| The development of unconventional tight oil and gas has been intensified year by year in recent years.Tight oil and gas reservoirs generally have the characteristics of low permeability,low porosity,high capillary pressure and high effective stress.During the development of tight reservoirs,crude oil can be displaced from matrix pores through imbibition,thereby improving displacement efficiency and increasing production.It can be seen that imbibition is very important in the recovery of unconventional tight oil and gas,and it is of great significance to improve the efficiency of unconventional tight oil and gas recovery,but there is still a lack of relevant in-depth research.In view of this,a series of studies on imbibition in tight oil and gas reservoirs were carried out in this thesis.At present,there are three main difficulties in the study of the imbibition process in tight reservoirs: 1.variable control in the experiment;2.quantitative description of the imbibition process;3.research on the microscopic mechanism of the imbibition process.Experiment,theory,and numerical simulation are used to overcome the three research methods: 1.through reasonable design of comparative experimental schemes in experimental research to enhance variable control in experiments;2.through theoretical research,a quantitative theoretical description formula is proposed to make up for the lack of quantitative description.Insufficient;3.through numerical simulation,using the simulation method that can describe the microscopic imbibition process,study the microscopic mechanism of the imbibition process in tight reservoirs.This thesis firstly carried out relevant experimental research,using nuclear magnetic resonance(NMR)to compare and study the process of spontaneous imbibition under normal pressure and spontaneous imbibition under pressure of different types of cores,and analyzed the flow trajectory of imbibition fluid after entering the core,and the pores of the core.In the process of imbibition,such as the expansion and closure,and the generation of micro-fractures,the influence of the characteristics of the core itself and the pressure conditions on the imbibition process is analyzed.A new imbibition theoretical model based on the capillary bundle model is proposed to quantitatively characterize the spontaneous imbibition process.Finally,in order to make up for the shortcomings of the aforementioned experimental methods and theoretical models that are difficult to describe the microscopic imbibition phenomenon in the core,a lattice The Boltzmann method(LBM)has carried out a numerical simulation study on the imbibition process in two micro-nano-scale pore networks: the first is the imbibition process in the ideal fractured pore network with different characteristics,and the second is According to the imbibition process in the pore network modeled by the real core data used in the experiment in this thesis,the flow of fluid in the pore network,the development characteristics of imbibition front,the distribution characteristics of residual oil and the imbibition process during the imbibition process are studied and analyzed.Sensitivity to related parameters,etc.These works can lay a foundation for the further establishment of imbibition theoretical models for unconventional tight oil and gas reservoirs considering complex pore and fracture structures and main influencing factors,and provide reference data for practical engineering applications.The main work and results of this thesis are as follows:1.Through the spontaneous imbibition experiments on different ordinary sandstone,tight sandstone and volcanic rock samples,the influence of the initial pore throat distribution and other factors on the spontaneous imbibition process was studied.The study shows that the relationship between imbibition length and the square root of time can be divided into three stages: linear segment,curve segment and low-slope linear segment;the spontaneous imbibition process is significantly affected by the distribution and scale of pore throats.Based on the experimental data,the general characteristics of imbibition in heterogeneous tight oil and gas reservoirs are analyzed.2.The pressure imbibition experiment was carried out on a group of shale samples taken out from adjacent wells,and the influence of clay minerals on the imbibition process was studied in combination with the measurement of NMR T2 spectrum.The results show that the imbibition rate and imbibition quality are lower when the content of illite and montane mixed layer is low.If the clay mineral swells with water,resulting in micro-cracks,the imbibition rate will increase.The effect of external pressure on imbibition is affected by factors such as capillary force,pore throat structure,and whether new microcracks are formed.Shale imbibition flooding is a comprehensive effect of multiple factors.The relationship between the main factors and the displacement efficiency is complicated.3.Based on the experimental data of spontaneous imbibition,and on the basis of previous work,a theoretical model based on the distribution of capillary bundles and pore throat scales is proposed,which can better explain the three stages of spontaneous imbibition.In contrast,this theoretical model is simple and effective.Compared with the previous theoretical models,this model takes into account the influence of different pore diameters in the capillary bundle on the imbibition process,and is more accurate for the third stage in which the imbibition speed changes from fast to slow in the imbibition process.describe.4.The Lattice Boltzmann Method(LBM)is applied to numerically simulate the imbibition process in the fractured pore network with ideal geometric structure.Imbibition when pressure gradient dominates in a fractured-pore network.The results show that the imbibed fluid first enters the small-scale pores and the front develops faster in the larger pores,which is opposite to the case of seepage.As the Knudsen number decreases,the dimensionless imbibition rate becomes faster and faster;as the connectivity of the pore network decreases,the imbibition develops more and more slowly.When the pore connectivity is less than or equal to 50%,the imbibition is difficult to develop.5.Based on the measured pore size distribution of the core used in this experiment,a pore network model is established,and the imbibition process is numerically simulated by LBM.Firstly,the main influencing factors in the imbibition process of this pore network model were determined through dimensional analysis,and then the influence of the main factors was analyzed through LBM simulation,and the variables such as displacement efficiency and displacement speed were established with the main influencing factors such as contact Fitting relationships for angles,connectivity,etc.There are still some areas for further improvement in the research of this thesis,such as the insufficient observation accuracy of the imbibition front in the experimental process,and the limited number of pore-throat structures involved in the numerical simulation calculation.In future research,microscopic observations with higher definition can be carried out in experiments,and more complex pore-throat and micro-crack structures corresponding to real cores can be simulated in numerical simulation calculations. |
PDF Full Text Request