| Waterflooding is mainly applied to increase the depleted inherent energy of the reservoir.During water flooding operation,water flow occurs mainly in the fracture system(artificial and natural fractures).Meanwhile,the injected water could spontaneously imbibe into the porous media and will assist to displace the remaining oil under the action of strong capillary force.However,due to the high start-up pressure gradients of the confined liquid in the pore space of the tight matrix.After long-term of waterflooding,most of the oil would still be left within the complex structure pore space.Therefore,how to improve imbibition recovery of tight matrix rock has become one of the key problems to achieve sustainable development in water flooding operation.Taking Chang8 extra-low permeability sandstone of Yanchang Formation of Triassic in FX area of Ordos Basin as the research object,on the basis of physical properties of the reservoir rock and the degree of spontaneous imbibition and drainage oil recovery,this paper puts forward the low-frequency wave assisted surfactant enhanced imbibition and oil recovery technology.Through single factor experiment,the wave frequency,wave intensity,wave time and surfactant system suitable for percolation and drainage of extra-low permeability sandstone are optimized.The influence of low-frequency wave assisted surfactant on percolation and recovery of extra low permeability sandstone is studied.With the help of advanced nuclear magnetic resonance and high-precision CT scanning technology,the micro-mechanism of low-frequency resonance wave assisted surfactant promoting oil absorption and drainage in extra-low permeability sandstone is revealed from the Micro-absorption of movable oil and remaining oil.The concrete conclusions are as follows:Spontaneous imbibition plays a vital role in water-flooding of the tight sandstone reservoir.The spontaneous imbibition recovery of core samples from the Fujian area can reach 5.24%-18.23%.Due to the thickness of the solid-liquid adsorbed layer(sub-micron magnitude of the porosity made a greater contribution to the imbibition recovery for the tight sandstone reservoir.Whereas nano-submicron of the porosity made less contribution to the imbibition recovery)and pore-throat connectivity effect(with the increase of permeability,the number of connected pore throat and the connectivity face porosity will increase exponentially).The higher of spontaneous imbibition recovery degree is in agreement with the greater permeability of matrix.According to the experiments of extra-low permeability(0.05×10-3?m2-0.25×10-3?m2)sandstone samples treated by spontaneous imbibition,low-frequency resonance wave imbibition,surfactant-imbibition and low-frequency resonance wave assisted surfactant imbibition,the oil recovery by spontaneous imbibition locates in the range of 4.55%-17.23%.The oil recoveries by low-frequency resonance wave,surfactant and low-frequency resonance wave-surfactant are found to be 0.49-4.41 percentage points 1.14-3.03 percentage points and2.75-6.76 percentage points higher than that by single spontaneous imbibition,respectively.The experiments also indicate that for a sandstone with a higher permeability and better pore connectivity,a higher oil recovery increase will be obtained by each of the resonance,surfactant and resonance-surfactant assisted imbibitions.According to the micro-distribution of the movable oil in the sandstone sample characterized by NMR,the movable oil by each the four kind of imbibition models locates in pores with sizes higher than sub-micrometers,while the movable oil in the pores with sizes higher than micrometers dominates the contributions to the total oil recovery due to the influence of the distribution of the irreducible water in the sample.Compared to the micro-distribution of the movable oil in the sandstone sample treated by spontaneous imbibition,the imbibition assisted by each of the low frequency resonance wave,surfactant and low frequency resonance wave-surfactant would reduce the lower limitation of the diameter(sub-micrometer)of the drainage channels for the movable oil and increase ratio of the movable oil in the pores with sizes higher than micrometers.And specially when affected by the combination of the induced inertial force(by low frequency wave),the reduction in the oil/water IFT and the increase in the water wettability(by surfactant),the water absorbing capacity(in the pores with sizes of sub micrometers)and the drainage of the oil(in the pores with sizes higher than micrometers)will be significantly increased,and finally the good synergy effect is observed compared with the single chemical and physical assisted methods.According to high-resolution CT scanning results,the connectivity oil phase in the pore space was separated by the water phase.Meanwhile,the snap-off phenomenon was clearly observed with the sharp growing number of oil blocks in the pore space.The micro-occurrence of remaining oil in pore space is very complex after spontaneous imbibition,which is mainly composed of‘network',‘cluster'and‘isolated'.It is also noteworthy that the network remaining oil will be transformed into‘cluster'and‘isolated'remaining type oil which has relatively simple structure during imbibition drainage process.Among the above three types of remaining oil,the network remaining oil has a fewer number and better connectivity in pore space which was considered to be the main remaining oil type after spontaneous imbibition.Furthermore,on the combined effect of reverse imbibition behavior,variable diameter pore geometry and Jamin's effect,the above-mentioned three remaining oil mainly enriched in large channels(over than the peak pore radius)although they have the different individual structure.Under the dual role of low-frequency wave and surfactant group,some of the network remaining oil in pore space has been destroyed and that transformed to cluster remaining oil.Meanwhile,the flow-ability of cluster remaining oil has been also improved.To best of our acknowledgement,the microcosmic synergistic mechanism list as below:the inertial force of low-frequency wave and the decreased interfacial tension will improve the shape change-ability and coalescence-ability of oil droplets in the pore space.Furthermore,this micro-behavior will help decrease the number of remaining oil blocks and the Jamin's effect as well.Afterward,the inertia force will increase owing to the coalesced large volume oil droplets.Since,the adhesion and interfacial tension of oil droplets will reduce by surfactant work.The breakthrough ability of oil droplets in the irregular channel has been improved and the separation of oil-water phase in the pore network is promoted as well.Therefore,remaining oil flow-direction has been changed and oil displacement efficiency is improved. |
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