Experimental Study Of Seismic Anisotropy In Artifitial Sandstone With Controlled Fractures

In fracturedl reservoirs,fractures usually serve as important storage places and migration channels for oil and gas resources.Previous studies have introduced a series of equivalent medium theories to describe the laws of elastic wave propagation in fractured medium.But these theories need to be verified and calibrated by experimental data.Due to the complex internal structure of natural rocks,parameters like fracture density fracture scale etc.are difficult to be controlled quantitatively.Therefore,most of the experimental studies on rock physics of fracture medium are based on artificial rock samples.At present,most of the rock physics experiments on the propagation law of elastic waves in fractured media have been conducted in high porosity range,while few studies have been conducted under the condition of low porosity.There is also a lack of studies on the influence of matrix properties(porosity,etc.)and partial saturation on the anisotropy of fractured rocks.In this paper,by improving the production process in Ding(2014),we have produced artificial sandstone samples with low porosity,and in which parameters such as porosity and fracture density of the samples can be quantitatively controlled.A group of artificial sandstone samples with the same fracture density but different matrix porosity were produced to study the influence of matrix porosity on seismic anisotropy under saturated water conditions.It was found that when the porosity increases from 14.1 % 30.4%,the P-wave anisotropy showed a large decrease.Meanwhile S-wave polarization anisotropy(SWS)changes very slightly(less than 0.2%)throughout the porosity interval.In order to study the applicability of the equivalent medium theory-Galvin’s(2015)theory,a set of low porosity artificial sandstone samples with different fracture densities were produced.P and S wave velocities were measured under dry and water saturated conditions.In view of the reasons leading to the difference between the predicted results and the actual measured results,Galvin’s(2015)theory was modified buy considering the fluidic action of matrix through squirt flow theory.The modified theory can give prediction results closer to experimental results.P and S wave velocity of a group of low porosity artificial sandstone samples with different fracture densities were measured under partial saturation conditions,in order to study the effects of water saturation on seismic wave velocity and anisotropy in fractured rocks.It is found that when the water saturation decreases from 100% to 0%,the P-wave anisotropy caused by cracks increases generally,while the S-wave anisotropy is not so sensitive the change of water saturation.The variation trend of P and S wave anisotropy with water saturation can be explained qualitatively by the assumed relationship between the water saturation in fractures and the water saturation in matrix.