Screening Of The Methods To Improve Oil Recovery For Ultra-high Water-cut Oilfield

After a long time of water flooding, most of the oil fields in the world,have entered into the high water cut period of development. The east Texas oilfield of America, oilfield of ford Geraldine, column salt field of Russian library,and so on,the water cut has been 97%, and the average recovery degree has been more than 50%,and the recovery degree of the east Texas oilfield has even been reached 77.5%.Most of oilfields in China have entered high water cut or ultra-high water cut, that is, the oil field water cut is up to 90% or more over, Daqing, Shengli, Dagang, Zhongyuan, Henan, Jianghan and so on 6 oil region, the average water cut oil well production has reached more than 80%.For the oilfield of water flooding development, as the different water cut stage, the water cut rising speed is not the same. As the low and medium water cut stage, the speed of the water cut increases with the increase of water cut, and the speed is greater than 5%;As the high water cut stage, such as the water cut reached 80%, the rising speed water cut is slowly, almost stable at 1%.When Oilfield goes into the development stage of ultra-high water cut, the development rule is different from the low water cut period, the well pattern changes.As the late water flooding sandstone reservoir, the object changes, and the effect of hydrodynamic adjustment is bad, and what is more,it is difficult to adapt to the underground reservoir.Though, the ultra-high water cut reservoir has the development characteristics of low oil recovery rate, high water consumption,more dispersed residual oil, worse development effect, worse deterioration of well conditions and worse development economic, from the point of producing reserves and production,the old oil fields of high water cut are still the main point of oilfield development, the annual oil production and the residual recoverable reserves still occupies a pivotal position in the country.Therefore, high water cut oilfield recovery is an important factor of restricting the national sustainable development in the current economical and technological conditions, and it will have the potential for further improve oil recovery.In this paper, we firstly study the characteristics and the situation of the ultra-high water cut reservoir at home and abroad, and we analysis the distribution features of the remaining oil.Then,we comprehensively summarize the basic principles,the development situation and the applicability of the seven kinds of methods for the ultra-high water cut reservoir to improve the oil recovery,such as:oil field water flooding, polymer flooding, nitrogen flooding, polymer flooding, gel flooding, water injection flooding, CO2 flooding,and we make a comprehensive system integration.1)For ultra-high water cut reservoir of water flooding development, which is at the high production stage, but there is still a lot of remaining oil, water drive can keep the reservoir pressure, increase formation energy significantly, improve the development effect greatly, improve the recovery factor effectively,with the layered water injection,adjustment of the injection-production system, cyclic water flooding technology.2)Nitrogen flooding has great potential to improve oil recovery. For ultra-high water cut reservoir, the vertical water flooding inside is serious, the water channeling is serious,too. In the process of further improve the mining efficiency, the problem of how to stop the flow of the water along the high permeability parts is must be solved. Liquid nitrogen foam flooding goes into the high permeability, low oil saturation, of the reservoir, and it can seal the reservoir and it can be induced into the low permeability,high oil saturation area to displace the crude oil.3)Polymer flooding, as one of methods to enhance oil recovery, it increases the injection fluid viscosity, change the flow rate of oil-water phase, thus,to expand the sweep volume. When water drives oil, as the water flow ratio is below 1, the flow ability of water is stronger than crude oil,then the flow of water causes the fingering phenomenon,the sweep efficiency is low. After adding polymer, the ability of solution into the formation reduces, the viscosity increases, the flow solution reduces. What is more, the flow resistance increases, and the sweep efficiency is improved.4)Gel flooding,anew kind of technology with displacement and profile,which developed based on the conventional polymer flooding.It can greatly improve the mobility ratio, ease the contradiction between the layers, and it has a good prospect to improve the recovery factor. Under certain pressure, the profile control agent in the formation channel, changes into the organic polymer gel to plug the high permeability layer, and the injected water changes, then it can avoid the phenomenon of "tongue" and injected water turned into middle and low permeable formation, so that the suction surface is uniform.5) Water and gas inject in turn, the technology has overcame the faults of "gas channeling" of air displacement, and it organically combine the water flooding and air displacement,and inject gas into the reservoir after water flooding, thus to reduce water phase permeability, thereby, it can reduce the mobility of water and expand the sweep volume of water. This technique is very suitable for low permeability and serious heterogeneity, big channel reservoir.And it can solve many technical problems, such as the low permeability of central and western regions in China, low permeability oilfield water flooding development, the local water shortage, the difficult of water injection, the serious deficit of formation,r and the pollution to the environment, etc.It can also promote the efficient development of oil and gas resources in the Midwest and in our country. In Zhongyuan, Tuha, Huabei these oilfields,there are all water injection, and there have been good effect.6)CO2 flooding can improve the recovery of ultra-high water cut reservoir, but also solve the problem of CO2 sequestration, and protect the atmospheric environment. The technology is suitable for conventional reservoirs, and it can obviously increase the recovery efficiency of the reservoir with low permeability and extra-low permeability. As CO2 is soluble into water, the viscosity of water increased by 20%～30%, and the migration performance increases 2-3 times; After CO2 is dissolved into oil, crude oil volume expands, and the viscosity is reduced by 30%～80%, inter-facial tension of oil-water reduces, thus to improve the efficiency of oil production rate, washing oil and collecting of the residual oil. Generally, CO2 flooding can improve oil recovery by 7%～ 7%, and prolong the well production life by 15～20 years. CO2 can be obtained from the emissions of industrial facilities, such as power plants, chemical fertilizer plants, cement plants, chemical plants, refineries, natural gas processing plants.On the one hand, It can reduce emissions of greenhouse gases, on the other hand, the increase production of oil and gas.Finally, combining the instances:1)Kumkol South oilfield, we analyze the problems of its development, optimize the way of development, the feasibility of the profile control with water plugging and polymer flooding, then we get the optimal solution;2)double river 7 oilfield, according to its geological model and fluid model, we combined the water flooding, polymer flooding, cross-linked polymer flooding, and compare the effect,then we get the optimal combination scheme;3)Liang Na pure 56 block oilfield, according to its development situation,we propose the water well profile control, step-down pressure of the oil well, nitrogen foam alternately, nitrogen water alternately, and we optimize the best solution of all kinds of situation;4)11 river sand block oilfield, according to the situation of development, we study the feasibility of polymer profile control and water alternating injection profile, as well as the comprehensive feasibility of the injection of polymer and water alternately,then we get the optimal solutions.