An adsorptional isotherm technique makes it possible to define quantitatively the hydrational tendencies of any formation, thereby eliminating the cumbersome rock classification schemes presently used in industry. Introduction Properties of some argillaceous rocks are known to be Properties of some argillaceous rocks are known to be altered by adsorption of water. Montmorillonitic rocks, for example tend to expand and crumble when contacted by low-salinity water.' Quantitative data are available on the behavior and properties of montmorillonitic sandstones and pure clays, but not for rocks that can be broadly categorized as shales. (A shale can be described as being fine grained, argillaceous, highly compacted, and partially dehydrated.) The purpose of this work was to observe the behavior and measure the properties of a broad range of shale types-montmorillonitic, illitic and chloritic-when exposed to water. Investigators working with clays and montmorillonitic rocks have found that expansion is caused by water or ion adsorption onto the electrically charged surfaces of clays. Some investigators postulate that on a molecular scale an "ordering" of the water molecules takes place on the clay, resulting in the water's being "rigid" or "captured". The degree of capture (or the extent of total hydration) is best represented by an adsorption isotherm. When the clay is dry it consists of layers that are close together. Adsorption produces a net flow of water that causes the layers to separate, thus inducing internal expansive stresses. Hydration reduces the clay's strength and density, and alters its electrical resistance. Since shales are essentially quartzitic-feldspathic rocks that contain clay, it was felt that the mechanism just described would be responsible for most of the property alterations that take place when adsorption property alterations that take place when adsorption occurs. Shales Studied Samples of 50 shales from formations ranging in depth from 300 to 16,000 ft were collected and studied. All shales were restored to their original state of hydration, and samples of 12 shales were preserved at the wellsite and subsequently tested with minimal exposure to weathering. Control samples deliberately exposed to the atmosphere adsorbed water and eventually fragmented. All samples tested adsorbed fresh water (tap water was used) and evidenced physical alteration. It would be inconvenient to present data on all 50 shales. Instead, results of tests on six shales from as many different locations and depths are presented as being typical. Table 1 shows compositions from X-ray analyses of these shales (designated A through F) and gives their locations and depths. Table 1 also shows the weight percent water of each shale when it is in equilibrium with an atmosphere having a relative humidity of 50 percent. Generally, for shales containing an appreciable amount of clay, the weight percent water observed under these conditions percent water observed under these conditions exceeded 3 percent.

Fluid penetration from water-based muds into shale formations results in swelling and subsequent wellbore instability. Particles in conventional drilling fluids are too large to seal the nano-sized pore throats of shales and to build an effective mudcake on the shale surface and reduce fluid invasion. This paper presents laboratory data showing the positive effect of adding commercially available, inexpensive, nonmodified silica nanoparticles (NP) (particle sizes vary from 5 to 22 nm) to water-based drilling muds and their effect on water invasion into shale.Six brands of commercial and nonmodified nanoparticles were tested and screened by running a three-step pressure penetration (PP) test (brine, base mud, nanoparticle mud). Two types of common water-based muds, a bentonite mud and a low-solids mud (LSM), in contact with Atoka shale were studied with and without the addition of 10 wt% nanoparticles. We found that a large reduction in shale permeability was observed when using the muds to which the nonmodified nanoparticles had been added. For the bentonite muds, the permeability of Atoka shale decreased by 57.72 to 99.33%, and, for the LSMs, the permeability of Atoka shale decreased by 45.67 to 87.63%. Higher plastic viscosity (PV) and lower yield point (YP) and fluid loss (FL) of the nanoparticle muds compared with base muds were also observed. We also found that nanoparticles varying in size from 7 to 15 nm and a concentration of 10 wt% are shown to be effective at reducing shale permeability, thereby reducing the interaction between Atoka shale and a waterbased drilling fluid.This study shows for the first time that it is possible to formulate water-based muds using inexpensive nonmodified and commercially available silica nanoparticles and that these muds significantly reduce the invasion of water into the shale. The addition of silica nanoparticles to water-based muds may offer a powerful and economical solution when dealing with wellbore-stability problems in troublesome shale formations.

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.

An industry and academic standard, Applied Drilling Engineering presents engineering science fundamentals as well as examples of engineering applications involving those fundamentals. Two appendices are included, along with numerous examples. Answers are included for every end-of-chapter question. Solutions to Chapter 8 (http://go.spe.org/ADEsolutions)

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