Effect of high-multiple water injection on rock pore structure and oil displacement efficiency

Experimental methods,including mercury pressure,nuclear magnetic resonance(NMR)and core(wateroil)displacement,are used to examine the effects of high-multiple water injection(i.e.water injection with high injected pore volume)on rock properties,pore structure and oil displacement efficiency of an oilfield in the western South China Sea.The results show an increase in the permeability of rocks along with particle migration,an increase in the pore volume and the average pore throat radius,and enhanced heterogeneity after high-multiple water injection.Compared with normal water injection methods,a high-multiple water injection is more effective in improving the oil displacement efficiency.The degree of recovery increases faster in the early stage due to the expansion of the swept area,and the transition from oil-wet to water-wet.The degree of recovery increases less in the late stage due to various factors,including the enhancement of heterogeneity in the rocks.Considering both the economic aspect and the production limit of water flooding,it is recommended to adopt other technologies to further enhance oil recovery after 300 PV water injection.

Nanoparticle-reinforced foam system for enhanced oil recovery(EOR):Mechanistic review and perspective

Boosted by economic development and rising living standards,the world's carbon dioxide emissions remain high.Maintaining temperature rises below 1.5℃ by the end of the century requires rapid global carbon capture and storage implementation.The successful application of carbon capture,utilization,and storage(CCUS)technology in oilfields has become the key to getting rid of this predicament.Foam flooding,as an organic combination of gas and chemical flooding,became popular in the 1950s.Notwithstanding the irreplaceable advantages,as a thermodynamically unstable system,foam's stability has long restricted its development in enhanced oil and gas recovery.With special surface/interface effects and small-size effects,nanoparticles can be used as foam stabilizers to enhance foam stability,thereby improving foam seepage and oil displacement effects in porous media.In this paper,the decay kinetics and the stabilization mechanisms of nanoparticle-reinforced foams were systematically reviewed.The effects of nanoparticle characteristics,including particle concentration,surface wettability,particle size,and type,and reservoir environment factors,including oil,temperature,pressure,and salinity on the foam stabilization ability were analyzed in detail.The seepage and flooding mechanisms of nanoparticle-reinforced foams were summarized as:improving the plugging properties of foams,enhancing the interaction between foams and crude oil,and synergistically adjusting the wettability of reservoir rocks.Finally,the challenges in the practical application of nanoparticle-reinforced foams were highlighted,and the development direction was proposed.The development of nanoparticle-reinforced foam can open the way toward adaptive and evolutive EOR technology,taking one further step towards the high-efficiency production of the petroleum industry.

Unraveling the influence of surface roughness on oil displacement by Janus nanoparticles

Janus nanoparticles(JNPs)possess great potential in recovering the residual oil from reservoirs,however,the fundamental interaction mechanisms among nanoparticles,the oil,and reservoir wall characteristics remain to be elucidated.In this work,models of oil trapping grooves with different geometric features are subjected to molecular dynamics simulations for investigating the influences of roughness parameters on oil displacement dynamics by JNPs.Four key surface geometry parameters and different degrees of surface hydrophobicity are considered.Our results indicate that JNPs hold an outstanding performance in displacing residual oil on weakly to moderately hydrophobic surfaces.Overall,smaller entry and exit angles,the larger aspect ratio of the oil trapping grooves,and a bigger tip length of the rough ridges lead to superior oil recovery.Among the key geometric parameters,the aspect ratio of the oil trapping grooves plays the dominant role.These insights about the interaction of surface properties and JNPs and the resulting trapped oil displacement could serve as a theoretical reference for the application of JNPs for targeted reservoir conditions.

Mechanisms of oil displacement by ASP-foam and its influencing factors

ASP-foam （ASPF） is a system prepared by injecting natural gas into the conventional alkali- surfactant-polymer （ASP） system. Foam can be formed in the porous media by the interaction of gas and surfactant in the ASP system. With the ASPF system, oil recovery is improved as the interfacial tension （IFT） is reduced to a relatively low level, and the swept volume is enlarged. In this paper, four surfactants were evaluated and characterized by IFT between ASP system and oil and the foaming performance. AI- kyl benzene sulfonate （ORS-41） was chosen as the surfactant to best reduce IFT between displacement fluids and oil and improve the foaming performance. The mechanisms of ASPF flooding were studied in this paper, the results show that the ASPF flooding not only enlarges the swept volume but also enhances the displacement efficiency. The effects of reservoir heterogeneity, the gas-liquid ratio of ASPF system, and the concentrations of polymer and surfactant on the displacement efficiency were studied. A field trial of ASPF flooding has also been conducted. Both the laboratory results and the field trial results show that the ASPF flooding can significantly increase the oil recovery, with a 30% increase in the proportion of the original oil in place recovered compared with water flooding.

The mechanism of hydraulic fracturing assisted oil displacement to enhance oil recovery in low and medium permeability reservoirs

Aiming at the technology of hydraulic fracturing assisted oil displacement which combines hydraulic fracturing,seepage and oil displacement,an experimental system of energy storage and flowback in fracturing assisted oil displacement process has been developed and used to simulate the mechanism of percolation,energy storage,oil displacement and flowback of chemical agents in the whole process.The research shows that in hydraulic fracturing assisted oil displacement,the chemical agent could be directly pushed to the deeper area of the low and medium permeability reservoirs,avoiding the viscosity loss and adhesion retention of chemical agents near the pay zone;in addition,this technology could effectively enlarge the swept volume,improve the oil displacement efficiency,replenish formation energy,gather and exploit the scattered residual oil.For the reservoir with higher permeability,this measure takes effect fast,so to lower cost,and the high pressure hydraulic fracturing assisted oil displacement could be adopted directly.For the reservoir with lower permeability which is difficult to absorb water,hydraulic fracturing assisted oil displacement with surfactant should be adopted to reduce flow resistance of the reservoir and improve the water absorption capacity and development effect of the reservoir.The degree of formation energy deficit was the main factor affecting the effective swept range of chemical agents.Moreover,the larger the formation energy deficit was,the further the seepage distance of chemical agents was,accordingly,the larger the effective swept volume was,and the greater the increase of oil recovery was.Formation energy enhancement was the most important contribution to enhanced oil recovery(EOR),which was the key to EOR by the technology of hydraulic fracturing assisted oil displacement.

Experimental investigation on stable displacement mechanism and oil recovery enhancement of oxygen-reduced air assisted gravity drainage

The effects of gravity,capillary force,and viscous force on the migration characteristics of oil and gas interface in oxygen-reduced air-assisted gravity drainage(OAGD)were studied through a two-dimensional visualization model.The effects of bond number,capillary number and low-temperature oxidation on OAGD recovery were studied by long core displacement experiments.On this basis,the low-temperature oxidation number was introduced and its relationship with the OAGD recovery was established.The results show that the shape and changing law of oil and gas front are mainly influenced by gravity,capillary force and viscous force.When the bond number is constant(4.52×10-4),the shape of oil-gas front is controlled by capillary number.When the capillary number is less than 1.68×10-3,the oil and gas interface is stable.When the capillary number is greater than 2.69×10-2,the oil and gas interface shows viscous fingering.When the capillary number is between 1.68×10-3 and 2.69×10-2,the oil and gas interface becomes capillary fingering.The core flooding experiments results show that for OAGD stable flooding,before the gas breakthrough,higher recovery is obtained in higher gravity number and lower capillary number.In this stage,gravity is predominant in controlling OAGD recovery and the oil recovery could be improved by reducing injection velocity.After gas breakthrough,higher recovery was obtained in lower gravity and higher capillary numbers,which means that the viscous force had a significant influence on the recovery.Increasing gas injection velocity in this stage is an effective measure to improve oil recovery.The low-temperature oxidation number has a good correlation with the recovery and can be used to predict the OAGD recovery.

Progress and prospect of carbon dioxide capture, utilization and storage in CNPC oilfields

The development history of carbon capture,utilization and storage for enhanced oil recovery(CCUS-EOR)in China is comprehensively reviewed,which consists of three stages:research and exploration,field test and industrial application.The breakthrough understanding of CO_(2) flooding mechanism and field practice in recent years and the corresponding supporting technical achievements of CCUS-EOR project are systematically described.The future development prospects are also pointed out.After nearly 60 years of exploration,the theory of CO_(2) flooding and storage suitable for continental sedimentary reservoirs in China has been innovatively developed.It is suggested that C7–C15 are also important components affecting miscibility of CO_(2) and crude oil.The mechanism of rapid recovery of formation energy by CO_(2) and significant improvement of block productivity and recovery factor has been verified in field tests.The CCUS-EOR reservoir engineering design technology for continental sedimentary reservoir is established.The technology of reservoir engineering parameter design and well spacing optimization has been developed,which focuses on maintaining miscibility to improve oil displacement efficiency and uniform displacement to improve sweep efficiency.The technology of CO_(2) capture,injection and production process,whole-system anticorrosion,storage monitoring and other whole-process supporting technologies have been initially formed.In order to realize the efficient utilization and permanent storage of CO_(2),it is necessary to take the oil reservoir in the oil-water transition zone into consideration,realize the large-scale CO_(2) flooding and storage in the area from single reservoir to the overall structural control system.The oil reservoir in the oil-water transition zone is developed by stable gravity flooding of injecting CO_(2) from structural highs.The research on the storage technology such as the conversion of residual oil and CO_(2) into methane needs to be carried out.

Preparation and Field Application of a Novel Micro-emulsion as Heavy Oil Displacement Agent

A novel micro-emulsion was prepared by mixing an oil-soluble viscosity reducer,which was screened to aim at improving the heavy oil properties of Shengli oilfield with water-soluble surfactant and co-surfactant.The static viscosity reduction and oil washing performance of the micro-emulsion were investigated,and the field application of the microemulsion used as heavy oil displacement agent was also reported.Results showed that the micro-emulsion exhibited excellent viscosity reduction performance for the studied heavy oil samples.When heavy oil was mixed with 0.5%of the micro-emulsion,a stable oil-in-water heavy oil emulsion could be formed.After the content of the micro-emulsion was increased to 3.0%,the oil removing rate reached up to 80%.Field application of the micro-emulsion to the Pai-601-Ping-115 well and the Pai-601-Ping-123 well was shown to be effective by increasing the periodic oil production up to 203 tons.

Experimental Research on the Millimeter-Scale Distribution of Oil in Heterogeneous Reservoirs

Oil saturation is a critical parameter when designing oil field development plans.This study focuses on the change of oil saturation during water flooding.Particularly,a meter-level artificial model is used to conduct relevant experiments on the basis of similarity principles and taking into account the layer geological characteristics of the reservoir.The displacement experiment’s total recovery rate is 41.35%.The changes in the remaining oil saturation at a millimeter-scale are examined using medical spiral computer tomography principles.In all experimental stages,regions exists where the oil saturation decline is more than 10.0%.The shrinkage percentage is 20.70%in the horizontal well production stage.The oil saturation reduction in other parts is less than 10.0%,and there are regions where the oil saturation increases in the conventional water flooding stage.

Synthesis of temperature-resistant and salttolerant surfactant SDB-7 and its performance evaluation for Tahe Oilfield flooding (China)

In order to improve the enhanced oil recovery of high-temperature and high-salt oilfields, a novel temperature-resistant and salt-tolerant surfactant （denoted as SDB-7） was synthesized and evaluated for the Tahe Oilfield （Xinjiang, China）, which is representative of high-temperature and high-salt oilfields. It has a central reservoir temperature of 140 ℃ and salinity of 22.6× 10^4 mg/L. The temperature-resistant and salt-tolerant performance, interfacial activity, oil displacement efficiency, aging properties, and adsorption properties of the synthesized surfactant were evaluated for Tahe Oilfield flooding. The results showed that the SDB-7 was temperature-resistant and salt-tolerant capacity of 140 ℃ and 22.6×10^4 rag/ L, respectively, oil displacement efficiency under static condition of 84%, and adsorption loss of 0.4 mg/ g （less than 1 mg/g-oil sand）. In the heat aging experiment （under the temperature of 140 ℃ for 60 days）, the oil-water interracial tension and oil displacement efficiency of SDB-7 were almost unchanged. The oil displacement experiments showed that, under the temperature of 140 ℃ and the salinity of 22.6× 10^4 mg/L, the surfactant SDB-7 can enhance oil recovery by 14.5% after water flooding,suggesting that SDB-7 has a promising application in high temperature and high salinity （HT/HS） reservoir.

Microscopic production characteristics of crude oil in nano-pores of shale oil reservoirs during CO_(2)huff and puff

The parameters such as pore size distribution,specific surface area and pore volume of shale rock samples are analyzed by low-temperature nitrogen adsorption experiment,and then the conversion coefficient between relaxation time(T_(2))and pore size is calibrated.Nuclear magnetic resonance experiments of CO_(2)huff and puff in shale samples are carried out to study the effects of gas injection pressure,soaking time and fractures on the oil production characteristics of shale pores from the micro scale.The recovery degrees of small pores(less than or equal to 50 nm)and large pores(greater than 50 nm)are quantitatively evaluated.The experimental results show that the recovery degree of crude oil in large pores increases rapidly with the increase of injection pressure under immiscible conditions,and the effect of injection pressure rise on recovery degree of large pores decreases under miscible conditions;whether miscible or not,the recovery degree of crude oil in small pores basically maintains a linear increase with the increase of injection pressure,and the lower size limit of pores in which oil can be recovered by CO_(2)decreases with the increase of gas injection pressure;with the increase of soaking time,the recovery degree of crude oil in large pores increases slowly gradually,while the recovery degree of crude oil in small pores increases faster and then decelerates,and the best soaking time in the experiments is about 10 h;the existence of fractures can enhance the recovery degrees of crude oil in small pores and large pores noticeably.

Imbibition mechanisms of high temperature resistant microemulsion system in ultra-low permeability and tight reservoirs

Lower-phase microemulsions with core-shell structure were prepared by microemulsion dilution method.The high temperature resistant systems were screened and the performance evaluation experiments were conducted to clarify the spontaneous imbibition mechanisms in ultra-low permeability and tight oil reservoirs,and to direct the field microfracture huff and puff test of oil well.The microemulsion system(O-ME)with cationic-nonionic surfactant as hydrophilic shell,No.3 white oil as oil phase core has the highest imbibition recovery;its spontaneous imbibition mechanisms include:the ultra-low interfacial tension and wettability reversal significantly reduce oil adhesion work to improve oil displacement efficiency,the nanoscale“core-shell structure”formed can easily enter micro-nano pores and throats to expand the swept volume,in addition,the remarkable effect of dispersing and solubilizing crude oil can improve the mobility of crude oil.Based on the experimental results,a microfracture huff and puff test of O-ME was carried out in Well YBD43-X506 of Shengli Oilfield.After being treated,the well had a significant increase of daily fluid production to 5 tons from 1.4 tons,and an increase of daily oil production to 2.7 tons from 1.0 ton before treatment.

Experimental characterization and mechanism of hydraulic pulsation waves driving microscopic residual oil

To clarify microscopic mechanisms of residual oil displacement by hydraulic pulsation wave,microscopic visualization experiments of hydraulic pulsation wave driving residual oil were carried out by using the microscopic visualization device of pulsating water drive.For the four types of residual oil left in the reservoir after water flooding,i.e.membrane,column,cluster,and blind end residual oils,hydraulic pulsation waves broke the micro-equilibrium of the interface by disturbing the oil-water interface,so that the injected water invaded into and contacted with the remaining oil in small pores and blind holes,and the remaining oil was pushed or stripped to the mainstream channel by deformation superposition effect and then carried out by the injected water.In the displacement,the pulsation frequency mainly affected the cluster and blind end remaining oil,and the hydraulic pulsation wave with a frequency of about 1 Hz had the best effect in improving the recovery.The pulsation amplitude value mainly affected the membrane and column residual oil,and the larger the amplitude value,the more remaining oil the hydraulic pulsation wave would displace.The presence of low intensity continuous flow pressure and holding pressure end pressure promoted the concentration of pulsating energy and greatly improve the recovery of cluster residual oil.The rise in temperature made the hydraulic pulsation wave work better in displacing remaining oil,improving the efficiency of oil flooding.

Geochemical characteristics of reservoir after water and polymer flooding in Xingshugang oilfield of Daqing

Collecting 44 oil-sand specimens of Pu-I Member in two inspection wells before and after polymer flooding in the thirteenth district of Xingshugang oilfield,with experimental analysis,the author obtained the data about oil viscosity,flow,oil saturation and oil displacement efficiency.The result shows that viscous oil predominates in the main remaining oil in Xingshugang oilfield after water flooding with a certain amount of low viscosity oil,high viscosity oil and heavy oil;after polymer flooding,the viscous oil is main ingredient.Compared with water flooding,the low mobility and poor oil can be spread by polymer flooding,expanding the affected area and improving sweep efficiency and oil recovery.The geochemical affecting factors of water flooding and oil displacement efficiency refer to reservoir flow,permeability and the viscosity of residual oil.In the reservoir with permeability from low to high,the polymer flooding efficiency is better than water flooding.It provides the basis for improving the water and polymer flooding efficiency of the Xingshugang oilfield.

Progress and prospects of carbon dioxide capture,EOR-utilization and storage industrialization

Carbon dioxide capture,EOR-utilization and storage(CCUS-EOR)are the most practical and feasible large-scale carbon reduction technologies,and also the key technologies to greatly improve the recovery of low-permeability oil fields.This paper sorts out the main course of CCUS-EOR technological development abroad and its industrialization progress.The progress of CCUS-EOR technological research and field tests in China are summarized,the development status,problems and challenges of the entire industry chain of CO_(2) capture,transportation,oil displacement,and storage are analyzed.The results show a huge potential of the large-scale application of CCUS-EOR in China in terms of carbon emission reduction and oil production increase.At present,CCUS-EOR in China is in a critical stage of development,from field pilot tests to industrialization.Aiming at the feature of continental sedimentary oil and gas reservoirs in China,and giving full play to the advantages of the abundant reserves for CO_(2) flooding,huge underground storage space,surface infrastructure,and wide distribution of wellbore injection channels,by cooperating with carbon emission enterprises,critical technological research and demonstration project construction should be accelerated,including the capture of low-concentration CO_(2) at low-cost and on large-scale,supercritical CO_(2) long-distance transportation,greatly enhancing oil recovery and storage rate,and CO_(2) large-scale and safe storage.CCUS-EOR theoretical and technical standard system should be constructed for the whole industrial chain to support and promote the industrial scale application,leading the rapid and profitable development of CCUS-EOR emerging industrial chain with innovation.

Experimental Study and Numerical Simulation of Polymer Flooding

The numerical simulation of polymer flooding is a complex task as this process involves complex physical and chemical reactions,and multiple sets of characteristic parameters are required to properly set the simulation.At present,such characteristic parameters are mainly obtained by empirical methods,which typically result in relatively large errors.By analyzing experimentally polymer adsorption,permeability decline,inaccessible pore volume,viscosity-concentration relationship,and rheology,in this study,a conversion equation is provided to convert the experimental data into the parameters needed for the numerical simulation.Some examples are provided to demonstrate the reliability of the proposed approach.

Production calculation of the second and tertiary recovery combination reservoirs under chemical flooding

Based on the analysis of the production composition of reservoirs developed by the second&tertiary recovery combination(STRC),the relationship between the overall output of the STRC project and the production level during the blank water flooding stage is proposed.According to the basic principle of reservoir engineering that the“recovery factor is equal to sweeping coefficient multiplied by oil displacement efficiency”,the formula for calculating the ultimate oil recovery factor of chemical combination flooding reservoir was established.By dividing the reservoir into a series of grids according to differen-tial calculus thinking,the relationship between the ultimate recovery factor of a certain number of grids and the recovery de-gree of the reservoir was established,and then the variation law of oil production rate of the STRC reservoir was obtained.The concept of“oil rate enlargement factor of chemical combination flooding”was defined,and a production calculation method of reservoir developed by STRC was put forward based on practical oilfield development experience.The study shows that the oil production enhancing effect of STRC increases evenly with the in crease of the ratio of STRC displacement efficiency to water displacement efficiency,and increases rapidly with the increase of the ratio of recovery degree at flooding mode conversion to the water displacement efficiency.STRC is more effective in increasing oil production of reservoir with high recovery degree.Through practical tests of the alkali free binary flooding(polymer/surfactant)projects,the relative error of the oil production calculation method of STRC reservoir is about±10%,which meets the requirements of reservoir engineering.

Micro-pore structure and oil displacement mechanism analysis for deep zone and low permeability reservoir in Mobei oilfield

The Mobei reservoir is a low-permeability-sandstone reservoir,due to differences in pore geometry,it can be divided into two independent reservoirs:A1 reservoir and A2 reservoir.For better understanding the water flooding development effects of Mobei reservoir,the mercury intrusion porosimetry,water flooding CT scanning and micro-CT scanning experiments are used in this study.The result shows that the reservoir has the strong heterogeneity which is weaken gradually from A1 reservoir to A2 reservoir.Reservoir pore radius is mainly distributed in the 100e200 microns,the throat radius is mainly distributed in the 1e3 micron.The water flooding core experiment in each reservoir shows a short water-free oil production period and rapid water cut after breakthrough.The A2 reservoir core flooding process is similar to piston displacement,the A1 reservoir core flooding process refers to the phenomenon(The fingering phenomenon in the process of core flooding in the A1 reservoir is obvious).The calculated water drive efficiency of the A2 reservoir is 61.2%,which is higher than 49.1%of the A1 reservoir.According to the CT scanning process,the Mobei oilfield has low micro displacement efficiency and the A1 reservoir has a smaller spread(sweep area)and higher residual oil saturation.

Displacement and development characteristics of fire flooding of vertical wells in old heavy oil areas

As the vertical-well fire flooding technology is industrially applied in the steam-injection old heavy oil areas of Xinjiang and Liaohe oilfields,its enhanced oil recovery potential is gradually clear.According to laboratory experiment,field test and reservoir engineering,the displacement characteristics of verticalwell fire flooding in the steam-injection old heavy oil areas are systematically investigated.Laboratory experiments and core data show that the vertical-well fire flooding has significantly high flooding ef-ficiency,no residual oil are remained in the firing front sweeping zone,and the lateral sweep efficiency and ultimate recovery can be achieved.The vertical-well fire flooding is a strategic replacement technology to enhance the recovery greatly.Development characteristics and advantages of areal and linear fire flooding are well investigated,and research results are applied in the design of industrial test plan of fire flooding in the Hongqian block,Xinjiang.The research shows that the linear well pattern has advantages of easy construction of ground facilities and management,less well of stage management,simple matching technology and easy achievement of purposeful control for fire front;the areal well pattern has advantages in reducing the air/oil ratio during the fire flooding period,increasing the total recovery rate of reservoirs and reducing the geological and reservoir management risks.To select well patterns of fire flooding,some factors such as geology,reservoir,fluid properties,oil price,and reservoir development degree should be mainly considered.In the Hongqian block,an improved linear well pattern with a combination of new wells and old wells is applied,this not only absorb experiences of linear well pattern pilot test,but also use advantages of areal well pattern.

The study on exploitation potential of original low-oil-saturation reservoirs

It is preliminary estimated that the proved geological reserves of original low-oil-saturation reservoirs(OLOSRs)of 54 blocks in China are more than 820 million tons.As same as the high water-cut stage of a reservoir after water flooding,the initial water cut of OLOSRS is higher than 80%.The industrial application of EOR technology could efficiently enhance oil recovery rate from 8.4% to 20.1% of reservoirs after water flooding.The suitable measurements of EOR to the OLOSRs probably have a profound impact on achieving higher oilfield reserves and production.In this paper,the main characteristics and reserves potential of OLOSRs have been studied by investigations and laboratory experiments.The relationship between initial oil saturation and displacement efficiency is also analyzed.The feasibility of EOR technologies for different OLOSRs is discussed.The results show that the OLOSRs could be divided into two categories(primary and secondary)by origin of low oil saturation.The two categories contain abundant reserves in China’s major oilfields,but their occurrence states of crude oil are quite different.If the average oil saturation of the OLOSR is defined at 40%,there are 13.1%-54.2%crude oil that could be recovered by using the existing EOR technology,especially in the primary OLOSRs with low permeability and the secondary OLOSRs with high permeability.