油气井富含CO_(2)酸性环境下Ca^(2+)对可降解铝镁合金的影响

Effects of Ca^(2+)on degradable aluminum-magnesium alloys in CO_(2)-rich acidic environment of wells

以铝镁合金为代表的可降解金属材料具有质量轻、强度高、施工完毕后易返排等特点,在油气田开发领域已用于制造射孔枪、封隔器、桥塞以及压裂滑套等井下工具。但在使用过程中存在降解后产生化学反应、残渣堵塞油气通道等问题。为此,对这种现象开展机理研究,实验模拟要点:①根据市场应用情况,优选了基础成分为Mg—8Al—0.7Zn—0.25Mn—1.0Ce的可降解铝镁合金作为实验试样材料。②基于华北油田冀中地区某井试油作业所测得的地层产液成分数据,配置与其成分相近的含CaCl_(2)等盐类的1#复配溶液,同时为了研究Ca^(2+)对于铝镁合金试样的影响,配置了不含CaCl_(2)的2#复配溶液。③为了模拟油气井富含CO_(2)酸性环境,往实验的水溶液中持续注入CO_(2)并保持20×10^(5) Pa左右的分压,使呈弱酸性。④为模拟近地层环境,实验温度为150℃、实验设定的CO_(2)分压为20×10^(5) Pa(25℃),在高温高压反应釜中分别装满经过充分除氧处理之后的1#、2#复配溶液,将可降解铝镁合金试样以挂片方式放入溶液,在不同的浸入时间后取出,观测试样的质量变化,采用扫描电镜(SEM,Scanning Electron Microscope)对试样表面附着物的形貌及其成分进行分析,从而研究Ca^(2+)对可降解铝镁合金的影响。研究结果表明:①在高温、弱酸环境、矿化度较高的油气井,使用可降解铝镁合金制作的井下工具可能会形成以CaCO_(3)为主、包含MgCO_(3)、Mg(OH)_(2)等的结晶残渣,堵塞油气通道。②对于150℃、富含CO_(2)、弱酸性的近似地层环境,往井筒中添加含Ca^(2+)的盐(如CaCl_(2)),欲采用这种方式来加速铝镁合金制成的井下工具的降解,反而不利于井下工具的有效降解,可能造成井筒的不洁或者堵塞。

Degradable metal materials represented by aluminum-magnesium alloys show characteristics of light weight,high strength,and easy flowback after utilization.They have been used to manufacture downhole tools such as perforating guns,packers,bridge plugs,and fracturing sleeves for oil and gas development.However,there exists some problems including chemical reaction after degradation and residues blocking oil and gas channels.To solve the problems,the mechanism was investigated by experiment simulation.Firstly,according to the market application,the degradable aluminum-magnesium alloy with Mg-8Al-0.7Zn-0.25Mn-1.0Ce as the basic composition was selected as sample for the experiment.Secondly,based on the measured composition of the formation fluid produced from a well in Jizhong area of Huabei Oilfield,a 1#compound solution containing CaCl_(2) with the similar composition was prepared.In addition,a 2#compound solution without CaCl_(2) was prepared to clarify the effect of Ca^(2+)on the sample.Thirdly,to simulate the CO_(2)-rich acid environment in wells,CO_(2)was continuously injected into the experimental aqueous solution to make it weakly acidic,with partial pressure of about 20 bar.Lastly,to simulate the formation environment,the experimental temperature was set to 150ºC,and the CO_(2)partial pressure was set to 20 bar(25ºC).The 1#and 2#solutions were respectively injected into the HT/HP reaction kettle after being fully deoxidized,and then the degradable aluminum-magnesium alloy samples were put into the solutions.After soaking for different times,the samples were taken out to observe the quality changes.The morphology and composition of the attachments on the surface of the samples were analyzed by using Scanning Electron Microscopy(SEM),so as to identify the effect of Ca^(2+)on the alloy.Results show that(1)in wells with high temperature,weak acid environment and high salinity,downhole tools made of degradable aluminum-magnesium alloys may form crystalline residues mainly composed of CaCO_(3),as well as MgCO3,Mg(OH)2,etc.,which can block the oil and gas channels;and(2)for the approximate formation environment(150ºC,CO_(2)-rich and weak acid),the injection of Ca^(2+)-containing salts(e.g.CaCl_(2))into wellbore to accelerate the degradation of downhole tools made of aluminum-magnesium alloys is not helpful to the effective degradation,and may cause uncleanness or blockage.