盐间页岩盐离子扩散对自发渗吸驱油的影响——以潜江凹陷潜江组页岩为例

The Influence of Salt Ion Diffusion in Intersalt Shale on Spontaneous Imbibition and Oil Displacement:A Case Study of Qingjiang Formation in Qingjiang Sag

潜江凹陷盐间页岩富含石盐矿物,遇到水后迅速发生盐溶解、扩散,目前对于盐离子扩散对自发渗吸驱油的影响尚不清楚。以潜江凹陷的钙芒硝质盐间页岩、白云质盐间页岩和泥质盐间页岩样品为研究对象,开展页岩油储层逆向自发渗吸实验,采用低场核磁共振测试仪监测盐间页岩的渗吸过程中的油相变化。实验结果显示渗吸水进入盐间页岩后,将孔隙中的油以油滴的形式置换、排驱出来,具有小孔吸水、大孔排油的特征。初期小孔原油排出速率较高,大孔或微裂缝中的原油含量基本不变,但随着小孔原油排出速率的下降,新生的大孔或微裂缝中原油逐渐减少。此外,渗吸过程中,骨架结构中石盐矿物迅速溶解,产生大量的溶孔、裂缝。然而,量纲分析结果表明渗吸速率与石盐矿物含量具有负相关关系,石盐矿物含量越高,渗吸速率越低。石盐矿物的溶解引起颗粒崩落,堵塞基质孔隙,对储层产生了伤害作用,反而不利于孔隙中原油向新生溶孔、裂缝中迁移。研究了盐间页岩油储层中的盐的溶解、扩散机制及其对油动态迁移特征的影响,对盐间页岩的压裂液返排制度的建立和提高页岩油产出具有重要意义。

the intersalt shale in Qianjiang Sag is rich in halite minerals,which are dissolved and diffused rapidly after encountering water.At present,the effect of salt ion diffusion on spontaneous imbibition and oil displacement is not clear.Thus,the samples of glauberite salt shale,dolomite salt shale,and argillaceous salt shale in Qianjiang Sag were taken as the research objects.The reverse spontaneous imbibition experiment of shale oil reservoir was carried out,and the oil phase change in the imbibition process of salt shale was monitored by low field NMR tester.The experimental results show that the oil in the pores was displaced and drained out in the form of oil droplets after the water seepage entered the salt shale,being characteristic of water absorption in small hole and oil drainage in large hole.In the early stage,the oil discharge rate of small hole was higher,and the crude oil content in the large hole or micro fracture remained largely unchanged;with the decrease in oil discharge rate of the small hole,crude oil in new large hole or micro fracture gradually decreased.In addition,in the process of imbibition,halite minerals in the skeleton structure dissolved rapidly,resulting in a large number of dissolved pores and cracks.However,the results of dimensional analysis show that there was a negative correlation between the imbibition rate and the content of halite minerals.The higher the content of halite minerals was,the lower the imbibition rate was.The dissolution of halite minerals caused particle collapse,blocked matrix pores,and caused damage to the reservoir.On the contrary,it was not conducive to the migration of crude oil from pores to new dissolved pores and fractures.The mechanism of salt dissolution and diffusion in intersalt shale oil reservoir and its influence on the characteristics of oil dynamic migration were revealed,which is of great significance to the establishment of fracturing fluid return system and the improvement of shale oil production.