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.

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.

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.

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.

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.