致密储层压驱焖井阶段渗吸机理分子模拟研究

THE SHUT-IN IMBIBITION MECHANISM OF HYDRAULIC FRACTURING-ASSISTED OIL DISPLACEMENT IN TIGHT RESERVOIRS BASED ON MOLECULAR SIMULATION

针对致密储层开发中存在的天然地层能量衰减快、驱替相有效波及体积不足等难题,考虑将压驱与焖井相结合的高效开发技术应用于致密储层.基于分子动力学方法从微观作用力角度分析致密储层压驱渗吸机理,进行致密储层压驱过程溶质动态迁移表征,并从分子尺度对渗吸阶段进行划分.采用分子模拟方法,构建致密储层壁面-油相-驱替相三相体系,分别从体系弛豫特征、浓度分布及扩散能力和介质间相互作用能等方面分析常规水力压裂后常压驱替条件和压驱高压条件渗吸过程特征性差异,从分子尺度阐释致密储层压驱渗吸机理.研究表明:相对于常规压裂后驱替而言,压驱条件下,驱替相分子扩散系数提高20.06%,与孔隙壁面的相互作用能提高2.3倍;驱替相分子吸附层数增加,油相解吸效果更为明显,渗吸换油效率提高38.73%.此外,渗吸效率随储层温度变化的过程存在峰值,具有先上升后降低的特征;且受到壁面润湿性的影响,壁面亲水性越强渗吸效率越高.从分子尺度将焖井渗吸过程划分为3个阶段:水分子优先靠近壁面;驱替相流体与油相分子竞争吸附,将吸附态油相剥离为游离态,使其远离壁面;压驱液溶质分子进入初始油相范围,进一步置换油相,提高渗吸效率.压驱技术高压注入压驱剂可快速补充地层能量,扩大驱替相流体波及体积并提高洗油效率,在二者的协同作用下可大幅度提高渗吸驱油效率.该项研究可为致密储层高效开发提供理论参考.

Aiming at the problems of insufficient effective sweep volume and rapid energy depletion of formations during the development process of tight reservoirs,it is considered that the technology of hydraulic fracturing-assisted oil displacement(HFAD)combining shut-in can be applied in the development of tight reservoirs.The molecular dynamics method is applied to analyze the imbibition mechanism of tight reservoir under HFAD condition from the perspective of microscopic forces,the dynamic migration of solutes under HFAD conditions in tight reservoir was described,and the imbibition stages were divided from the molecular scale.A three-phase system of tight reservoir wall-oil-displacement phase is constructed by molecular simulation method.The characteristic differences of conventional development condition and HFAD condition were analyzed from the aspects of relaxation characteristics,concentration distribution,diffusion capacity,and interaction energy between media.The imbibition mechanism of tight reservoir under HFAD condition was explained from the molecular level.The result shows that:compared with conventional development condition,the molecular diffusion coefficient of displacement phase under HFAD condition increases by 20.06%,the interaction energy with the wall increases by 2.3 times.The number of adsorption layers of displacement phase molecules increases,and the desorption effect of oil phase becomes more obvious,the imbibition displacement efficiency increases by 38.73%.In addition,the imbibition efficiency has a peak value as the reservoir temperature changes,which first increases and then decreases.Moreover,the imbibition efficiency is influenced by the wall wettability,the stronger the wall hydrophilicity,the higher the imbibition efficiency.The imbibition process can be divided into three stages from the molecular scale:water molecules preferentially approach the wall surface;The displacing phase fluid competes with the oil phase molecules for adsorption,stripping the adsorbed oil phase into a free state and keeping it away from the wall;The solute molecules of HFAD liquid enter the initial oil phase range to further displace the oil phase and improve the imbibition efficiency.High-pressure injection of HFAD agent in HFAD technology can quickly replenish formation energy,expand the displacement phase fluid sweep volume and improve oil washing efficiency.With the synergistic effect of the two,the imbibition displacement efficiency can be greatly improved.This study can provide a theoretical reference for efficient development of tight reservoirs.