Experimental measurements of fluid-dependent anisotropy in synthetic porous rocks with controlled fracture parameters

Abstract

In this study, a new construction process was used to create synthetic rock samples with controlled fracture parameter. The new construction process provides synthetic rocks which have more realistic mineral composition, porous structure, cementation and pressure sensitivity. The synthetic rock samples contain fractures which have controlled distribution, diameter, thickness and fracture density. In this study, fracture diameter is about 4mm and thickness of fractures is about 0.06mm, the fracture density in the two fractured rock samples is about 3.45%. SEM images show well-defined penny shaped fractures with size 4mm in length, 0.06mm in width. X-ray CT images are used to determine the distribution of aligned fractures in the two fractured rock samples. The Rock samples were saturated with air, water and oil, and P- and S-wave velocities were measured in ultrasonic measurement system. The laboratory measurement results show the P-wave anisotropy is strongly influenced by saturated fluid, and the P-wave anisotropy parameter ε has much larger value in air saturation, than in water and oil saturations. The S-wave anisotropy decreases when saturated with oil, and this may be caused by fluid viscosity. In the direction perpendicular to the fractures (the 0° direction), shear wave splitting is negligible, and similar to the blank sample without fractures, as expected. In the direction parallel to the fractures (the 90° direction) shear wave splitting is significant. The fractured rock samples show significant Pand S-wave anisotropy caused by the fractures and controlled by the saturated fluid.