牛东裂缝型火山岩致密油藏渗吸采油机理

Mechanism of Imbibition and Oil Recovery of Fractured Volcanic Reservoir in Niudong

牛东火山岩油藏为裂缝型火山岩致密油油藏,发育裂缝、气孔,基质渗透率低。油藏开发初期利用天然能量进行弹性生产后,需补充地层能量,常规注水水驱存在注入水沿裂缝串流、基质难以注水等问题。为此提出裂缝型火山岩致密油油藏注水吞吐采油方法,为研究注水吞吐中的油水置换机理,进行渗吸及核磁共振扫描实验(NMR),结果表明:牛东火山岩油藏裂缝和气孔为主要储集空间,裂缝沟通孔洞为主要渗流通道;毛管力主导的渗吸作用可将基质中的油置换出来,渗吸初期(前20 h)为快速渗吸阶段,后期为低速阶段,启示现场焖井时间可控制在24 h左右;注入水能驱替裂缝及裂缝沟通的气孔中的原油,故裂缝型岩心采出程度高于孔隙型岩心;核磁共振常温常压渗吸实验得到渗吸可动油孔径下限为纳米级,微孔渗吸速率最高,但原油含量少,渗吸采出程度占比小。可见牛东火山岩油藏在转换开发方式时,应采取体积压裂造缝以沟通含油孔洞,并充分发挥水驱和渗吸双重作用,最大限度提高火山岩油藏采收率。

Niudong reservoir is of a volcanic rock type that composed of dense volcanic rocks with fracture,pore,and low matrix permeability.After elastic production with natural energy in initial stage of reservoir development,stratigraphic energy needs to be supplemented.There are some problems in conventional water flooding,such as the runaway of injected water along fractures,and difficult water injection int the matrix.To study the mechanism of oil-water displacement in water-injection stimulation,a method of huff-and-puff water injection for this type of reservoir was put forward.The results of permeation and nuclear magnetic resonance scanning experiments show that the fracture and pore of Niudong volcanic reservoir are the main reservoir space,the fracture communication pore is the main seepage channel,and the oil in matrix can be displaced and seeped by capillary force-dominated seepage.The initial stage(first 20 hours)is a rapid imbibition stage,the later stage is a low-speed stage,and the onsite soaking time can be controlled in about 24 hours.The injected water can displace crude oil in the pore where fracture and fracture communicate,so the recovery degree of fractured cores is higher than that of porous cores,the lower limit of movable oil pore size is nano-scale and the micro-pore imbibition rate is the highest by nuclear magnetic resonance atmospheric temperature and pressure imbibition experiment.However,the content of crude oil was low,and the proportion of seepage recovery was small.It can be seen that when Niudong volcanic reservoir transformed into its development mode,volume fracturing should be adopted to communicate oil-bearing holes,and give full play to the dual roles of water flooding and infiltration to maximize the recovery of volcanic reservoir.