酸化对富有机质页岩孔隙结构影响的试验研究

Experimental study on the influence of acidification on organic-rich shale pores structure

随着能源的日益紧缺,页岩气开采尤为迫切。页岩致密低渗,为提高产量必须利用增产措施提高储层渗透率。酸化预处理可有效改善岩石孔隙结构,增强孔隙的连通性,进而提高页岩渗透率及产气量。本试验采用质量分数3%氢氟酸和10%盐酸的混合酸对湖南牛蹄塘组页岩样品进行了酸化浸泡处理,通过超景深显微镜观察了不同酸化时长下页岩层理、裂隙和孔隙结构特征的演化规律,统计了页岩孔隙直径、频次、频率及孔隙度随时间的变化趋势,分析了酸化对页岩内部气体流动的影响。实验结果表明:①牛蹄塘组页岩中的脆性矿物沿层理发育,酸化处理后长石及石英矿物强烈溶蚀,可形成沿层理发育的裂隙;②随酸化时长增加,页岩表面石英、长石的出露面积先增大后减小,酸化后期,脆性矿物与周围矿物间孔隙明显增加,孔隙向深部腐蚀且呈现连通的趋势;③根据不同酸化时长页岩孔隙统计特征可见酸化分为两阶段:酸化初期为产生新生小孔阶段,酸化后期孔隙增大阶段;④酸化使页岩原始致密的单一孔隙结构转变为裂隙和孔隙并存的双重介质结构。孔隙度随酸化时间的增加呈先提高后降低的趋势,现场生产中应综合考虑储层特性和酸岩反应,设计最优作用时间。

With the increasing shortage of energy sources,extraction of shale gas is getting particularly urgent.Shale is tight and low-permeability,thus,stimulation treatment is necessarily needed to increase the reservoir permeability for commercialized production of the shale gas.Acidification pretreatment can effectively improve the shale pore structure and enhance the connectivity among shale pores,thus increases the shale permeability and gas production.In this experiment,the acidity treatment to the Niutitang formation shale samples are carried out under the mixture of hydrofluoric acid and hydrochloric acid,the mass fractions of which are 3% and 10% respectively.The evolution law of beddings,fractures,and pore structure on acidized shale samples for different durations are observed by an optical microscope.The trends of pores diameter,pore quantity,pore frequency and porosity of shale changing with time are summarized,and the influence of acidification on the shale gas flow is analyzed.Experimental results show that the brittle minerals in the shale of the Niutitang formation develop along bedding,and feldspar and quartz are strongly dissolved during the process of acidization,which can form fractures along the bedding;the exposed area of quartz and feldspar increase during the initial stage of acidization and decrease during the later stage of acidization,and at the later period of acidification,the pores between brittle minerals and other surrounding minerals enlarged obviously and are eroded deeply,which presentes a connecting tendency;according to the statistical characteristics of pores in the acidized shale,the acidification can be divided into two stages,new pores generation stage and pores enlargement stage;acidification transforms the structure of the shale samples from single-pore structure to a dual-medium structure made up with fissures and pores.While acidizing,the porosity of the shale samples increases at first and then decreases.The characteristics of the reservoir and the acid-rock reaction should be comprehensively considered in the production of shale gas to design an optimal operating time.