Experimental Research On Rock Physics And AVO Characteristics Of Tight Sandstone Reservoirs

Tight sandstones reservoir is a research hotspot and difficulty in unconventional geophysical exploration.Rock physical analysis and AVO(Amplitude Versus Offset)technology are the basis of reservoir fluid detection.However,tight sandstone has the characteristics of low porosity,low permeability,complex pore structure and large variation of clay content.These characteristics make the existing rock physical models no longer applicable to tight sandstone,and thus bring great challenges to reservoir recognition.Therefore,through the combination of high-pressure rock physics experiment and seismic physical simulation,it is important to carry out the related research of tight sandstone reservoir petrophysics and AVO characteristics,which is very important for precise exploration of tight sandstone reservoirs.First,it is found that the shear modulus of the natural tight sandstone sample increases or decreases after the saturation of water,which is not in conformity with the Gassmann's theory.It is also found that this phenomenon is related to the pore structure,pore type and effective pressure of tight sandstone.Under in-situ pressure,tight sandstone dominated by primary pores is more likely to show shear weakening,while secondary pores are more likely to show shear hardening.When the saturation velocity is predicted by Gassmann's theory,the influence of shear modulus change should be considered,otherwise the prediction error will be caused.Second,according to the distribution pattern of partially saturated tight sandstone fluid and the fluid flow at micro and mesoscale scale,a new physical model of rock(MJGW)is built based on the Mavko–Jizba–Gurevich relationship and White model,and a wet proportional parameter(WR)is introduced.The experimental velocity test results of tight sandstone samples show that the new model can explain the dependence of tight sandstone velocity on saturation and effective pressure,and the partially saturated fluid flow mechanism in different pore structures under high and low pressure conditions.It is also found that the shear modulus of tight sandstone is also dependenton saturation and pressure,and the lower the pressure,the more sensitive the shear modulus is to saturation.Third,the samples of tight sandstone with different clay content and clay type have been successfully developed,and the effect of fluid on the elastic modulus of clay bearing tight sandstone is studied.The shear modulus of tight sandstones rich in hydrophilic clay will decrease obviously after saturated fluid.By using the DKT(differential Kuster–Toks?z)model with dual pore structure,the reduction of shear modulus caused by hydration is well simulated by controlling the content of flaky cracks.It provides a suitable rock physical model and corresponding rock physical explanation for the prediction of shear wave in tight sandstone with clay.Last,on the basis of DKT and new MJGW rock physical model,the seismic physical simulation and AVO characteristic analysis of tight sandstone with different mud content and gas saturation are carried out.It is found that the change of clay content not only affects the size of the amplitude,but also changes the type of AVO.The presence of clay also affects the AVO characteristics of different saturation.Using the prestack synchronous inversion,we find that the shear wave impedance has certain indication ability for gas saturation,which further verifies the importance of shear wave information in the identification of tight sandstone fluid.