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页岩气开采过程中,目前最重要的增产方法是水平井大规模水力压裂技术[1]。向储层注入含有酸、破胶剂、阻垢剂等添加剂的压裂液后,压裂液与页岩会发生多种化学反应[2]。国内外学者已经对给定条件下的压裂液-页岩作用开展了诸多研究,但不同地区的页岩组分、储层环境和压裂液成分并不相同,导致研究结果存在差异,主要体现在页岩结构改变形式、返排液组分演化规律等方面[3-4]。水力压裂结束后产生的返排液中含有各种有机物、放射性元素、有害元素等[5-6],现有技术很难经济合理的处置[7],如果发生泄漏,将给地下水环境带来严重的污染风险。
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。水力压裂过程中,向储层注入压裂液前通常采用浓盐酸预处理井筒[8],pH的变化可能影响压裂液-页岩作用。储层埋深不同时,环境温度也随之改变,从而影响页岩中矿物的氧化还原速率、溶解速率和溶解度。地层水和回用返排液的存在可以改变压裂液离子强度[9],影响反应中各元素的溶出。同时,压裂液-页岩作用还受压裂液与页岩接触面积的影响。此外,为提高水力压裂效率,压裂液中含有多种添加剂,这可能导致压裂液化学性质的变化,对压裂液-页岩作用产生影响。中国页岩气开发仍处于初始阶段,已经开展的研究中考虑的仅是部分环境因素和压裂液成分的影响,与实际水力压裂过程中的压裂液-页岩作用有所差别,因此研究还不充分。
为揭示储层环境和压裂液成分对压裂液-页岩作用的影响,本文选择四川某页岩气产区的龙马溪组页岩,通过室内实验研究了不同环境因素(初始pH、温度等)和压裂液成分(过硫酸铵、柠檬酸等)影响条件下,页岩组分在液-岩作用过程中的释放特征。以期为探究储层环境中的压裂液-页岩作用机理提供一定依据,对预测返排液组分和防治地下水污染具有重要意义。
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
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摘要: 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。Abstract: 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.
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Key words:
- shale /
- hydraulic fracturing /
- environmental factor /
- additive /
- fluid-rock interaction
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表 1 龙马溪组页岩主要矿物组分
Table 1. Main minerals of the Longmaxi Formation shale
组分Components 分子式Molecular formula 含量/%wt Content 石英 SiO2 28 钾长石 KAlSi3O8 3.5 钠长石 NaAlSi3O8 7 方解石 CaCO3 17 白云石 CaMg(CO3)2 5 黄铁矿 FeS2 2 伊利石 K0.6Mg0.25Al2.3Si3.5O10(OH)2 19.5 绿泥石 Mg5Al2Si3O10(OH)8 10.3 伊/蒙混层 混层比:10 7.7 表 2 实验中不同环境变量的设置
Table 2. Designed environmental variables in experiments
变量
Variables编号
No.初始pH
Initial pH温度/℃
Temperature离子强度/(mol·L−1)
Ionic strength固液比/(g·L−1)
Solid-liquid ratio初始pH #1—#4 2.0、4.0、7.0和9.0 25±1 超纯水 5 温度 #5—#7 6.2 25±1、50±1和70±1 超纯水 5 离子强度 #8—#10 6.2 25±1 0.09、0.17和0.42 5 固液比 #11—#13 6.2 25±1 超纯水 1、5和10 表 3 实验中的模拟压裂液成分配制
Table 3. Simulated fracturing fluid compositions in experiments
编号No. 模拟压裂液成分Simulated fracturing fluid compositions A(对照组) 超纯水 B 过硫酸铵(0.006%wt)+超纯水 C 柠檬酸(0.004%wt)+超纯水 D 过硫酸铵(0.006%wt)+柠檬酸(0.004%wt)+乙二醇(0.002%wt)+碳酸钾(0.01%wt)+超纯水 -
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