The physical phenomenon of changing of volume of solid bodies under the effect of shearing stresses is known as the dilatancy. The effect of dilatancy on the physical state and properties of rock formations with static loads was widely studied in the first half of the XX century in the leading scientific centers of the United States, Great Britain and Russia. As a result of these studies it has been shown that in static state the anomaly of deformation behavior of rock formations resulting from dilatancy processes becomes significant when the intensity of single axis load exceeds ⅓…⅔ of the ultimate stress limit: it results in faster grow of transversal deformations causing the effect of decompaction of the deformed medium. This decompaction represents an inter-granular micro-fracturing is to a large extent irreversible and results in changing of properties of the formations – density, strength, porosity, filtering permeability, velocity of propagation of elastic waves and deformation characteristics.
Despite the significant dependence of the properties of rock formations on the degree of their dilatance decompaction, this effect was not considered suitable for practical application, as a significant changing of properties of formations was observed directly prior to the destruction. Research of dilatance processes with dynamic (impulse) loads performed by the Institute of Geophysics of the National Academy of Sciences of Ukraine showed totally different results: with a sufficiently high non-uniformity of the stress state the dilatancy processes take place with stresses not exceeding 3…5 % of the ultimate stress limit, causing irreversible change of density, porosity, strength and permeability of formation. The value of the threshold pressure of dilatancy decompaction depends on genetic parameters of formation. In pure, low-porosity sandstones, quartzites, granites, dolomites, limestone, salts, highly-metamorphized coal and other similar formations a significant decompaction begins immediately from the moment of pressure pulse reception, i.e. at very low (compared to rock formation strength) stresses.
Dilatance decompaction of formations has the nature of extension of micro- and macro-defects of its structure due to the increase of bearing pads and fracture with nonzero opening, which can be considered as an artificial porosity of the formation. It is important to note that, unlike the fracturing processes, the dilatance-caused increase of porosity is uniform throughout the whole deformed volume resulting in uniform changing of the geological and technical properties of the formation.
The dilatance technology has been developed for application in various branches of mining industry. The most favorable conditions for its application are provided in oil-and-gas industry as a method of intensification of underground fluids extraction.
RESEARCH INTENSITY OF THE TECHNOLOGY
The dilatance technology is a science-intensive technology, its development and subsequent approbation took more than 40 years. On the basis of the research activities in 1971-2010, 21 methodological manuals have been issued and 9 monographs published, the latest – “Protection of boring casings in the process of demolition works in wells” – has been published in 2009, and the monograph “Dilatance technologies of degassing of coal measures” is scheduled for publishing next year. Most of the designs and engineering solution are patented by certificates of authorship of the USSR, Russia and Ukraine.