Rock Physics Experimental And Modeling Studies For Reservoir Rocks At Broad Frequency Band

Reservoir prediction and hydrocarbon detection used in seismic exploration are based on the understanding of lithology,physical properties,and fluid properties of target strata.In order to understand the petroleum system and meet the resource demand,rock physics analysis,which links between reservoir properties and seismic attributes,has already become an integral part of oil and gas exploration.The field seismic and well logging data show that the rocks under fluid saturation are dispersive.That is to say,the elastic parameters of fluid saturated rocks are frequency-dependent.However,the empirical and theoretical models of rock physics applied in actual production are developed mostly based on the high-frequency ultrasonic data measured in the laboratory.Furthermore,these models lack verification by laboratory data measured at intermediate(well logging)and low(seismic)frequencies.Considering the differences of dispersion and attenuation mechanisms between different frequency bands,it is maybe problematic to directly use ultrasonic data to interpret well logging and seismic data.Therefore,it is necessary to directly obtain the elastic moduli of fluid saturated rocks at intermediate and low frequency bands in the laboratory.To this end,a broad frequency band measurement system has been built,including a high-frequency ultrasonic transmission system,an intermediate frequency differential acoustic resonance spectroscopy system,and a low frequency forced oscillation system.The effect of frequency,pressure,and fluid properties on elastic parameters of rocks has been studied by using this system.Differential acoustic resonance spectroscopy(DARS)utilizes the resonance frequency shift caused by the perturbation sample to estimate the acoustic properties of the loaded sample in the intermediate frequency band around kHz.Based on the Green function method,the modified perturbation equation is derived under the impedance boundary conditions and the corresponding multi-points parameter estimation method is developed.The newly constructed multi-physics finite element numerical model,coupling the sound field,solid mechanics and electrostatic field,matches the real physical system.The DARS system has been optimized based on the numerical simulation results.The experimental tests show that the improved system has higher experimental accuracy and wider measurement frequency band at[500–2500]Hz.The Multi-frequency band elastic parameters measurement system is used to study the dispersion and attenuation of elastic moduli of fluid saturated rocks under in-situ conditions.The effective operating frequency for the system is[2–200,10~6]Hz by combining the high-frequency ultrasonic transmission method and the low-frequency forced oscillation method.The newly designed piezoelectric ceramic oscillator with stable low frequency characteristics and the new process for sample preparation enhance the reliability of the system,and improve the measurement accuracy.Experimental results on the low porosity and low permeability tight sandstone show that the squirt flow mechanism for the undrained/unrelaxed transition is a dispersion and attenuation mechanism that cannot be ignored even in the seismic frequency band.The comparison between the experimental data and the theoretical models reveals that the main reason for the dispersion and attenuation of elastic properties for the brine or glycerin saturated rock is due to the fluid flow induced by the pressure gradient between different pores with different compressibilities.Increasing pressure and decreasing fluid viscosity will cause the characteristic frequency to move to the lower frequency range,resulting in the occurrence of the squirt flow phenomenon for the tight sandstone saturated with high viscosity fluid at seismic frequency band.The new wide frequency band model combines the drained/undrained model under fluid percolation mechanism with the undrain/unrelaxed model under squirt flow mechanism.All the pore structure parameters in the model are estimated from the experimental data without any fitting parameters.Furthermore,the distribution for crack aspect ratio,which matches the pore structure distribution,replaces the characteristic crack aspect ratio,making the model closer to the real rock.The simulation results illustrate that the wide frequency band model shows two ladder-like distribution and has two transitions,which are a drained/undrained transition and an undrain/unrelaxed transition.The laboratory data can be well modelled by the new wide frequency band model.