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.     

Gas Injection Into Fractured Reservoirs Above Bubble Point Pressure

Abstract

Among all enhanced oil recovery (EOR) scenarios, gas injection seems to be promising for implementation in naturally fractured reservoirs. The use of CO₂ has received considerable interest as a method of EOR but a major drawback is its availability and increasing cost. Therefore, an alternative gas like CH₄ or N₂ must be considered to meet the economic considerations. To investigate the efficiency of oil recovery by CO₂, N₂, and CH₄ injection in fractured carbonate rock, a series of experiments was designed. Both miscible and immiscible schemes for gas injection were carried out on a low-permeable outcrop carbonate rock that was surrounded by fracture, established with a novel experimental method. The experiments aimed to investigate the potential of oil recovery by secondary and tertiary gas injection under high-temperature conditions. The matrix block was saturated using a recombined mixture of Iranian live oil, and by pumping water into the annular space, the space between rubber sleeve and outer jacket, high overburden pressure was exerted to obtain the desired homogeneous saturation. Using a back-pressure regulator, the pressure was kept above the bubble point pressure. The inlet was attached to a constant pressure pump injecting gas or water above the bubble point pressure, and the overburden pressure was removed gradually and the inlet fluid inflated the rubber sleeve. The amount of produced water from the annular space was recorded to estimate the distance between the rubber sleeve and sand face. This distance creates the fracture surrounding the core. Gas was injected into the fracture at pressures above the bubble point of the oil. Oil recovery as a function of time was monitored during the experiments. Results from both secondary and tertiary gas injection experiments indicate that CO₂ injection at elevated pressure and temperature is more efficient than N₂ and CH₄ injection.     

Black Oil and Compositional Reservoir Simulation for Increasing the Recovery Performance of an Iranian Fractured Carbonate Reservoir

Abstract

Naturally fractured reservoirs contain a significant amount of world oil reserves. Accurate and efficient reservoir simulation of naturally fractured reservoirs is one of the most important, challenging, and computationally intensive problems in reservoir engineering. Black oil and compositional reservoir simulators have been used to determine the reservoir management and production strategies to increase the oil recovery from a low-porosity, low-permeability fractured carbonate reservoir, with an average matrix permeability of 0.8 md, average fracture permeability of 500 md, and an average matrix porosity of 10%. This reservoir is a candidate for an enhanced oil recovery (EOR) process, because the reservoir production rate has been declined due to increasing the water cut as a result of rising the water oil contact. The injection techniques that have been considered in this study for black oil model include (a) gas injection, (b) water injection, and (c) simultaneous water alternating gas injection and for the compositional model include (a) dry gas injection, (b) CO₂ injection, and (c) N₂ injection. Simulation results show that CO₂ injection has the maximum oil recovery between the EOR scenarios.     

低渗致密气藏、凝析气藏开发难点与对策

Low permeability tight gas reservoirs and condensate gas reservoirs account for a rather high proportion ofdomestic gas reserves, but many of them have low productivity. So it is significant to develop these reservoirs effi-ciently for continuous and stable development of China′s petroleum industry. Around the problems of the developmentof deep low permeability tight gas reservoirs and condensate gas reservoirs, this paper makes an analysis on the geo-logic and development characteristics of these reservoirs and presents ten proper technologies. Finally, five technicalmeasures for the development of such gas reservoirs are proposed in detail. These are deep fracturing technology,treatment technology of accumulated liquids in condensate gas well and near well bore, gas injection technology whenthe formation pressure is lower than the maximum condensate pressure, phase behavior analysis technology in porousmedia of low permeability tight condensate gas reservoir and other gas reservoir engineering technologies.     

Application of Gas Injection and Recycling to Enhance Condensate Recovery

Condensate loss is one of the major challenges in the gas condensate reservoirs. It results in a reduction of gas permeability and gas productivity. Different methods have been proposed to mitigate the condensate dropout and increase gas and condensate productivity. One of these techniques is gas recycling. Moreover, dry gases such as methane, nitrogen, and carbon dioxide are good candidates for this purpose. The authors investigate the effect of gas recycling and gas injection on an Iranian gas condensate reservoir. First, a literature review of the subject matter was carried out, followed by an introduction of the field. Afterward, steps performed for construction of dynamic model were described. Then, specified production scenario for development plan of the field was presented. A sensitivity analysis was carried out to determine the optimum number of additional wells to meet the target rate of the field. Consequently, injection scenarios including gas recycling, methane injection, nitrogen injection, and CO₂ injection at different injection rates were studied. Results indicated that nitrogen injection and gas recycling can improve recovery factor and plateau time rather than other cases. At high injection rates, gas recycling showed the highest condensate recovery (47%) among these scenarios.     

A Review of the Potential for CO2 Sequestration and Enhanced Gas Recovery in an Iranian Gas Condensate Reservoir From a Fluid Properties Point of View

The use of carbon dioxide (CO₂) in enhanced oil recovery is a mature well-practiced technology; however, enhancing gas recovery (EGR) through the injection of CO₂ is yet to be tested in the field. Depleted natural gas or gas condensate reservoirs are becoming important targets for CO₂ injection. The study of injection of CO₂ for EGR needs accurate fluid modeling of CO₂ behavior in the gas reservoir conditions; especially the interaction of CO₂ and reservoir fluid should be taken into account to obtain the best conditions for injection (Oldenburg and Benson, 2001 Oldenburg, C. M. and Benson, S. M. Carbon sequestration with enhanced gas recovery. First National Conference. May14–17, Washington, DC.  [Google Scholar]). The purpose of this study is to evaluate the efficiency of CO₂ injection in one of Iranian gas condensate reservoirs by modeling of reservoir fluid properties and different behaviors of its mixture with CO₂ to have a proper estimation of injection condition and possibility of problems such as gravity segregation, viscous fingering, and liquid drop out at reservoir conditions. PVTi module of Eclipse software was used to perform modeling based on laboratory pressure-volume-temperature analysis of well number X of one of Iranian gas condensate reservoirs.     

牙哈5凝析气藏开发方式优化

凝析气藏作为一种特殊的油气藏类型,由于特殊的流体相态特征决定了其开发难度要比一般油气藏复杂得多,开发中不仅要考虑天然气的采出程度,而且还要兼顾凝析油的采出程度.牙哈5气藏为富含凝析油气藏,通过组分数值模拟对不同的开采方式进行对比,得出采用衰竭、注水和注气3种开采方式的开发效果.注水和注气是为了保持地层压力,防止凝析油在地层内大量析出,从而提高凝析油的采收率.对比研究表明,注水和注气开采牙哈5凝析气田要优于衰竭式开采.对注水和注气两种开采方式而言,由于牙哈5气田储层自身的特点,在提高凝析油采出程度方面注气开采方式又要优于注水开采方式.预测结果表明,采用大井距注气开发方式对于延缓气窜可以起到一定作用,从而有利于提高注气效率,增加凝析油采出程度.     

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。     

凝析气藏开发方式浅析

凝析气藏的开发不同于一般气藏,除了要考虑天然气采收率外,更重要的还需要考虑提高凝析油采收率的问题。针对衰竭式、保持压力和部分保持压力3种开发方式,研究了纯凝析气藏和带油环凝析气藏开发方式的选择,并进行举例论证。     

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).     

天然气组分的水合常数、水合热及理论溶解度

天然气在水中的溶解度是天然气地球化学定量研究中的基础参数。欲应用理论公式计算天然气组分在水中的溶解度,关键在于建立天然气组分的水合平衡常数与温度的关系。本文利用天然气组分的溶解度数据,标定了甲烷、乙烷、丙烷、N₂、CO₂和H₂S的水合常数与温度的关系,从而建立了上述组分的理论溶解度公式。经验证,至少在下述分压范围内,上述气体的溶解度计算值与实测值基本吻合:甲烷≤60MPa;乙烷≤5MPa;丙烷≤3MPa;N₂≤50MPa;CO₂≤5MPa;H₂S≤5MPa.天然气的水合热是研究气水合物形成的重要参数。研究表明,天然气组分的水合反应符合一般的化学反应规律。根据计算,上述气体的水合热分别为:甲烷,-17.741;乙烷,-10.471;丙烷,-9.441;CO₂,-17.520;N₂,-15.418;H₂S,-11.845(负号表示水合反应为放热反应,水合热的单位:kJ/mol).从理论上证明了当压力趋向无穷大时,气体的溶解度趋向一极限值。     

X区块注CO2和N2提高反凝析油采收率机理研究

通过对X区块原始地层流体的相态分析,确定了该区块为近临界凝析气藏。该区块一直采用衰竭式开采方式,气藏原始地层压力为27MPa,自开采以来至2012年8月地层压力下降为10MPa,已低于露点压力,反凝析加剧,地层污染加重,并造成大量的凝析油损失。利用该凝析气田附近所具有的高纯CO2气藏气,探索注CO2气开发方式改善和提高凝析油气的采收率具有技术进步意义。通过对CO2和N2两种注入气与地层流体膨胀实验数据的对比研究,得到CO2与地层流体的增溶膨胀能力、混相能力等相态配伍性较好且优于N2。在CO2与目前地层凝析油混相能力研究中发现在19.3MPa下CO2能与目前凝析油达到混相,CO2与目前地层凝析油容易达到混相,故开采时只要通过先期注CO2把目前地层压力提高至19.3MPa以上,就可实现混相驱提高凝析油的采收率。     

牙哈凝析气藏二次注气抑制反凝析机理及相态特征

为解决凝析气藏天然气回注率不足,地层压力低于露点压力而出现的反凝析损失等问题,以塔里木盆地牙哈凝析气藏反凝析损失监测井地层流体取样器(MDT)取样为基础,运用高温高压PVT相态实验测试和模拟技术,建立了牙哈凝析气藏二次循环注气抑制反凝析损失机理的相态特征研究方法,对提高牙哈凝析气藏凝析油采出程度的可行性和有效性做出了评价。通过目前地层压力下反凝析油和剩余凝析气体系的实验室再现,分别测定了其色谱组成、相态特征和p-丁相态图;分别针对反凝析油和地层剩余凝析气,开展了注气增溶膨胀实验、多次接触抽提实验和注气抽提实验,对地层剩余平衡凝析气还开展了加注干气传质扩散过程非平衡相态行为实验;分析了二次循环注气抑制反凝析损失、降低其反凝析油饱和度、使凝析油产生相态反转的相行为机理;给出了二次循环注气开发时应尽量使其注气压力高于露点压力的建议,当注入0.82 PV时,凝析油累积采出程度将提高13.55%。该研究成果为牙哈凝析气藏的增产提供了重要技术支撑。     

低渗致密气藏凝析气藏开发难点与对策

我国低渗、致密气藏和凝析气藏的储量占相当大的比例, 而其中相当部分处于低产状态开发好这类气藏对石油工业持续稳定发展具有十分重要的意义。围 绕深层低渗致密气藏、凝析气藏开发问题.首先分析了低渗致密气藏的地质特征和开发特征.提出了低渗致密气藏开发的十项配套工艺技术.最后重点建议了五项技术措施即深度压裂改造技术.凝析气井井筒和近井地带积液的处理技术开发后期低于最大凝析压力条件下的注气技术。低渗致密凝析气藏多孔介质油气体系相态分析技术和某些适合于低渗致密气藏和凝析气藏的气藏工程分析技术。     

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.     

CO2压裂后近缝带烃类的相态特征

CO₂压裂与常规水力压裂相比，除具有滤失小、易返排、低伤害等常规优点外，还能降低烃类的露点压力，对易凝析的中间组分产生很强的抽提作用，有效解除了由于温度压力的变化所造成的近缝带凝析油污染。这为提高凝析气藏采收率、制定合理有效的开发手段提供了新的途径。以中原油田白庙凝析气田为例，建立了数值计算模型，研究了CO₂压裂停泵时刻其CO₂量不同时的凝析气藏近缝带烃类的相态变化及各点凝析油饱和度的变化情况。研究结果表明：CO₂气体对凝析气藏烃类的相态影响是CO₂压裂优于常规水力压裂的重要原因，且CO₂量不同时对烃类相态的影响程度存在显著差异。这为现场实际结果的解释及优化凝析气藏CO₂压裂的气体用量和压裂规模提供了理论依据。     

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。     

东海平湖油气田中高渗凝析气藏开发效果分析

东海平湖油气田放鹤亭始新统平湖组气藏,属中高渗砂岩中低凝析油含量凝析气藏,1999年3月投入生产,采用衰竭式开采,至今已有8个年头,目前仍处于开发稳产阶段,日生产天然气量在140×104m3左右。通过对平湖油气田凝析气藏开发生产状况跟踪研究发现,其采气速度高、主力气藏气井无水采气期普遍较长、气油比随着压力下降而上升、凝析油含量随着压力下降而下降、天然气采收率将优于开发方案设计值,同时也发现,防止气井出水、出砂是中高渗凝析气藏开发中应予以高度重视的问题。平湖油气田凝析气藏开发,在技术上和经济上都取得了较好的效果,可以借鉴于类似凝析气田的开发实践中。     

Experimental study into the effectiveness of the partial gas cycling process in the gas-condensate reservoir development

In this study a laboratory study was carried out with the purposes of estimating the effectiveness of the gas cycling process during gas-condensate reservoir development. Specific laboratory equipment was constructed to carry out an experimental investigation by modelling the gas injection and reservoir depletion process. Volumetric properties, fog up, liquid drop-out condition, and retrograde dew point pressures of gas-condensate fluids were investigated. The reservoir fluid was recombined based on samples from Azerbaijan natural gas-condensate reservoir. Results are describing gas injection effect on condensate/gas ratio and condensate recovery in each pressure step. These have provided an understanding on the influence of the retrograde dew point pressure on optimum gas injection stage. The analyses also assure that as condensate/gas ratio decreases continually during the gas injection process, which causes the reservoir fluid PVT characteristics to be changed accordingly furthermore the reservoir pressure can be depleted in order to vaporize the liquid phase and to maintain the reservoir fluid to be in a single phase state during the reservoir exploitation. It was seen to be closely related with retrograde dew point pressure and fog up pressures. Therefore, in terms of incorporating colloidal systems into the subject focusing more on the region between fogging up and retrograde condensation gave interesting results.     

注干气开发凝析气藏注采压力系统研究——以塔里木盆地大涝坝2号构造凝析气藏为例

为有效提高凝析油的采收率,获得更大的经济效益,在开发凝析油含量较高的凝析气藏时多采用注干气保压方式.注干气开发凝析气藏时,对注采压力系统的研究尤其重要,但相关的技术方法较少.经过研究,建立了一种用于分析注采压力系统的方法:以系统测试资料为基础,利用一点法公式、无阻流量与地层系数的关系,确定出气藏的平均产气能力及注气能力方程,再结合垂直管流方程、液滴重力平衡方程对注干气开发凝析气藏注采压力系统进行研究.以塔里木盆地大涝坝2号构造凝析气藏为例,对注干气开发该气藏时的注采压力剖面、注采平衡系统、废弃压力及停喷压力进行了计算与分析.实例应用结果表明,该方法简便实用.