Research On Seepage Mechanisms Of Pore-Scale Elastic Microspheres By Experiment And Modeling

With the rapid development of Chinese economy,the demand for oil resources grows constantly.However,the difficulty of oil exploration and development is ever increasing.Therefore,enhancing the oil recovery of developed oilfields becomes more and more important.The technology of depth profile control and flooding using pore-scale elastic microspheres(MPEMs),as an enhanced oil recovery(EOR)technology with wide developing prospect,can effectively improve waterflood development effect and enhance oil recovery for the special characteristics of MPEMs.The seepage of MPEMs in porous media is an extremely complex issue and the scientific and reasonable understaning and description of seepage mechanisms and EOR mechanisms of MPEMs is the key problem determining the application prospect of this technology.In addition,the transport phenomenon of particles in porous media exists in a large variety of industrial and environmental processes like water flooding of oil reservoirs,water filtration,soil contamination,chemical separation,etc.and has always been the research focus of energy and environment field.Therefore,the research on seepage mechanisms of MPEMs by experiment and modeling is of great practical significance and theoretical value.Firstly,according to the micron-scale characteristic of rock pores and pore-throats in sanstone reservoirs and based on the synthesis theory of polymer microspheres,four kinds of micron-grade pore-scale elastic microspheres(MPEMs)with different particle size and size distribution were controllably prepared by suspension polymerization technique,and via the characterization methods of polymer materials,the structure and property of MPEMs and the rheological property of MPEMs suspension were studied systematically.Secondly,using sandpacked model displacement experiments,the migration and plugging properties of MPEMs were studied;then the matching factor between particle size of MPEMs and pore-throat size of sand-packed models was introduced and used to characterize their matching relationship;based on this matching relationship,the plugging and diversion properties of MPEMs were evaluated and a design method for MPEMs deep profile control and flooding was proposed.Thirdly,according to the pore-throat characteristics of actual reservoirs and the particle size characteristics of MPEMs,a 3D transparent micromodel with a real-time microimage collection,data test and analysis system were constructed and used to reveal the pore-scale transport,profile control and flooding mechanisms of MPEMs in porous media.At last,according to the seepage rule of heterogeneous porous media and tracer transport theory,a novel dual-tracer method of accessing the heterogeneity of oil reservoirs and evaluating the profile control and flooding performance of MPEMs by simultaneously injecting ionic chemical tracer(KBr)and fluorescent nonparticle tracer(Cdot)was proposed;by building heterogeneous single-tube sand-packed models which considered the crossflow between layers,this dual-tracer method was vertified experimentally,and used to identify the heterogeneity of oil reservoirs and to evaluate the profile control and flooding performance of MPEMs.The results show that MPEMs are spherical particles with a typical single-peak distribution characteristic obeying Weibull Distribution;MPEMs have good viscoelasticity,excellent swelling property,high thermal stability,and high tolerance to salinity,acid and alkali;MPEMs almost do not cause corrosion to the injection equipment and pipeline due to the pH value(6.5~7.5)of MPEMs suspension;MPEMs hardly increase the viscosity of injected water,and under the influence of MPEMs mass fraction,temperature,shear rate and salinity,the viscosity change of MPEMs suspension is not large,indicating that MPEMs have good mobility and injectivity in the profile control and flooding process.MPEMs can migrate and plug in porous media gradually relying on their own elasticity,and lead to fluctuation of injection pressure;only when the particle size of MPEMs matches with the pore-throat size of reservoir rock can a better plugging performance be obtained;based on the evaluation criteria of plugging rate more than 85%,the optimal matching factor is 1.35~1.55.MPEMs prefer to move into and plug the high-permeability layer with an optimal matching factor,and can reduce the its permeability greatly;however,the reduction of permeability of low-permeability layer with a high matching factor is extremely small,indicating that MPEMs have a certain protective action on the low-permeability layer.These results provide vertification for the importance of particle-pore matching relationship.The pore-scale transport mechanisms of MPEMs in porous media are mainly embodied as deposition mechanism on pore-surfaces,plugging pore-throats mechanism through capture-,superposition-,and bridgeplugging,interception mechanism with MPEMs particulate filter,and remigration mechanism in pore-throats through four steps of capture-plugging,elastic deformation,steady migration,and deformation recovery;the pore-scale profile control and flooding mechanisms of MPEMs in porous media are mainly embodied as selective plugging mechanism in large channels,fluid diversion mechanism after plugging,oil drainage mechanism due to negative pressure produced by the breakthrough of MPEMs and oil droplets converging into oil flow mechanism;and these results provide vertification for the advancement and scientificity of MPEMs depth profile control and flooding technology.The breakthrough curves of KBr and Cdot under homogeneous condition are almost coincidence with each other;however a separate exists in the breakthrough curves of KBr and Cdot under hetrogeneous condition and the integral fv values of KBr and Cdot have large difference.At a certain injection rate,analyzing the output rule of KBr and Cdot and the change law of their integral fv values before and after the injection of MPEMs can identify the reservoir heterogeneity and evaluate the oil displacement performance of MPEMs;this dual-tracer method provides a practical and effective method for the evaluation of MPEMs profile control and flooding performance when considering the crossflow characteristic between layers of oil reservoirs.