Displacement characteristics of CO_(2)flooding in extra-high water-cut reservoirs

Carbon dioxide(CO_(2))flooding is a widely applied recovery method during the tertiary recovery of oil and gas.A high water saturation condition in reservoirs could induce a‘water shielding’phenomenon after the injection of CO_(2).This would prevent contact between the injected gas and the residual oil,restricting the development of the miscible zone.A micro-visual experiment of dead-end models,used to observe the effect of a film of water on the miscibility process,indicates that CO_(2)can penetrate the water film and come into contact with the residual oil,although the mixing is significantly delayed.However,the dissolution loss of CO_(2)at high water-cut conditions is not negligible.The oil-water partition coefficient,defined as the ratio of CO_(2)solubility in an oil-brine/two-phase system,keeps constant for specific reservoir conditions and changes little with an injection gas.The NMR device shows that when CO_(2)flooding follows water flooding,the residual oil decreasesdnot only in medium and large pores but also in small and micro pores.At levels of higher water saturation,CO_(2)displacement is characterized initially by a low oil production rate and high water-cut.After the CO_(2)breakthrough,the water-cut decreases sharply and the oil production rate increases gradually.The response time of CO_(2)flooding at high watercut reservoirs is typically delayed and prolonged.These results were confirmed in a pilot test for CO_(2)flooding at the P1-1 well group of the Pucheng Oilfield.Observations from this pilot study also suggest that a larger injection gas pore volume available for CO_(2)injection is required to offset the dissolution loss in high water saturation conditions.

Influence of reservoir heterogeneity on immiscible water-alternating-CO_(2)flooding:A case study

Currently,limited studies of immiscible water-alternating-CO_(2)(imWACO_(2))flooding focus on the impact of reservoir heterogeneity on reservoir development outcomes.Given this,using the heterogeneous reservoirs in the Gao 89-1 block as a case study,this study conducted slab core flooding experiments and numerical simulations to assess the impact of reservoir heterogeneity on imWACO_(2)flooding efficiency.It can be concluded that imWACO_(2)flooding can enhance the sweep volume and oil recovery compared to continuous CO_(2)flooding.As the permeability difference increases,the difference in the swept volume between zones/layers with relatively high and low permeability increases.To optimize the exploitation of reservoirs in the Gao 89-1 block,the optimal timing and CO_(2)injection rate for imWACO_(2)flooding are determined at water cut of 40%and 10000 m^(3)/d,respectively.A short injection-production semi-period,combined with multiple cycles of water and CO_(2)injection alternations,is beneficial for enhanced oil recovery from imWACO_(2)flooding.

Effect of CO_(2)flooding in an oil reservoir with strong bottom-water drive in the Tahe Oilfield,Tarim Basin,Northwest China

The dissolution and diffusion of CO_(2)in oil and water and its displacement mechanism were investigated by laboratory experiment and numerical simulation for Block 9 in the Tahe oilfield,a sandstone oil reservoir with strong bottom-water drive in Tarim Basin,Northwest China.Such parameters were analyzed as solubility ratio of CO_(2)in oil,gas and water,interfacial tension,in-situ oil viscosity distribution,remaining oil saturation distribution,and oil compositions.The results show that CO_(2)flooding could control water coning and increase oil production.In the early stage of the injection process,CO_(2)expanded vertically due to gravity differentiation,and extended laterally under the action of strong bottom water in the intermediate and late stages.The CO_(2)got enriched and extended at the oil-water interface,forming a high interfacial tension zone,which inhibited the coning of bottom water to some extent.A miscible region with low interfacial tension formed at the gas injection front,which reduced the in-situ oil viscosity by about 50%.The numerical simulation results show that enhanced oil recovery(EOR)is estimated at 5.72%and the oil exchange ratio of CO_(2)is 0.17 t/t.

Seismic dynamic monitoring in CO_2 flooding based on characterization of frequency-dependent velocity factor

The phase velocity of seismic waves varies with the propagation frequency, and thus frequency-dependent phenomena appear when CO2 gas is injected into a reservoir. By dynamically considering these phenomena with reservoir conditions it is thus feasible to extract the frequency-dependent velocity factor with the aim of monitoring changes in the reservoir both before and after CO2 injection. In the paper, we derive a quantitative expression for the frequency-dependent factor based on the Robinson seismic convolution model. In addition, an inversion equation with a frequency-dependent velocity factor is constructed, and a procedure is implemented using the following four processing steps： decomposition of the spectrum by generalized S transform, wavelet extraction of cross-well seismic traces, spectrum equalization processing, and an extraction method for frequency-dependent velocity factor based on the damped least-square algorithm. An attenuation layered model is then established based on changes in the Q value of the viscoelastic medium, and spectra of migration profiles from forward modeling are obtained and analyzed. Frequency-dependent factors are extracted and compared, and the effectiveness of the method is then verified using a synthetic data. The frequency-dependent velocity factor is finally applied to target processing and oil displacement monitoring based on real seismic data obtained before and after CO2 injection in the G89 well block within Shengli oilfield. Profiles and slices of the frequency-dependent factor determine its ability to indicate differences in CO2 flooding, and the predicting results are highly consistent with those of practical investigations within the well block.

Screening and field application of microbial-flooding activator systems

This study aims to further enhance the oil recovery of reservoirs in the Zhong-2 Block of the Gudao Oilfield by identifying the most effective microbial-flooding activator systems and applying them in the field.We began by analyzing the structure of the reservoirs'endogenous microbial communities to understand the potential impact of microbial flooding.This was followed by determining commonly used activator systems based on their abilities to stimulate oil-displacement functional bacteria.Through laboratory experiments on oil displacement efficiency and sweep characteristics,we determined the optimal activator injection method(injection ratio)and the requisite bacterial concentration for maximal microbial-flooding efficacy.Finally,we selected the optimal activator systems and applied them to field tests.Our findings suggest the target block is highly receptive to microbial-flooding.In terms of performance,the activator systems ranked as No.3>No.4>No.1>No.2.Interestingly,a deep activator system,when compared to the top-performing No.3 system,exhibited a higher bacterial concentration peak and longer peaking duration.Optimal oil displacement effects were observed at a 1:4 vol ratio between the No.3 activator and deep activator systems,with bacterial concentrations of up to 106 cells/mL or above.Field tests with the selected activator systems,following a specific injection protocol,demonstrated a notable increase in oil production and a reduction in water cut.

Key Problems and Countermeasures in CO_(2)Flooding and Storage

Based on literature research in combination with the practice of CO_(2)flooding and storage in Jilin Oilfield,this study assesses the key problems in CO_(2)flooding and storage,proposing the corresponding countermeasures from five aspects of CO_(2)gas source condition,namely geological condition evaluation,scheme design incoordination with other production methods,economic and effectiveness evaluation,together with dynamic monitoring and safety evaluation.The results show that CO_(2)flooding is the most economic and effective CO_(2)storage method.In eastern China,inorganic origin CO_(2)gas reservoirs are widely developed and are especially the most enriched in the Paleozoic carbonate rock strata and the Cenozoic Paleogene–Neogene system,which provide a rich resource base for CO_(2)flooding and storage.In the future,CO_(2)generated in the industrial field will become the main gas source of CO_(2)flooding and storage.The evaluation of geological conditions of oil and gas reservoirs is the basis for the potential evaluation,planning scheme design and implementation of CO_(2)flooding and storage.CO_(2)storage should be below the depth of 800 m,the CO_(2)flooding and storage effects in lowpermeability oil reservoirs being the best.CO_(2)geological storage mechanisms primarily consist of tectonic geological storage,bound gas storage,dissolution storage,mineralization storage,hydrodynamic storage and coalbed adsorption storage.The practice of CO_(2)flooding and storage in Jilin Oilfield demonstratesthat the oil increment by CO_(2)flooding is at least 24%higher than by conventional water flooding.The most critical factor determining the success or failure of CO_(2)flooding and storage is economic effectiveness,which needs to be explored from two aspects:the method and technology innovation along with the carbon peaking and carbon neutrality policy support.After CO_(2)is injected into the reservoir,it will react with the reservoir and fluid,the problem of CO_(2)recovery or overflow will occur,so the dynamic monitoring and safety evaluation of CO_(2)flooding and storage are very important.This study is of great significance to the expansion of the application scope of CO_(2)flooding and storage and future scientific planning and deployment.

Characteristics and mechanism of smart fluid for sweep-controlling during CO_(2) flooding

A smart response fluid was designed and developed to overcome the challenges of gas channeling during CO_(2)flooding in low-permeability,tight oil reservoirs.The fluid is based on Gemini surfactant with self-assembly capabilities,and the tertiary amine group serves as the response component.The responsive characteristics and corresponding mechanism of the smart fluid during the interaction with CO_(2)/oil were studied,followed by the shear characteristics of the thickened aggregates obtained by the smart fluid responding to CO_(2).The temperature and salt resistance of the smart fluid and the aggregates were evaluated,and their feasibility and effectiveness in sweep-controlling during the CO_(2)flooding were confirmed.This research reveals:(1)Thickened aggregates could be assembled since the smart fluid interacted with CO_(2).When the mass fraction of the smart fluid ranged from 0.05%to 2.50%,the thickening ratio changed from 9 to 246,with viscosity reaching 13 to 3100 mPas.As a result,the sweep efficiency in low-permeability core models could be increased in our experiments.(2)When the smart fluid(0.5%to 1.0%)was exposed to simulated oil,the oil/fluid interfacial tension decreased to the level of 1×10^(-2)mN/m.Furthermore,the vesicle-like micelles in the smart fluid completely transformed into spherical micelles when the fluid was exposed to simulated oil with the saturation greater than 10%.As a result,the smart fluid could maintain low oil/fluid interfacial tension,and would not be thickened after oil exposure.(3)When the smart fluid interacted with CO_(2),the aggregates showed self-healing properties in terms of shear-thinning,static-thickening,and structural integrity after several shear-static cycles.Therefore,this fluid is safe to be placed in deep reservoirs.(4)The long-term temperature and salt resistance of the smart fluid and thickened aggregates have been confirmed.

Profile improvement during CO_2 flooding in ultra-low permeability reservoirs

Gas flooding such as CO2 flooding may be effectively applied to ultra-low permeability reservoirs, but gas channeling is inevitable due to low viscosity and high mobility of gas and formation heterogeneity. In order to mitigate or prevent gas channeling, ethylenediamine is chosen for permeability profile control. The reaction mechanism of ethylenediamine with CO2, injection performance, swept volume, and enhanced oil recovery were systematically evaluated. The reaction product of ethylenediamine and CO2 was a white solid or a light yellow viscous liquid, which would mitigate or prevent gas channeling. Also, ethylenediamine could be easily injected into ultra-low permeability cores at high temperature with protective ethanol slugs. The core was swept by injection of 0.3 PV ethylenediamine. Oil displacement tests performed on heterogeneous models with closed fractures, oil recovery was significantly enhanced with injection of ethylenediamine. Experimental results showed that using ethylenediamine to plug high permeability layers would provide a new research idea for the gas injection in fractured, heterogeneous and ultra-low permeability reservoirs. This technology has the potential to be widely applied in oilfields.

Modeling CO_2 miscible flooding for enhanced oil recovery

The injection of fuel-generated CO2 into oil reservoirs will lead to two benefits in both enhanced oil recovery （EOR） and the reduction in atmospheric emission of CO2. To get an insight into CO2 miscible flooding performance in oil reservoirs, a multi-compositional non-isothermal CO2 miscible flooding mathematical model is developed. The convection and diffusion of CO2-hydrocarbon mixtures in multiphase fluids in reservoirs, mass transfer between CO2 and crude, and formation damages caused by asphaltene precipitation are fully considered in the model. The governing equations are discretized in space using the integral finite difference method. The Newton-Raphson iterative technique was used to solve the nonlinear equation systems of mass and energy conservation. A numerical simulator, in which regular grids and irregular grids are optional, was developed for predicting CO2 miscible flooding processes. Two examples of one-dimensional （1D） regular and three-dimensional （3D） rectangle and polygonal grids are designed to demonstrate the functions of the simulator. Experimental data validate the developed simulator by comparison with 1D simulation results. The applications of the simulator indicate that it is feasible for predicting CO2 flooding in oil reservoirs for EOR.

Effect of stress sensitivity on displacement efficiency in CO_2 flooding for fractured low permeability reservoirs

Carbon dioxide flooding is an effective means of enhanced oil recovery for low permeability reservoirs. If fractures are present in the reservoir, CO2 may flow along the fractures, resulting in low gas displacement efficiency. Reservoir pore pressure will fluctuate to some extent during a CO2 flood, causing a change in effective confining pressure. The result is rock deformation and a reduction in permeability with the reduction in fracture permeability, causing increased flow resistance in the fracture space. Simultaneously, gas cross flowing along the fractures is partially restrained. In this work, the effect of stress changes on permeability was studied through a series of flow experiments. The change in the flowrate distribution in a matrix block and contained fracture with an increase in effective pressure were analyzed. The results lead to an implicit comparison which shows that permeability of fractured core decreases sharply with an increase in effective confining pressure. The fracture flowrate ratio declines and the matrix flowrate ratio increases. Fracture flow will partially divert to the matrix block with the increase in effective confining pressure, improving gas displacement efficiency.

CO_2-triggered gelation for mobility control and channeling blocking during CO_2 flooding processes

CO2 flooding is regarded as an important method for enhanced oil recovery （EOR） and greenhouse gas control. However, the heterogeneity prevalently dis- tributed in reservoirs inhibits the performance of this technology. The sweep efficiency can be significantly reduced especially in the presence of ＂thief zones＂. Hence, gas channeling blocking and mobility control are important technical issues for the success of CO2 injection. Normally, crosslinked gels have the potential to block gas channels, but the gelation time control poses challenges to this method. In this study, a new method for selectively blocking CO2 channeling is proposed, which is based on a type of CO2-sensitive gel system （modified polyacry- lamide-methenamine-resorcinol gel system） to form gel in situ. A CO2-sensitive gel system is when gelation or solidification will be triggered by CO2 in the reservoir to block gas channels. The CO2-sensitivity of the gel system was demonstrated in parallel bottle tests of gel in N2 and CO2 atmospheres. Sand pack flow experiments were con- ducted to investigate the shutoff capacity of the gel system under different conditions. The injectivity of the gel system was studied via viscosity measurements. The results indi- cate that this gel system was sensitive to CO2 and had good performance of channeling blocking in porous media. Advantageous viscosity-temperature characteristics were achieved in this work. The effectiveness for EOR in heterogeneous formations based on this gel system was demonstrated using displacement tests conducted in double sand packs. The experimental results can provide guideli- nes for the deployment of theCO2-sensitive gel system for field applications.

Integrated Techniques of Underground CO_2 Storage and Flooding Put into Commercial Application in the Jilin Oilfield, China

Global warming touches everybody＇s nerve, and direct reason for sharp increasing of CO2 in the atmosphere results mainly from the use of fossil fuel in power generation and other industries. How can humans return this ＂devil＂ to underground, and keep a peaceful environment for human？ Scientists from all over the world have been exploring them.

Experimental study on asphaltene precipitation induced by CO_2 flooding

The effects of CO2 pressure,temperature and concentration on asphaltene precipitation induced by CO2 were studied using a high-pressure vessel,interfacial tensiometer,Fourier transform infrared （FTIR） and drill core displacement experimental apparatus.The results indicated that the content of asphaltene in crude oil decreased,and the interfacial tension between a model oil and distilled water increased,with an increase of CO2 pressure,decrease of temperature and increase of molar ratio of CO2 to crude oil when CO2 contacted crude oil in the high pressure vessel.The content of asphaltene in sweepout oil and the permeability of test cores both also decreased with an increase of CO2 flooding pressure.The main reason for changes in content of asphaltene in crude oil,in interfacial tension between model oil and distilled water and in the permeability of the test core is the precipitation of asphaltene which is an interfacially active substance in crude oil.Precipitation of asphaltene also blocks pores in the drill core which decreases the permeability.

低渗砂岩油田CO_(2)驱化学机理及提高采收率研究

针对低渗砂岩油藏进行了CO_(2)驱开发技术研究,分析了CO_(2)驱油化学机理及主要影响因素。基于目标油藏流体特征进行了PVT拟合,确定其CO_(2)驱最小混相压力,明确了不同压力及注入时机对CO_(2)驱采收率、气油比、含水率及驱动压差等的影响规律,探究了CO_(2)泡沫驱在提高采收率方面的效用。结果表明:24.5 MPa为目标区域CO_(2)驱的最小混相压力,采收率会随着压力的升高而增加,28 MPa时CO_(2)驱提高采收率可达30.57%。气体突破时间、总采收率与CO_(2)注入时机密切相关,CO_(2)注入越早,越有利于采收率的提高,出口含水率为60%时注入可提高采收率39.13%。CO_(2)泡沫驱可以在一定程度上起到提高采收率的效用。

CO_(2)驱硅酸凝胶封窜体系的制备与性能

开发易注入、可形成有效封堵的裂缝封窜技术,对于二氧化碳驱提高采收率技术具有重要意义。本文对硅酸乙酯进行改性提高其耐温性和不溶性,再乳化形成易注入的O/W乳液。考察了乳化剂类型和乳化剂用量对胶凝过程的影响。明确了pH值、温度和矿化度等因素对凝胶性能的影响,并通过人造岩心评价了该硅酸凝胶体系的封堵性能。结果表明,20%Span-80+80%Tween-80组成的复合乳化剂乳化效果最好,当乳化用量为1%时,乳化稳定性最好,粒径均一,易注入。当环境pH小于7时,可诱导乳液形成凝胶;随着温度的升高,成胶强度增强,但在150℃高温下,成胶强度下降。升高矿化度可加速凝胶的生成,但对凝胶强度影响较小。动力学分析表明,H+浓度是影响凝胶产生的关键因素,因此可在CO_(2)驱替酸性环境中,实现地层封堵。岩心实验结果显示,该凝胶颗粒可显著降低地层渗透率,封堵效果大于80%,可满足二氧化碳驱防气窜要求。

CO_(2)驱油封存泄漏风险管理系统及应用研究

CO_(2)驱油封存技术在提高原油采收率的同时能实现大规模CO_(2)封存。然而,驱油封存过程伴随着多种CO_(2)泄漏风险。针对以往CO_(2)泄漏风险管理系统的缺乏,特别是缺少基于在线监测的管理系统,难以支撑动态的风险管理,研究基于CO_(2)驱油封存泄漏风险管理体系的构建,开发了集成多环境实时风险识别和评估、多空间风险预测、多层级风险预警和全过程风险控制的动态CO_(2)泄漏风险管理系统,并应用于鄂尔多斯盆地延长石油CO_(2)驱油封存示范项目。案例应用研究表明,所开发的CO_(2)泄漏风险管理系统可以全空间动态识别CO_(2)驱油封存过程的各种泄漏风险,有效支撑泄漏风险的动态管理,为CO_(2)驱油封存项目提供全面及时的安全保障。

胜利油田CO_(2)高压混相驱油与封存理论技术及矿场实践

针对胜利油田开展CO_(2)驱油与封存面临的原油轻烃含量低混相难、储层非均质性强波及效率低、易气窜全过程调控难等问题,通过室内实验、技术攻关和矿场实践,形成CO_(2)高压混相驱油与封存理论及关键技术。研究发现,提高地层压力至1.2倍最小混相压力之上,可以提高原油中的中重质组分混相能力,增大小孔隙中的原油动用程度,均衡驱替前缘,扩大波及体积。通过超前压驱补能实现快速高压混相,采用梯级气水交替、注采耦合、多级化学调堵等技术全过程动态调控渗流阻力,实现采收率与封存率的协同最优。研究成果应用于高89-樊142 CCUS(二氧化碳捕集、利用与封存)示范区,区块日产油由254.6 t提高至358.2 t,预计实施15年可提高采出程度11.6个百分点,为CCUS规模化应用提供理论和技术支撑。

吉林特低渗油藏长岩心CO_(2)驱替微观动用规律研究

针对松辽盆地莫里青油田特低渗储层动用困难的问题,利用物理模拟实验和核磁共振技术相结合的实验方法,开展长岩心CO_(2)驱替研究。结果表明:岩样CO_(2)驱替驱油效率介于72.30%~80.40%。大孔喉(>33 ms)平均赋存占比为33.65%,小孔喉(<33 ms)平均赋存占比为18.01%;1 PV的CO_(2)驱替后,岩样大孔喉平均相对采出程度为80.67%,小孔喉平均相对采出程度为17.45%;5 PV的CO_(2)驱替后,岩样大孔喉平均相对采出程度为95.68%,小孔喉平均相对采出程度为39.82%,大PV驱替可有效动用小孔喉的油。研究成果可为莫里青油田储层CO_(2)驱替提供理论支撑,同时也为同类油藏开展注气先导试验提供科学指导。

原油-CO_(2)相互作用机理分子动力学模拟研究

CO_(2)的众多驱油机理已经被广泛认同,但受油藏因素影响,不同油藏条件下CO_(2)驱的效果差异较大。因此,需要进一步深化研究CO_(2)与原油的微观相互作用机理,明确不同油藏条件下CO_(2)的驱油方式,最大限度挖潜CO_(2)驱的潜力。利用分子动力学模拟方法研究了组分、温度、压力对油滴-CO_(2)相互作用的影响。求取动力学参数,量化表征油滴-CO_(2)间的相互作用,厘清了不同条件下二者的微观相互作用规律。模拟结果显示,色散力是主导CO_(2)-烷烃分子相互作用的主要作用能,二者相互作用主要包含两方面:一是CO_(2)分子克服烷烃分子间的位阻作用向油滴内部溶解扩散,二是CO_(2)分子对油滴外层分子的萃取吸引作用。随着烷烃分子链长减小、温度降低和压力增加,油滴溶解度参数和CO_(2)配位数增加,油滴外层分子的弯曲度减小,二者的相互作用增强。研究结果认为,在温度较低、压力较高的轻质和中轻质油藏中,应尽可能地实现CO_(2)混相驱和近混相驱,在温度较高、压力较低的中质和重质油藏中,应充分发挥CO_(2)非混相驱的溶解降黏、膨胀原油体积和补充能量的优势。研究结果能够为室内研究和现场实施CO_(2)驱油提供理论指导。

高温环境下模拟CO_(2)驱采出液中304不锈钢的腐蚀行为

现阶段随着CO_(2)驱油技术的普遍使用,采出液中CO_(2)的含量不断上升,使得采出系统中金属管道的腐蚀程度逐渐增大。为了掌握304不锈钢在高温环境下CO_(2)驱采出液中的腐蚀行为,室内配制了模拟CO_(2)驱采出液,测试了304不锈钢在高温环境下不同含量CO_(2)、不同腐蚀时间下的金属腐蚀速率,并对不同腐蚀时间下金属表面的微观形貌和腐蚀产物组成进行了表征。结果表明,随着CO_(2)含量和腐蚀时间的不断上升,金属的腐蚀速率上升;随着腐蚀时间的上升,金属表面的腐蚀产物逐渐增加,发生了局部区域析氢腐蚀过程;腐蚀产物膜主要由Fe_(2)O_(3)、Cr_(2)O_(3)和FeCO_(3)组成。