Study On The Influence Of Water Shield Barrier On CO₂ Flooding Effect

Low permeability sandstone reservoirs play an increasingly important role in the development of oil and gas resources in China.However,due to the narrow pores and throats,the way of the traditional water flooding possesses many issues,such as difficult water injection,high seepage resistance and so on.While,CO₂ gas boasts high mobility and strong solubility in crude oil,which decreases the interfacial tension between CO₂ and crude oil and makes it easy to enter low permeability layer.The pilot test results in high water cut reservoir indicate that CO₂flooding is a promising replacing technology.The formation water is stored on the reservoir surface in the form of water film,nevertheless,the influence of water shield barrier effect on CO₂ flooding cannot be ignored.And so far,the micro and macro equilibrium mechanism of oil,gas and water three-phase system has not been fully clarified,which is the basic scientific problem restricting CO₂ flooding to ameliorate the oil displacement effect.Taking Y oilfield as the research object,this paper studied the micro-oil flooding process and macro-oil flooding effect of CO₂-crude oil under the condition of water shield,and laid a certain theoretical foundation for the promotion and application of CO₂-oil flooding process in the field.The phase characteristics of CO₂-crude oil were studied under the water shield barrier effect through the experiment of CO₂-oil-water interaction.A one-dimensional mathematical model was established to figure out the diffusion and mass transfer of CO₂ in water film.The influence law of different water content on minimum miscible pressure was analyzed by core displacement experiment.The effects of water shield and no water shield on CO₂ immiscible displacement process were investigated by means of microscopic visualization simulation model.The characteristics of CO₂displacement under different degrees of water shield barrier effect were studied in the way of designing a long sand-packed pipe model with three sampling points at equal intervals.The effect of water shield on CO₂ displacement in different displacement modes was studied by combining well pattern core model with multi-point pressure acquisition system.Through the above experiments,the following understandings were obtained:（1）Under the same CO₂ injection volume,with the increase of water content,the amount of dissolved CO₂ in crude oil is smaller,which leads to the slow growth of volume expansion multiple of crude oil,the decline of density and viscosity,and the decrease of CO₂ dissolving ability of crude oil.（2）The thicker the water film is,the greater the contact resistance between CO₂-crude oil contact is,and the longer the time to break through the water film is.（3）Under certain conditions,the minimum miscible pressure of CO₂-crude oil increases with the rise of core average water content.（4）Microscopic visualization model experiments manifest that the existence of formation water can delay gas channeling,change the seepage direction,produce different types of remaining oil,further expand the swept volume,and the recovery efficiency of CO₂ flooding is increased by 12.28%compared with that of no water condition.（5）The higher the barrier degree of water shield is,the weaker the ability of CO₂ to extract light components of crude oil is,and the best effect of CO₂ flooding is when the barrier degree of water shield is 50%.（6）In the well pattern core model experiment,the oil recovery of CO₂flooding after water flooding to 98%water cut was 6%higher than that of CO₂ flooding without water;CO₂ huff and puff after water flooding to water cut of 98%can improve the recovery factor by 4.83%compared with anhydrous CO₂ huff and puff.However,due to the high barrier degree of water shield,the amount of CO₂ dissolved in water increases and the amount dissolved in crude oil decreases,resulting in the volume expansion capacity of crude oil is weakened,the elastic potential energy becomes smaller,and the cyclic of huff and puff recovery becomes progressively worse than that of anhydrous CO₂huff and puff.