Study On Microscopic Mechanism Of Frequency-dependent Electroosmosis And Streaming Potential In Reservoir Rock

The study of frequency-dependent electroosmosis and streaming potential in reservoirrock can help geophysicist understand the mechanism of seismo-electric prospecting,electric-seismo prospecting and electrokinetic logging and in hydrocarbon reservoirs. Butbecause of the complexity of the microscopic pore structure of the actual reservoir rock andfluid properties, the study on microscopic mechanisms of frequency-dependentelectroosmosis and streaming potential in reservoir rock have not yet achieved keybreakthrough, and there is no real theoretical system or mathematical description about it upto today, which limited the promotion and application of related technologies. Combined withthe theory of double layer potential and electrochemistry transfer dynamics in the porousmedia, this paper established the mathematical equations to describe the microscopicmechanisms of frequency-dependent electroosmosis and streaming potential based on thecapillary model.Firstly, the theory of electrokinetic coupling was introduced in water bearing reservoirrocks based on the capillary model. The paper utilized the theory to study the fluidelectroviscosity effect in rocks, and analyzed the influence of the rock pore structure,formation water salinity, cation exchange capacity and exciting source frequency on theapparent viscosity coefficient.Secondly, based on the straight capillary model and combined with the double layerpotential, this paper derived the equation to describe the velocity distribution in capillary bythe water flow obeying Navier-Stokes equation under the frequency-dependent electric field.Then, combined with the theory of electrochemistry transfer dynamics, this paper obtained theanalytical expression of electroosmotic velocity to reveal the microscopic mechanism of theelectroosmotic flows in rocks. Then, we emphasized on the macroeffect of electroosmotic flows including electroosmotic Darcy velocity and electroosmotic pressure coefficient inunsealed rocks and sealed rocks, respectively, and investigated them changing with theporosity, solution concentration, cation exchange capability and harmonic frequency bymathematical simulation.Thirdly, this paper set out from the microscopic pore structure of water-oil bearingargillaceous sandstone and established a new capillary with water wettability. Based on thecapillary model, the theory of frequency-dependent electroosmosis theory in water bearingreservoir rocks expanded to water-oil bearing argillaceous sandstone. We discussedelectroosmotic Darcy velocity and electroosmotic pressure coefficient in unsealed rocks andsealed rocks, respectively, changing with water saturation and irreducible water saturation.Fourthly, according to the microscopic adsorption characteristics on the solid-liquidinterface in oil-water bearing reservoir rock, we introduced the concepts of “water film” and“oil film” into the straight capillary model, and created a new capillary model including sixkinds bundle of capillary tubes with different physical properties that can realize toquantitatively describe reservoir wettability, and gave the conversion formulas betweenmacroscopic transport parameters of reservoir rocks and microscopic parameters of capillarymodel. Then, the Darcy law about seepage velocity and the Ohm Law about electric currentdensity were derived in every kind capillary, and the mathematical method was established todescribe the frequency-dependent streaming potential in water-oil bearing reservoir rocks.Then, we investigated the influences of reservoir wettability, porosity, saturation, cationexchange capacity and formation water salinity on the effective permeability andfrequency-dependent streaming potential coupling coefficients by mathematical simulation.