Study On Alternate Oil Displacement Mechanism Between Active Water And CARBON Dioxide In Low Permeability Sandstone

The development of low permeability sandstone reservoir is difficult:during water injection development,the water absorption capacity of water injection wells is low,it is difficult to stabilize production,and the phenomenon of viscous fingering and gas channeling is serious during gas injection development.The field development practice shows that water gas alternate displacement technology is an effective method to improve crude oil recovery,but its mechanism is not well understood at present.In this paper,the oil displacement experiments of different displacement modes(active water,CO₂ and active water CO₂)of low-permeability sandstone are carried out,and the alternating macro oil displacement mechanism is studied.At the same time,nuclear magnetic resonance technology and core slice fluorescence analysis technology are used to compare and analyze the oil-water distribution and crude oil occurrence state before and after oil displacement under different oil displacement modes,so as to clarify the alternating micro oil displacement mechanism of active water and CO₂ of low-permeability sandstone.The following work has been completed:(1)Based on the literature investigation on the physical and chemical properties of CO₂and active water,the EOR mechanism of CO₂ and active water is preliminarily clarified;Betaine type surfactant and heavy alkyl benzene petroleum sulfonate were selected to prepare the composite system active water,and the interfacial tension test experiment was carried out on the active water prepared with different proportions of the two surfactants.The experimental results show that the final interfacial tension of the composite system of 60%heavy alkyl benzene petroleum sulfonate and 40%betaine type surfactant is the best active water system.(2)Based on three groups of cores with similar physical properties of the same reservoir,oil displacement experiments of different displacement modes(active water,CO₂ and active water CO₂)are designed and carried out.The experimental results show that the final recovery of active water CO₂ alternative oil displacement is the highest,which is 33.97%and 10.41%higher than that of active water and CO₂ alone;Compared with active water flooding,the combined action of gas and water during active water CO₂ alternative flooding prolongs the anhydrous oil production period and reduces the water cut.Compared with CO₂ flooding,the combined action of gas and water during active water CO₂ alternative flooding delays the gas channeling time.(3)The core nuclear magnetic resonance T₂ spectrum test in the state of"saturated water centrifugation saturated oil after displacement"is carried out.Based on the test results,the recovery degree of crude oil in different sizes of pores is determined,and the effect of active water CO₂ alternative oil displacement on the improvement of crude oil recovery in different sizes of pores is quantitatively evaluated.The research results show that under the action of active water plugging large pores,carbon dioxide can produce smaller crude oil in the pore throat.Under the combined action of active water and CO₂,the crude oil in the pores of different sizes in the core is relatively evenly exploited,and the oil displacement efficiency of the two media is higher.(4)The micro occurrence state of crude oil in the core in the state of"saturated oil after displacement"is characterized by core fluorescent thin section analysis technology.Based on the characterization results,the occurrence state types of movable oil and the proportion of various types of crude oil in the core before and after oil displacement under the three displacement modes are compared and analyzed.The research results show that the active water can improve the slit,light fog,cluster The degree of production of intergranular adsorbed residual oil.It can also inhibit the transformation of other types of remaining oil to slit type.Carbon dioxide can use pores smaller than activated water to improve the utilization degree of clustered residual oil.Carbon dioxide injection into crude oil will expand the volume of crude oil,increase the elastic energy,and greatly improve the utilization degree of free residual oil,which can play a better role in coordination with activated water.The research results provide a theoretical basis for enhanced oil recovery by alternate displacement of active water and CO₂ in low permeability reservoirs,and have important guiding significance for the design of actual development scheme of alternate displacement of active water and CO₂ in low permeability reservoirs.