岩心驱替实验法测定低渗透油藏 CO2近混相最小混相压力

CO_2在驱油过程中能否与原油达到混相,直接影响驱油效果和最终采收率。因此需对CO_2在低渗透油藏中与原油的混相条件及近混相区域中最小混相压力进行系统研究。选取YC油田低渗目标区块,以室内油藏物理模拟为基础,通过均质和非均质长方形岩心实验,在评价影响CO_2驱油效果的渗透率、采收率、驱替速度、气油比和渗透率级差的基础上,建立了测定最小混相压力的岩心驱替实验法。结果表明,该方法重复性好,可模拟低渗超低渗孔隙介质以及油藏非均质等性质,在模拟YC目标区块油藏非均质条件下测得CO_2与原油的最小混相压力为18.5 MPa,与数值模拟软件计算结果相同,比传统细管实验测值17.8 MPa高出0.7 MPa。两种方法所测结果基本一致。用岩心驱替实验法可以探究渗透率及其非均质性和驱替速度等不同因素对最小混相压力的影响规律。该岩心驱替实验方法可用于确定CO_2近混相驱最小混相压力,为深化CO_2近混相驱油机理的认识及矿场应用提供技术基础和理论指导。     

挥发性油藏CO2驱动态混相特征

挥发性油藏地层能量充足,原始地层压力高,常规水驱开发难以实施。CO_2驱以其良好的驱油特性在该类油藏中得到了应用,但由于挥发性原油气油比高,溶解气中甲烷含量高,导致CO_2驱混相压力高,使得其驱油效果受到一定的影响。通过室内实验和数值模拟,研究挥发性油藏注CO_2过程中的动态混相特征,并剖析衰竭开发转CO_2驱界限。结果表明:挥发性油藏存在着适度衰竭转CO_2驱"脱气降混"机理,即随着地层压力的降低,原油中甲烷成分部分脱出,有助于CO_2驱最小混相压力的降低。另外,其脱气降混程度与其原油类型和溶解气油比有关,原油越接近于凝析油,气油比越高,混相压力降低程度越大;反之,原油越接近于黑油,气油比越低,混相压力降低程度越小。结合动态混相机理,提出了挥发性油藏衰竭开发转CO_2驱界限,即气油比越高,其转驱界限越低,脱气后CO_2混相驱补充地层能量幅度越小;反之,转驱界限越高,补充地层能量幅度越大。     

低渗透油藏 CO2混相条件及近混相驱区域确定

为了探究CO_2在低渗透油藏中与原油的混相条件及在近混相条件下的驱油效果,采用室内物理模拟方法,通过均质、非均质长方体岩心实验,在评价影响CO_2驱油效果的渗透率、岩心长度、渗透率级差和压力因素的基础上,借助采收率与各影响因素参数指标,分析非混相、近混相和混相不同阶段的曲线特征,建立了近混相驱区域的确定方法。采用该岩心实验方法,在模拟油藏条件下,CO_2与原油的最小混相压力为18.5 MPa左右,比传统细管实验确定的17.8 MPa高出0.7 MPa,同时根据驱油曲线特征,划分了CO_2非混相、近混相和混相区域,并根据驱油效率确定出近混相驱的压力区域为16.5数18.5 MPa。建立的最小混相压力岩心测定方法和近混相驱区域划定的方法,为进一步深化CO_2近混相驱油机理的认识及YC油田CO_2矿场驱油方案的设计提供了参考。     

非离子表面活性剂分子结构对CO2驱混相压力的影响

为揭示非离子表面活性剂对油/CO_2界面张力和最小混相压力(MMP)的影响,利用高温高压界面流变仪,采用下悬滴法,研究了温度和非离子表面活性剂分子结构对超临界CO_2/油界面张力和MMP的影响规律。结果表明,随着温度的升高,超临界CO_2/油界面张力和MMP逐渐增大,月桂醇聚氧丙烯醚(C12PO6)降低CO_2驱混相压力的幅度先增大后减小,60℃时的降幅最大(25.6%)。直链脂肪醇碳数由12增至18时,超临界CO_2/油MMP逐渐增加,C12PO6降低CO_2驱混相压力的效果最好。聚氧丙烯醚基团(亲CO_2端)降低界面张力和CO_2驱混相压力的效果好于聚氧乙烯醚基团。随聚氧丙烯醚聚合度增加,超临界CO_2/油MMP先降低后增加,聚合度为6时的MMP最小(13.22 MPa)。降低CO_2驱混相压力效果最好的表面活性剂分子结构为碳十二醇聚氧丙烯醚-6(C12PO6)。     

稠油火驱产出流体色谱指纹特征燃烧状态判识方法

为认清稠油火驱过程中原油与尾气物理化学性质的变化规律,采用室内三维物理模型开展了稠油火驱实验,并对火驱高温氧化后的原油与尾气开展了饱和烃、芳香烃、烯烃、尾气组分等方面的色谱指纹特征研究。研究表明:稠油火驱后原油物性变好,族组分中饱和烃、芳香烃含量升高;饱和烃色谱指纹图中正构烷烃含量明显增加,轻重比增大;原油中新生成了烯烃化合物,与同碳数饱和烃成对出现且同碳数烯烃与饱和烃比值始终小于0.5;芳香烃色谱指纹图中菲系列化合物发生了明显的甲基转移与脱甲基作用;尾气多维气相色谱图中可见烯烃、氢气、一氧化碳等火驱高温氧化的特征组分。该研究可作为火驱过程中指示燃烧状态的有效技术手段,为稠油火驱燃烧状态的判识提供了支持。     

热作用条件下烟道气与轻质原油混相规律

通过细长管混相驱替实验开展高温高压条件下烟道气与不同类型轻质原油的热混相规律研究。在高温高压条件下烟道气可以与轻质原油实现混相驱替;相同温度条件下,烟道气驱油效率与压力呈近线性关系;相同压力条件下,随温度增加驱油效率先平缓增加,然后急速增加,驱油效率快速达到90%以上,实现混相驱,驱油效率急速增加过程与稀油轻质组分随温度增加发生蒸馏相变有着密切关系;相同压力条件下,原油越轻,烟道气与原油的最小混相温度越低,越容易实现混相,注空气热混相驱替开发效果越好;高温高压条件下轻质原油与烟道气的混相更多体现的是超临界状态的高温相变特征,与常规高压条件下CO₂的接触抽提混相存在较大差异。     

低渗油藏非纯 CO2近混相驱及实现条件研究

为明确非纯CO_2近混相驱的压力区域和实现条件,综合采用细管实验和细管模拟方法,分别从物理化学和工程应用的角度,建立了基于驱油效率和界面张力的非纯CO_2-原油体系近混相压力区间的综合界定方法,在传统的最小混相压力(MMP)附近划分了一个近混相区域。研究发现,该区域对应的驱油效率为85%~95%,对应的界面张力约为0.001~0.05 mN/m。以秦皇岛29-2E-5井1段为例,该目标油藏典型井在不同CO_2纯度下的近混相压力区间为26.52~32.9 MPa,约为MMP的0.8~1倍,实现近混相驱的CO_2纯度下限为64%。     

延长油田致密砂岩油藏CO2驱油机理研究

CO_2驱是提高低渗透油田产量、缓解温室效应的有效途径。针对鄂尔多斯盆地油藏压力系数低、原油轻质组分含量高的特点,通过PVT和最小混相压力等测试分析方法,揭示了低压、低孔、低渗油藏CO_2驱提高采收率主要机理。开展了CO_2注入储层与无机、有机物作用后的沉淀研究,表明CO_2在无机盐溶液中不会形成沉淀堵塞孔隙,CO_2与有机质作用后沉积点高于油藏压力,且注入压力越高,CO_2在地层原油中的溶解能力越强,目标区块CO_2注入后不易形成沥青质沉淀。物模驱替实验结果表明,均质岩心的采出程度明显高于非均质岩心,且随着岩心非均质性的增加,水驱采出程度、气驱采出程度及最终采出程度均明显下降。     

原油组分对CO2最小混相压力的影响

为研究原油组分对CO2与原油最小混相压力的影响程度,在不同压力条件下开展两组细管驱油实验。结果表明,当原油中组分C5-C9和C10-C14含量降低,C15-C19、C20-C24和C25-C33含量增加时,CO2与原油最小混相压力升高至36 MPa,较第1组实验最小混相压力上升约5.88%。     

深层缝洞型油藏烃气混相驱可行性及影响因素

针对塔河油田深层缝洞型油藏,基于室内相态实验,采用经验公式法、拟三元相图法和细管模拟法计算烃气与原油最小混相压力,并通过数值模拟,研究了油藏前期注入氮气和原油品质对烃气混相驱的影响。研究结果表明：3种方法计算得到的原油最小混相压力远小于地层实际平均压力,研究区深层缝洞型油藏烃气混相驱具有较好的可行性;油藏前期注入氮气对烃气混相驱影响显著,注入氮气波及的储集层区域,烃气与原油的最小混相压力变高,氮气含量与注入烃气含量比大于1.208时,无法形成混相;原油品质对烃气混相驱影响显著,原油中的轻组分含量越高,烃气与原油最小混相压力越低,原油中的重组分含量越高,烃气混相驱最终采收率越低。     

The extraction effect of CO2 injection on the flow properties of crude oil

During CO₂ flooding, extraction of lighter hydrocarbons from crude oil makes the remaining oil hard to be recovered. In this work, we design a new experimental method to characterize the effect of CO₂ extraction on crude oil. The experimental results show that, the volume of extracted hydrocarbons increases as system pressure increases. The hydrocarbons with wider carbon number can be extracted from crude oil at high pressures. Moreover, the wax precipitation and viscosity of the remaining oil increase with increasing pressure. This study is expected to provide the basic understanding of the mechanisms of CO₂ flooding for enhanced oil recovery.     

二氧化碳—原油多相多组分渗流机理研究

二氧化碳与烃类体系的多相多组分渗流机理,对于深入理解实际油田注二氧化碳的驱替特征、提高采收率及地质埋存等都具有非常重要的意义。应用细管和多次接触实验以及包含相间传质的多相多组分CO₂驱油藏数值模拟模型,系统研究了CO₂—原油体系的相变规律以及多组分体系的变相态渗流特征。结果表明,,CO₂能够大量蒸发C₁₁以下的烃组分,甚至能够蒸发C₃₂等重烃组分;CO₂气驱过程是一个蒸发与凝析的混合过程, 混相带出现在气驱前缘附近;温度越高,CH₄和N₂含量越大,最小混相压力越大。     

X油藏注气混相驱可行性实验研究

针对X底水油藏油井注水后综合含水上升过快的问题,利用HB70/300型高压物性分析仪开展了该区块原油相态特征实验、注气相态特征实验,并运用细管法开展了注CO₂最小混相压力实验。对比分析了CO₂和N₂两种性质气体注入前后原油的相态特征变化,确定了该区块原油注CO₂最小混相压力,为X油藏注气提高采收率可行性提出依据。实验结果表明,X油藏原始地层压力为46.01 MPa,原油饱和压力为11.06 MPa,注N₂后饱和压力上升迅速,在原始地层条件下难以实现混相,表现出典型的非混相特征;注CO₂后饱和压力上升较平缓,细管法测得的最小混相压力为28.03 MPa,说明利用CO₂可实现CO₂的混相驱替,而且最终的驱替效果比较理想。说明该油藏可开展注CO₂混相驱,为进一步的开发方案调整提供了依据和合理的建议。      

Changes in the properties of conventional crude oil before and after CO2 flooding

Abstract

In order to enhance oil recovery of a conventional oil reservoir by CO₂ flooding, the changes in the properties of the crude oil before and after CO₂ flooding are systematically investigated by on-site sampling and experimental testing. The results show that, after CO₂ flooding, the light hydrocarbons of the produced crude oil is increased and the heavy hydrocarbons of the produced crude oil is decreased due to the deposition of resins and asphaltenes in the pores of the formation. In addition, the produced fluid (a mixture of oil and water) has a high water separation rate, the oil- water interface has a high tension value, and the crude oil has a high acid value and a low viscosity. The conclusions can provide a certain guidance for high-efficiency development of a conventional oil reservoir by CO₂ flooding.     

Economical Evaluation of CO2-EOR Projects in the Middle East

Abstract

Recently, there is a growing interest the in oil industry to utilize carbon dioxide (CO₂) to enhance oil production from mature reservoirs. Conversely, there is a rising global attention to reduce CO₂ emissions from burning fossil fuels due to environmental concerns. Synchronization between these two objectives is promising through CO₂ Capture and Storage (CCS) projects where CO₂ is captured from large emission sources and then storedin safe geological structures. Economical evaluation of CO₂-EOR projects is a crucial measure in order to ensure a project's viability.

In this study, an efficient model was developed to predict the economics of CO₂-EOR projects. The developed model consists of five modules that are linked together to allow for fast prediction of CO₂-EOR economics.

The model was used to predict the economics of a case study where CO₂-EOR application is considered for a Middle Eastern reservoir. Moreover, the case study was subjected to sensitivity analyses to evaluate the effects of several parameters on the various economical components of CO₂-EOR projects.     

Oil recovery and CO2 storage in CO2 flooding

In this study, the interfacial tension (IFT) of crude oil-carbon dioxide mixtures was measured to determine the minimum miscibility pressure. CO₂ flooding with sand packs, long cores, and heterogeneous cores was conducted to investigate the oil recovery and storage efficiency. The experiment results show that the interfacial tension decreases linearly with increasing pressure at two different pressure ranges. Under immiscible condition, the oil recovery and storage efficiency are increased by 30.1% and 52.4% when the injection pressure is increased from 13 to 22 MPa, and improved by 16.3% and 22.04% when the permeability is decreased from 270 to 10 mD, respectively. Under miscible condition, increase of injection pressure can only lead to much slower increase of oil recovery and storage efficiency, and permeability almost has no influence on oil recovery and storage efficiency. The oil recovery and storage efficiency can be remarkably reduced by heterogeneity. Water alternating CO₂ injection can improve the oil recovery and storage efficiency by 35.5% and 13.55%, respectively, compared with continuous injection.     

Asphaltene Deposition under CO2 Injection: An Experimental Study of the Bangestan Reservoir of Ahwaz Oilfield,SW Iran

Abstract

It is essential that precipitation of asphaltenes is recognized early in the planning stage of any CO₂ enhanced oil recovery (EOR) project so that appropriate testing can be performed to evaluate whether there will be a negative impact on reservoir performance. This article presents detailed evaluations of slim tube data that were obtained during CO₂ injection using a medium-gravity Iranian crude oil.

A crude oil from Bangestan reservoir of Ahwaz oilfield containing 18.2% asphaltenes with ~31.5 °API gravity was flooded by purified CO₂ (>96% CO₂) in a slim tube apparatus under 2,700 psi at 110°C. We were going to determine the minimum miscibility pressure (MMP) of the sample oil under injection of CO₂ flood, but when a CO₂ slim tube test was performed for this oil at 2,700 psi, less than half of the saturated oil in the tube was recovered, which implied that the displacement process was immiscible. At this pressure, the asphaltene deposition in the slim tube apparatus was so severe that even a pressure gradient of 6,200 lb/in² was not able to displace any fluid through the capillary tube. Therefore, we abandoned MMP determination with this sample and investigated the problem.

Due to the high percentage of asphaltenes in the sample, using the slim tube MMP as an apparatus for determining minimum miscibility pressure of CO₂ and sample oil can be misleading.     

Influence of pressure and CO2 content on the asphaltene precipitation and oil recovery during CO2 flooding

During CO₂ flooding, the crude oil is treated with CO₂, and meanwhile it is displaced by CO₂. Based on the two processes, the influence of pressure and CO₂ content on the asphaltene precipitation and oil recovery efficiency are systematically investigated by indoor simulation experiment. With the increase of the pressure or CO₂ content during CO₂ treatment, the amount of asphaltene precipitation can be increased to a certain value. Correspondingly, the degrees of the changes of oil-water interface, the compositions of crude oil, and reservoir permeability are positively correlated with the amount of asphaltene precipitation. However, during the process, the oil recovery has an optimal value due to the combined action of asphaltene precipitation and the improvement of flow performance of the crude oil. These conclusions can provide a basis for high efficiency development of low permeability oil reservoirs by CO₂ flooding.