pressure on the oil-water interface is: Pull Where: Pg-The density of water, kg/m H--The depth of oil-water interface, m p-The pressure on the oil-water interface, MPa This kind of driving force that relies on hydrostatic pressure of supply area to displace oil is led water drive. For most reservoirs, water supply is always less than the oil production. Apart from a few small oil and gas reservoirs, in the whole development stage, it is rare to see the reservoirs with natural water drive, and water injection is the main development scheme. The majority of oil and gas reservoirs in China are developed by water Flooding 2. 2 The elastic energy of formation and the fluid in it The crude oil in formation is long-tcrm under the original formation pressure, after the well is put into production, because of the drop of reservoir pressure, the tluid in the reservoir will expand, and then part of oil is driven to the bottom of wel The physical quantity that characterizes the magnitude of clastic energy of fluid is compacti factor of it is the reciprocal of bulk modulus of elasticity, and it shows the relative change of fluid vo when unit pressure changes. It is expressed as the following formula C i aL (1-1 In the formula above V is volume ol fluid, because tluid volume v is a decreasing function of pressure p, so its derivative is negative. In order to make C positive, a negative sign need to be added in front of the formula. Also, because tluid volume is function of temperature T, marked as V(p, T). so there appears a partial derivative in the formula, and CL is isothermal compressibility Normally, during development there is little change in temperature of reservoir, so the impact of temperature on the liquid volume is not considered generally. The coefficient of compressibilit of water is considered as a constant within the reservoir pressure variation range. Its magnitude is about(3. 7-5)x 10- MPa". the coefficient of compressibility is relevant to the natural gas content dissolved in it, and it has a big range, its magnitude usually is(7140)x10"MPa- While the coefficient of compressibility of gas is rather bigger than that of oil and water, an it cannot be considered as constant with pressure, When the temperature of reservoir is a constant formula(1-1)can be simplified as I<I1 (1-2) When the coeflicient of compressibility of fluid changes little, and it can be approximately considered as a constant, the formula above can be also be expressed as ⊥I (1-3)