鄂尔多斯盆地长7段页岩油渗吸驱油现象初探

鄂尔多斯盆地长7段陆相页岩油储层孔渗小,连通性差,通过单纯的水驱作用难以采出,盆地页岩油的开发主要通过大规模体积压裂增加油水置换面积进而增加采收率。开发实践与室内实验证明,储层流体与井筒流体之间存在渗吸置换现象,且通过油水渗吸置换采出的页岩油占比为15%~40%,为页岩油的有效开发提供了新的思路。通过岩心外边界敞开实验,对比不同渗透率岩心的吸水排油能力,定量研究孔隙半径、界面张力、岩心渗透率等因素对渗吸置换有效性的影响。实验显示,小于10 μm的孔隙中采出的原油占渗吸采油量的56%~80%;当界面张力为1.18 mN/m时,页岩油储层的渗吸采收率最大;在界面张力较小（小于2 mN/m）时,渗透率与渗吸采收率成正比关系,而当界面张力大于4 mN/m时,渗透率与渗吸采收率没有明显的相关性。     

二氧化碳对原油中胶质沥青质作用的可视化研究

为明确CO_2驱油过程中胶质沥青质沉淀原因,从原油中分离出胶质和沥青质分别与正庚烷和甲苯配置成胶质模拟液和沥青质模拟液,并用2种模拟液分别与CO_2组成不同摩尔分数的二元体系,通过高压显微固相沉淀实验,观察2个体系中固相颗粒的变化规律,探究CO_2对胶质沥青质的作用机理。实验表明:胶质模拟液-CO_2体系中CO_2的摩尔分数达到12.50%时,胶质颗粒发生沉淀,随CO_2含量增加沉淀半径增大;当CO_2摩尔分数增至35.00%时,胶质沉淀开始沉积,发生聚集;当CO_2的摩尔分数大于50.00%时,降低体系压力至泡点压力以下,CO_2先从正庚烷液相中析出,后从液状胶质聚集体中析出。沥青质模拟液-CO_2体系在泡点压力处颗粒半径最大;随CO_2含量增加,沥青质颗粒沉淀半径增大;当CO_2摩尔分数增至60.00%时,沥青质颗粒发生聚集形成沉积。该项研究对于分析CO_2驱过程中胶质沥青质对沉淀的贡献情况、沉淀生成的主要原因以及如何减小沉淀对生产造成的伤害具有重要指导作用,对于增强原油的运移能力进而提高储层原油采收率具有理论指导意义。     

注CO2过程中沥青质沉淀对低渗储层的伤害及 对润湿性的影响*

沥青质沉淀是注CO_2提高采收率过程中普遍存在的问题,常导致储层孔喉堵塞和润湿性改变。利用岩心驱替实验并结合核磁共振技术,定量表征了不同CO_2注入压力下沥青质沉淀对低渗透储层的伤害程度和引起的润湿性变化。结果表明,随着CO_2注入压力的增加,采出原油的黏度和沥青质含量不断减小,剩余油中的沥青质含量不断增加。CO_2注入压力分别为9.1、16.2、24.1 MPa时,岩心的渗透率损失率为2.40%、7.41%、8.32%,岩心驱替后的混合润湿指数下降了0.04、0.12和0.14,表观接触角增加5°、12°、19°。CO_2注入压力越高,沥青质沉淀越严重,对低渗储层的孔隙度和渗透率伤害越大。图8表3参26     

致密储层不同二氧化碳注入方式下的沥青质沉积特征及其对渗透率的影响

注CO₂ 引起的原油沥青质沉积对致密储层造成的伤害严重影响其开发效果。为明确不同CO₂ 注入方式下 的沥青质沉积特征及其对储层渗透率的影响,对致密岩心分别进行了CO₂ 驱替和吞吐两种注入方式的实验,通 过核磁共振和扫描电镜等,分析了两种注入方式下的沥青质沉积特征、原油采出程度以及对致密岩心渗透率的影响。结果表明,两种CO₂ 注入方式下的致密岩心均会产生沥青质沉积,且主要以膜状吸附的方式沉积在孔隙表面。受CO₂ 与原油相互作用时间的影响,吞吐方式下的沥青质沉积量大于驱替方式,且吞吐方式下的沉积孔 径范围高于驱替方式。沥青质沉积对致密岩心渗透率的伤害程度与原油的产出方向有关。驱替方式下沥青质 沉积对岩心正向（CO₂ 注入方向）渗透率的伤害程度较大,而吞吐方式下沥青质沉积对岩心逆向渗透率的伤害程 度较大。在围压为10、5MPa时,CO₂ 驱替和吞吐方式下的沥青质沉积对岩心正向渗透率的伤害程度（正向渗透 率平均降幅）分别为7.05%和1.67%,对岩心逆向渗透率的伤害程度（逆向渗透率平均降幅）分别为0.41%和 2.66%。受注采模式和流动机制的影响,CO₂ 吞吐方式下的采出程度低于驱替方式。研究结果对于致密储层CO₂ 驱沥青质沉积不同方向上储层伤害程度的认识及注入方式的优选具有一定的指导意义。     

致密砂岩油藏CO2吞吐沥青质沉积对储层的伤害特征

针对注CO₂提高原油采收率过程中易产生沥青质沉积的现象,以致密砂岩天然岩心和储层原油为研究对象,利用CO₂吞吐以及核磁共振等实验手段,开展了致密砂岩油藏CO₂吞吐过程中沥青质沉积对储层的伤害特征研究。实验结果表明：原油的沥青质含量越高,CO₂吞吐过程中沥青质的沉积率越大;随着实验压力的升高,沥青质沉积率先增大后减小,当压力为25 MPa时,沥青质沉积率最大;CO₂吞吐过程中沥青质沉积对储层渗透率的伤害程度较大,而对孔隙度的伤害程度则相对较小;沥青质主要沉积在大孔隙中,且油样中沥青质的含量越高,对岩心大孔隙的堵塞程度就越大;沥青质沉积可以使岩心进口端面的润湿性由亲水性向亲油性转变;沥青质沉积会影响CO₂吞吐实验的采收率,沥青质含量越高,采收率越小。在致密砂岩油藏注CO₂吞吐过程中,应采取相应的抑制沥青质沉积措施,以提高CO₂吞吐措施的效果。     

CO2驱沥青质沉积量对致密砂岩油藏采收率的影响机理

为揭示CO_2驱过程中沥青质沉积量对致密砂岩油藏采收率的影响机理,选取3种不同沥青质含量的原油样品及相同渗透率的石英砂环氧树脂胶结人造岩心,模拟实际地层的温度、压力进行室内驱替实验,并通过核磁共振技术分析沥青质沉积对不同尺度孔喉中原油采收率的影响。实验结果表明:原油中沥青质含量越大,驱替过程中其沉积量越大;同时,岩心样品中的沥青质沉积量也随着CO_2注入量的增加而增加;驱替实验的最终采收率受沥青质沉积量的影响,沉积量越大,采收率越低。核磁共振技术测试结果表明,沥青质沉积对较大孔喉(2.0~200.0 ms)的采收率无明显影响。但是,对于较小孔喉(0.1~2.0 ms),沥青质沉积会产生一定程度的堵塞效应,导致剩余油难以采出,最终影响整体的采收率。     

致密轻质油藏不同CO2注入方式沥青质沉积及储层伤害特征

致密轻质油藏注CO₂会引发沥青质沉积现象。为厘清不同CO₂注入方式下沥青质沉积特征及其对储层的伤害机理,以鄂尔多斯盆地延长组7段储层轻质原油为例,在明确CO₂注入量和压力对沥青质沉淀量影响的基础上,通过开展不同注入压力下CO₂吞吐和驱替实验,辅以核磁共振在线扫描技术,研究了不同注气方式下沥青质在岩心中的沉积特征,定量评价了不同注气方式下沥青质沉积对储层物性、润湿性和孔隙结构的伤害程度,从微观孔隙尺度剖析了沥青质沉积对储层的伤害机理。研究结果表明,沥青质主要在岩心入口端大量沉积,且越接近岩心出口端沉积量越小,驱替方式下的沥青质沉淀量和沉积区域大于吞吐方式;两种注入方式下孔隙度变化率相差较小,但驱替方式下的渗透率伤害率远高于吞吐方式;沥青质沉积引发岩石润湿性向亲油反转,润湿反转指数随注入压力的升高而增大,且驱替方式下的润湿反转指数大于吞吐方式;微观尺度下沥青质主要在大孔隙中沉积,但吞吐方式下大孔隙（0.092 μm≤T₂<4.500 μm）堵塞率随注入压力的升高而增大,小孔隙（0.009 μm≤T₂<0.092 μm）堵塞率则先下降、后上升;驱替方式下小孔隙和大孔隙堵塞率均随注入压力的增加而增大。     

塔河油田高压注氮气对沥青质沉积的影响

针对塔河油田注N_2提高采收率中可能发生沥青质沉积伤害储层的问题,通过注气相态分析和黏度测试分析了注N_2对原油组分、性质的影响,采用岩心驱替装置通过注气吞吐沥青质沉积实验研究了注N_2对储层渗透率的影响,以裂缝性岩心缝宽为指标评价了沥青质沉积对储层的伤害程度,提出了减少沥青质沉积的措施。研究结果表明,注N_2吞吐会破坏原油的原有平衡状态,使油中重质组分含量增加,轻质组分减小,原油黏度增大;注气轮次、裂缝宽度和生产井井底流压均会影响沥青质的沉积程度,井底流压高于饱和压力时沥青沉降情况不明显,井底流压低于饱和压力时沥青质沉积程度增大。在现场注气生产中应保持合理的生产压差预防沥青质沉积,选择合适的化学解堵剂有效解除沥青质堵塞。     

不同压力下原油和CO_2的相互作用研究

本文研究了不同压力下原油与CO2的相互作用,及其对原油与CO2间的界面张力和采收率的影响.首先,在可视化高压饱和单元中,进行一系列不同平衡压力下的原油-CO2饱和测试来确定沥青质沉降的初始压力.然后,采用对称悬滴形状分析法来研究在27 ℃下、平衡压力时的原油-CO2原油体系的界面张力.通过可视高压视窗可以观察到实际地层条件下原油-CO2之间的界面相互作用.研究发现,平衡压力低于门限压力时,原油-CO2体系的平衡界面张力随着平衡压力呈线性降低.平衡压力足够高,原油中的轻质组分会很快从油相中进入CO2相,即强烈的轻质组分抽提.通过饱和实验发现,发生强烈的轻质组分抽提时的压力高于沥青质沉降的初始压力.最后,做了不同注入压力下的CO2的驱替实验,研究了原油与CO2的相互作用对采收率的影响.岩心实验表明,注入压力在适当的范围时,CO2采收率随注入压力的增大而迅速增加.     

注CO_2对超低渗储层的渗透性伤害研究

为深入研究注入CO_2对超低渗储层的伤害情况,采用室内实验手段分别考察了注CO_2引起的微观孔道堵塞和原油沉积伤害。研究表明,注CO_2会引起岩石物性、原油性质的变化,两种作用的叠加加剧了储层的渗透率伤害。CO_2与岩石的溶蚀过程以150 h为分界点而分为两个阶段,渗透率伤害主要发生在第一阶段,渗透率降幅达到11. 5%,第二阶段仅为1. 71%,溶蚀反应生成高岭石、重碳酸盐等次生矿物、以及盐霜反应析出的结晶都会堵塞孔道; CO_2与原油作用使原油发生组分分异,导致沥青质沉积,在1～5. 5 MPa压力区间内沥青质沉积最为严重,渗透率越高沉积量越多,沉积物堵塞孔道降低储层有效渗透率。研究揭示了注CO_2引起的储层伤害机理,形成的认识对于该类油藏开发和方案编制具有重要的指导意义。     

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.     

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。     

CO2 miscible flooding in low permeability sandstone reservoirs and its influence on crude oil properties

Miscible CO₂ injection process has become widely used technique for the enhanced oil recovery in low permeability reservoirs. Core flooding experiments and field test of CO₂ miscible flooding in low permeability sandstone reservoirs and its influence on crude oil properties was studied. The results showed that CO₂ miscible flooding in low permeability sandstone reservoirs can enhance oil recovery both in laboratory study and field test. The permeability of sandstone reservoirs decreased during CO₂ miscible flooding due to the precipitation of asphaltene of crude oil. The precipitation of asphaltene lead to a reduction of asphaltene content and the apparent viscosity of crude oil. A further study on inhibitors and removers for asphaltene deposits from crude oil should be investigated to prevent and remove asphaltene deposits in low permeability sandstone reservoirs.     

Modeling of Asphaltene Deposition During Miscible CO2 Flooding

The authors present the results of numerical tests and simulations to investigate and analyze the likelihood of asphaltene precipitation and deposition during CO₂ flooding in a reservoir. The effects of asphaltene precipitation on oil properties such as oil viscosity and density during miscible CO₂ flooding process were elaborated by using Winprop software of Computer Modeling Group. Also oil properties change during CO₂ miscible flooding by numerical slim tube were investigated by a compositional simulator (GEM). A fluid sample of Saskatchewan Reservoir that had been flooded miscibly with CO₂ was chosen for performing the sensitivity analyses. The results showed that asphaltene precipitation reduces the oil viscosity and density that is in favor of production increasing. On the other hand asphaltene deposition causes resistance in oil production due to porosity and permeability reduction. The competition between these two effects declares the positive or negative effect of asphaltene on recovery that could be different for each reservoir. The results also show that decreasing the rate of CO₂ injection leads to an increase in asphaltene deposition near the injective well. Due to this phenomenon in higher injecting rates the increment in well bottom-hole pressure becomes less.     

Influence of injection pressure and injection volume of CO2 on asphaltene deposition

To further improve the oil displacement effect by CO₂ flooding, the trends and conditions of asphaltene deposition under different injection pressures and injection volumes of CO₂ were studied by SDS solid phase deposition testing system, high temperature and high pressure microscope, and P-X phase diagram. When the mole fraction of CO₂ in crude oil increases to a certain value, asphaltene deposition appears. The lower the pressure, the lower the mole fraction of CO₂ in crude oil causing the asphaltene deposition there is. After the onset of asphaltene deposition, the degree of deposition increases with an increase in pressure. The amount of the deposited asphaltene under miscible displacement is the highest, under near-miscible displacement is the second highest, and under immiscible displacement is the lowest. When the dissolution of CO₂ in crude oil reaches the saturation point, the asphaltene deposition becomes slow. Besides, it is feasible to prevent or reduce the asphaltene deposition by adjusting the thermodynamic parameters according to the phase behaviors of the CO₂-crude oil system. The experimental results can provide theoretical basis for optimization design of the parameters of CO₂ flooding.     

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.     

Asphaltene Precipitation During Different Production Operations

Asphaltene precipitation due to enhanced oil recovery (EOR) methods or natural depletion is a serious technical problem at petroleum industry. The authors present the result of asphaltene precipitation during associated gas injection, CO₂ injection, and natural depletion in reservoir condition. In addition, the effect of variations in operation pressure, injection gas concentration, and production rate on asphaltene precipitation and difference between slope of precipitation graph due to various method of EOR or natural depletion were investigated. The results revealed that temperature has an efficient role on result of asphaltene deposition through associated gas and CO₂ injection. By decreasing temperature, the amount of asphaltene precipitation due to associated gas injection was increased. In fact, recovery of gas injection was decreased at lower temperatures, hence; solubility has an important rule on asphaltene precipitation.     

The Impact of CO2 Injection and Pressure Changes on Asphaltene Molecular Weight Distribution in a Heavy Crude Oil: An Experimental Study

Abstract

This work concerns observing the pressure as well as CO₂ mole percentage effects on asphaltene molecular weight distributions at reservoir conditions. A high-pressure, high-temperature asphaltene measurement setup was applied, and the amount of precipitated asphaltene at different pressures as well as CO₂ mole percentage in an Iranian heavy crude oil was measured. Moreover, the asphaltene molecular weight distributions during titration of crude oil with different n-alkanes were investigated. The gel permeation chromatography (GPC) apparatus was used for characterization of asphaltene molecular weight under different conditions. It has been observed that some thermodynamic changes such as pressure depletion above the bubble point increase the average molecular weight of asphaltene and cause the asphaltene molecular weight distributions changes from a bimodal curve with two maxima to a single maxima curve. One the other hand, below the bubble point, pressure reduction causes a decrease in the average molecular weight of asphaltene and also causes the shape of asphaltene molecular weight distributions to restore, which might be due to dissolution of asphaltene aggregates. An interesting result is that asphaltene molecular weight distribution at the final step of pressure reduction tests, ambient condition, shows approximately the same trend as the distribution of asphaltene molecular weight obtained at reservoir condition. This behavior explains the reversibility of the asphaltene precipitation process under pressure depletion conditions. In the case of CO₂ injection, the graphs of asphaltene molecular weight distributions always show a single modal trend and shift toward larger molecular weight values when CO₂ mole percentage increases. The results of this work can be imported to thermodynamic models that use polydisperse data of heavy organic fractions to enhance their performance at reservoir conditions. The distributions obtained by this method are good indicators of asphaltene structures at reservoir conditions.     

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.     

降低启动压力的表面活性剂体系在朝阳沟油田的应用

针对朝阳沟低渗透油田注水过程中注水压力上升较快、欠注严重的问题,进行了朝阳沟油田降压增注表面活性剂体系的筛选工作,最终确定表面活性剂体系的配方为0.2%石油磺酸盐类表面活性剂T702-40^#+0.5%Na₂CO₃。实验结果表明,该表面活性剂体系与原油间平衡界面张力能够达到2×10^-2mN/m,耐温、抗盐性好,与朝阳沟油田注入水和地层水配伍性好,能够使岩石的润湿性发生反转,比水驱提高采收率5%左右。该表面活性剂体系驱替计算得出的可流动渗透率值约比水驱可流动渗透率值大15%,具有明显的降低启动压力的作用,并进行了表面活性剂体系降低启动压力的机理分析。朝阳沟油田朝82-152井区矿场试验结果表明,该表面活性剂体系能够降低启动压力,使油层吸水能力提高,使低渗透储层动用比例提高,7口油井累积增油1768t。