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作者简介:

李晓骁(1994—),男,河北沧州人,在读博士研究生,从事油田化学及提高采收率方面的研究。E-mail:544382061@qq.com。

通讯作者:

岳湘安(1957—),男,北京人,教授,博导。E-mail:yuexa@139.com。

中图分类号:TE357.45

文献标识码:A

文章编号:1009-9603(2021)06-0094-07

DOI:10.13673/j.cnki.cn37-1359/te.2021.06.012

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目录contents

    摘要

    针对稠油油藏储层特征与开发现状,基于静态泡沫性能与驱油性能评价,复配了3种具备不同性能的表面活性剂,并研究了不同表面活性剂辅助CO2驱油效果,明确了表面活性剂的主控性能。实验结果表明,表面活性剂S1, S2和S3辅助CO2驱油对渗透率级差为3.0的非均质岩心水驱后采收率增幅分别为19.7%,13.2%和15.2%,优于直接注入 CO2的驱油效果,说明表面活性剂可以提高稠油油藏高含水阶段 CO2驱油效果。其增油机理主要为表面活性剂驱油贡献及其与后续CO2产生的泡沫的驱油贡献。表面活性剂乳化能力越强,乳化现象越明显,驱油效率增幅越大;静态发泡能力越强,与CO2在岩心中越容易产生泡沫;强发泡能力-弱稳定性表面活性剂与CO2产生的泡沫更容易实现深部调驱。随着非均质岩心渗透率级差从 3.0增至 9.0,3种表面活性剂辅助 CO2驱油效果均不同程度地下降,其中S1辅助CO2驱油的采收率增幅降至12.6%,相比于S2和S3,S1对岩心非均质性变化的适应性较强。因此,对于目标油藏,具备强乳化、强发泡能力且兼顾弱泡沫稳定性的复配表面活性剂S1辅助CO2驱油效果最佳,对非均质性变化的适应性更强。

    Abstract

    In light of the characteristics and development status of heavy oil reservoirs,three surfactants with different prop- erties were compounded based on the evaluation of static foam properties and oil displacement performance. The oil dis- placement efficiency of CO2 flooding assisted by different surfactants was studied,and the dominating performance of the surfactants was clarified. The experimental results show that the recovery factor of the heterogeneous core with a permeabili- ty ratio of 3.0 grows by 19.7%,13.2%,and 15.2%,respectively after CO2 flooding assisted by surfactants S1,S2,and S3, which is superior to the oil displacement by direct CO2 injection. This demonstrates that the surfactants can improve the CO2 flooding of heavy oil reservoirs in the high water-cut stage. The mechanism of enhanced oil recovery(EOR)is mainly manifested as the oil displacement contributed by surfactants and the foam induced by the surfactants with subsequent CO2. When the emulsifying capability of a surfactant is stronger,the emulsification will be more evident,and the increase in oil displacement efficiency will be larger;stronger static foaming ability makes it easier to foam with CO2 in the core;the foam formed by CO2 with the surfactant of strong foaming ability but weak stability is more favorable for deep profile control. As the permeability ratio of the heterogeneous core increases from 3.0 to 9.0,the oil displacement efficiency of CO2 flooding as- sisted by the three surfactants declines to varying degrees. The increase in the recovery factor of S1-assisted CO2 flooding reduces to 12.6%,and compared with S2 and S3,S1 is more adaptable to the heterogeneous changes in the core. Therefore, for the target reservoir,the compound surfactant S1 with strong emulsification,strong foaming ability,but weak stability per- forms best in assisting CO2 flooding for EOR.

  • 稠油油藏注水开发采收率普遍较低,水驱后转注 CO2驱油技术具有大幅度提高采收率的潜力[1-2]。室内研究结果表明,CO2在稠油中具有较好的溶解和萃取能力,具有降低原油黏度、膨胀增容、与原油多次接触混相降低界面张力等优点[3-6]。但是,多数矿场实践证实,长期注水开发后生产井采出液含水率上升,普遍进入高含水甚至特高含水阶段,转注 CO2后普遍过早突破发生窜流现象,造成增油有效期短,提高经济效益有限[7-10]

  • 目前,表面活性剂是辅助 CO2驱油提高采收率的常用助剂。一方面,表面活性剂与 CO2接触可以产生泡沫,具有封堵高渗透通道、扩大波及体积的功能;另一方面,表面活性剂在驱油过程中也具有降低界面张力、乳化分散原油和改善润湿性的功能[11-15]。但是,多数研究往往强调表面活性剂达到超低界面张力的能力和增强 CO2 泡沫的稳定性[16-18],对就地生成泡沫后的运移与封堵特征的研究较少。另外,由于储层孔喉结构的复杂性,并不能保证表面活性剂在油藏运移过程能够与CO2实现完全接触并生成泡沫,泡沫破灭或者未与 CO2接触的表面活性剂溶液,其驱油过程对采收率贡献程度与表面活性剂的主控性能也往往重视不足。因此,根据苏北盆地洲城油田某区块的储层物性和原油特征,通过对表面活性剂的油水界面张力和乳化能力以及与 CO2的发泡能力和泡沫的稳定性评价,优选出具有明显性能差异的表面活性剂作为CO2驱油助剂;基于物理模拟实验,对表面活性剂辅助驱油的主控性能、驱油性能与辅助调驱机理进行研究,并分析了表面活性剂辅助CO2驱油对非均质性变化的适应性,以期为矿场应用提供理论依据。

  • 1 实验器材与方法

  • 1.1 实验器材

  • 实验仪器主要包括:Haake RheoStress6000流变仪、JJ200B 界面张力仪、蔡司 Axiovert200 高倍光学显微镜、Waring-Blender 搅拌器、THE-Ⅲ高温乳化动态评价仪和ISCO泵等驱替设备。

  • 研究区原油经脱水脱气处理后,原油密度为 0.904 g/cm3,地面原油黏度为 253.2 mPa·s,胶质、沥青质和蜡的质量分数分别为 23.10%,1.77% 和 11.74%,具有中高黏度、高密度和高胶质的特征。实验用油为由研究区地面脱气原油与煤油复配的模拟油,油藏温度(75℃)下黏度约为32 mPa·s。

  • 实验所用表面活性剂主要包括阴离子型(SDS, SDBS,AOS 和 AES)、两性离子型(LAB,RA 和 BS-12)和非离子型(Tween-80 和 AEO),均为由北京现代东方精细化学品有限公司生产的分析纯。

  • 实验用水为根据研究区储层地层水配制的模拟地层水,其矿化度为7 443 mg/L,水型为Na2SO4型。

  • 实验所用物理模型采用无机胶结,根据研究区储层的物性研发[19]。所用岩心包括柱状均质岩心和两层非均质岩心。其中,均质岩心的岩心长度为 30 cm,半径约为2.5 cm,渗透率为1 800 mD,孔隙度约为 28%;非均质岩心的尺寸为 30 cm×4.5 cm×4.5 cm,高渗透层和低渗透层厚度相近,高渗透层渗透率为 1 800 mD,低渗透层渗透率分别为 200,300, 400和600 mD,整体孔隙度为22%~26%。

  • 1.2 实验方法

  • 表面活性剂驱油性能评价  表面活性剂驱油性能评价主要是评价油水界面张力和乳化能力。利用JJ200B界面张力仪在75℃下稳定20 min,测定界面张力,所测界面张力为动态界面张力,取其平衡后稳定值作为该体系的界面张力。表面活性剂乳化能力通过自主研发的 THE-Ⅲ高温乳化动态评价仪进行测试,采用乳化系数来表征表面活性剂的乳化能力[20-22]

  • 泡沫静态性能评价  采用Waring-Blender搅拌法评价表面活性剂的发泡能力和泡沫稳定性。将 100 mL 表面活性剂溶液加入到 Waring-Blender 搅拌器中,以 3 500 r/min 的转速搅拌 1 min,将全部泡沫与液体迅速倒入 1 000 mL量筒中,记录发泡高度并计算泡沫半衰期,来表征不同表面活性剂的发泡能力与泡沫稳定性。

  • 泡沫封堵性能评价  为评价表面活性剂与CO2 在油藏中接触后对封堵能力与运移产生的影响,采用多测点岩心装置对CO2与表面活性剂生成的泡沫封堵性能进行测试。多测点岩心装置共设置5个压力点(分别距离注入端5,10,15,20和25 cm)。实验步骤主要包括:①按照图1连接实验仪器,将干燥好的岩心称重,抽真空12 h后,饱和模拟地层水8 h,再次称重计算孔隙度和孔隙体积;设定围压为 20 MPa,测定岩心的水相渗透率。②注入一定体积的表面活性剂段塞,后续注入 CO2直到压差稳定。③ 以 0.3 mL/min 的流速进行水驱直至压差达到稳定,记录各点压力,并计算阻力系数和残余阻力。

  • 驱油潜力评价按照图1 连接实验仪器,依据 SY/T5345—2007[23]对不同表面活性剂辅助 CO2的驱油潜力进行评价。实验步骤主要为:①将岩心模型干燥后称重,抽真空饱和模拟地层水 8 h,再次称重,计算孔隙度和孔隙体积。②设定围压为 20 MPa,地层温度下饱和油,达到束缚水状态后置于岩心夹持器中老化 12 h 以上,并计算原始含油饱和度。③对岩心模型进行水驱,以 0.3 mL/min 的流速驱替至含水率为 98%,记录采出油体积并计算水驱采收率。④水驱结束后,注入不同段塞表面活性剂。⑤岩心夹持器出口端设置回压阀压力为 10 MPa,注入CO2直至不出油为止。

  • 图1 表面活性剂辅助CO2驱油实验流程

  • Fig.1 Experimental process of surfactant-assisted CO2 flooding

  • 2 表面活性剂复配与性能评价

  • 2.1 表面活性剂的初选与优化

  • 9种常用低浓度表面活性剂静态发泡性能评价结果(图2)表明,当表面活性剂质量分数为 0.1% 时,SDBS,AOS,LAB,RA 和 AEO 的静态发泡能力较强,而 SDS,SDBS,AES,BS-12 和 Tween-80 的泡沫稳定性相对较好。

  • 图2 不同表面活性剂的静态发泡性能

  • Fig.2 Static foam properties of different surfactants

  • 单一表面活性剂往往难以达到最优效果,通常通过复配方式筛选适合油藏的表面活性剂[24]。按照初始发泡能力与稳定性,统一质量分数为0.5%对不同表面活性剂进行复配和筛选,得到发泡能力与稳定性具有明显差异的 3 种复配表面活性剂。其中,表面活性剂 S1 由质量分数为 0.4% 的 LAB 和 0.1% 的 Tween-80 组成,属于两性-非离子型,初始发泡体积为 832 mL,形成泡沫的半衰期为 289 s,为强发泡能力-弱泡沫稳定性;表面活性剂 S2 由质量分数为 0.4% 的 SDBS 和 0.1% 的 RA 复配,属于阴离子-两性型,初始发泡体积为763 mL,形成泡沫的半衰期425 s,为强发泡能力-强泡沫稳定性;表面活性剂 S3 由质量分数为 0.2% 的 SDS 和 0.3% 的 AES 组成,属于阴离子型,其初始发泡体积为447 mL,形成泡沫的半衰期为 402 s,为弱发泡能力-强泡沫稳定性。

  • 2.2 复配表面活性剂性能评价结果

  • 动态油水界面张力测试结果(图3a)表明,3 种复配表面活性剂的动态油水界面张力及其与原油的乳化能力具有明显差异。其中,S1的动态油水界面张力平衡后稳定值约为4.0×10-1 mN/m;S3的动态油水界面张力最低可达9.2×10-2 mN/m,平衡后稳定值约为 1.2×10-1 mN/m,两者均属于低界面张力体系;而 S2 的动态油水界面张力相对最低,平衡后稳定值为8.2×10-3 mN/m,属于超低界面张力。

  • 图3 不同表面活性剂的动态油水界面张力和乳化油率曲线

  • Fig.3 Dynamic curves of oil-water interfacial tension and emulsified oil rates of different surfactants

  • 乳化油率测试结果(图3b)表明,S1,S2和 S3的最大乳化油率分别为 68.7%,33.6% 和 62.9%,S1 和 S3达到最大乳化油率的稳定时间较长,而S2达到最大乳化油率的时间较短。经乳化系数计算[22],S1, S2 和 S3 的乳化系数分别为 0.635,0.312 和 0.559。说明复配表面活性剂 S1和 S3与原油的界面性能具有低界面张力和强乳化能力的特点;S2可以改善油水界面张力至超低界面张力,但其乳化能力相对最弱。

  • 3 不同性能表面活性剂辅助CO2驱油效果

  • 在注入 CO2之前注入表面活性剂,一部分表面活性剂与 CO2充分接触后生成泡沫,达到调剖的效果;而一部分泡沫在油藏运移过程中破灭或者未与 CO2接触的表面活性剂,具有驱油的作用。为此,分别研究了不同性能表面活性剂溶液及其与CO2生成的泡沫对水驱后岩心进一步提高采收率的潜力。

  • 3.1 表面活性剂的驱油贡献

  • 均质长岩心水驱后注入不同段塞的表面活性剂,3种表面活性剂的驱油效率增幅(图4)表明,3种表面活性剂均能进一步提高水驱后的驱油效率,随着表面活性剂段塞注入量的增加,驱油效率增幅先增加后趋于稳定,基本在 0.5 PV 达到最大。从 3 种表面活性剂注入过程中的采出液微观照片(图5)可以看出,表面活性剂 S1和 S3的乳化能力相对较强,岩心中的乳化作用相对容易发生,相应采出液的油水乳化现象也更加明显,所形成乳状液滴数量更多、粒径尺寸分布更广。结合 3 种表面活性剂对水驱后的驱油效率增幅,表面活性剂 S1 和 S3 的增油效果优于S2,较小的段塞注入量即可以实现较高的驱油效率,说明相比超低界面张力,表面活性剂对原油的乳化作用对驱油效率增幅贡献更大,乳化能力越强驱油效果越好。这主要是因为:①表面活性剂可以改善界面活性,增强原油的流动能力,降低原油的启动阻力,实现降压增注增油的效果。②水驱后,残余油以吸附在岩石壁面的油膜为主,表面活性剂可以使残余油膜乳化分散,降低残余油膜的剥离难度,乳化能力越强,驱油效率增幅越大[2224]。因此,表面活性剂与原油的乳化作用是驱油过程的主控性能,乳化能力越强,驱油效果越好。

  • 图4 不同表面活性剂对岩心水驱后的驱油效率增幅对比

  • Fig.4 Comparison between increases in oil displacement efficiency after water flooding in core by different surfactants

  • 3.2 表面活性剂辅助CO2的封堵效果

  • 由阻力系数和残余阻力系数分析结果(图6)可见,随着段塞注入量增加,3种表面活性剂与 CO2对岩心整体的阻力系数和残余阻力系数均整体呈现先增加后趋于稳定的趋势。其中,表面活性剂S1和 S2与CO2产生的阻力系数和残余阻力系数随注入量变化趋势基本相同,阻力系数和残余阻力系数相对较大;而 S3 在段塞注入量较小时,阻力系数相对较低,当段塞注入量达到 0.5 PV 后,所产生的阻力系数与 S1和 S2基本相同,但残余阻力系数相对较小。这说明,虽然 3 种表面活性剂的静态发泡能力具有明显的差异,但是与 CO2均能在岩心中发泡产生阻力,阻力系数与残余阻力系数相差较小。因此,考虑相同段塞注入量的条件下,静态发泡能力较强的 S1 和 S2 与 CO2产生的泡沫封堵效果更好。结合残余阻力系数在岩心中的分布(图7)可知:相比表面活性剂 S1,静态泡沫稳定性更好的 S2 和 S3,与 CO2 接触后,压力上升段在距离岩心注入端更近的部位 (5和10 cm)产生的残余阻力系数更大,而岩心中后部位的残余阻力系数下降明显;而静态泡沫稳定性最差的表面活性剂S1,在岩心中后部位残余阻力系数相对更大。这说明对于表面活性剂与CO2在多孔介质中的发泡和运移来说,泡沫稳定性并非越高越好。静态泡沫稳定性越强,在岩心注入端产生残余阻力系数更大,这也意味着在注入端 CO2与表面活性剂接触体积更大,运移到岩心中后部位后与 CO2 的有效接触体积变弱,使得岩心中后部位的残余阻力系数增加并不明显;相反泡沫稳定性越差,表面活性剂在注入端的损耗相对较小,能够运移到岩心中后部位并与 CO2在岩心中运移后接触发泡,造成岩心中后部位的残余阻力系数明显地增加。因此,与 CO2发泡能力更强、形成泡沫稳定性较差的表面活性剂 S1,有利于进入实际油藏的深部,实现液流转向的调驱要求。

  • 图5 不同表面活性剂注入过程中的采出液微观照片

  • Fig.5 Microscopic photos of produced fluids during injection of different surfactants

  • 图6 岩心整体阻力系数和残余阻力系数与表面活性剂段塞注入量的关系

  • Fig.6 Relationship of overall resistance coefficient and residual resistance coefficient of core with slug injection rates of surfactants

  • 图7 不同注入段塞表面活性剂的残余阻力系数在岩心中的分布

  • Fig.7 Distribution of residual resistance coefficients of surfactants with different injection slugs in core

  • 3.3 表面活性剂辅助 CO2对岩心非均质性变化的适应性

  • 由表面活性剂辅助 CO2驱油对渗透率级差为 3.0时非均质岩心水驱后采收率增幅(图8)可见:水驱后直接注入 CO2提高采收率的幅度仅为 5.7%;而 3种表面活性剂辅助CO2,无论是在表面活性剂段塞的注入过程,还是CO2驱替过程,采收率均有一定幅度的提高。S1,S2和S3与CO2的采收率增幅分别为 19.7%,13.2% 和 15.2%,其中 S1 在注入过程具有较好的驱油能力,与 CO2产生的驱油效果最佳。结合 S1与S3的驱替过程有明显的乳化现象,说明了乳化能力更好的表面活性剂对稠油高含水状态更容易乳化分散;同时,原油乳化后形成了数量多、粒径尺寸广的乳化油滴,一部分乳液暂堵孔喉产生贾敏效应形成了微观绕流,提高了 CO2对剩余油的接触面积。另外,表面活性剂与 CO2接触后就地生成了泡沫,对窜流通道产生阻力作用并扩大了波及面积,有效地延缓了 CO2窜流时间,并延长了 CO2与剩余油的接触时间,有效地提高了采收率[25-26]

  • 图8 不同表面活性剂对渗透率级差为3.0的非均质岩心水驱后驱油效果

  • Fig.8 EOR in heterogeneous cores with permeability ratio of 3.0 after water flooding by different surfactants

  • 综上所述,表面活性剂具有驱油与辅助 CO2调驱的双重效应。考虑实际储层的非均质性更加复杂,选择渗透率级差分别为 3.0,4.5,6.0 和 9.0 的非均质岩心进行表面活性剂辅助CO2驱油实验。结果 (图9)表明,随着非均质岩心渗透率级差的增加,表面活性剂辅助 CO2对水驱后采收率的增幅逐渐下降。当渗透率级差小于6.0时,表面活性剂辅助CO2 驱油仍然具有较好的效果;但当渗透率级差继续增大,其整体采收率也逐渐下降。当非均质岩心的渗透率级差达到9.0时,3种表面活性剂的采收率增幅分别为 12.6%,10.2% 和 7.8%。这说明,表面活性剂辅助 CO2对岩心非均质性的变化具有一定的适应性。随着渗透率级差的增加,非均质程度增强,表面活性剂辅助CO2驱油对水驱后采收率的增油效果也相应下降。此外,表面活性剂 S1辅助 CO2驱油对水驱后采收率的增油效果仍然高于S2和S3,这表明随着渗透率级差的增大,岩心非均质性增强,具备强乳化、强发泡能力且兼顾弱泡沫稳定性的表面活性剂辅助CO2驱油对岩心非均质性变化的适应性最好,实现驱油-调驱的协同作用最佳。

  • 图9 表面活性剂辅助CO2驱油不同非均质岩心采收率增幅

  • Fig.9 Increase in recovery factor of different heterogeneous cores with surfactant-assisted CO2 flooding

  • 4 结论

  • 表面活性剂可以与CO2产生驱油-调剖作用,对稠油油藏高含水期具有明显的增油效果。3种表面活性剂辅助 CO2对渗透率级差为 3.0 的非均质岩心的采收率增幅分别为 19.7%,13.2% 和 15.2%,高于直接注入CO2的采收率增幅(5.7%)。表面活性剂驱油过程中,其乳化作用对驱油效率的贡献明显优于界面张力,乳化能力越强,驱油效率增幅越高;与 CO2形成的泡沫应该具有静态发泡能力强、形成泡沫稳定性弱的特征,有利于在岩心中运移到中后部位与CO2发泡,在岩心中后部位残余阻力系数更高,能够实现深部调驱;并且随着非均质岩心渗透率级差增大,表面活性剂辅助 CO2驱油的采收率增幅逐渐下降。

  • 强乳化、强发泡能力且兼顾弱泡沫稳定性的表面活性剂 S1辅助 CO2驱油的驱油效果最佳,其驱油机理主要包含表面活性剂驱油贡献及其与后续CO2 产生的泡沫调驱贡献,两者可以产生协同驱油-调驱效果;同时,随着岩心非均质性的增强,对非均质性变化的适应性较好。

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