Synchronous injection-production energy replenishment for a horizontal well in an ultra-low permeability sandstone reservoir: A case study of Changqing oilfield in Ordos Basin, NW China

It is difficult to build an effective water flooding displacement pressure system in the middle section of a horizontal well in an ultra-low permeability sandstone reservoir.To solve this problem,this study proposes to use packers,sealing cannula and other tools in the same horizontal well to inject water in some fractures and produce oil from other fractures.This new energy supplement method forms a segmental synchronous injection-production system in a horizontal well.The method can reduce the distance between the injection end and the production end,and quickly establish an effective displacement system.Changing the displacement between wells to displacement between horizontal well sections,and point water flooding to linear uniform water flooding,the method can enhance water sweeping volume and shorten waterflooding response period.The research shows that:(1)In the synchronous injection and production of horizontal well in an ultra-low-permeability sandstone reservoir,the water injection section should select the section where the natural fractures and artificial fractures are in the same direction or the section with no natural fractures,and the space between two sections should be 60?80 m.(2)In addition to controlling injection pressure,periodic water injection can be taken to reduce the risk of re-opening and growth of natural fractures or formation fracture caused by the gradual increase of water injection pressure with water injection going on.(3)Field tests have verified that this method can effectively improve the output of single well and achieve good economic benefits,so it can be widely used in the development of ultra-low permeability sandstone reservoirs.

Study on Injection-Production System Adjustment of Fault Block Reservoir Based on Equilibrium Displacement

This paper proposed a method of injection-production system adjustment to solve the problem that the water flooding effect was restricted because of the horizontal and vertical contradictions during the development process of fault block reservoirs. Considering the heterogeneity of reservoir, the Buckley-Leverett water flooding theory was applied to establish the relationship between the recovery and cumulative water injection. In order to achieve the goal of vertically balanced recovery of each section, the calculation method of vertical sectional injection allocation was proposed. The planar triangular seepage unit was assumed and sweep coefficients of different oil-water distribution patterns were characterized using multi-flow tube method. In order to balance and maximize the plane sweep coefficient, the calculation method of plane production system optimization was obtained. Then the injection-production system stereoscopic adjustment method based on equilibrium displacement was proposed with vertical sectional injection allocation and plane production system optimization. This method was applied to injection and production adjustment of BZ oilfield in southern Bohai. The effect of water control and oil increase was obvious. This method can greatly improve the effect of water flooding of offshore fault block reservoirs with the adjustment of injection-production system.

Microscopic experiment on efficient construction of underground gas storages converted from water-invaded gas reservoirs

Based on the microfluidic technology,a microscopic visualization model was used to simulate the gas injection process in the initial construction stage and the bottom water invasion/gas injection process in the cyclical injection-production stage of the underground gas storage(UGS)rebuilt from water-invaded gas reservoirs.Through analysis of the gas-liquid contact stabilization mechanism,flow and occurrence,the optimal control method for lifecycle efficient operation of UGS was explored.The results show that in the initial construction stage of UGS,the action of gravity should be fully utilized by regulating the gas injection rate,so as to ensure the macroscopically stable migration of the gas-liquid contact,and greatly improve the gas sweeping capacity,providing a large pore space for gas storage in the subsequent cyclical injection-production stage.In the cyclical injection-production stage of UGS,a constant gas storage and production rate leads to a low pore space utilization.Gradually increasing the gas storage and production rate,that is,transitioning from small volume to large volume,can continuously break the hydraulic equilibrium of the remaining fluid in the porous media,which then expands the pore space and flow channels.This is conducive to the expansion of UGS capacity and efficiency for purpose of peak shaving and supply guarantee.

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.

Change of phase state during multi-cycle injection and production process of condensate gas reservoir based underground gas storage

Based on the differences in production mode and operation process between gas storage and gas reservoir,we established a phase balance test procedure and a theoretical simulation model of phase balance during multi-cycles of injection and production of underground gas storage(UGS)rebuilt from condensate gas reservoir to study the phase characteristics of produced and remaining fluids during multi-cycles of injection and production.Take condensate reservoir gas storage as example,the composition of produced fluid and remaining fluid,phase state of remaining fluid,retrograde condensate saturation and condensate recovery degree in the process of multi-cycles of injection-production were studied through multi-cycle injection-production experiment and phase equilibrium theory simulation.The injected gas could greatly improve the recovery of condensate oil in the gas reservoir,and the condensate oil recovery increased by 42% after 5 cycles of injection and production;the injected gas had significant evaporative and extraction effects on the condensate,especially during the first two cycles;the condensate oil saturation of the formation decreased with the increase of injection-production cycles,and the condensate oil saturation after multi-cycles of injection-production was almost 0;the storage capacity increased by about 7.5% after multi-cycles of injection and production,and the cumulative gas injection volume in the 5 th cycle increased by about 25%compared with that in the 1 st cycle.

4D-stress evolution of tight sandstone reservoir during horizontal wells injection and production: A case study of Yuan 284 block,Ordos Basin,NW China

To investigate the 4D stress change during injection and production in tight sandstone reservoirs, a multi-physical fields modeling method is proposed considering the reservoir heterogeneity, hydraulic fracture and complex injection-production system. The 4D stress evolution of tight sandstone reservoir in Yuan 284 block of Huaqing oilfield, Ordos Basin,during injection-production in horizontal well network is investigated by modeling coupled flow and geomechanics. Results show:(1) Induced by injection and production, the 3D stress increases near the injectors but decreases near the producers, and the horizontal stresses are distributed in obvious strips along their respective stress directions.(2) The horizontal stress difference is the highest at the horizontal wellbore beside injectors during injection and production, while it is the lowest in undeveloped zone between the injectors, and the orientation of maximum horizontal principal stress changes the most near the injectors, which is distributed radially.(3) The hydraulic fracture in re-fracturing well was observed to be asymmetrical in geometry and deflected as the stress changed. The results provide theoretical guidance for horizantal well network modification and re-fracturing optimization design in tight sandstone reservoir.

Simulation of pore space production law and capacity expansion mechanism of underground gas storage

One-dimensional gas injection storage building and one-cycle injection-production modeling experiment,and two-dimensional flat core storage building and multi-cycle injection-production modeling experiment were carried out using one-dimensional long core and large two-dimensional flat physical models to find out the effects of reservoir physical properties and injection-production balance time on reservoir pore utilization efficiency,effective reservoir capacity formation and capacity-reaching cycle.The results show that reservoir physical properties and formation water saturation are the main factors affecting the construction and operation of gas-reservoir type underground gas storage.During the construction and operation of gas-reservoir type gas storage,the reservoir space can be divided into three types of working zones:high efficiency,low efficiency and ineffective ones.The higher the reservoir permeability,the higher the pore utilization efficiency is,the smaller the ineffective working zone is,or there is no ineffective working zone;the smaller the loss of injected gas is,and the higher the utilization rate of pores is.The better the reservoir physical properties,the larger the reservoir space and the larger the final gas storage capacity is.The higher the water saturation of the reservoir,the more the gas loss during gas storage capacity building and operation is.Optimizing injection-production regime to discharge water and reduce water saturation is an effective way to reduce gas loss in gas storage.In the process of multiple cycles of injection and production,there is a reasonable injection-production balance time,further extending the injection-production balance period after reaching the reasonable time has little contribution to the expansion of gas storage capacity.

Numerical simulation-based correction of relative permeability hysteresis in water-invaded underground gas storage during multi-cycle injection and production

By conducting relative permeability experiments of multi-cycle gas-water displacement and imbibition on natural cores,we discuss relative permeability hysteresis effect in underground gas storage during multi-cycle injection and production.A correction method for relative permeability hysteresis in numerical simulation of water-invaded gas storage has been worked out using the Carlson and Killough models.A geologic model of water-invaded sandstone gas storage with medium-low permeability is built to investigate the impacts of relative permeability hysteresis on fluid distribution and production performance during multi-cycle injection and production of the gas storage.The study shows that relative permeability hysteresis effect occurs during high-speed injection and production in gas storage converted from water-invaded gas reservoir,and leads to increase of gas-water transition zone width and thickness,shrinkage of the area of high-efficiency gas storage,and decrease of the peak value variation of pore volume containing gas,and then reduces the storage capacity,working gas volume,and high-efficiency operation span of the gas storage.Numerical simulations exhibit large prediction errors of performance indexes if this hysteresis effect is not considered.Killough and Carlson methods can be used to correct the relative permeability hysteresis effect in water-invaded underground gas storage to improve the prediction accuracy.The Killough method has better adaptability to the example model.

Experiments on nitrogen assisted gravity drainage in fractured-vuggy reservoirs

Visual models of fractured-vuggy reservoirs were designed and manufactured to conduct experiments of nitrogen assisted gravity drainage(NAGD). The impacts of flooding pattern, gas injection rate, well type, and displacement direction(vertical or horizontal) on development performances and remaining oil distribution were studied. The results show that during NAGD, the sweep scope is decided by the connections between producer and reservoir, and the local sweep efficiency is decided by fracture-vuggy configuration. The homogenous fractured reservoir has higher oil recovery, and the bigger the aperture of fracture is, the higher the recovery. The main regions of remaining oil due to poor connectivity and gas-oil gravity difference include blind fractures and vugs below the connected fractures, the bottom of vugs, and the narrow and low-angle fractures. The accumulation of remaining oil in the bottom of reservoir is easily formed and controlled by the connections between producers and reservoir. The higher the gas injection rate and the stronger the fracture heterogeneity, the earlier the gas channeling and the lower the oil recovery of the producer will be.Horizontal wells have the best development effect, so horizontal well can be applied in fractured-vuggy reservoirs without bottom water.Producers should be preferentially drilled at low structural position. Gas channeling firstly occurs in the producer at high structural position, and it should be shut in timely to improve the utilization of injected gas.

Experiments on the influences of well pattern on water flooding characteristics of dissolution vug-cave reservoir

Based on the similarity criterion, volcanic rock samples were taken from outcrops to make experimental models. Water flooding experiments of five-spot well pattern, nine-spot well pattern, five-spot to nine-spot well pattern, the relationship between relative well and fracture positions, and injection rate in dissolution vug-cave reservoirs with/without fractures were carried out to analyze variation regularities of development indexes, find out development characteristics of water flooding with different well patterns and sort out the optimal water flooding development mode. For dissolution vug-cave reservoirs without fractures, five-spot well pattern waterflooding has very small sweeping area, serious water channeling and low oil recovery. When the well pattern was adjusted from five-spot to nine-spot well pattern, oil recovery could be largely improved, but the corner well far from the injector is little affected. In dissolution vug-cave reservoirs with fractures, when the injector and producer are not connected by fractures, the fractures could effectively connect the poorly linked vugs to improve the development effect of water flooding. Whether there are fractures or not in dissolution vug-cave reservoirs, the development effect of nine-spot well-pattern is much better than that of five-spot well pattern and five-spot to nine-spot well pattern, this is more evident when there are fractures, and the edge well has better development indexes than corner well. At the high-water cut stage of water flooding with nine-spot well pattern, the oil recovery can be further improved with staggered line-drive pattern by converting the corner well into injection well. It is helpful to increase the oil production of corner well of nine-spot well pattern by increasing the injection rate, and improve ultimate oil recovery, but the water-free production period would be greatly shortened and water-free recovery would decrease.

A new approach of proration-injection allocation for water-flooding mature oilfields

This paper presents a new method of injection-production allocation estimation for water-flooding mature oilfields.The suggested approach is based on logistic growth rate functions and several type-curve matching methods.Using the relationship between these equations,oil production and water injection rate as well as injection-production ratio can be easily forecasted.The calculation procedure developed and outlined in this paper requires very few production data and is easily implemented.Furthermore,an oilfield case has been analyzed.The synthetic and field cases validate the calculation procedure,so it can be accurately used in forecasting production data,and it is important to optimize the whole injection-production system.

A new method used for reservoir production performance analysis

Water flooding type-curves,recently receiving increasing attentions,play an essential role in evaluation of OOIP and prediction of future production.The study proposed a method modified form Weibull model and water flooding type-curves to analyze the reservoir production performance,including predication of oil production and water injection rates.The results indicate that the method can estimate and provide solutions for oil production,water injection rate and injection-production ratio with a well accuracy.A case study is conducted to demonstrate the applicability and accuracy of model by field data.The matching results illustrate a good accordance between calculated values and field data.