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作者简介:

刘小波(1978—),男,四川广安人,高级工程师,在读博士研究生,从事油气田开发研究工作。E-mail:liuxiaobo660.slyt@sinopec.com。

中图分类号:TE357.45

文献标识码:A

文章编号:1009-9603(2020)03-0113-07

DOI:10.13673/j.cnki.cn37-1359/te.2020.03.015

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目录contents

    摘要

    正理庄油田樊142块为典型的低孔、特低渗透滩坝砂油藏,弹性开发递减快,采收率低。为研究特低渗透滩坝砂油藏CO2混相驱的可行性,优选正理庄油田樊142块的樊142-7-斜4井组开展CO2混相驱先导试验。室内研究结果表明,CO2具有较好的降黏和膨胀地层原油的作用,能够大幅度提高采收率。利用数值模拟,进行CO2驱恢复地层能量优化设计,井组采用反七点法井网、周期注采、合理地层压力保持在 1.3MMP,可获得较好的采收率和经济效益。现场试验效果证实,CO2具有较好的注入能力,井组地层压力显著恢复,井组混相阶段注气换油率为0.44 t/t,采收率提高6.2%,取得较好的增产效果,可见CO2混相驱是提高特低渗透油藏采收率的有效手段。

    Abstract

    Block Fan142 in Zhenglizhuang Oilfield is a typical low porosity,extra-low permeability beach-bar sand reser- voir with the rapid production decline and low recovery during the elastic development period. In order to study the feasibil- ity of CO2 miscible flooding in the extra-low permeability beach-bar sand reservoirs,a pilot test was carried out in Well Group F142-7-X4 of the Block Fan142. The results show that CO2 has a good effect on reducing viscosity and swelling for- mation oil,which can dramatically improve oil recovery. By using numerical simulation,the formation pressure may be re- stored through the optimization design of CO2 flooding. The higher recovery and economic benefit of the well group may be obtained at the conditions of the inverted seven-point pattern,the cyclic injection and production,and maintaining forma- tion pressure at 1.3MMP. The field test results show that CO2 has a good injection capacity and the formation pressure of the well group is significantly restored. A good stimulation performance is achieved. The oil exchange rate by gas injection of the well group at the miscible phase is 0.44 t/t and oil recovery factor of well group is increased by 6.2%. The CO2 miscible flooding is an effective means to improve the recovery of the extra-low permeability reservoirs.

  • 胜利油区低渗透油藏资源丰富,已探明石油地质储量为12.2×108 t。按照渗透率大小,将胜利油区低渗透油藏细分为3大类:渗透率小于3 mD的油藏定义为致密油藏,渗透率为3~10 mD的油藏定义为特低渗透油藏,渗透率为 10~50 mD 的油藏定义为一般低渗透油藏。同时结合开采特征,根据沉积类型和储层展布状况,特低渗透油藏又可分为浊积岩、滩坝砂和砂砾岩油藏等。特低渗透油藏占低渗透油藏探明石油地质储量的40%以上,开发面临的困境为注水难度大(注入压力为30 MPa)、启动压力高(20 MPa)、注水作用距离短和弹性开发采收率低 (8%左右),因此油藏注不进采不出矛盾突出[1-2]

  • CO2驱是有效的提高原油采收率技术,提高采收率幅度可达 15%[3-5],其中 CO2混相驱具有更好的驱油效果[6-8]。中国自 20 世纪 60 年代初开始关注 CO2驱油理论与技术的发展,针对低渗透油藏,在大庆、吉林、胜利、中原和华东等油田开展 CO2驱矿场试验,并取得了一些成果和经验[9-11]。但对于油藏埋藏较深、地层温度大于 140℃的特低渗透滩坝砂油藏能否达到 CO2混相驱开发尚无成功的实践认识。为研究特低渗透滩坝砂油藏CO2混相驱的可行性,优选正理庄油田樊142块开展先导试验,通过室内研究,明晰 CO2驱油机理,优化 CO2驱注采设计,从而为先导试验、工业化推广提供室内实验依据,现场应用取得了良好的开发效果。

  • 1 区块概况

  • 樊 142 块位于济阳坳陷东营凹陷博兴洼陷金家-正理庄-樊家鼻状构造带中部,是一个东南高、西北低的单斜构造,主要含油层系为沙四段上亚段,油藏埋深为 2 900~3 200 m,探明含油面积为 34.2 km2,石油地质储量为 1 605×104 t。发育陆相断陷盆地滩坝砂体,包括坝砂和滩砂 2 种沉积微相。坝砂物性远好于滩砂,储层非均质性强,平均孔隙度为13.1%,平均渗透率为2.3 mD,平均孔喉半径为 0.28 μm,属低孔、特低渗透、微孔喉储层[12]。樊 142 块自2005年投入开发,主要为油井压裂天然能量开采,生产半年后油井产量递减迅猛,截至 2013 年 6 月注气前,总油井数为 66 口,累积产液量为 33.7× 104 m3,平均单井日产油量为 1.2 t/ d,采出程度为 4.4%,采油速度为0.4%,标定采收率仅为10%,地层亏空大。

  • 在樊 142块沙四段北部坝砂区转注的 8口井进行注水开发试验以补充地层能量,但注水中又出现注入压力高、注水井欠注、采油井受效有效期短、含水率上升快及注水见效不均衡等问题。因此,2013 年 6月优选樊 142-7-斜 4井组开展 CO2混相驱先导试验,井组包括1口注气井6口采油井(图1)。

  • 2 室内实验结果

  • 为了研究 CO2混相驱技术在樊 142块特低渗透滩坝砂油藏的可行性,选取樊 142 块采油井的原油和岩心样品,开展地层原油与 CO2体系的相态特征及长细管模型混相实验和长岩心模型驱替实验。

  • 图1 樊142-7-斜4井组沙四段油层顶面构造及井位分布

  • Fig.1 Top structure and well location of Es4 member in Well Group F142-7-X4

  • 2.1 体系相态特征

  • CO2与地层原油体系相态特征研究,对于混相驱替过程是相当重要的。CO2驱提高原油采收率的基本原理为:通过 CO2在原油中的溶解使原油体积膨胀提高产能,使地层原油黏度和界面张力降低提高流体的流度,通过 CO2与地层原油的多次接触混相提高原油采收率[13-14]

  • 注入CO2后地层原油的黏度和膨胀量测试结果 (图2)表明,地层原油黏度随CO2溶解量的增加而降低,当 CO2溶解量达到 200 m3 /t 时,地层原油黏度下降了 43%,地层原油膨胀量达 34%。说明注入的 CO2对樊142块地层原油有很好的降黏效果,能够有效提高地层原油的流动性;注入 CO2可使地层原油体积大幅度膨胀,原油中溶解的CO2越多,体积膨胀系数越大,从而越有利于提高驱油效率。

  • 图2 CO2溶解量与地层原油黏度和膨胀量的关系

  • Fig.2 Relationship between the CO2 solubility and oil viscosity as well as oil expansion volume

  • 2.2 混相压力

  • 最小混相压力是判断能否实现混相驱的关键指标,通过混相压力实验能确定 CO2与地层原油体系的最小混相压力(MMP),判断樊 142 块油藏能否实施CO2混相驱,从而为矿场实施提供决策性依据。

  • 通过室内长细管模型实验,可测定不同驱替压力下 CO2的驱油效率。当驱替压力小于 31.65 MPa 时,驱油效率大幅度提高;当驱替压力大于 31.65 MPa 时,驱油效率增幅变缓(图3)。用该方法确定樊 142 块原油与 CO2体系的最小混相压力为 31.65 MPa,低于原始地层压力(43.4 MPa),说明樊 142 块能够进行CO2混相驱开发。

  • 图3 樊142块最小混相压力实验中驱油效率与驱替压力的关系

  • Fig.3 Relationship between oil displacement efficiency and displacement pressure in minimum miscible pressure test of Block Fan142

  • 2.3 驱油效率

  • 为了进行 CO2驱注气方式和驱油效率研究,共进行了 4 次不同驱替方式的长岩心驱替实验,一是完全水驱,二是完全水驱后连续CO2驱,三是初始连续 CO2驱后转长期水驱,四是初始连续 CO2与水交替注入驱后转长期水驱。长岩心驱替实验结果表明,在目前的地层温度和压力下,完全水驱采收率为 33.50%,完全水驱后连续 CO2驱的最终采收率为 85.64%,初始连续 CO2驱后转长期水驱的最终采收率为 79.58%,初始连续 CO2与水交替注入驱后转长期水驱的最终采收率为 81.56%,说明 CO2驱油可以获得比水驱更高的采收率[15]

  • 分析不同压力下采收率与注入量的关系(图4) 可知,最终采收率随压力的增大而增加。因此为提高注气效果,应保持在较高压力下进行CO2驱。

  • 图4 樊142块采收率与注入量的关系

  • Fig.4 Relationship between oil recovery and injection rate of Block Fan142

  • 3 开发技术设计

  • 3.1 井网与井距

  • 中外低渗透油藏CO2驱的井网形式主要有反七点法井网、反九点法井网、五点法井网和线性井网等[16-21]。针对樊 142 块滩坝砂油藏滩坝交互、储层非均质性强的特性,尽量采用多向注采对应井网,利用 ECLIPSE 软件模拟对比五点法、反七点法和反九点法3种井网下油藏开采15 a的最终采收率。模拟结果表明,3 种井网下的最终采收率分别为 21.1%,22.4% 和 22.5%,反九点法井网和反七点法井网开发效果均优于五点法井网。根据特低渗透油藏 CO2驱油技术极限井距计算图版[22],当生产压差为 18 MPa 时,计算得到井组技术极限井距为 450 m。

  • 3.2 注采方式

  • CO2驱有多种注采方式[23],运用数值模拟方法,模拟周期、脉冲和恒速3种CO2注采方式(表1)下的原油波及效率。模拟结果表明,周期注采的波及效率最高(图5)。周期注采在关井浸泡时,由于CO2的扩散和溶解作用,原油体积增大,黏度降低,能够提高波及系数,从而改善非均质储层的驱油效果。

  • 通过室内长细管模型实验,确定樊 142 块 CO2 驱的最小混相压力为31.65 MPa,根据樊142-7-斜4井组测压资料,井组注气前地层压力仅为14.2 MPa,因此,应采用超前注气的开发方式[24],先期连续注气以补充地层能量。

  • 表1 不同CO2注采方式实验设计

  • Table1 Experimental design of different CO2 injection and production methods

  • 图5 不同注采方式下的波及效率

  • Fig.5 Comparison of sweep efficiencies under different injection and production methods

  • 3.3 压力保持水平

  • 选择合适的注气时机,保持合理的地层压力,对后期采收率有重要影响[25]。结合油藏开发实际,地层压力的保持水平,既要考虑油藏采收率,又要追求换油率及无因次利润的最大值。地层压力为 1.0~1.1MMP 时的换油率最高,之后换油率随地层压力的升高有所降低(图6);地层压力为1.3MMP时的无因次利润最高(图6)。因此,以无因此利润最高为目标,确定 CO2驱合理地层压力应为 1.3MMP,即41 MPa。

  • 图6 樊142块换油率和无因次利润随地层压力的变化曲线

  • Fig.6 Curves of oil exchange rate and dimensionless profit of Block Fan142 at different pressure levels

  • 4 开发效果

  • 结合老井生产情况,优选樊 142-7-斜 4井组开展先导试验,该井组采用反七点法注采井网,其中6 口采油井均压裂,注气井未压裂。井组注采井距为 243~676 m,平均为 442 m,满足技术极限井距,各采油井注采井距的差异易观察不同注采井距采油井的受效特征。试验设计 2 个阶段:一是压力恢复阶段,只注不采,采油井关井恢复地层压力。该阶段重点做好注气井的注入能力及地层压力和注气前缘的监测,以指导下一步生产。二是全面气驱阶段,地层压力达到 1.3MMP 后,对采油井开井求产。该阶段的重点工作为做好注气井的压力监测及采油井的压力监测、产量变化和产出气的组分监测等,预警气窜风险。

  • 4.1 压力恢复阶段

  • 4.1.1 注入情况

  • 试验井组自 2013年 6月开始注气,压力恢复阶段历时 42 个月,累积 CO2注入量为 1.9×104 t。注气速度为 15~30 t/d,油压为 25~32 MPa,注入过程中注入压力呈下降趋势,说明原油与CO2互溶,气液两相变单相。注气初期启动压力为9.5 MPa,吸气指数为0.17 t/(d·MPa·m),注气井吸气指数随CO2不断注入逐渐升高。与同区块滩坝砂注水油藏对比,由于气体扩散性好于水,且CO2具有膨胀、降黏和降低界面张力等特性,CO2驱的波及系数和洗油效率均好于水驱[26],在注入量相当的条件下,樊142块的注气压力低于注水压力,吸气指数是吸水指数的2倍。

  • 注气井吸气剖面显示,纵向各小层的吸气量整体与渗透率成正比,干层也有一定的吸气能力,且随着注入时间的推移,渗透性相对好的油层段吸气量下降,渗透性相对差的干层吸气量增加,逐渐趋于平衡。

  • 4.1.2 压力变化情况

  • 对注气井和采油井实施直读+存储式连续压力监测,持续跟踪地层压力变化情况,建立试井解释模型[27],结合数值模拟,对试验井组压力恢复速度、压力恢复状况、前缘等进行充分认识[28]。试井压力监测数据显示,地层压力显著恢复,平均地层压力由注气前的 14.2 MPa 恢复至 39.0 MPa,上升了 24.8 MPa。在地层压力小于最小混相压力之前,随着地层压力升高,压力变化率整体呈上升趋势;地层压力大于最小混相压力后,压力变化率减缓。

  • 受沉积微相控制,各井点压力恢复不均衡,根据压力检测数据,并结合试井解释结果,确定了井区的压力混相范围。东部坝砂区3口采油井注气后压力变化大,压力上升快,压力高,平均为 41 MPa,压力变化率主要为 30~50 kPa/d,其中樊 142-6-2 井压力变化率主要为30~80 kPa/d(图7a)。西部滩砂区 3 口油井注气压力变化率小,压力低,平均为 25.5 MPa,压力变化率平均小于 10 kPa/d,其中樊 142-7-3 井压力变化率长时间小于 10 kPa/ d(图7b)。试井压力监测资料显示,东部3口油井于2015 年陆续达到最小混相压力,达到混相的时间与注采井距相关,井距小的油井首先达到混相。依据地层压力恢复情况,指导油井在 2016 年 12 月后进入全面气驱开发阶段。

  • 4.2 全面气驱阶段

  • 4.2.1 采油井见效特征

  • 采油井受效不均衡,产量变化情况与压力变化情况相同,西部滩砂区采油井受效不明显,东部坝砂区采油井增产效果显著,初期单井自喷日产油量为5.6~7.6 t/d,平均单井日产油量为6.5 t/d,产油量远大于注气前泵抽的1.9 t/d,较试验前提升2.9~4.0倍,平均增产 3.4倍,峰值产量为采油井投产初期产量的 0.8 倍。由于 CO2可以超越孔道中水的阻碍进一步驱替残余油[29],采油井含水率略有下降。井组稳产期为 21 个月,累积增油量为 8 600 t,注气年换油率为0.34 t/t,累积换油率为0.22 t/t,混相后阶段注气换油率为0.44 t/t(图8)。

  • 图7 樊142-7-斜4井组采油井压力恢复曲线

  • Fig.7 Pressure build-up curve of wells in well group F142-7-X4

  • 4.2.2 采油井受效影响因素

  • 从采油井受效情况看,井组气驱效果主要受沉积相、注采井距、储层厚度、物性、亏空、地应力方向等因素的影响。东部坝砂区受效采油井储层厚度大、物性好、注气前累积生产原油较多。沉积相为影响井组气驱效果的主控因素,坝砂区注采井距为 243~494 m 的采油井均明显受效。受注采井距影响,CO2浓度前缘首先到达樊 142-6-2井,其次是樊 141-1井,最后到达最远的樊142-6-3井。

  • 通过监测注气井对应受效采油井CO2浓度及单井产量变化发现,位于注气井北偏东 65°的樊 142-6-3 井于 2018 年 7 月气油比升高,气窜风险大的采油井方向与研究区的主应力方向一致。

  • 图8 樊142-7-斜4井组东部坝砂区采油井综合开发曲线

  • Fig.8 Comprehensive production curve of wells in east beach-bar sand area of well group F142-7-X4

  • 4.2.3 采收率预测

  • 井组内6口采油井,若不实施注气开发,运用递减法预测井组 30 a 累积产油量为 4.64×104 t(图9),采收率为 14.2%;井组注气开发预测 30 a 累积产油量为 6.65×104 t(图9),采收率为 20.4%。井组注气开发采收率提高6.2%。

  • 图9 樊142-7-斜4井组月度采油曲线

  • Fig.9 Monthly production curve of well group F142-7-X4

  • 4.3 存在问题

  • 依据樊 142-7-斜 4 井组开发实践,目前 CO2混相驱开发面临3个问题:①CO2混相驱驱油机理有待进一步深化。地质研究表明,井组油层连通性较好,CO2混相驱开发中采油井仅坝砂区3口井见到注气效果,物性较差的滩砂区 3 口采油井未受到注气影响,全面气驱过程中气驱前缘的演化规律有待进一步验证。②采油井合理的工作制度有待进一步探索。3口受效井中,为最大限度地求产,对樊141-1及樊 142-6-3井实施自喷转抽生产。泵抽井生产压差放大,气油比上升较快,多次注采调配,气油比下降的同时,影响到采油井产量,截至 2018 年 12 月,自喷采油井与下泵采油井在 CO2混相驱期间累积增油量相当。樊 142-7-斜 4井组实践证实,保持较小的生产压差,采油井井底压力保持较高压力,保证大部分油藏处于混相状态,有利于采油井稳产、延缓气窜。混相能力与注采井距和生产压差的关系须进一步深化研究。③CO2混相驱开发效益问题须进一步评价。樊 142-7-斜 4 井组先期采用弹性开发,地层压力下降较大,为达到混相驱开发,前期压力恢复阶段历时较长,累积注气量较大,影响累积换油率及经济效益。应加强压力恢复阶段的注入能力分析,缩短关采油井恢复地层压力的时间。

  • 5 结论

  • 室内研究及樊 142-7-斜 4 井组实践结果均表明,樊 142 块高温高压特低渗透滩坝砂油藏能够实现 CO2混相驱开发,注气井不压裂亦能够满足配注要求,且随着累积注气量的增加,吸气指数呈上升趋势。

  • CO2混相驱保持的合理地层压力为 1.3MMP,持续的地层压力监测是保障试验成功的关键。

  • 井组气驱效果主要受沉积相、注采井距、储层厚度、物性、亏空和地应力方向等因素的影响,其中沉积相为主控因素,采油井气驱受效后沿地应力方向易产生气窜,该方向应适当拉大注采井距。

  • 注CO2以来,井组阶段累积增油量为8 600 t,预测采收率提高6.2%。

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