Chapter 1 The basic principle of percolation mechanics in porous medium T'he tlow of fluid( liquid, gas and their mixture )in porous medium is called seepage flow Solid body containing pores or solid collection is called porous medium. Large pores are called cavern and its diameter is from 2mm to lens of meters; the porous media with micron diameter called pores, as for the fractures that divide the solid into non-contact parts, its width has a wide range, while the throats connecting pores are rather narrow, No matter whether they are cavern fractures or pores, there is no strict detinition at home and abroad at present. Sandstone limestone are commonly seen as porous media; shale, igneous and metamorphic rocks with cracks that are scen as porous media too. The process of oil and gas development is the procedure that the i. gas and water flow from the reservoir to the well bottom and then from bottom to the surface Only the seepage principles of oil, gas, water and their mixture are mastered can more and more oil and gas be produced from reservoir. Percolation mechanics in porous medium seepage and gas is the science that studies movement principles of oil, gas, water and their mixture in Most of pores in sandstone reservoir are intergranular and they are more evenly distributed compared with the fractures. Shale, igneous and metamorphic rocks are fractured reservoir and the distribution of fractures is of great random. Sandstone is known as the porous medium: while shale with cracks, igncous and metamorphic rocks are known as fractured medium. Limestone dolomite and some sandstone with both pores and fractures are known as double medium. Th rocks talked above are the main reservoir of oil and gas of industrial valuc in china According to the current teaching program of relevant speciality, fluid statics in and character of fluid and porous mediu is introduced in the course of oil and gas reservoir physics. In order suit the needs of petroleum engincering and without loss of generality, the static distribution of oil, gas and water in reservoir is introduced first in this chapter, and several oil displacement forces are introduced focusing on teaching the concepts. On this basis, the basic principle of seepage-Darcy's law is introduced and then is used to analy ze a variety of practical 1.1 Static conditions in reservoirs 1. 1 The distribution of oil and gas in reservoirs Most of oil and gas reservoirs remain a state of relative equilibrium before development. the distribution of oil, gas and water in reservoir is associated with the properties of rocks and fluid in it. If the reservoir contains oil, gas and water at the sae time. as the lightest component, gas