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作者简介:

郑文宽(1989—),男,山东东营人,高级工程师,博士,从事CO2驱提高采收率研究。E-mail:zhengwenkuan.slyt@sinopec.com。

中图分类号:TE357

文献标识码:A

文章编号:1009-9603(2023)02-0086-08

DOI:10.13673/j.cnki.cn37-1359/te.202209037

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目录contents

    摘要

    非均质、黏性指进是决定CO2驱开发效果的关键因素,而调整注采方式是控抑气窜、扩大波及的有效手段。目前该方向的研究以数值模拟为主,亟需开展 CO2驱不同注采方式的物理模拟研究,以进一步明确 CO2驱连续气驱、轮换开采和注采耦合等模式的动用机制、开发特征和油藏适应性。通过CO2驱相似物理模拟实验,对比CO2连续气驱、轮换开采和注采耦合模式的开发特征差异,并分析CO2驱不同注采模式的适用性。实验结果表明:连续气驱模式下不同渗透率区域开发效果差异大,低渗透率区域见气后的增油潜力较低,适合相对均质油藏开发;轮换开采模式通过采油井交替开启,改变注采井间主流线方向,有效改善非均质储层中低渗透率区域的开发效果,适合强非均质油藏或气窜后的开发调整阶段;注采耦合模式利用注采交替引起的压力场交替变化,可以较均衡的提高全区波及范围,提高难动用边角区储量动用率,适合弱非均质油藏或前期开发阶段。

    Abstract

    The heterogeneity and viscous fingering are the key factors to determine the development effect of CO2 flooding. The adjustment of injection-production modes is an effective means to control gas channeling and expand the swept area. At present,research on the above directions mainly focuses on the numerical simulation. Therefore,it is urgent to carry out physical simulation research on different injection-production modes of CO2 flooding,so as to further clarify the production mechanism,development characteristics,and reservoir adaptability of modes of CO2 flooding such as continuous gas injec- tion,rotation production,and injection-production coupling. In this paper,the development characteristics of these modes were compared through similar physical simulation experiments of CO2 flooding,and the applicability of different injectionproduction modes of CO2 flooding was analyzed. The results show that the development effect in areas with different permea- bility is different under the continuous gas injection,and the potential for enhanced oil recovery after gas breakthrough is very low in low-permeability areas,so CO2 continuous gas flooding is suitable for developing relatively homogeneous reser- voirs. For the rotation production mode,the directions of the mainstream line between injection-production wells can be changed by opening production wells alternately,so as to effectively improve the development effect of low-permeability ar- eas in heterogeneous reservoirs. Therefore,this mode is suitable for strongly heterogeneous reservoirs or the development adjustment stage after gas channeling. The injection-production coupling mode can expand the swept area of the whole res- ervoir in a more balanced way by alternating injection-production to cause alternating changes in the pressure field,so as to improve the reserves producing ratio in the corner area of the reservoir. Therefore,it is suitable for weak heterogeneous reservoirs or the initial development stage.

  • 中外CO2驱提高采收率实践表明,非均质、黏性指进是决定 CO2驱开发效果的关键因素[1-3]。非均质严重、黏性指进强时气体波及系数明显降低,从而导致水气无效-低效循环,驱油效率降低。近年来胜利油田水驱及CO2驱矿场实践表明,轮换开采、注采耦合等模式是控抑驱替相窜进、扩大波及、提高驱油效率的一种有效手段[4-6]

  • 目前中外对轮换开采、注采耦合技术的研究主要集中于水驱油藏,在气驱领域研究较少。崔传智等应用数值模拟方法分析了注采调控开发效果影响因素,明确了水驱条件下注采调控技术的适用条件[7-11]。王瑞等借助数值模拟方法研究了注采耦合模式下注水和采油过程中各特征区液流方向的变化,研究结果表明,注采耦合技术可以扩大波及面积约为 10%,有效提高断角滞留区剩余油的动用程度[12-15]。郑文宽等应用物理模拟与数值模拟相结合的方法,明确了 CO2驱轮换开采和注采耦合在气驱中的作用机理[16-18]

  • 调整注采模式是抑制气窜、扩大波及的有效手段,但现阶段仍然存在一些突出的难题需要解决,例如不同注采模式下的提高采收率效果及适应性等尚不明确[19-23]。目前针对 CO2驱轮换开采、注采耦合的研究多以数值模拟为主,尚不能很好地表征不同注采模式的扩大波及机理,研究成果的可靠性受到限制,因此需要开展物理模拟研究解决上述问题。笔者开展了 CO2驱相似物理模拟实验,对比连续气驱、轮换开采、注采耦合的开发效果差异,分析不同注采模式的提高采收率机制,并明确了连续气驱、轮换开采、注采耦合的适用油藏,以期为胜利油田低渗透油藏CO2驱开发技术研究及示范应用提供技术支撑。

  • 1 CO2驱物理模拟相似准则体系

  • 根据 CO2驱油、气、水三相渗流特点,采用以下基本假设:①考虑油、气组分的相间传质,水组分不参与传质;②考虑流体压缩性,忽略岩石压缩性;③ 不考虑吸附作用;④不考虑注入流体与油藏、岩石的化学反应;⑤油藏为恒温环境,且能瞬时达到热力学平衡状态。

  • CO2驱渗流模型由质量守恒方程、运动方程、相平衡方程、辅助方程、初始及边界条件组成。

  • 质量守恒方程为:

  • tϕj nijρjSj+j nijρjvj-ϕj SjDijnijρjnij=0
    (1)
  • 运动方程为:

  • vj=-KKrjμjpj+ρjg
    (2)
  • 相平衡方程为:

  • kijm=nijnim
    (3)
  • 辅助方程为:

  • i nij=1
    (4)
  • 初始及边界条件为:

  • po(x,y,z,t=0)=pg(x,y,z,t=0)=pw(x,y,z,t=0)=pi
    (5)
  • Sg(x,y,z,t=0)=0
    (6)
  • n1,j(j=o,w,g,t=0)=0
    (7)
  • (8)
  • pinj r=rw,t-ppro r=rw,t=Δp
    (9)
  • 考虑几何、动力、运动及时间相似,对(1)—(9) 式采用量纲分析法推导CO2驱物理模拟相似准则体系,如表1所示。

  • 2 实验方案设计

  • 2.1 实验参数

  • 根据 CO2驱物理模拟相似准则体系,可将实际油藏按比例缩小至实验室物理模型,并实现油藏开发与物理模拟实验主要注采参数之间的相互换算,其换算原则是使得实际油藏开发和物理模拟实验中的 π1—π16 这 16 个相似准数取值相同。以史深 100 油藏为目标区块,根据实际油藏的物性及注采参数,利用相似准则计算得到物理模拟实验的模型参数和注采参数(表2)。

  • 2.2 实验装置

  • 实验采用高温高压三维物理模拟装置,主要包括:恒温箱、大模型夹持器、高压恒速恒压泵、压力传感器、三相流体流量计量器等,其中大模型夹持器的直径为400 mm、厚度为60 mm。

  • 表1 CO2驱物理模拟相似准则体系

  • Table1 Similarity criteria of physical simulation of CO2 flooding

  • 表2 实际油藏开发与物理模拟实验参数

  • Table2 Parameters of actual reservoir development and physical simulation experiments

  • 2.3 非均质物理模型

  • 设计非均质物理模型分为 4 个区域,渗透率分别为 5,10,20,40 mD。将 70~140 目的砂子、石英砂、环氧树脂、固化剂、酒精等按照不同的比例混合放置平铺在4个区域,然后放入岩心制备装置,最大压实压力为 60 MPa,加压 12 h 以上压制而成,并在物理模型内设置 1注 4采的五点法井网。布井完成的非均质物理模型如图1所示。

  • 2.4 对比实验方案

  • 设计相同压力、温度下的连续气驱、轮换开采、注采耦合3个方案进行对比实验。实验压力保持水平为 20 MPa,高于最小混相压力,确保全程全域混相驱替;实验温度为 80℃。其中,连续气驱实验为中心井连续注气,生产井 P1,P2,P3 和 P4 井全部开井生产。轮换开采实验为中心井连续注气,生产井按照 P1,P2,P3和 P4井的顺序轮换生产;循环 2轮,其中第 1 轮生产井见气时转下一口井,第 2 轮生产井不出油时转下一口井。注采耦合实验为注采井轮换开启,注采不见面:注入阶段中心井注气,生产井P1,P2,P3和P4井全部关闭,直至模型达到1.3倍最小混相压力;采出阶段中心井关闭,生产井 P1, P2,P3 和 P4 井全部开井生产,直至模型达到 0.7 倍最小混相压力;依次循环至不出油为止。

  • 图1 非均质物理模型

  • Fig.1 Heterogeneous physical model

  • 3 实验结果分析

  • 3.1 连续气驱实验

  • 连续气驱实验的产油速度及气油比随累积注入气量的变化曲线如图2 所示,可以看出 CO2连续气驱见气前,产油速度不断升高,峰值达到 17.57 mL/h,对应的累积注入气量为0.44 HCPV,此时的气油比为 297.58 m3 /m3;继续注入,当累积注入气量约为 0.5 HCPV 时开始见气,对应产油速度快速降低,见气时累积采出程度约为 43.2%。至驱替实验结束,产油速度降至 2.64 mL / h,累积采出程度为 51.9%。驱替压差由见气前的峰值为 1.25 MPa,下降至驱替结束时的 0.24 MPa;单位注气量所能获得的采出程度,由采油速度峰值对应的 43.26%/ HCPV,下降至驱替结束时对应的 25.55%/HCPV,气驱效率降低约40%。

  • 图2 连续气驱实验产油速度及气油比随累积注入气量的变化曲线

  • Fig.2 Curves of oil production rate and gas-oil ratio with injected gas volume in continuous gas injection experiment

  • 对不同渗透率区域的单井进行分析,各生产井见气时间及见气、气窜时的累积产油量如表3所示,可以看出对于非均质油藏,低渗透率区域的井见气较晚,见气时累积产油量较高,但见气以后的增油潜力很低,这主要是因为高渗透率井气窜后,在高渗透率区域形成快速通道,CO2形成无效循环,抑制了低渗透率区域的流动。4口井(渗透率由高至低)对总产量的最终贡献占比分别为 28.1%,26.7%, 23.7% 和 21.5%。因此,若要提高采收率,需延缓高渗透率层位的见气时间。

  • 表3 连续气驱实验生产井见气时间及见气、气窜时的累积产油量统计

  • Table3 Gas breakthrough time and cumulative oil production at gas breakthrough and gas channeling of production wells in continuous gas injection experiment

  • 3.2 轮换开采实验

  • 轮换开采模式的特征是注入井保持高压注入,生产井轮换开采。轮换开采实验的产油速度及气油比随累积注入气量的变化曲线如图3 所示,可以看出第 1 轮轮换开采期间累积注入气量少,产油速度高,气油比很低,在累积注入气量为0.30 HCPV时采出程度达到 27.7%,单位注气量所能获得的采出程度达到 92.3%/HCPV,气驱效率很高;第 2 轮轮换开采期间注入气量多,采出程度增幅变缓,在累积注入气量为 2.84 HCPV 时采出程度提高了 34.4%,单位注气量所能获得的采出程度为 12.1%/HCPV,气驱效率明显降低,表明该阶段有明显气窜现象发生,采油效果变差。由此可见,轮换开采模式有助于提高气体利用率,但首轮效果要明显好于第 2 轮效果。相较于连续气驱实验,轮换开采实验在见气后虽然也出现了气油比大幅增高、产油速度大幅降低的现象,但是随着油井的轮换开采,注采井间主流线的不断变化,起到了一定的抑制气窜和聚集能量的作用,气油比依次下降且增速变缓,产油速度依次增大且递减速度变缓,在一定程度上可以提高气驱效率、延长开发周期。轮换开采实验最终采出程度为62.1%,较连续气驱实验提高10.2%。

  • 图3 轮换开采实验产油速度及气油比随累积注入气量的变化曲线

  • Fig.3 Curves of oil production rate and gas-oil ratio with injected gas volume in rotation production experiment

  • 对不同渗透率区域的单井进行分析,各生产井第1轮及第2轮末的累积产油量如表4所示,可以看出相较于连续气驱实验,4 口井(渗透率由高至低) 的累积产油量均有了不同幅度的提高,提高幅度分别为 1.0%,1.4%,24.2% 和 30.3%;对总产量的贡献占比分别为 25.1%,24.0%,26.0% 和 24.7%;说明相较于连续气驱实验,轮换开采实验增加的产量主要为难动用的低渗透率区域贡献。这主要是因为连续气驱形成的渗流场在注采井之间,一旦有高渗透通道就极易发生严重气窜,但在轮换开采模式下,关井区域内的积聚能量会逐渐增大,从而使得注气驱动力的作用方向发生变化,即由在注采井连线间的渗流区内部作用,转变为在渗流区和滞流区之间作用,从而使液流方向随之发生改变。因此,轮换开采可以调整气驱方向,改善难动用的低渗透率区域的开发效果,从而提高注采井网的整体开发效果。

  • 表4 轮换开采实验各生产井累积产油量统计

  • Table4 Cumulative oil production of production wells in rotation production experiment

  • 3.3 注采耦合实验

  • 注采耦合模式的特征是注采井交替开启,注采井不见面。注采耦合实验的采出程度及气油比随累积注入气量的变化曲线如图4 所示,可以看出注采耦合实验见气早于连续气驱实验,累积注入气量约为0.24 HCPV时即见气,初期气油比上升较快,见气时采出程度约为 29.1%,大幅低于连续气驱实验见气时采出程度(43.2%);见气后气油比上升速度明显低于连续气驱实验,待耦合 43 个周期,累积注入气量达 2.0 HCPV 时,气油比也仅为 5 311 m3 /m3,远低于连续气驱和轮换开采,此时累积采出程度约为60.2%,较连续气驱提高8.3%。

  • 对不同渗透率区域的单井进行分析,各生产井见气时及气窜时累积产油量如表5 所示,可以看出相较于连续气驱实验,注采耦合实验 4 口井的增产幅度较为平均,累积产油量提高幅度分别为15.3%, 15.1%,15.4% 和 17.0%;4口井对总产量的贡献占比分别为 28.0%,26.5%,23.6%和 21.7%。这主要是因为注采耦合模式并没有改变非均质储层中的流线强弱分布,而是利用注采交替引起的压力场变化,通过高压扩容、传质降黏等作用均匀地扩大波及范围。

  • 图4 注采耦合实验采出程度及气油比随累积注入气量的变化曲线

  • Fig.4 Curves of recovery and gas-oil ratio with injected gas volume in injection-production coupling experiment

  • 表5 注采耦合实验各生产井累积产油量统计

  • Table5 Cumulative oil production of production wells in injection-production coupling experiment

  • 使用核磁共振技术测试主流线上连续气驱与注采耦合模式开发后的剩余油饱和度分布(图5)。对于大中孔隙,注采耦合与连续气驱的采出程度接近,都约为 90%;对于小孔隙,注采耦合的采出程度约为 85%,要明显高于连续气驱的 75%。这主要是因为注采耦合通过反复的憋压-泄压引起压力场交替变化,强化了弹性能释放过程,使得CO2可以通过溶解扩散进入这些常规气驱难以进入的微小孔喉,并挤出或带出其中的原油,从而提高了微观驱油效率。

  • 图5 主流线上不同开发模式的核磁共振T2

  • Fig.5 T2 spectra of main stream lines under different development modes

  • 注采耦合实验下单次耦合周期的产油量变化如图6 所示,可以看出注采耦合模式的单次耦合周期产油量是先增大后减小,这主要是因为注采耦合是注采井不见面的,能量从注入井到采出井的积聚及传递需要一定时间。注采耦合在第 10 个耦合周期的单次耦合周期产油量达到峰值,此时累积产油量为 29.5 mL,采出程度为 20.0%。单次耦合周期产油量峰值过后 2 个周期开始见气,此后单次耦合周期产油量缓慢下滑但仍相对稳定,产油量下降阶段的累积产油量为 58.9 mL,最终采出程度为 60.2%,说明注采耦合开发有较长的稳产期,产油量下降阶段也可以提供较大的增产潜力。

  • 图6 注采耦合实验单次耦合周期产油量变化曲线

  • Fig.6 Oil production curve of single coupling period in injection-production coupling experiment

  • 3.4 适用性分析

  • 不同注采模式下的非均质区域的开发效果对比结果如图7所示。根据该开发特征,对连续气驱、轮换开采、注采耦合 3 种不同气驱开发模式的适用性进行分析和总结。对于相对均质油藏,各区域注采情况较为均衡;由于连续气驱开发周期短、工作制度简单,因此推荐采用连续气驱模式开发。对于弱非均质油藏,不存在明显的低渗透难动用区;由于注采耦合开发可以较均衡的提高全区波及范围,同时动用难动用的边角区储量,因此推荐采用注采耦合模式开发。对于强非均质油藏,低渗透难动用区开发效果差,限制了整个油藏的开发效果提高; 由于轮换开采可以调整气驱方向,使液流方向发生改变,动用低渗透难动用区的储量,从而实现扩大波及、控抑气窜,因此推荐采用轮换开采模式开发。对于开发中前期/气窜前的开发调整,此时尚未形成注采井间优势通道的直接流线沟通,可通过注采耦合改变压力场强弱分布,实现流线均匀扩展。对于开发后期/气窜后的开发调整,此时已形成注采井间优势通道的直接流线沟通,可通过轮换开采使得液流转向,扩大非主流线波及。

  • 图7 不同注采模式下非均质区域开发效果对比

  • Fig.7 Comparison of development effect in heterogeneous areas under different injection-production modes

  • 4 结论

  • 非均质油藏在连续气驱开发模式下,不同渗透率区域开发效果差异大;当高渗透率区域井气窜后,CO2在高渗透率区域形成无效/低效循环,抑制低渗透率区域的流动,使得低渗透率区域井见气以后的增油潜力较低。轮换开采模式通过采油井的转换改变注采井间主流线方向,使得注气驱动力的作用由在注采井连线间的渗流区内部作用转变为在渗流区和滞流区之间作用,增加的产油量主要为难动用的低渗透率区域贡献。注采耦合模式利用注采交替引起的压力场变化,高压扩容、传质降黏,可以较均衡的提高全区波及范围,提高难动用边角区的储量动用率。对于相对均质油藏,推荐采用连续气驱模式开发;对于弱非均质油藏或前期开发阶段,推荐采用注采耦合模式开发;对于强非均质油藏或气窜后的开发调整阶段,推荐采用轮换开采模式开发。

  • 符号解释

  • Dij ——i组分在j相中的扩散速度,m2 /s;

  • g——重力加速度,m/s2

  • i——不同组分编号;

  • jm——不同的相,o为油相,g为气相,w为水相;

  • kjm i ——i组分在j相和m相中的平衡比;

  • K——储层绝对渗透率,D;

  • K-——特征渗透率,D;

  • Krj ——j相的相对渗透率;

  • KxKyKz ——对应xyz方向的渗透率,D;

  • L——特征长度,m;

  • nij ——i组分在j相中的摩尔分数,%;

  • nim——i组分在m相中的摩尔分数,%;

  • n1,j ——CO2j相中的摩尔分数,%;

  • pi ——原始地层压力,MPa;

  • pinj ——注入井井底流压,MPa;

  • pj ——j相的压力,MPa;

  • ppro——采油井井底流压,MPa;

  • p——注采压差,MPa;

  • r——距离模拟井的径向距离,m;

  • rw——井筒半径,m;

  • Sj——j相的饱和度;

  • t——模拟时间,s;

  • vj ——j相的渗流速度,m/s;

  • x——x方向坐标值,m;

  • y——y方向坐标值,m;

  • z——z方向坐标值,m;

  • γ——重度差,N;

  • μj ——j相的黏度,mPa·s;

  • ρj ——j相的密度,kg/m3

  • ϕ——孔隙度;

  • ϕ-——特征孔隙度。

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