Percolation Network Study On The Electrical Properties Of Rocks And Applications

Rock electrical resistivity parameters are crucial for reservoir identifications,water saturation evaluation,and reservoir characteristic analysis.However,the knowledge on influence factors and mechanism on rock resistivity are relatively rare,leading to the lack of the methodology in complex reservoir evaluation.It is necessary to launch the fundamental research on rock resistivity properties.Percolation theory was initially used for study the micro-and macro-properties of the random system.The percolation network,born from the incorporating the percolation theory into the random pore network,was widely used in physics,petrophysics and transport properties of rocks.In recent years,a large number of studies have shown that the methodology,based on percolation theory,has huge advantages on analyzing the effects of rock microstructure on their macroscopical transport properties,which is theoretically and practically significant to understand the fluid flow in rocks and guide the well-logging interpretation.On the basis of the percolation theory,this study focuses on the electrical resistivity of rocks using numerical simulation,laboratory experiment and analysis.Finally,the theoretical research results are simplified for the actual logging interpretation based on the engineering geological condition.The main research works,results and understandings are listed as follows:(1)percolation network was built for comparing the pore-throat size,spatial distribution and the microscopic fluid flow rule with different pore-scale heterogeneity,the effects of pore-scale heterogeneity on the microscopic flow rules were analyzed according to the simulation results.The results show that:for relatively homogeneous pore-throat size distribution,the pore-throat spatial distribution and microscopic flow rule in percolation pore network can be represented by the capillary buddle model;for heterogeneous pore-throat size distribution(in tight rocks),the pore-throat spatial distribution and microscopic flow become disorder(radius of pore-throat size is not related to the corresponding flow flux).(2)The electrical resistivity of three kinds of reservoir rock(i.e.,single porous medium,matrix-vug rocks and fractured rocks)were studied.On the basis of the resemblance and common character of microscopic pore-structure in different kinds of rocks,the modeling and simulation methods of three kinds of percolation networks(i.e.,percolation pore network,dual-pore percolation network and fracture network)were introduced.(3)The effects of micro structure on the electrical property of reservoir rock were studied numerically.According to the electrical conductive mechanism,simulation results and percolation theory,formation factor/resistivity index model for three kinds of reservoir rock were derived,and compared with corresponding experimental data of rock samples for validations.The research results show that the percolation "universal" power law can be used for studying the electrical property of different kinds of reservoir rocks.For water-wet rocks,the resistivity index curve is bent in the log-log plot("non-Archie" phenomenon),the form of the resistivity index curve is related to pore connectivity,pore-size distribution and the water-film thickness.The "non-Archie" phenomenon is widely observed in dual-pore rocks.(4)For meeting the practical well-log interpretation,based on the microstructure of rocks and practical engineering situation,the resistivity index model was simplified to(Li-Bernabe-Tang)LBT model,which can be directly used for well-log interpretation.The LBT model can be used for single-pore and dual-pore porous media,and simultaneously inherits the extensive adaptability of percolation theory(can also be used for tight oil/gas rocks).The LBT model is validated by comparing to interpreted results of the nuclear magnetic resonance(NMR)logging.Compared with the traditional Archie's equations,the fitting results of experimental data using LBT method impoved 0.2%?13.2%,the interpretation accuracy of water saturation using LBT method impoved 10%?28%.Depending on all the studies and experiments above,the relations between pore microstructure,water saturation and macroscopical electrical propertiy of rocks were further clarified and described quantificationally.The proposed quantitative models have extensive adaptability and clear physical meanings.The overall research methodology,combining the percolation theory,numerical simulations and experimental research,is developed in this study.The research results,obtained from the methodology,have "universal" adaptability and can be easily extended for the actual application,enriching and developing the rock physics theory and technology.