Research And Application Of Ultrasonic Attenuation Characteristics Of Cave - Type Carbonate Rock

According to statistics, the carbonate oil and gas reserves occupy about50%of total reserves.60%oil and gas production are from carbonate reserves, so the development of carbonate reservoir is the focus of exploration and development of oil&gas reserviors. During the exploitation of homogeneous formation, acoustic logging data is the most important and direct field data to represent and predict the rock mechanical parameters.The wave velocity recorded by first wave trigger mechanism reflects the acoustic velocity propagation along the path of largest wave impedance, which can not accurately reflect the rock pore structure information. Because of the variety of carbonate rocks pore shape, the multi-scale development of pore space and the complex of pore distribution, the results show the formation porosity and strength parameters prediction are far away from actual values.Numerous studies show that the ultrasonic kinetic parameters are more sensitive than dynamics parameters as the pore structure changes. Thus, this article mainly studys the relationship between attenuation coefficient and strength parameters. For physics experiments are hard to analyze the single factor effect, this article builds numerical model in different pore shapess sizes, distributions, densities. The models were used to conduct ultrasonic transmission simulation and uniaxial compression simulation. Research on the effect of single factor to attenuation and strength parameters points out the applicable conditions using attenuation to predict strength parameters.And the authors also had research on numerical models of complex pore structure and physical models. Under this condition, the author verified the feasibility of predicting strength parameters based on attenuation coefficient, which can instruct the predicting of strength parameters. The main achievements are listed as follows:(1) Pore shape has little effect on the function law between attenuation coefficient, uniaxial compressive strength and elastic modulus; the aspect ratio of pore shape shows good power functions with attenuation coefficient, uniaxial compressive strength, elastic modulus and Poisson’s ratio. There is a critical aspect ratio value in0.86-1. When aspect ratio is greater than the critical value, the Poisson’s ratio increases with the increasing of attenuation coefficient. When aspect ratio is less than the critical value, Poisson’s ratio decreases with increasing of attenuation coefficient, and the greater the aspect ratio deviates from threshold, the faster the Poisson’s ratio changes.(2) Pore size is the main factor that affects the correlation between attenuation coefficient and strength parameters. And different frequencies vibration source has different critical pore size. When the pore size is larger than the critical value, the attenuation coefficient can not reflect the change of strength parameters. When vibration sources frequency is50KHz or250KHz, the respectively critical sizes are2.4mm and3.6mm, respectly.(3) Pore distribution has little effect on the correlation between strength parameters and attenuation coefficient. Under different distribution, the RMSE of strength parameters measuring by attenuation coefficients is small, and the RMSE increases with increasing porosity.(4) Pore density has little effect on the correlation between strength parameters and attenuation coefficient. The RMSE of the results is very small when applying attenuation coefficient to predict the strength parameters. A porousity of9%is the threshold of strength parameters prediction. When porosity is greater than9%, the prediction accuracy will reduce.(5) The combinations of different pore size have little effect on the function law between attenuation coefficient, uniaxial compressive strength, elastic modulus and Poisson’s ratio.(6) For complex pore construct models, attenuation coefficient and uniaxial compressive strength show a power function law. And the relations between attenuation coefficient, elastic modulus and Poisson’s ratio are quadratic polynomial function laws. For complex pore construct models, the error of strength parameters predicted by attenuation coefficient is small.(7) Physical experimental studys proved the function laws established by numerical simulation are correct, and the prediction error generated by physics experimental is greater than numerical simulation, but the prediction results can meet the engineering requirements.