Shale Alteration by Water Adsorption

Abstract

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.