Meso-mechanical Characteristics Of Mahu Conglomerate And Its Influence On Fracture Propagation

Mahu conglomerate reservoir is the largest conglomerate reservoir in the world with proven geological reserves of more than 10×10~8 t.The conglomerate reservoir is dense,so it must be developed by means of horizontal wells and volumetric fracturing.Due to the complex mineral components and fabric characteristics of conglomerate and its strong heterogeneity,the regularity of fracture propagation is poor,thus restricting the efficient development of conglomerate reservoir.Therefore,it is urgent to carry out in-depth research on the mesoscopic scale to master the mesoscopic mechanical properties of conglomerate and their influence on fracture propagation.Based on Mahu conglomerate as the research object,using image,X-ray fluorescence spectrum,scanning electron microscope(SEM),etc.,the mesoscopic characteristics of gravel contents,fractal dimensions and cementation types of the Mahu conglomerate were analyzed.By means of micron-indentation,the mesoscopic mechanical properties of conglomerate matrix and gravels were tested.Using the uniaxial compression,the ultrasonic wave velocity test,the acoustic emission monitoring and CT scans,the fracture propagation law of conglomerate fractured under pressure was studied,and the meso-mechanical characteristics of conglomerate and the influence of meso-structure on macroscopic mechanical properties and fracture propagation were obtained.The main understandings arrived at are as follows:(1)The meso-structure of Mahu conglomerate is complex,which can be characterized by its matrix,gravels and cementation.The roundness of gravels is good,which is in the range of rounded to well rounded.The gravel size distribution has good fractal characteristics,and the proportion of gravels between 2 mm and 6mm is higher.The fractal dimension is influenced by both of the proportion of fine gravel and the gravel content.The gravel content varies widely,ranging from less than 10%to more than 60%.According to the gravel content,the conglomerate can be classified into matrix-supported,gravel-supported and mixed-supported.The conglomerate can be classified into carbonate cemented and clay cemented according to different cements.(2)The mechanical properties of matrix,gravels and cements exist obvious difference and control the overall mechanical properties and fracture law of conglomerate.The compressive strength of conglomerate decreases with the increase of gravel contents.When the gravel content is the same,the compressive strength of carbonate cemented conglomerate is greater than that of clay cemented conglomerate.When the gravel content is small(0%～20%),the conglomerate is supported by matrix,and its overall mechanical properties are controlled by the matrix.The larger the elastic modulus of the matrix is,the larger the elastic modulus of the conglomerate is.When the gravel content is high(40%～60%),the conglomerate is supported by gravels,and its overall mechanical properties are controlled by gravels.The larger the elastic modulus of gravels is,the larger the elastic modulus of conglomerate is.(3)The fracture evolution of conglomerate can be divided into five stages:original crack closure,elastic deformation,microcrack initiation,fracture propagation and post-fracture deformation.Influenced by the mesoscopic fabric characteristics of conglomerate,the fracture evolution characteristics of different conglomerates varies and are controlled by the cementation strength and gravel contents.(4)The complexity of fractures includes tortuosity of a single fracture and the distribution density of multiple fractures,which are characterized by the fracture fractal dimension and the fracture intersection index respectively.The fractal dimensions of single fractures in Mahu conglomerate range from 2.097 to 2.358,with an average of2.188.The fracture intersection index ranges from 3.50 to 11.60,with an average of7.08.Compared with homogeneous rocks,the tortuosity of single fractures and the distribution density of multiple fractures are larger and more complex.(5)For clay-cemented conglomerate,with the increase of gravel content,the fracture propagation stress decreases,and the gravel-surrounding fractures transform to a co-development of gravel-surrounding fractures and gravel-penetrating fractures.The fractal dimension and intersection index increase,and the fracture complexity increases.(6)For carbonate-cemented conglomerate,the fracture propagation with the change of gravel contents can be divided into two stages.In the first stage,as the gravel content increases from 20%to 40%,the fracture propagation stress decreases,and the gravel-surrounding fractures transform to a co-development of gravel-surrounding fractures and gravel-penetrating fractures.The fracture fractal dimension and fracture intersecting index increase,and the complexity of the fractures increases.In the second stage,when the gravel content increases from 40%to 60%,the fracture propagation stress increases,and the co-development of gravel-surrounding and gravel-penetrating fractures changes to gravel-penetrating fractures.The fracture fractal dimension and fracture intersection index decrease,and the fracture complexity decreases.