This paper presents data showing the positive impact of adding nanoparticles to water-based drilling muds and their effect on fluid penetration into hard and soft shale. Use of present-day water-based muds during drilling can produce fluid penetration from the mud into shale formations resulting in swelling and wellbore instability. The nanometer sized pore throat diameters of shales are too small for conventional drilling fluid particles to invade and build an internal or external mud cake.
Four field muds in contact with Atoka and Gulf of Mexico (GOM) shales were studied with and without the addition of nanoparticles. Penetration of fluids into the shales was shown to decrease dramatically when nanoparticles were properly sized and applied.
Results show that nanoparticles reduce the permeability of the Atoka shale by a factor of 5 to 50. Similar results are obtained for the GOM shale. When nanoparticles were used, water penetration into Atoka shale was reduced by 98% as compared to sea water.
Measurement of shale pore sizes and scanning electron micrographs of the Atoka shale taken after exposure to nanoparticle dispersions, show that the nanoparticles are indeed small enough to penetrate and plug the shale pores and pore throats. This plugging of pore throats by the use of nanoparticles offers a powerful and economically viable new solution for controlling wellbore stability problems in troublesome shales.
Maintaining wellbore stability is one of the most critical aspects of drilling. Water invasion into shale formations weaken the wellbore and causes problems such as hole collapse and stuck pipe. Shales account for 75% of the all footage drilled (Steiger 1992) and are responsible for 90% of wellbore stability problems.
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