Experimental Study On The Effect Of Fracture Aperture On Seismic Wave Characteristics At Different Frequencies

In fractured oil and gas reservoir,fracture aperture is an important parameter.To study the effect of fracture aperture on seismic response is helpful to detect fractures by using seismic attribute parameters,and useful for guiding the exploration of fractured reservoirs.In this paper,the equivalent medium models with different fracture apertures are constructed.Ultrasonic pulse transmission method and stress-strain method are used to observe the variation of P-and S-wave velocity,anisotropy and attenuation under different fracture aperture and test frequency,and the effect of fracture aperture and frequency on seismic wave propagation is analyzed.Four transducers with different dominant frequencies are to test a group of fracture models constructed with soft material of the same diameter and different thicknesses(0.1-0.3mm)embedded in the matrix.The variation of the velocity,amplitude and anisotropy of the P-and S-wave and the effect of the fracture aperture on P-wave attenuation are observed.The results are compared with Hudson Theory and Kawahara Theory.The results show that when the fracture density is constant,with the increase of fracture aperture,the P-wave and S-wave velocity decrease,and the anisotropy of P-wave and S-wave increase.The scattering caused by the soft inclusions in the fracture has more significant influence on the P-wave and two slow S-waves which propagation direction are perpendicular to the fracture direction.The test frequency increases,the velocity of P-and S-wave increases,and the anisotropy decreases.The variation of P-wave velocity and anisotropy estimated by Hudson theory is consistent with the experimental results,and it fits well when the test frequency is relatively low(0.1 MHz).The S-wave velocity and anisotropy estimated by Hudson theory do not change with the fracture aperture.The variation of P-wave velocity and attenuation estimated by Kawahara theory is consistent with the experimental results.As the fracture aperture increases,the estimated P-wave velocity decreases and the attenuation increases.Kawahara’s estimated P-wave velocity is close to the measured velocity at relatively low frequency(0.25 MHz),and its attenuation is close to the measured P-wave attenuation at relatively high frequency(0.5 MHz and 1 MHz).The models used for ultrasonic testing are cut into cylinders,and the Young’s modulus and Poisson’s ratio of the matrix model made by pure epoxy and two fracture models with different apertures are tested by strain-stress method at low frequency(1-3000Hz).The results show that with the increase of testing frequency,the Young’s modulus of pure epoxy matrix model sample increases slightly,while the Poisson’s ratio decreases,which are close to the ultrasonic test results.However,the Young’s modulus values of the two fracture aperture models are abnormally high,deviating greatly from the ultrasonic results,and the repeated test results are not improved.It is believed to be related to the axial bonding between model and reference aluminum sample and the bonding of strain gage.