If oil volume Iactor B, is lead in. and another formula of compressibility of oil Co can be educed from formula(1-2)as dB dp p>P, his is because when the formation pressure is lower than the saturation pressure, volume factor is the increasing function of pressure, so there is no negative sign in front of formula, The negative sign is needed when the formation pressure is higher than the saturation pressure Similar rock particles stand compression of formation pressure, when the formation pressure comparing with that of liquid and gas, so it can be omitted. The formation lies deep underground it stands overburden force F. and this force is balanced by the sum of the product of rock frame stress Um multiplied by the contact area of cap formation and reservoir bed Am plus of the product of formation pressure hydrostatic pressure of lluid in reservoir pores )p multiplied by the cross- section arca of pores A. the formula is shown below F=U +BA the process of oil rccovery, the formation pressure will drop; especially arround the bottom hole, the pressure drop even more. Because the overburden acting force Fis a constant, the stress that the rock stands will increase which causes its deformation and the decreasing pore tion of porosity and permeability. Figure 1-5 is the deformation ligure of rock (a Before deformation (b)After deformation If the water injection pressure is increased, the stress that the rock frame stands will decro and rock frame will recover which makes the pore volume increase. In fact. this process is irreversible, the increase of rock Frame stress will cause plastic deformation, such as the destruction of cementation texture, and it will not recover after the build up of TO Ien the reservoir is not deep, the process can be considered as reversible, i, e, elast and the crror is not great. When the depth of reservoir is more than 4000m or it is abnormal pressure reservoir, the change of porosity and permeability with pressure cannot be ignored. The experience of low permeability oil and gas field development has shown that the change of