Research On The Theory And Numerical Simulation Of Seismoelectric Effect Of The Porous Media

The seismoelectric effect has important application value in oil logging and other fields,but there are still some shortcomings in the basic research of seismoelectric effect,including physical models and basic principles.Based on the fundamental principles of seismoelectric effect,in this dissertation,the coupling equations of seismoelectric effect and the corresponding coupling coefficients of different types of porous medium are deduced,and numerical simulation on the model of seismoelectric logging while drilling in different types of porous medium is carried out.The main research contents are summarized as follows.1.A new method was used to study the microscopic mechanism of the electrokinetic conversion of slit-like saturated pores under time-harmonic conditions.The current balance condition is used to solve the electric field in the existing researches on the electrokinetic effect in the pore under the time-harmonic condition.The acoustic field and the electric field in the pore are decoupled in this dissertation,the acoustic field is solved separately to calculate the streaming current,and then the electromagnetic field is directly solved with the streaming current as the excitation source.The conduction current and streaming current in the pore are calculated.The calculation results show that the streaming current and conduction current are not equal,which proves that the current balance condition is not valid under the time harmonic condition.The magnitude of viscous force and electric force in the pore are also compared,which proves the rationality of the proposed decoupling method.A new analytical model for the electrokinetic conversion of saturated porous media has been established.The existing theory to study the seismoelectric effect of saturated porous media is mainly Pride theory.Pride theory adopts the assumption of thin electric double layer and uses the volume averaging method to approximate the Nernst-Planke equation.The electrokinetic coupling coefficient obtained is only applicable to the case of thin electric double layer.At the same time,Pride theory ignores the change of seepage velocity with the radial position of the pore when calculating the streaming current.As a result,the influence of pore size on the electrokinetic coupling coefficient cannot be accurately reflected.In this dissertation,the flux averaging method is proposed to obtain the effective net residual charge density in porous medium,and the effective net residual charge density is further used to express the streaming potential coupling coefficient under the action of steady acoustic field and the streaming current coupling coefficient under the action of time-varying acoustic field.The flux averaging method adopted in this dissertation does not need to limit the condition of thin electric double layer,so it is applicable to both thin and thick electric double layer cases.A flume experiment was also carried out to investigate the effect of salinity on the seismoelectric effect,and the thin and thick electric double layer cases are investigated,and the experimental result is compared with the simulation result,which verifies the correctness of the theory.The change of seepage velocity with pore location is not ignored when solving the streaming current,so the proposed method can more accurately reflect the influence of pore size on the electrokinetic coupling coefficient.The coupling equations of the acoustic field and electric field in saturated porous medium is deduced,and the model of electrokinetic logging while drilling in saturated porous media is numerically simulated.The reciprocity principle is used to calculate the electric field signal in the electrokinetic logging,and the influence of the electrokinetic conversion at different positions in the formation on the received seismoelectric signals is examined,which proves that the resolution of the electrokinetic logging is very high.At the same time,the frequency-velocity correlation characteristics of the acoustic field and the electric field under the excitation of multipole sources are also investigated.It is proved that receiving electric field can avoid the influence of the drill collar wave to a certain extent in the electrokinetic logging while drilling in saturated porous medium.2.A new analytical model for the electrokinetic conversion of gas phase continuous unsaturated porous media has been established.The existing research on the seismoelectric effect of unsaturated porous media does not consider the influence of gasliquid interface,nor can it explain the non-monotonic change of streaming potential coupling coefficient with saturation.In this dissertation,the influence of gas-liquid interface on the seismoelectric effect in unsaturated porous media is considered,and the effective net residual charge density corresponding to the solid-liquid interface and gasliquid interface in the porous media is solved,and the streaming potential coupling coefficient under the action of steady acoustic field and the streaming current coupling coefficient under the action of time-harmonic acoustic field are derived respectively.The change of streaming potential coupling coefficient with water saturation under steady condition is mainly studied,and compared with several existing theories.The change trend of streaming potential coupling coefficient with saturation obtained according to this dissertation is consistent with the experimental measurement of Allègre,which provides a theoretical explanation for the non-monotonic change of streaming potential coupling coefficient with saturation.The new coupling equations of seismoelectric effect in gas phase continuous unsaturated porous media are deduced,and the model of electrokinetic logging while drilling in unsaturated porous media is numerically simulated.The Hertz vector potential is introduced,and the electromagnetic field is treated as a timevarying field.The frequency-velocity correlation characteristics of acoustic field and electric field in electrokinetic logging while drilling under multipole sources excitation are investigated,and the influence of drill collar wave is analyzed.It has been proven that in electrokinetic logging while drilling of unsaturated porous formation,receiving electric field can to some extent avoid the interference of drill collar waves.3.The microscopic mechanism of the electrokinetic conversion of gas phase discrete unsaturated pores is analyzed.At present,the research on the seismoelectric effect of unsaturated porous media mostly adopts the model of gas-liquid two-phase continuous distribution,and lacks the mathematical description and theoretical research on the microscopic mechanism of the seismoelectric effect of unsaturated porous media in the case of discrete gas phase.The existing research also does not consider the effect of electric double layer at the gas-liquid interface on the seismoelectric effect in the unsaturated porous media.The influence of electric double layer at the pore wall and the gas-liquid interface is considered,and the microscopic principle of the seismoelectric effect of unsaturated porous media is theoretically studied.Firstly,the change of gaswater two-phase flow pattern with saturation in unsaturated pores is studied.The equivalent principle of series circuit is proposed to calculate the effective streaming current and conductivity in pores under different flow patterns,and then the streaming potential coupling coefficient in gas phase discrete unsaturated pores under the excitation of steady acoustic field is derived.The phenomenon of non-monotonic variation of the streaming potential coupling coefficient in unsaturated porous media with saturation was explained from a microscopic perspective.4.A new analytical model for the electrokinetic conversion of oil-wetted porous medium with oil-water two-phase flow has been established.At present,the researches on the seismoelectric effect of porous medium are mainly aimed at water-wetted porous medium,such as water-saturated porous medium and water-wetted two-phase flow porous medium.There is no corresponding research on the seismoelectric effect of oilwetted porous medium,especially the oil-wetted porous medium containing oil-water two-phase flow.Based on the capillary pore,the oil-wetted pore model with two immiscible,continuous and parallel phases of oil and water is adopted.The two-phase flow is regarded as an equivalent fluid.The govern equation of electrokinetic conversion is derived,and the streaming potential coupling coefficient under the excitation of steady acoustic field and the streaming current coupling coefficient under the excitation of timeharmonic acoustic field are calculated respectively.The new coupling equations of seismoelectric effect in oil-wetted porous media containing two phase flow are deduced,and the model of electrokinetic logging while drilling in two-phase flow oil-wetted porous media is numerically simulated.The influence of drill collar wave on the acoustic field and electric field is investigated,which has a certain guiding role for the selection of tools of electrokinetic logging while drilling.The simulation results demonstrate that in the seismoelectric logging while drilling of two-phase oil wetted porous formation,receiving electric field can to some extent avoid the interference of drill collar waves.