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作者简介:

秦国伟(1978—),男,山东沂水人,副教授,博士,从事油田化学与油气田开发工程工作。E-mail:qgw126@126.com。

中图分类号:TE377

文献标识码:A

文章编号:1009-9603(2022)05-0126-07

DOI:10.13673/j.cnki.cn37-1359/te.202107006

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目录contents

    摘要

    常规排水采气和控水采气技术难以解决致密气藏的低孔、低渗透和小孔喉等问题,为此采用转相技术制备适合致密气藏的纳米排水剂来改变岩石表面性质,提高致密气藏的排水效果。基于自制纳米排水剂和致密岩心,利用现代测试技术,探究纳米排水剂对岩心薄片及其粉末的作用机制,评价其对岩心润湿性及微观形貌的影响,并探讨纳米排水剂的排水采气作用机理。结果表明:纳米排水剂与岩心表面并未发生化学作用,仅通过静电和氢键等作用吸附于岩心表面形成疏水膜,从而降低岩心表面粗糙度,减小岩心孔隙中渗流阻力,达到排水采气的目的。同时纳米排水剂可使储层润湿性发生反转,即毛管压力变为阻力导致排出的地层水很难再次渗流至储层中,延长了排水采气有效期。

    Abstract

    The conventional drainage gas recovery and water-controlled gas recovery techniques are difficult to solve the problems of low porosity,low permeability,and small pore throats in tight gas reservoirs. For this reason,the phase inver- sion technology is utilized to prepare nano-drainage agents suitable for tight gas reservoirs to change the rock surface char- acteristics and improve the drainage effects of tight gas reservoirs. On the basis of self-prepared nano-drainage agents and tight cores,this paper explored the action mechanism of nano-drainage agents to core slices and their powder by modern testing technology and evaluated the effects on core wettability and microscopic morphology. The drainage gas recovery ac- tion mechanism of nano-drainage agents was then discussed. The results show that instead of reacting chemically with the core surface,the nano-drainage agent is only adsorbed on the core surface by electrostatic and hydrogen bonding effects to form a hydrophobic film,which can reduce the core surface roughness and the flow resistance in the pores to achieve drain- age gas recovery. At the same time,the nano-drainage agent can reverse the core wettability in the reservoir,namely that the capillary pressure becomes a resistance to prevent the discharged formation water from flowing into the reservoir again, extending the validity period of drainage gas recovery.

  • 天然气是一种优质清洁能源,在“碳达峰、碳中和”政策下已引起关注和重视。中国低渗透致密气藏中的天然气资源丰富,高效开发该类气藏对石油工业的持续稳定发展具有十分重要的意义[1-2];但因其储层具有低孔、低渗透和小孔喉等特性,一般情况下无自然产能或产能较低,需经压裂后方能投产[2-8]。同时,在天然气开采过程中水体的弹性能量会使气藏中的地层水沿高渗透带窜流至储层,由于储层的亲水性以及毛管压力作用导致孔隙中部分气体被水封堵形成死气区,且窜流至储层的地层水会导致气井出水,从而降低了气井产量,影响了气藏采收率[9-14]。针对气井出水问题,目前主要采用排水采气和气井控水处理方法[12-18]。其中排水采气处理方法主要排除气井积液以恢复正常生产,但其较高的工艺成本及现场应用条件限制,并不利于气藏开发[17];气井控水处理方法主要采用高黏度和高强度的堵水体系封堵储层中的地层水,难以满足致密气藏储层的需求[18]

  • 纳米材料因具有能耗小、无污染、黏度低、粒径小等优势得到广泛应用[19-25],但主要集中于制备方法、稳定性、传热特性、强化传热机理等方面研究[19],在油气开采方面也主要涉及纳米微球调剖堵水、聚硅材料降压增注、纳米乳液改善水驱、纳米膜驱油以及纳米阻聚堵水等[20-22]。虽然纳米材料具有改变岩石孔隙结构及其表面润湿性的特性[23-26],但其对岩石表面作用机理以及如何影响岩石表面性质等方面研究较少。为此,笔者采用转相技术制备纳米排水剂,利用致密岩心结合现代测试技术,探究纳米排水剂对致密岩心薄片及岩心粉末的作用机制,分析其对岩心薄片表面性质的影响规律,并探讨纳米排水剂的排水采气机理,从而为纳米材料在致密气藏中的应用奠定了理论基础。

  • 1 实验部分

  • 1.1 实验器材

  • 实验材料主要包括改性硅油(化学纯,无锡市品创化工科技有限公司),乳化剂(化学纯,广州凯晟化工科技有限公司),低分子醇类、低分子有机酸类(分析纯,天津市富宇精细化工有限公司),氯化钠、氯化钙、氯化镁(分析纯,天津市盛奥化学试剂有限公司),某油田致密岩心等。

  • 实验仪器主要包括 BSA423S型电子天平(北京赛多利斯科学仪器有限公司)、JB200-D型电动搅拌机(上海沪析实验有限公司)、JY92-ⅡN型超声波细胞粉碎机(宁波新芝生物科技股份有限公司)、FE20 型台式 pH计(上海梅特勒-托利多仪器有限公司)、 UV2600i 型紫外分光光度计(北京京科瑞达科技有限公司)、DZF 型真空干燥箱(北京科伟永兴仪器有限公司)、JC2000C 型接触角测量仪(上海中晨数字技术设备有限公司)、BSA423S 型扫描电镜(北京赛多利斯科学仪器有限公司)、250XⅠ型 X 射线电子能谱仪(Thermo 公司)、ZetaPALS 型 Omni 多角度粒度及高灵敏度 Zeta电位分析仪(美国布鲁克海文仪器公司)、5700型傅立叶红外光谱仪(美国尼高力仪器公司)等。

  • 1.2 实验方法

  • 1.2.1 纳米排水剂制备

  • 根据纳米乳状液的主要组成,采用转相技术制备纳米排水剂,结合单因素和多因素优化法明确其最佳配方,实验的具体步骤包括:①室温下,按照油相与水相的质量比为 12.5∶87.5 称取适量改性硅油和蒸馏水。②将改性硅油加入乳化器中,以 500 r/ min 的转速搅拌,再依次加入 61% 改性硅油质量的乳化剂和27%改性硅油质量的低分子醇类助乳剂,继续搅拌 25 min,使之充分混合均匀。③将 10% 的蒸馏水加入乳化器中,以 500 r/min 的转速搅拌 5 min后,再加入低分子有机酸类,调节乳化器中溶液 pH 值至 6.4,同速下持续搅拌 20 min。④最后加入 6%改性硅油质量的氯化钠和剩余的蒸馏水,以500 r/min 的转速搅拌 30 min,并将乳化器静置 60 min,消泡后即可得到纳米排水剂。

  • 1.2.2 纳米排水剂对致密岩心作用测试实验

  • 利用 X 射线光电子能谱仪(XPS)、Omni 多角度粒度及高灵敏度 Zeta电位分析仪、傅立叶红外光谱仪(FT-IR)、接触角测量仪和扫描电镜(SEM)等测试技术探讨纳米排水剂对致密岩心作用机制。将1 块致密岩心(孔隙度为 9.60%,渗透率为 0.095 mD) 洗油干燥后,按实验要求一部分切割为若干岩心薄片,另一部分研磨成岩心粉末备用;同时用蒸馏水配制不同质量分数的纳米排水剂,再进行相关测试实验分析。

  • XPS测试 将1块岩心薄片不做任何处理,另1 块浸泡于质量分数为100%的纳米排水剂7 d后,再置于 60℃真空干燥箱内烘干,对 2块岩心薄片分别进行XPS测试。

  • Zeta 电位测试 将每组质量为 0.5 g 的岩心粉末分别浸泡于质量分数为 0,25%,50%,100% 的 4 种纳米排水剂中(每种质量为 4.0 g),7 d 后置于 60℃真空干燥箱内烘干;再向烘干的 4组岩心粉末中分别加入 4.0 g 蒸馏水,利用超声波细胞粉碎机 (频率为 40 kHz)作用 45 min 并静置 24 h;最后取溶液上部澄清部分进行Zeta电位测试。

  • FT-IR 测试 将每组质量为 1.0 g 的岩心粉末分别浸泡于质量分数为 0,100% 的 2 种纳米排水剂中(每种质量为 8.0 g),7 d 后置于 60℃真空干燥箱内烘干;并将烘干的 2 组岩心粉末分别与溴化钾按质量比为 1∶100 混合,研磨均匀后压成圆薄片。利用傅立叶红外光谱仪获取波长为 400~4 000 cm-1 内 2组岩心粉末的傅立叶红外光谱。

  • 接触角和SEM测试 按实验要求取8块岩心薄片均分为 4 组,将每组岩心薄片分别浸泡于质量分数为 0,25%,50%,100% 的纳米排水剂中,7 d 后置于60℃真空干燥箱内烘干;再将烘干后的岩心薄片进行接触角和喷金处理后SEM测试,其中接触角测试中利用表面涂有白凡士林载玻片与岩心薄片进行对比。

  • 2 实验结果与讨论

  • 2.1 纳米排水剂表征

  • 将制备的纳米排水剂采用涂膜法进行傅立叶红外光谱表征。从图1可知,波长3 423 cm-1 处的宽峰是由纳米排水剂中的水引起;波长 2 970 和 1 650 cm-1 处分别为长链烷基和仲氨基(—NH2)的特征吸收峰;波长1 460 cm-1 处为饱和碳氢键(C—H);波长 1 260 cm-1 处为硅甲基键(Si—CH3)中的碳氢键(C— H)伸缩振动;波长 1 090 cm-1 处为硅氧键(Si—O)和醚键(—CH2—O—CH2—)伸缩振动。分析认为纳米排水剂中引入一些亲水基团,既增加了水中纳米排水剂的分散性,又提高了稳定性;同时也有利于纳米排水剂在储层岩石表面铺展形成一层疏水膜。

  • 图1 纳米排水剂傅立叶红外光谱图

  • Fig.1 FTIR spectrum for nano-drainage agents

  • 由纳米排水剂粒径分布(图2)可知,其粒径中值为 5.37 nm,说明纳米排水剂的粒径小且分布均匀,与致密气藏孔喉大小匹配性较好。经测试纳米排水剂的 Zeta 电位为 47.93 mV,表明其稳定性强且带正电。

  • 图2 纳米排水剂粒径分布

  • Fig.2 Particle size distribution of nano-drainage agents

  • 2.2 岩心薄片表面元素特征

  • 利用 XPS 分析纳米排水剂处理前后岩心薄片表面元素变化。由图3可知纳米排水剂处理前岩心薄片表面的主要元素为 Si,O 和 C;处理后岩心薄片表面的主要元素为 Si,O,C和 N,其中 C元素较处理前的强度有所增强,主要是因为岩心薄片表面存有一层纳米排水剂。图3a 中 C1s 结合能为 284.5 eV,以外来污染碳的 C1s 结合能 284.8 eV 为基准,经荷电校正后得到纳米排水剂处理前后岩心薄片表面不同元素的高分辨率图(图4)。

  • 图3 纳米排水剂处理前后岩心薄片表面XPS全扫描谱图

  • Fig.3 Wide-scan XPS spectra for core slice surface before and after nano-drainage agent treatment

  • 由图4a和图4b可知纳米排水剂处理前岩心薄片表面 Si2p 和 O1s 结合能分别为 102.9 和 532.25 eV,对应二氧化硅中的 Si 和 O 元素,表明纳米排水剂处理前岩心薄片表面主要成分为二氧化硅。纳米排水剂处理后岩心薄片表面Si2p和O1s结合能分别为102.3和532.4 eV,对应改性硅油中的Si—O键,且 Si和 O元素只有单峰,说明其表面只存在改性硅油中的 Si—O 键,分析认为岩心薄片表面吸附了纳米排水剂的主要成分并形成一层致密膜。

  • 由图4c 可以看出纳米排水剂处理后岩心薄片表面 C 元素具有双峰,C1s 结合能为 284.8 和 286.5 eV,分别对应改性硅油中的硅甲基键(Si—CH3)和醚键(—CH2—O—CH2—),表明纳米排水剂中引入亲水基团增强了其分散性。

  • 由图4d 可知纳米排水剂处理后岩心薄片表面同时存在伯氨基(—NH—)和仲氨基(—NH2),对应 N1s结合能为399.7 eV;而质子化的—NH2 + 和—NH3 + 对应 N1s 结合能为 401.9 eV,分析认为在配制纳米排水剂过程中改性硅油的仲氨基与低分子有机酸类发生酸碱反应产生结合能[26]。通过对比纳米排水剂处理前后岩心薄片表面的元素特征,可以看出各元素结合能并未发生明显位移,故推断纳米排水剂与岩心薄片表面并未发生化学作用。

  • 2.3 岩心粉末电性特征

  • 由图5 可知,纳米排水剂处理前岩心粉末带负电(Zeta 电位为-10.7 mV),但经处理后电性发生改变,即随着纳米排水剂质量分数的增大,岩心粉末负电性逐步转为正电性。当纳米排水剂质量分数为40%(岩心粉末与纳米排水剂质量比为5∶16)时, Zeta电位为 0,说明质量分数高于 40%时,岩心粉末表面负电吸附位完全被纳米排水剂占满并呈正电性,证实了纳米排水剂通过静电作用吸附于岩心表面,从而影响岩心表面微观形貌以及表面性质。

  • 2.4 岩心粉末官能团变化特征

  • 由图6可知,纳米排水剂处理前波长469和766 cm-1 处为 Si—O 键对称伸缩振动,但经纳米排水剂处理后波长 469 cm-1 处的波峰向右移至 463 cm-1 处且吸收强度增强,说明纳米排水剂与岩心粉末表面间产生氢键作用。同时,处理后波长 798 cm-1 处为 Si—O 键对称伸缩振动,波长 1 097 cm-1 处出现的新吸收峰为 Si—O键伸缩振动,波长 2 970 cm-1 处为纳米排水剂中长链烷基特征吸收峰,进一步证实岩心粉末表面存有纳米排水剂。

  • 2.5 岩心薄片表面性质变化

  • 纳米排水剂对岩心薄片表面作用表明其与岩心未发生化学作用,但通过静电、氢键等作用吸附于岩心薄片表面,从而影响其表面性质。

  • 图4 纳米排水剂处理前后岩心薄片表面主要元素高分辨率图

  • Fig.4 High-resolution images for main elements of core slice surface before and after nano-drainage agent treatment

  • 图5 不同质量分数纳米排水剂作用下岩心粉末Zeta电位

  • Fig.5 Zeta potential for core powder under different mass fractions of nano-drainage agents

  • 图6 纳米排水剂作用前后岩心粉末傅立叶红外光谱图

  • Fig.6 FTIR spectra for core powder before and after nano-drainage agent treatment

  • 2.5.1 润湿性反转

  • 采用标准盐水配制不同质量分数的纳米排水剂,再测试不同质量分数纳米排水剂下涂有凡士林载玻片和岩心薄片表面的接触角。由图7和图8可知,纳米排水剂既降低了涂有凡士林载玻片亲油表面的接触角,又增加了岩心薄片亲水表面的接触角,且随着质量分数的增加,其变化程度降低。纳米排水剂质量分数分别为0,25%,50%,100%时,亲油表面接触角分别为88.5°,76.5°,57.5°,55.5°;亲水表面接触角分别为 18.0°,88.5°,90.1°,92.7°。由此可见,纳米排水剂可以使储层表面润湿性发生反转,毛管压力由动力变为阻力导致排出的地层水很难再次渗吸至储层中,从而提高了排水采气的有效期。分析认为纳米排水剂中有亲水和疏水等不同基团,当储层表面具有亲水特征时,纳米排水剂中的亲水基团吸附于其表面,疏水基团则指向孔隙内形成疏水膜,导致储层表面润湿性反转。

  • 图7 不同质量分数纳米排水剂作用下涂有凡士林载玻片接触角

  • Fig.7 Contact angles for glass slides with vaseline under different mass fractions of nano-drainage agents

  • 图8 不同质量分数纳米排水剂作用下岩心薄片接触角

  • Fig.8 Contact angles for core slices under different mass fractions of nano-drainage agents

  • 2.5.2 微观形貌特征

  • 由图9 可知,经质量分数为 0 的纳米排水剂处理的岩心薄片表面存在许多细小的孔隙和岩屑,且岩屑边缘比较尖锐;但随着纳米排水剂质量分数的增加,岩心薄片表面细小孔隙逐渐减少,且岩屑边缘变得光滑,岩心薄片表面整体更加平整光滑。分析认为纳米排水剂吸附于岩心薄片表面并形成一层疏水膜,由此改变岩心薄片表面微观结构即降低岩石表面粗糙度,增加其光滑程度,减小孔隙中流动阻力,从而达到排水采气的目的。

  • 图9 不同质量分数纳米排水剂作用下岩心薄片电镜扫描照片

  • Fig.9 SEM for core slices under different mass fractions of nano-drainage agents

  • 2.6 排水采气机理探讨

  • 在天然气开采过程中,由于储层岩石亲水性和多孔介质的毛管压力作用,气藏中的地层水沿高渗透带窜流至储层中并发生水锁效应,导致储层中气体未能及时有效排出,降低了气藏采收率。纳米排水剂与储层岩石表面之间产生分离压,使岩石表面的束缚水分离变为自由水,从而更易流动。同时,纳米排水剂与致密岩心作用后,宏观上岩心表现为润湿性发生反转,减缓了排出的地层水再次渗吸至储层中,增加了水相再次渗吸阻力,延长了排水采气有效期;微观上纳米排水剂在岩心表面形成一层疏水膜,改变其微观结构,降低水相排出阻力,提高了排水效果。但纳米排水剂与岩心表面未发生化学作用,仅通过静电作用改变岩心表面带电性,同时Si—O键的特征峰证实纳米排水剂与岩心表面存在氢键作用。综上所述,纳米排水剂主要通过静电、氢键等作用吸附于岩心表面,亲水基团指向岩心表面,疏水基团向外形成疏水膜,使得岩心表面变得更均匀、光滑且具有疏水性。

  • 3 结论

  • 采用转相技术合成了一种用于致密气藏排水采气的纳米排水剂,其具有粒径中值小、粒径分布均匀和稳定性强等特点。纳米排水剂与岩心表面未发生化学作用,但通过静电、氢键等作用吸附于岩心表面并影响岩心表面性质。其吸附于岩心表面形成一层疏水膜,使润湿性发生反转,亲油表面接触角降低了 33.0°,亲水表面接触角增加了 74.7°。润湿性反转使毛管压力成为水相渗吸阻力,延长了排水采气的有效期。纳米排水剂与岩石表面之间产生的分离压降低了束缚水饱和度;同时在岩石表面形成疏水膜,从而降低了岩石表面粗糙度,减少了流体的流动阻力,提高了致密气藏的排水效果。

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