional shale inhibition method through either chemical inhibition or invert emulsions will not be enough to satisfy the increasingly stricter environmental requirement.

This paper focuses on the technique and the performance results of nanoparticles as a physical shale inhibitor by plugging the pores and micro-cracks in shale and preventing water invasion into the shale. The companion paper (Part II) presents the formulations and performance tests of this novel water-based drilling fluid. Pressure and water penetration tests were performed to evaluate water invasion rates into various shale core samples, with initial brine permeabilities varying form less than 1 to over 100,000 nD, due to microfractures. Permeability reduction was used as a proxy of water invasion reduction into shale and thus the effectiveness in plugging pores and microfractures in shale by the nanoparticles. many orders of permeability reduction were consistently observed for drilling fluids with nanoparticles.

Test results showed that altough nanoparticles alone may be effective in preventing water invasion into shale samples with no microcracks, the combination of properly formulated mud and nanoparticles of appropriate material, size and concentration is the key to preventing water flow into shale play core samples with or without microcracks. This new water-based drilling fluid with nanoparticles provides an entirely different type of shale inhibition by physically plugging the shale micro-cracks and pores while meeting the strictest environmental regulations for shale play drilling.