Experimental Investigation Of Ultrasonic And Electrical Properties Of Organic-rich Shale

As an unconventional energy source,shale gas is playing a more and more important role due to the sustained growth of global energy consumption.In order to achieve the purpose of economical and efficient exploitation of shale gas reservoirs,it is necessary to identify the"Sweet Spot"of shale gas reservoirs.The content of organic matter and clay minerals in shale are important indicators for evaluating shale gas reservoirs,rock drillability and fracturing.It is of great significance to study the influence of composition,water saturation and confining pressure on the acoustic wave propagation characteristics and electrical parameters in laboratory experiments for identify the "Sweet Spot" in shale gas reservoirs.Based on the theory of petrophysics and rock mechanics,We chose The Chang 7 member of Triassic Yanchang Formation continental shale in Ordos Basin and the Longmaxi Formation marine shale in the Sichuan Basin as the object of our research.During the process,the influence of confining pressure and water saturation on shale acoustic wave propagation and electrical parameters were designed and developed,the response characteristics of acoustic wave propagation on shale components was explored.Besides this,the effect of organic matter on the acoustic and electrical parameters of shale,the response mechanism of the acoustic and electricity parameters of shale to composition,water saturation and confining pressure was revealed and the applicability of the rock acoustic electrical model in shale is investigated.First of all,this paper revealed the response characteristics of acoustic wave propagation to shale components.The influence of the shale composition on the acoustic wave propagation are as follows:organic matter,pyrite,clay(from the strongest to the weakest);When the content of organic matter increased from 1.47%to 8.89%,the compressional/shear wave velocity of shale is respectively reduced by 8%and 20%.The higher organic matter contained,the more times of diffracted waves passing through the shale and the slower propagation velocity are;The organic matter enhances the absorption of the high frequency of the compressional/shear wave,and the center frequency of the compressional/shear wave decreases linearly with the increase of the organic matter content.The effects of confining pressure and 3%KCl solution on the acoustic wave propagation characteristics of shale were analyzed.With the increase of confining pressure,the cracks of dry and saturated shale are closed,the pore compressed,and the velocity of compressional wave and shear wave increased;The growth rate of the compressional wave velocity of dry shale is greater than that of the shear wave,while the saturated shale shows the opposite trend;We also carried out frequency spectrum analysis of longitudinal and shear waves of dry shale with confining pressure.The results demonstrated that when the confining pressure increases,the growth rate of the dominant frequency amplitude is much larger than the growth rate of the wave velocity.3%KCl solution gets into the organic pore,reduce the mechanical properties of shale.When using Gassmann equation to calculate the mechanical properties of water saturated shale,We should take the influence of hydration,Longmaxi shale measured water saturated shale shear modulus and bulk modulus were calculated 85.7%,82.6%intoconsiderations.The connectivity of organic matter pores and its influence on the electrical parameters of shale are discussed.The vertical/parallel bedding Longmaxi shale samples were soaked in the solution of four types solution to analyse vertical/parallel bedding shale resistivity changes with the immersion time.The results showed that with the increase of soaking time,the difference between vertical/parallel bedding shale resistivity is decreased,when immersed in 48h,the shale resistivity of vertical and parallel bedding was almost the same.The results showed that with the increase of soaking time,the vertical/parallel bedding shale resistivity difference decreases,when immersed 48h vertical/parallel bedding shale resistivity was almost the same,the solution can enter into the pores of shale organic matter,and the organic matter pores are connected with each other.Besides,the applicability of Archie equation in shale was discussed.Five Longmaxi formation shales were taken for the test.The water saturation was established by the method of self-imbibition(3%KCl solution).The relationship between the I-Sw accords with Archie formula.In dualporosity structure,I-Sw curve in shale is bent down at high water saturation.The dielectric constant increases significantly when the water saturation is greater than 50%.The surface and interlayer of clay minerals are filled with water molecule,and the water molecule enters into the surface and pores of organic matter,which makes the polarity of the interface increases and the dielectric strength increases rapidly.