Experimental Study On Ultrasonic Velocity And Anisotropy Of Organic Rich Shales

Rock physics studies the physical properties of rocks and their changing rules.Oil and gas rock physics focuses on the relationship between rock ultrasonic velocity,elastic parameters and their mineral composition,microstructure,providing a basis for oil and gas geophysical exploration and engineering applications.Shale gas reservoirs are characterized by ultra-low porosity,abundant organic content,and strong anisotropy.It is of great significance to understand the wave velocity and anisotropy of shale gas reservoirs for shale gas exploration and development.In this paper,the organic-rich shale in different regions were selected.The elastic wave velocities and anisotropy parameters of these cores under different confining pressures and fluid saturations were experimentally tested.The influencing factors and the changing rules are analyzed,and the shale wave velocity and anisotropy rock physics model were established.The experiments showed that:1.The wave velocity of shale was mainly affected by the content of clay and organic matter,and the effect of porosity on the wave velocity is low;2.Both the layered arrangement of clay and organic matter and the large number of micro-cracks along the bedding direction were the two main factors that caused anisotropy of shale.3.With increasing confining pressure,shale wave velocity,Young's modulus,compression strength would increase,anisotropy wold decrease,and the magnitude of anisotropy decrease was proportional to organic matter content.4.The higher the organic matter content,the more stress of the wave velocity in the direction of vertical bedding;5.As shale water sensitivity saturation increases,the velocity of P wave would increase and S wave would decrease and Vp/Vs ratio would increase accordingly.For shale,Vp/Vs ratio is sensitive to water saturation.Regression of experimental data yielded empirical formulas for shale wave velocity and anisotropy:1)Without confining pressure condition:Vp=4948.8-23.1×C-75.×TOCy=2838-9.9×C-31.8×TOC?=0.0617×TOC+0.0308×10-2×C-0.0886?=0.0697×TOC+0.0348×10-×C-0.12712)Under 40MPa confining pressure condition:Vp=4858.5-15.9×C-55.65×TOCVs=2912.4-7.8×C-27.32×TOC?=0.0298×TOC+0.0894×10-3×C-0.0279...