The Performance of Various Injecting Gases into Fractured Retrograde Gas Reservoirs for Revaporization of Liquid Drop-Out

Abstract

Lean gas cycling and gas injection scenarios are widely used to avoid liquid formation and to revaporize the liquid drop-out in gas condensate reservoirs. In this study, the performance of methane, carbon dioxide, and nitrogen as injecting gases into fractured retrograde gas reservoirs for revaporization of condensates is discussed. The investigation is performed from two perspectives: comparison of the condensate revaporization potency of injecting gases and comparison of the mixing performance of injecting gases. The results show that carbon dioxide has the best performance for condensate revaporization, whereas nitrogen has a greater diffusion coefficient (i.e., the best mixing performance).     

The Effect of Nitrogen Injection on Production Improvement in an Iranian Rich Gas Condensate Reservoir

In gas condensate reservoirs, after a decrease in pressure below the dew point pressure, condensate formation would occur. One of the common and practical methods for prohibiting the rapid pressure drop in the reservoir and reduction of liquid dropout is gas injection. In this study, experimental measurement, the full field reservoir model and an economical evaluation have been performed to investigate the effect of nitrogen injection on an Iranian Southwestern gas condensate reservoir. At first, swelling and CVD tests were carried out to determine the swelling factor and changing the saturation pressure by nitrogen. Then, the full field simulation (history matching and prediction) was performed and finally an economical evaluation has been made. The results show that injecting more volumes of nitrogen cause to decrease the liquid dropout, increase the dew point pressure, and condensate ultimate recovery, but this process is not an economical project.     

An investigation of different gas injection scenarios as enhanced condensate recovery method in a naturally fractured gas-condensate reservoir

In gas condensate reservoirs, when the bottom hole pressure falls below the dew point pressure condensates are formed. This causes hydrocarbon liquid saturation around the well bore region. This phenomenon is called condensate blockage or condensate banking. Condensate recovery reduction due to condensate banking near the well bore region is an important problem in gas condensate reservoirs. The common method to prevent the condensate banking is gas cycling (reinjection of produced dry gas) into the reservoir that can contribute to the condensate vaporization, thus increasing the condensate recovery factor. The objective of this study was to find a suitable replacement for gas cycling. For this purpose, an investigation on the effects of injection of different types of gases (CO₂, N₂, and C₁) on enhance condensate recovery factor and pressure maintenance were performed. This research was done on one of the Iranian gas condensate reservoirs through a compositional simulator. The two-parameter Peng-Robinson equation of state (EOS) and Lohrenz-Bray-Clark correlation were used to model reservoir fluid properties through regression on the pressure-volume-temperature (PVT) experimental data. A fracture network was distributed over the reservoir, so a dual porosity/dual permeability model was selected for better evaluation of the fracture system. Then, various scenarios of natural depletion and CO₂, N₂, C₁, and gas cycling injection were studied. The results showed that CO₂ injection scenario being associated with the highest efficiency compared to that of other gases.     

Enhancing recovery and sensitivity studies in an unconventional tight gas condensate reservoir

The recovery factor from tight gas reservoirs is typically less than 15%, even with multistage hydraulic fracturing stimulation. Such low recovery is exacerbated in tight gas condensate reservoirs, where the depletion of gas leaves the valuable condensate behind. In this paper, three enhanced gas recovery (EGR) methods including produced gas injection, CO₂ injection and water injection are investigated to increase the well productivity for a tight gas condensate reservoir in the Montney Formation, Canada. The production performance of the three EGR methods is compared and their economic feasibility is evaluated. Sensitivity analysis of the key factors such as primary production duration, bottom-hole pressures, and fracture conductivity is conducted and their effects on the well production performance are analyzed. Results show that, compared with the simple depletion method, both the cumulative gas and condensate production increase with fluids injected. Produced gas injection leads to both a higher gas and condensate production compared with those of the CO₂ injection, while waterflooding suffers from injection difficulty and the corresponding low sweep efficiency. Meanwhile, the injection cost is lower for the produced gas injection due to the on-site available gas source and minimal transport costs, gaining more economic benefits than the other EGR methods.     

层间矛盾对循环注气凝析气藏开发效果的影响评价

塔里木盆地某凝析气田经过12年循环注气开发,由于古近系和白垩系层间矛盾突出,导致凝析油采收率降低的现象日益严重。通过数值模拟研究发现气窜通道是高渗透层古近系,在地层压力低于露点压力后,大量凝析油析出滞留在低渗透层白垩系,大大影响凝析油采收率。同时建立四种方案来研究不同注气层段对开发效果的影响,通过对比发现在底部低渗透层实施注、采,凝析油采收率可达到最高,最大程度消减层间矛盾的影响。因此,该凝析气田中、后期的开发调整方向主要是针对低渗层的剩余油的挖掘。     

不同注入气对凝析气相态的影响

凝析气藏开发过程中最大的问题是由于地层压力低于露点压力以下而造成凝析油损失,注气是防止凝析油析出并提高采收率的较好方法,但目前不同注入介质对凝析气相态影响还未进行系统研究,以一个真实的凝析气藏为例,使用自行开发的PVTCOG软件和PR状态方程,研究和对比了在凝析气藏定容衰竭不同阶段,注干气、氮气及二氧化碳气对凝析气相态的影响。研究表明不同种类的气体对凝析气露点、凝析油饱和度及采收率有明显不同的影响。     

牙哈2-3凝析油气田产量变化特征及预测

牙哈2-3凝析油气田是目前我国发现的最大整装凝析油气田，采用循环注气开发，由于其凝析油含量高、相态变化复杂，给生产动态的预测带来了很大困难。为此，在对产量变化特征和影响产量的敏感性因素进行的基础上，预测了注气结束后凝析油的含量。研究表明：地层中析出的凝析油饱和度较低，不会形成气液两相流动；储层中的反凝析现象对天然气产量影响小，而凝析油产量则受反凝析和处理量的限制，凝析油含量下降符合指数递减规律。经综合分析后预测：到2009年注气结束时，凝析油采出程度将达到28.79%，高于开发方案所设计的25.89％。     

Enhanced Natural Gas and Condensate Recovery by Carbon Dioxide and Methane Flooding

Abstract

The authors quantitatively investigates the recovery efficiency, pattern behavior, and relative permeability of (a) condensate following supercritical carbon dioxide (CO₂) injection, methane (CH₄) injection, and the injection of their mixtures; and (b) natural gas of various compositions following pure supercritical CO₂ injection. A high-pressure high-temperature experimental laboratory was established to simulate reservoir conditions and to perform relative permeability measurements on sandstone cores. This work is part of an integrated enhanced natural gas and condensate recovery project conducted for a local reservoir in Western Australia. This data will help the operators develop operational and design strategies for their present and future EOR projects.     

凝析气井无阻流量影响因素分析

在凝析气藏开发中，单井的绝对无阻流量与合理产能是比较重要的参数，它直接影响到气井开发的可行性及整个气田的开发规模。本文针对凝析气藏开发中经常存在的反凝析的现象，首先从理论上对影响凝析气井的产能因素进行了分析，然后利用气藏工程方法和单井数值模拟方法进行了系统的研究，确定了影响凝析气井产能的主要因素，提出凝析气井产能求取的可行性方法。     

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循环注气是提高凝析气藏开发效果及凝析油采收率的有效措施，而循环注气能否使已经存在于地层中的凝析油重新蒸发是人们普遍关注的问题。作者以柯克亚凝析气藏Ｘ４２—Ｘ５１层系为例，对循环注气开采过程中注入气对凝析油的再蒸发作用进行了数值模拟研究，并对Ｘ５１（３）循环注气试验区现场注气后的情况进行了具体分析，得出了凝析气藏循环注气可以使地层中的部分凝析油重新蒸发采出地面的结论。建议当地层压力低于露点压力时，采用保持压力循环注气开采，可提高凝析油的回收率。     

大张坨凝析气藏板52井产能试井资料分析与评价

经对板52井历次产能试井资料进行分析和评价,指出稳定试井方法在高凝析油含量气井测试中的不足;通过现场实施,指出适用于高凝析油含量气井的产能试井方法,为同类气藏的产能测试提供了借鉴和指导依据。     

凝析气藏循环注气三元开发机理与提高凝析油采收率新技术

塔里木盆地牙哈凝析气藏2000年开始采用循环注气开发技术提高凝析油采收率。但随着注气时间的延长,气油比受气窜的影响呈快速上升趋势。气藏历年的动态监测显示,注入的干气与地下凝析气并不能完全混相。室内多相流体PVT实验证实,干气与凝析气存在明显的界面,明确了驱替作用下干气和凝析气的非混相特征。基于瞬时平衡假设的相态和渗流理论无法准确描述其运动规律。熵增数学模型确定了不同储层物性特征对不同性质流体达到相平衡的影响;微元数学模型确定了重力作用对干气和凝析气渗流规律的显著作用;二维机理模型明确了3种作用力在注气井和采气井之间的作用范围。现场试验数据反映了气藏尺度的干气—凝析气运动规律,室内实验数据与数学模型分析反映了孔隙尺度和岩心尺度的开发机理,从而揭示了凝析气藏循环注气"扩散—驱替—重力分异"三元开发机理,明确了重力分异作用是影响循环注气效果的主要因素。基于循环注气三元开发机理,建立考虑扩散、重力分异与储层结构等多因素耦合的非平衡渗流数学模型,提高了循环注气开发过程中的流体分布预测精度,并建立了凝析气藏注气重力辅助立体驱替提高采收率新方法。该技术在塔里木盆地牙哈凝析气田得到了成功应用。     

凝析油可采储量计算经验公式

在凝析气藏的降压开采过程中，当地层压力低于露点压力后，地层中将出现气、液两相。对于中高凝析油含量的中高渗砂岩凝析气藏，在消耗式开发方式下，凝析油采收率分别与原始凝析油含量、原始气油比、天然气采收率以及初期凝析气井米采气指数(反映储集层物性)之间的相关性良好。根据对大港油田凝析气藏矿场资料的研究，得出4种计算凝析油采收率的经验公式，为计算凝析气藏的凝析油可采储量提供了便捷有效的方法。图4表1参10     

柯克亚凝析油气藏开发方法数值模拟研究

柯克亚凝析气田上油组西四二-西五一气藏是低渗透凝析气藏。为了合理开发该类气藏,进行了布井方式和水平井开采对比研究。在保持西四二-西五一凝析气藏构造主体格局、储集层基本特征与流体相态性质不变的前提下,建立相似的高低渗透混合分布的理想模型。数值模拟研究表明,在相对高渗区布井既有利于高渗透区油气的采出,也有利于低渗透区油气的采出;水平井的单井产能高,生产压差小,可明显提高干气采收率和凝析油采收率,并有效缩短开采年限。     

凝析气藏合理注气时机研究

凝析气藏的开发方式主要分为压力衰竭式和注气保压式,而目前对于后者合理注气时机的研究甚少,不清楚什么时候注气最有利于凝析气藏的开发:若过早注气,注气量较大,经济效益较低;而过晚注气,则有可能导致凝析油污染严重,产能及采收率均较低。为此,选取某凝析气井的真实岩心,利用长岩心驱替实验研究其合理的注气时机;并在此基础上,运用响应曲面法和数值模拟的方法来综合评价合理的注气时间。研究结果表明:①对于凝析油含量较高的凝析气藏,应在地层压力降低至露点压力附近时进行注气,即可避免析出大量凝析油而导致气井产能和采收率大幅降低;②对于凝析油含量较低的凝析气藏,合理的注气时机为地层压力低于露点压力且高于最大反凝析压力区域,这样既能获得相对较高的凝析油采收率,又具有较高的经济效益。结论认为:对于凝析气藏的开发,注气越早开发效果越好,但不一定都能获得较高的经济效益。     

缝洞型凝析气藏衰竭开采影响因素实验

为探究凝析气藏衰竭式开发效果的影响因素,选取塔里木盆地某区块凝析气藏为研究对象,通过对碳酸盐岩岩心人工造洞,开展不同降压速度、不同缝洞位置的凝析油和天然气采收率实验,分析气油比及采出气组分的变化规律。实验结果表明：降压速度与凝析油采收率、天然气采收率正相关;缝洞在目的层上部时采收率最大,在侧部时次之,在下部时采收率最小;气油比呈现先降低后升高的趋势,碳酸盐岩缝洞孔隙结构会造成凝析气体系露点压力上升;采出气中甲烷含量与凝析油采收率负相关。研究成果对缝洞型凝析气藏的开发方式选取和提高开采效果具有重要的指导意义。     

解除低渗凝析气井近井污染研究现状及进展

解除近井反凝析堵塞和水锁是深层低渗凝析气藏开发必须解决的难题。通过对国内外文献的调研，分析了凝析气藏近井带反凝析油堵塞和反渗吸水锁形成的机理及其对气井产能的影响，总结了解除凝析气井近井堵塞提高产能的实验和理论研究成果以及形成的主要技术手段。结果认为：低含凝析油的凝析气藏，高渗储层均可能由于反凝析和水锁的存在而严重影响气井产能；高临界凝析油流动饱和度和高含水饱和度导致反凝析影响严重。解除近井反凝析堵塞和反渗吸水锁的主要机理是延缓反凝析出现和加速反凝析油和地层水的蒸发；凝析气注入可反蒸发凝析油中的重烃；注甲醇可有效解除反凝析油和水锁的双重堵塞。将向近井带注入化学溶剂、注气和加热等方法结合起来，可能是解除低渗凝析气井近井堵塞研究的发展趋势。     

凝析气井含动能项的油管流压梯度计算

对凝析气井同一井流状况，使用包含和忽略动能项的方程计算了油管流压梯度，并将两种计算结果进行对比，从中得到了一些启迪。在凝析气田的开发过程中，获得最高的液烃收率是其主要目的之一，地面回收液烃的各种流程与常规采气流程有很大差异，这就给地面录取资料带来极大困难，为此探索了如何在凝析气井中取全取准资料的方法，并对井流偏差系数进行求解，获得了凝析气井井流的准确信息。     

柯克亚凝析油气田先导试验区循环注气开发

柯克亚凝析油气田西五一(3)凝析油气藏循环注气先导试验区从1994年9月2日开始注气,至1999年6月结束,累积注气382ｘ108ｍ3,累积采气3．47ｘ105ｍ3,注采比1．05,累积采凝析油6．83ｘ104ｔ．循环注气维持了地层能量,遏制了地下反凝析的进一步恶化,改善了地层的渗流能力,大大提高了凝析油的采收率。截止到1999年6月,凝析油采出程度达38．6％,比衰竭式开发的采收率提高17．6个百分点,开发效果良好。     

大张坨凝析气藏循环注气开发

对于凝析油含量高,并具有小油环的大张坨凝析气藏,对于采用什么样的开发方式可提高凝析油的采收率,进行了大量的室内实验和多组分数值模型研究,确定采用循环注气开发。于1995年1月开始循环注气,至今已正常注气18个月,见到了气油比下降并稳定的效果。预计可实现凝析油的采收率由35%提高到60.2%的开发方案。