Experimental study on permeability of low permeability rock and the effective stress based on Boltzmann method
1 Introduction
1.1 The study of low permeability rock properties on the importance of engineering application
The accurate description of the physical and mechanical properties for the low permeability rock is of great significance to the reasonable explanation and solution of a series of Earth Science and environmental engineering problems. These engineering problems are including the stability of large structure foundation, the construction and the safety problem of underground structure, the safe storage of the nuclear waste underground, the exploitation of unconventional (low permeability) oil and gas reservoirs and the geological storage of carbon dioxide etc.[1]. How to accurately understand and quantitatively characterize the physical and mechanical properties of low permeability rocks, and to explain the inherent laws of physical mechanics behind the apparent property has become a hot research topic around the world. It is very important for the safe and reasonable application of these projects that studying the physical and mechanical properties of low permeability rock[2].
In recent years, with the development of a series of large-scale water conservancy projects and underground engineering, the study on the physical and mechanical properties of engineering geological materials has been more and more important and urgent. These projects are including water conservancy dam construction, underground structure construction, tunnel excavation and construction, coal mine roadway construction, the subway project construction, the city underground engineering construction， for strategic energy reserves and the disposal of highly radioactive nuclear waste underground chamber construction and so on. Most of these projects will meet low permeability rock[3]. For example, the construction of the dam of the conservancy project in the water makes the upstream water level rise and the pore water pressure increases in the lower part of the dam. If this part of rock stratum is low permeability rocks, it can block the flow of pore water and make the hydraulic pressure gather. The consequences may be the instability of dam body[4]. The location of the underground chamber for energy storage and nuclear waste disposal is generally in the extremely low permeability of the salt rock or clay mineral layer. In these locations it will storage the oil and gas resources and prevent the leakage of nuclear waste. In order to ensure the safety and effectiveness of these projects, it is necessary to study the physical and mechanical properties of engineering geological materials and especially the properties of low permeability rock under the hydraulic mechanical coupling[5].
1.2 Study scientific problems faced by low permeability rock
1.2.1 The Importance and accurate measurement of permeability
The property of permeability is to describe the ability of the fluid which will flow through the internal pore structure of the rock. The low permeability rock as the name implies that the ability of the rock to allow the fluid to pass is very low. The study on the permeability of rock begins in the exploitation industry of the oil and gas. The permeability of the rock is regard as the most important property for describing reserves[6]. The conventional oil and gas reservoirs have high permeability, which means that they do not need or need only a little reservoir reform. Under certain conditions of pressure, the oil and gas resources can flow from the reservoir to the production wells, and then output.
However, the low permeability rock stratum can economy output must by the reservoir transformed including the hydraulic fracturing, the acidizing treatment, heat treatment and so on. Stankiewicz points out the importance of the multidisciplinary cooperation on the development of oil and gas and the multidisciplinary cooperation is considering the va
