Research Of Pore-fluid Effect And Rock Mechanical Properties In Porous Rock

The attenuation mechanism of elastic wave caused by the pore fluid flow effect is often called as“Wave-Induced Fluid Flow(WIFF)”mechanism.In this paper,we use Biot-Consistent Macroscopic(BCM)model,which is established according to Lagrangian continuum mechanics,to quantatively research the attenuation and dispersion characteristics of the effective drained bulk moduli.The mechanical parameters involved in BCM model are the inherent mechanical properties of the rock itself,and are"true"mechanical properties parameters which are independent of time and frequency.Therefore,when the observed effective drained bulk moduli are well simulated with BCM model,the corresponding mechanical parameters of the rock properties can be predicted,and it supports us to analyze the effect of rock mechanical properties on the attenuation and dispersion of the effective bulik moduli with different rock samples data.At last,the prediction results of rock mechanical properties from BCM model can be naturally used in the research of a new dynamic fulid substitution method.The specific research content can be summarized as the following aspects:Firstly,in order to verify the mechanical strictness of BCM model,we re-derive the Lagrangian and dissipation functions of BCM model under the assumption of isotropic medium and local linear interactions.The re-derivation proves that BCM model is a pure macroscopic mechanical model,and pays attention to the equivalent effect of the microscopic stuctures and fluid flow patterns in seismic frequency band,and it has clear and definite physical meaning,and can naturally and strictly bridge the microscopic rock physics experiments and seismic exploration.BCM model can simultaneously discribe WIFF attenuation mechanism and non-WIFF attenuation mechanisms(i.e.,the friction between rock grains),reveal their essential causes,and explain the frequency-dependent effect of the effective moduli observed in various rock physical experiments and the attenuation and dispersion of seismic wave observed in seismic exploration.Secondly,for the pore fliud flow effect,we research and analyze the values of the elements of the mechanical parameter matrix Q in BCM model,which represents the macro-elastic coupling inside the heterogeneous rocks,and detailly classify the primary factors of WIFF attenuation mechanism,strictly and effectively explain the attenuation phenomenon observed in sandstone rock physical experiments.And it demonstrates that the classification method of WIFF mechanism according to matrix Q is rigorous,clear and effective.More importantly,the proof procedure of the research of WIFF classification reveals that it is very necessary,strict and concise to establish the macro-mechanical properties for porous rock.Thirdly,in the study of rock macroscopic mechanical properties,we improve the previous algorithms for the prediction of rock mechanical properties based on BCM model.Compared with previous studies,the new data fitting method focuses on extracting the primary information of the attenuation and dispersion of the media,i.e.,the attenuation peaks and attenuation characteristic frequency.The peak values and number of the observed attenuation peaks is closely related with the heterogeneities of rock,and they determine the selection of the variables?_J describing the local characteristic of rock in the BCM model.A reasonable selection of?_J is critical for the explanation of attenuation peaks in actual observations and the prediction of rock mechanical properties.The published data of ten heterogeneous sandstone samples are used to predict the corresponding rock mechanical properties,and the prediction results are systematically analyzed.The analysis results prove that whether there is a single attenuation peak or multiple attenuation peaks,the BCM model can reasonably explain the attenuation spectrums and dispersion spectrums,and the prediction results of rock mechanical properties can well indicate the primary factors of the attenuation mechanism.At last,considering the effection of fluid viscosity on drained and saturated bulk modulus,we propose a new dynamic fluid substitution method based on BCM model.In the example application,this new method was used to predict the dispersion and attenuation characteristics of the saturated bulk modulus of the Berea sandstone sample with different water saturations,and the P-wave velocity dispersion and attenuation characteristics in a multi-scale heterogeneous sandstone sample saturated with different types of fluids.