龙马溪组页岩组分在液-岩作用过程中的释放特征

Releasing characteristics of the Longmaxi Formation shale components during fluid-rock interactions

压裂液-页岩作用存在于整个水力压裂过程中,受页岩组分、储层环境和压裂液成分的共同影响,导致返排液组分十分复杂.探究储层环境中的压裂液-页岩作用机理,对预测返排液组分和防治地下水污染具有重要意义.本文通过室内实验研究了环境因素和压裂液成分对龙马溪组页岩组分在液-岩作用过程中释放特征的影响.结果表明,酸性环境有利于碳酸盐矿物溶解,溶液pH主要受碳酸盐溶解控制,最后保持在6.8-9.3;温度升高(从25℃升高到70℃)有利于黄铁矿氧化,增加碳酸盐矿物溶解量;高离子强度环境有利于金属元素溶出,促进难溶化合物和碳酸盐矿物的溶解;高固液比环境有利于碳酸盐矿物溶解和黄铁矿氧化,促进微量元素溶出.溶液初始pH和离子强度对元素溶出的影响显著,温度和固液比的影响较小.压裂液中的过硫酸铵和柠檬酸均有利于黄铁矿氧化,产生的H+能够促进碳酸盐矿物溶解.过硫酸铵在反应初期迅速消耗,而柠檬酸始终在促进方解石溶解.添加剂主要通过影响黄铁矿氧化和碳酸盐矿物溶解控制金属元素溶出量,柠檬酸对元素溶出的影响程度较高.同时,添加剂会增加离子强度,促进金属元素溶出.

Fracturing fluid-shale interactions are involved in the whole process of hydraulic fracturing and affected by shale compositions, reservoir environment and fracturing fluid compositions, resulting in complex components of reverse drainage. Exploring the mechanisms of fracturing fluid-shale interactions in reservoir environment are of great significance for predicting components of reverse drainage and preventing groundwater pollution. In this study, the influence of environmental factors and fracturing fluid compositions on releasing characteristics of the Longmaxi Formation shale components during fluid-rock interactions were studied through laboratory experiments. The results showed that the acidic environment was beneficial to the dissolution of carbonate minerals. The solution pH was mainly controlled by the dissolution of carbonates and finally maintained in the range of 6.8-9.3. The increase of temperature(from 25 ℃ to 70 ℃)enhanced the oxidation of pyrite and increased the dissolved amount of carbonate minerals. The high ionic strength environment was beneficial to the dissolution of metal elements and promoted the dissolution of insoluble compounds and carbonate minerals. The high solid-liquid ratio environment facilitated the dissolution of carbonate minerals and the oxidation of pyrite and enhanced the dissolution of trace elements. The initial pH and the ionic strength of the solution had significant effects on the dissolution of elements, while the temperature and the solid-liquid ratio had a weak effect. Both ammonium persulfate and citric acid in the fracturing fluid were beneficial to the oxidation of pyrite and the H+ produced from the oxidation could intensify the dissolution of carbonate minerals. Ammonium persulfate was consumed rapidly in the initial stage of the reactions while citric acid promoted the dissolution of calcite. Additives controlled the dissolved amount of metal elements mainly by affecting the oxidation of pyrite and the dissolution of carbonate minerals.Citric acid had a higher degree of influence on the dissolution of elements. At the same time,additives increased the ionic strength and consequently enhanced the dissolution of metal elements.