Microscopic Mechanism Study Of Imbibition In Porous Media Based On Lattice Boltzmann Method

Imbibition in porous media is a typical immiscible fluid flow phenomenon in which a wetting fluid displaces a non-wetting fluid.The phenomenon is widespread in nature and plays an important role in many engineering and industrial processes,such as oil and gas resource exploration,CO₂ sequestration and groundwater contamination remediation.Due to the complex pore structure of porous media and instability at the fluid interface,imbibition in porous media is often accompanied by a noncompact fluid interface and incomplete pore filling phenomenon,which in turn affects the macroscopic displacement efficiency.Therefore,it is of great significance to accurately characterize the microscopic pore structure of rocks,to gain an in-depth understanding of fluid flow characteristics and distribution states at the pore scale,and to clarify the microscopic imbibition mechanism under the influence of multiple factors.In this thesis,based on the digital model constructed by CT scanning or modelling,and the pore-scale numerical simulation with the lattice Boltzmann method,the influences of microfracture structure,geometric characteristics of granular porous media and imbibition boundary conditions on the global evolution of fluid interface,local pore filling characteristics,imbibition rate and ultimate recovery factor during spontaneous imbibition are comprehensively investigated.Thus,the microscopic imbibition mechanism under the influence of some key factors is deeply revealed.On this basis,the displacement pattern and pore filling mechanism during forced imbibition in natural reservoir rocks under the completely wetting and viscously unfavorable conditions are further investigated,and the in-situ fluid distribution as well as the two-phase fluid geometry during the whole imbibition process are comprehensively evaluated.The main results obtained are as follows.（1）The geometric structure of the microfracture hardly affects the evolution of the imbibition front in the initial stage,but strongly influences the overall morphological characteristics of the fluid interface during the counter-current spontaneous imbibition.Therefore,the distribution of microfractures can significantly improve the overall nonwetting fluid recovery,but their different geometries have different effects on the imbibition rate.（2）Grain shape only has a more significant influence on the imbibition interface during counter-current spontaneous imbibition,while the grain packing pattern influences the evolution of drainage and imbibition interfaces and the recovery factor.Spontaneous imbibition with different boundary conditions has different imbibition rates and ultimate recovery factors.The larger the open face in contact with water,the faster the imbibition rate,so spontaneous imbibition with the AFO boundary condition has the fastest imbibition rate.The contact between the open face and the oil makes the oil phase to be discharged mainly by co-current imbibition,which facilitates the continuation of spontaneous imbibition,and therefore,spontaneous imbibition with the TFO-free boundary condition has the highest ultimate recovery factor.（3）The dominant displacement patterns and pore filling events during forced imbibition in porous media are significantly different under different injection flow rates.As the capillary number,expressed in logarithms,varies from-2.5 to-4.0,the flow characteristics of the invading fluid change from leading films on the solid surface of large pores to corner flow along the pore corners of different-size pores,the dominant pore filling event changes from ganglion dynamics to snap-off trapping,and the morphology of the displacement front changes from fingering to a rough but uniform characteristic.These changes at the two-phase interface determine the fluid distribution and fluid structure,controlling the displacement efficiency,interface area and fluid connectivity.This work reveals the fluid interface evolution dynamics,pore filling mechanism and fluid distribution characteristics in the pore space during the imbibition in porous media,clarifies the microscopic imbibition mechanism under the influence of multiple factors such as pore structure and operating conditions,and establishes the correlation between microscopic flow mechanism and macroscopic displacement efficiency,which provides a theoretical basis for efficient development of petroleum resources,safe storage of CO₂and effective purification of groundwater contaminants.