| In recent years China’s oil and gas exploration and development technology have focused on innovations for exploiting low-permeability or ultra-low-permeability reservoirs.Therefore,realizing efficient use and development is currently the main research trend.Low-permeability reservoirs generally demonstrate the following poor physical properties:high capillary pressure,the large resistance of Jamin effect,etc,resulting in insufficient water-injection,which greatly restricts the efficiency of conventional water flooding technology.Meanwhile,gas flooding is regarded as a widely used EOR technique because of its ideal injection,but gas breakthroughs have resulted in poor mobility control in serious heterogeneity reservoirs,contributing to poor displacement efficiency.Large amounts of oil are left behind after primary and secondary recovery processes.Moreover,chemical flooding（e.g.ASP or SP）with the oil-water interfacial tension dropped to an ultra-low value of the order of 10-3m N/m,with the addition of polymer to increase the viscosity of the oil displacement agent,demonstrates better conformance control and improves sweep efficiency.However,this technique is not recommended for use in low-permeability reservoirs due to injection difficulties in addition to polymer degradation,especially in high-temperature and high-salt reservoirs.These issues have made the exploration of low-permeability oil reservoirs challenging from a technical standpoint.Therefore,it is crucial to formulate effective chemical flooding agents that can improve sweep efficiency and oil displacement efficiency.Under these conditions,for the residual oil in low-permeability reservoir to be extracted,ultra-low interfacial tension（IFT）foam flooding has been proven to be an efficient EOR technique for low-permeability,high-temperature and high-salinity reservoirs,improving both sweep efficiency and oil displacement efficiency.In this study,building on the principle of compatibility with crude oil,a novel temperature-resistant,salinity-resistant,ultra-low interfacial tension（IFT）surfactant was formulated,based in molecular structure design theory,which was then used as foaming agent.Meanwhile,the molecular structure of the amphoteric surfactant was structurally characterized by FT-IR,1HNMR and TGA methods.Following that,experiments at the condition of Wen X field(temperature 84℃,salinity 86072mg/L,permeability 27.6×10-3μm2)were carried out to evaluate its properties.The results indicate that the ultra low transient minima of dynamic IFT for the newly developed surfactant can reach to a magnitude of 10-5m N/m.The foam volume of 0.10%TH161 is measured at 448m L,the half-life of foam came to 2567s,and the surfactant exhibited high foaming performance,which solves the discrepancy between ultra-low IFT and foaming efficiency in current foaming agents.Meanwhile,the injection capacity,blocking ability,and displacement efficiency of the foaming agents were evaluated.The results show that the newly formulated ultra-low IFT surfactant has ideal injectivity in low-permeability(0.1×10-3μm2～50×10-3μm2)reservoirs,that the critical permeability is 2.27×10-3μm2,the pressure gradient of flooding is 0.085MPa/m,and the displacement efficiency is16.16%.In addition,a series of core flooding tests were conducted to study the oil displacement efficiency based on the order of magnitude of equilibrium and dynamic IFT,as well as rock wettability and permeability.These two crucial factors are evaluated and the critical permeability value in low-permeability reservoirs through which the residual oil droplets can be activated is given quantitatively.The results show that dynamic interfacial tension is the key to mobilizing trapped oil droplets in low-permeability reservoirs.And the more hydrophilic the rock surface is,the lower the critical permeability becomes.When the wetting contact angle is 18°,44°,78°and142°,the corresponding critical permeability is 33.38×10-3μm2,6.13×10-3μm2,1.97×10-3μm2and 0.15×10-3μm2 respectively.Moreover,foam blocking ability were evaluated by single-core flooding tests.It shows that TH161 with a wide range of gas/liquid ratio（0.3:1.0～3.0:1.0）exhibit a high resistance factor that indicates a good mobility control ability.The combination of these characteristics can greatly improve the sweep efficiency.The parallel dual-core flooding tests were conducted to assess the oil displacement efficiency of low-IFT foam EOR for low permeability oil recovery from heterogeneous reservoir conditions.It shows enhanced oil recovery efficiency in comparison with water flooding by more than 15%OOIP.Hence,the optimum concentration of the foaming agent was determined to be is 0.10%,while the proper gas/liquid ratio is 1.0:1.0 and the injection slug is 0.3PV.Finally,the mechanism of residual oil on ultra-low interfacial tension foam flooding is evaluated through the microscopic visualization foam flooding experiments.The results show that the residual oil left behind after water flooding are mainly columnar and form isolated islands which are mainly stripped by the way of piston pressure.This greatly improves the spread efficiency and improves the oil washing efficiency while the sweep efficiency is increased by 13.0%.Additional evaluations were performed on the mechanism of the foam pressure to extract and separate crude oil and how the process differs between ultra-low ITF foam-flooding from the ordinary foam-flooding.To summarize,the newly formulated surfactants exhibit remarkable abilities and can be used for EOR in high-temperature,high-sality,low permeability reservoirs. |
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