Study On Multi-frequency Complex Resistivity And Saturationmodel Of Fractured Rocks

Fractured oil and gas reservoirs have very large reserves and production worldwide and is an important exploration and development target.However,fractured reservoirs have more complex pore space structure and strong anisotropy than homogeneous porosity reservoir,so the logging interpretation and evaluation of fractured reservoir is much more difficult than homogeneous porosity reservoirs.Especially the calculation of saturation.In well logging,the identification of fluid and the calculation of saturation mainly depend on the resistivity measured by electric well logging.The saturation calculation mainly depends on the famous Archie’s law.The quantitative evaluation of homogeneous reservoir saturation according to the relevant parameters(cementation exponent m,saturation exponent n)of different reservoirs.The common method for calculating the saturation of fractured rock is based on Archie’s law for homogeneous rock,which only uses the cementation exponent m to examine the influence of fracture on the conductivity of rock with 100%water content.The research on the influence of fracture on saturation exponent n when considering different oil and gas saturation has been paid attention to,but the research results are less.The existing models of dielectric saturation are mainly based on the relative dielectric constant at single frequency,without considering the effect of frequency and fractures.Few models of complex resistivity saturation include both frequency and water saturation.At present,electric well logging is mainly to measure the resistivity of a certain(or several)frequency.The theory and measurement methods of complex resistivity logging are accumulating and will be industrialized in the near future.In order to make full use of the real part and imaginary part of complex resistivity and the information on rock structure and fluid saturation contained in the measured signals of different frequencies,it is necessary to consider the effect of mineral surface wettability in the study of complex resistivity saturation model,and the importance of wettability has begun to appear.The research on saturation model of complex resistivity for fractured rock will be the development direction of fractured reservoir logging evaluation.Based on the basic theory of rock complex resistivity and the saturation model of homogeneous rock,the complex resistivity saturation model of fractured rock is established.Then the digital core containing flat ellipsoidal fracture is constructed for numerical simulation.And we constructed artificial fractured rocks and measured its complex resistivity.The multi-frequency complex resistivity saturation model for fractured rocks is verified by experiment and numerical simulation.Finally,a method of inversion of rock water saturation by genetic algorithm using rock complex resistivity is proposed.The paper specifically covers the following aspects:Firstly,a multi-frequency complex resistivity saturation model for fractured rocks is proposed on the basis of dielectric mixing law and the analysis of rock dispersion mechanism.The model mainly considers the effects of water saturation,fracture parameters and rock wettability on the complex resistivity of rock.Secondly,the digital core with ellipsoidal fracture is constructed.Calculation of complex resistivity in a wide frequency range(100Hz-1GHz)under different saturation states of ellipsoid fractured rock by numerical simulation using finite element method.The reliability of the model is proved by comparing the relationship of resistivity and relative permittivity of water-saturated rock with fracture parameters.Then the complex resistivity dispersion characteristics of homogeneous rock samples and fractured rock samples with different fracture parameters are studied through complex resistivity experiments in the middle and low-frequency range(100Hz-1MHz).First,artificially construct cores with different fracture parameters,such as different fracture width,density,and fracture dip angle.Then the two-electrode method was used to measure the complex resistivity in the middle and low-frequency range(100Hz-1MHz)of fractured rocks.The relationship between water saturation S_W and the resistivity index I_R,water saturation S_w and the dielectric constant index I_ε at 100 Hz,1 kHz and 10 kHz and rock samples with different fracture widths,densities,and dip angles were mainly studied.The experimental results show that the addition of fractures increases the electrolyte and conductive paths in the rock,which decreases the real part of the complex resistivity of the rock in the real non-dispersive region and decreases the magnitude of the imaginary part of the complex resistivity in the dispersive region.As the fracture width,density and dip angle become larger,the real part of the real non-dispersive zone rock complex resistivity decreases,the imaginary part of the dispersive zone complex resistivity amplitude decreases,the time constant decreases and the characteristic relaxation frequency increases.The numerical simulation results also had the same pattern.The slope of I_R-S_W和I_ε-S_W curves increases with the increase of fracture inclination.The fracture not only makes the characteristic relaxation frequency of water-saturated rocks larger,but also makes the characteristic relaxation frequency range of rocks in the same saturation range wider,so that the I_R-S_W curves of homogeneous rocks at 100 Hz,1 kHz and 10 kHz are basically not separated,and the I_ε-S_W curves are separated to a small extent,and the I_R-S_W curves of fractured rocks at 100 Hz,1 kHz and 10 kHz are separated,and the separation of I_ε-S_W curves at 100 Hz,1 kHz and 10 kHz becomes larger.By comparing the experimental results with the numerical simulation results,it is found that the calculated results of the complex resistivity saturation model of fractured rocks proposed in this paper can better describe the experimental phenomena,which indicates that this model is reliable and suitable for the fractured rocks with regular morphology.Finally,the method of inversion of rock water saturation by genetic algorithm using rock complex resistivity is proposed.Then we used actual fractured rock complex resistivity data to invert rock water saturation with good results.Through the study of fractured rock complex resistivity,the reliability of oil and gas identification and quantitative evaluation of water saturation in fractured reservoirs has been improved.It provides a certain basis for the exploration and development of oil and gas reservoirs in fractured reservoirs.