Experimental And Model Studies On Anisotropy Of Shales

Shale gas reservoirs have a wide range of burial and a large amount of resources,which has become one of the most important support in our country energy strategic policy.The velocity of an ultrasonic wave,elastic parameters and anisotropic properties of rock has a strong relationship with shale's mineral composition,microstructure,reservoir brittleness and fracability.The research on finding the dominant factors which influence the shale reservoir elastic properties,and establishing the relationship between the shale anisotropic and the fracability has important significance for guiding the exploration and development of shale reservoir.In this paper,the research object is Longmaxi Formation shale in Fuling area.The ultrasonic wave velocity testing and physical property parameter testing have been carried out successively,and the main influencing factors of wave velocity anisotropy coefficient have been obtained.Taking tight sandstone outcrops and the ordinary shale outcrops as comparisons,the backscatter scanning electron microscopy testing and the digital mineral composition testing were carried out to analyze the mechanism of the anisotropic which induced by the target reservoir structure.According to the physical property test and analysis results above,an anisotropic petrophysical model was established and it satisfied the elastic property of the target reservoir.Furthermore,the uniaxial compression testing of shale samples under acoustic emission monitoring,and the silent fracturing agent fracturing testing were carried out to evaluate the fracability of the samples.At last,the relationship between anisotropic parameters and rock fracture forming ability is established.The experimental results show that Longmaxi shale in Fuling has a strong anisotropy and heterogeneity,and the degree of anisotropy is positively correlated with its porosity and organic matter content.It is concluded that the combination of the clay-organic matter is the main controlling factor affecting the elastic properties and the anisotropy of this shale.Therefore,regarding kerogen as the background medium and clay minerals as the additive phase,the clay-kerogen combination model is established.Then,the bond transformation is used to rotate the whole model,in order to let the directional arrangement mode consistent with the actual sedimentary situation of the stratum.Subsequently,adding the inorganic mineral matrix into this combination as an additional phase,and then inorganic pores and organic pores are sequentially added to characterize the anisotropy of the real reservoir to the greatest extent.The error between the wave velocity calculated from this final model and the measured value in the laboratory is within 10%.This has proved the final model is highly reliable.The tight sandstone outcrop is developed with directionally arranged microcracks,and the directionally arranged microcracks are the main controlling factors of its anisotropy.The higher the anisotropy coefficient of the longitudinal wave is,the stronger the rock fracture fracability is.And for the shale outcrops,they are characterized by pore structure with randomly arranged microcracks.The content of kerogen-clay bond is the main controlling factor of anisotropy.The lower the value of longitudinal wave anisotropy coefficient is,the stronger the rock fracability it has.