Study On Oil Recovery Law Of Water Huff And Puff,Gas Huff And Puff And Combined Huff And Puff In Different Tight Oil Reservoirs

Tight sedimentary tuff oil reservoirs and tight volcanic oil reservoirs are currently important areas for increasing reserves and production in the Santanghu Basin.However,the special sedimentary environment and formation process make the two types of reservoirs significantly different from conventional tight sandstone reservoirs in pore structure,fluid mobility and oil recovery characteristics.In addition,limited by the extremely low pore throat size and seepage capacity of tight oil reservoirs,huff and puff method has become an important means to improve the recovery factor of tight oil reservoirs.Among them,water huff and puff is the most widely used,and N₂,CO₂ and multiphase fluid combined huff and puff can also improve the recovery factor of tight oil reservoirs.However,the applicability,oil recovery law,mechanism and influencing factors of the above processes in tight sedimentary tuff oil reservoirs and tight volcanic oil reservoirs are still unclear.In view of this,through a large number of tests and physical simulation experiments,this paper has conducted a comprehensive study on the pore structure,fluid mobility and huff and puff oil recovery law of different fluid of tight sedimentary tuff oil reservoirs and tight volcanic oil reservoirs,with a view to providing a reference for the efficient development of the two types of tight oil reservoirs.Firstly,the pore structure characteristics of two types of tight oil reservoirs were comprehensively characterized by casting thin sections,scanning electron microscopy,porosity and permeability analysis,high-pressure mercury injection and nuclear magnetic resonance,and the relationship between nuclear magnetic resonance T₂ value and pore radius of two types of rocks was established.The fluid movable saturation,the lower limit of movable pore throat radius,the lower limit of effective seepage pore throat radius and the lower limit of water flooding movable pressure gradient of two types of tight oil reservoirs were obtained by centrifugal method,mercury injection method and water flooding method.Based on the regression method and pore production characteristics,the relationship between pore structure and fluid mobility of two types of tight oil reservoirs was obtained.Secondly,through the spontaneous imbibition and forced imbibition experiments of glass etching model,multi-size plunger cores and full-diameter cores,the phenomena,law and microscopic pore production characteristics of imbibition oil recovery of the two types of tight oil reservoirs were clarified,and the rock sample size effect law of imbibition oil recovery in two types of tight cores were obtained.Based on the stable imbibition oil recovery characteristics of tight sedimentary tuff,the rock sample size effect model of its spontaneous imbibition recovery factor was established.Based on the pore structure characteristics of the two types of tight rocks and the theory of imbibition oil recovery,the fundamental reasons for the great difference in imbibition oil recovery between them were revealed.Thirdly,the physical simulation experiment platform of tight oil reservoir water huff and puff monitored by online nuclear magnetic resonance was used to fully simulate the whole process of water huff and puff of the two types of tight oil reservoirs.The law,microscopic pore production characteristics,and the oil production of elastic displacement and imbibition displacement in each cycle of water huff and puff oil recovery of their matrix pores were obtained.Meanwhile,the effects of water injection pressure,soaking time and reinjection pressure were studied.Fourthly,the matrix-fracture dual medium seepage system after hydraulic fracturing in tight oil reservoirs was simplified simulated by combining full-diameter cores and creating fractures,and the effect of water injection pressure on water huff and puff oil recovery of the two types of tight oil reservoirs under the influence of fractures was studied,and the great advantages of water huff and puff oil recovery over fracture pressure were clarified;On the basis of analyzing the disadvantages of constant pressure water huff and puff oil recovery in tight oil reservoirs,the advantages of stepwise pressure boosting water huff and puff oil recovery in tight oil reservoirs were proposed and verified.By changing the core fracture density,the quantitative relationship between the oil recovery effect of water huff and puff and fracture density in tight oil reservoirs was obtained.The reasons for the low flowback rate of injected water after water huff and puff in tight oil reservoirs were summarized from matrix pores,fractures and reservoir non-constant volume.Finally,based on the analysis of the shortcomings of the traditional huff and puff oil recovery physical simulation experimental device,the huff and puff oil recovery physical simulation experimental device for tight oil reservoirs was improved to enhance the experimental efficiency and stability.The new device was used to study the oil recovery law and pore production characteristics of N₂ and CO₂ huff and puff in two types of tight oil reservoirs,and the effects of rock sample size effect,soaking time,injection pressure（including over-fracture pressure injection）and stepwise pressure boosting on CO₂ huff and puff oil recovery of tight sedimentary tuff were studied.Based on the characteristics of single-phase fluid huff and puff,the law,pore characteristics and influencing factors of multiphase fluids alternating huff and puff of N₂ and water,CO₂ and water,and CO₂ and N₂ were studied.The research results of this paper can provide a solid basis for understanding the pore structure,fluid mobility and oil recovery law,mechanism and influencing factors of different huff and puff fluids and huff and puff methods of tight sedimentary tuff oil reservoirs and tight volcanic oil reservoirs,and can also provide reference for the development of similar reservoirs.