Study On Chemicals Loss And Application Boundary Of Asp Flooding In Daqing Second-Class Oil Layers

According to the classification criteria for late high water-cut reservoirs established by Daqing Oilfield,compared with the major reservoirs,Second-Class reservoirs have the following features in both vertical and horizontal directions:larger changes of sedimentary environment,smaller developing scale of sand body,increased number of layers,thinner single layer,lower permeability,larger content of clay mineral,more serious heterogeneity in horizontal and vertical directions.The major reservoirs of Daqingoilfieldhavebeenmostlyemployedpolymerflooding,and alkaline-surfactant-polymer?ASP?flooding is going to be mainly applied to the Second-Class reservoirs with relatively large remaining geological reserves.Laboratory study and field test showed that ASP flooding can enhance recovery for about 20%on the basis of water flooding.The migration of ASP composite system in the reservoirs is a very complex physical and chemical seepage process.The relatively poor physical properties of Second-Class reservoirs must have a greater impact on the performance of ASP flooding.The Second-class reservoir simulation physical model was applied to study the rule of variation of chemical concentration and properties of ASP compound system along with the migration distance in Second-Class reservoirs and its effect on oil recovery;in addition,the paper also explored the application limit of ASP composite system in Second-Class reservoirs.The results showed that:The characteristics of Second-class oil reservoir is the thickness of single layer is thin?1² m?,the effective permeability is low(100¹000×10^-3?m²),the plane and vertical heterogeneity is serious,the clay mineral content is high.The reservoir pore radius of this kind of oil layer is small,the distribution of pore throat is uneven,and the seepage ability is poor,so compared with First-class type of reservoir,oil and gas production capacity of Second-class oil layers was poor.By investigating the resistance coefficient and residual resistance coefficient of the ASP system in the core with permeability of(600⁸00×10^-3?m²),the polymer molecular weight and concentration were selected as1200¹600 and 2000mg·L^-1.According to the ASP compound system oil displacement experiment,the loss rule of components of different migration distance is made clear,the chemicals loss of HABS,alkali,polymer is 60%,37%and 28%respectively in the top 25%distance,the total loss of each component is more than 75%.The invalid loss of ASP system near the injection well has little the contribution to improve oil recovery.The rule of the oil displacement efficiency and recovery percent of reserves of ASP compound system along the distance is made clear,the oil displacement efficiency along the distance is in the range of 17%²7%,the recovery percent of reserves of ASP system increases with the increase of the migration distance.The rate of increase of recovery percent of reserves is closely related with the effective distance of the ASP compound system and residual oil saturation.Better oil recovery can still be recovered using ASP flooding in the lowest permeability of Second-Class oil layers.However,the agent loss including surfactant,alkali and polymer can be aggravated in long-distance migration of the ASP system.The agent loss will lower ASP flooding efficiency compared with the experiments in core-scale flooding.The long-distance ASP flooding experiment shows that ultra-low IFT can only be maintained in the distance of 30%percentage,and the viscosity loss rate can reach 95%after 10 meters migration for ASP system.The flooding efficiency of ASP system for secondary oil layers is affect by concentration,viscosity,IFT,and migration distance.There is a contradictory between superior property of ASP system and its maintaining distance,which needs to be solved for enhancing oil recovery using ASP flooding in Second-class oil layers.