Nonmodified Silica Nanoparticles Decrease Water Invasion Into Atoka Shale
Authors: Wilson Adam 
Published: February 2015

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 146979, ’Decreasing Water Invasion Into Atoka Shale Using Nonmodified Silica Nanoparticles,’ by J. Cai, SPE, China University of Geosciences; M.E. Chenevert, SPE, and M.M. Sharma, SPE, The University of Texas at Austin; and Jim Friedheim, SPE, M-I Swaco, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The complete paper was published in the March 2012 issue of SPE Drilling & Completion, Pages 103-112.
Shales account for 75% of the all footage drilled and are responsible for 90% of wellbore-stability problems. Maintaining wellbore stability is one of the more critical aspects of oil and gas drilling. The main cause of shale instability for both soft and hard shales is water absorption and subsequent swelling and sloughing of the wellbore. Laboratory data shows the positive effect of adding commercially available, inexpensive, nonmodified silica nanoparticles (particle size varies from 5 to 22 nm) to water-based drilling muds and their effect on water invasion into shale.
Experimental Methods
Materials Used . Nanoparticles. A total of 22 different types of nanoparticles were obtained from four manufacturers. Of these samples, only seven—NP-G, NP-D, NP-A, NP-E, NP-B, NP-C, and NP-F—were nonmodified, nonacidic, and commercially available (Table 1).
Shale Properties. Shale is a sedimentary rock that mineralogically consists of clays, quartz, and other silicate and carbonate minerals. Because of their high clay content, shales tend to absorb water from a water-based mud, which results in swelling and sloughing of the wellbore.
In this study, a hard and preserved Atoka shale was used. Great care was taken to preserve the shale at its native water activity. The shale was not exposed to air for any period of time over a couple of minutes. It was stored in oil, and, during preparations for testing, it was stored in a controlled-humidity desiccator. Use of shales that have dried out or that are reconstituted is not acceptable for the tests that were run in this study.
Muds Used. Three types of water-based muds were used in this study: a freshwater mud (FWM), a thick bentonite mud, and a low-solids mud (LSM). It was found that, in all tests, the presence of 10 wt% concentration of nanoparticles reduced the permeability of the shale.
Test Equipment for Measuring Shale Permeability. A pressure penetration (PP) test was used to measure shale permeability. The test consisted of three steps. Seawater was first flowed through the shale sample, followed by a base mud, and then by a base mud that contained 10 wt% nanoparticles.
The experimental setup (Fig. 1) consisted of several devices used to achieve a continuous flow of the test fluid across the top face of the shale sample while the simulated pore fluid was kept in contact with the lower face of the shale. The simulated pore fluid was a 4 wt% solution of sea salt that had a 0.98 water activity. A test fluid was flowed across the top of a shale sample (Fig. 2) at a constant pressure (approximately 250 psi), and the buildup of fluid pressure in a small sealed chamber located at the bottom of the shale was recorded.