| In view of the characteristics of low-permeability reservoirs and the unique challenges in oil recovery,some key problems of in-situ emulsification displacement and conformance control technology were investigated based on the technical ideas of the enhanced oil recovery(EOR)that differ from conventional reservoirs.In this work,it overcame the technical bottleneck of oil-water emulsification and emulsion stability evaluation and testing method,and initially revealed the mechanisms and related rules,which were useful to establish the groundwork for the development of viable in-situ emulsification displacement and conformance control technologies in low-permeability reservoirs.A novel testing instrument for oil-water emulsification and emulsion stability based on rotating method was introduced,which was characterized by a preparation condition of precise controllability and stability for emulsification,an identifiable and visible interface between emulsions and non-emulsified oil(or water),the reliability accuracy and repeatability of the measured parameters.A parameter,emulsification index(EI),was used to quantitatively characterize the emulsifying capability based on the relationship curve between the measured emulsified oil rate and the rotating rate.It is proved that EI value shows a positive function with the emulsifying capability,and presents a high identification for the slight differences between oil-water systems.Another parameter,stability index(SI),was also designed to characterize the emulsion stability based on the measured dynamic curve of emulsified oil rate in demulsification.The experimental results show that SI value also shows a positive function with the emulsion stability,which presents a high identification for the minor differences bwtween oil-water systems.The main-controlling factors and related rules of oil-water emulsifying capability and emulsion stability were deepened and quantified by emulsification and emulsion stability evaluation tests using 5 kinds of crude oils and 13 types of oil displacement agents.The results show that interfacial tension(IFT)is an important factor affecting the emulsifying capability and emulsion stability when the IFT is in the high IFT range.However,in the range of relatively lower IFT values,the oil-water IFT is not the main controlling factor to affect the emulsion stability.The effect of hydrophilic lipophilic balance(HLB)values of different types of surfactants on the emulsifying capability and emulsion stability is different.The increasing HLB value of nonionic surfactants is useful to improve the emulsifying capability and emulsion stability.But for the ionic surfactants,including anionic,anionic-nonionic and zwitterion surfactants,the emulsifying capability presents a non-monotonic correlation of decreasing previously and then increasing with the growing of HLB values,and the emulsion stability shows a monotonic decreasing function with HLB values.Besides,the effect of crude oil active components on the oil-water emulsifying capability and emulsion stability of various types surfactants are different.The results show the emulsifying capability of ionic surfactants increases first and then decreases with the increase of petroleum acid value,monotonically increases with the increase of resin and asphaltene content,and monotonically decreases with the increase of wax content.Also,the emulsify capability of nonionic surfactants show unapparent sensitivity in the variation of crude oil components.In addition,the emulsifying capability of alkali shows a monotonic increase along with the petroleum acid,resin and asphaltene content and decrease with wax content.Besides,the emulsion stability of typical surfactants presents monotonically increasing functions with the increasing of petroleum acid,resin and asphaltene content and decreasing with wax.These experimental rules provide a clear technical direction for the preparation of oil displacement agent using oil-water emulsification characteristics as the main index according to various reservoir conditions and crude oil compositions.The oil film and micro-heterogeneous residual oil displacement tests were systematically performed using three kinds of oil displacement agents with different EI,SI,and IFT.The results show that the oil displacement agent with strong emulsifying capability,high EI value,presents a profitable oil displacement efficiency that higher than 80%for both low and medium-high viscosity oil film,and also can detach oil film efficiently at a low flow rate.In addition,the oil displacement agent with strong emulsifying capability can significantly improve microscopic sweep efficiency and mobilize micro-heterogeneous residual oil by in-situ emulsification compared with the agent with the ultra-low ITF but weak emulsifying capability.Besides,given the results of flooding test after water flooding in the one-dimensional columnar low-permeability cores,it can be obtained that the the of oil displacement efficiency is improved by strong emulsifying capability agent more than twice as much as by non-strong emulsifying agent,indicating the EI contributes significantly more to residual oil displacement than IFT.Therefore,it can be concluded that the strong emulsifying capability,a large EI value,is the main-control behavoir of surfactants to improve oil displacement efficiency in low-permeability reservoirs dominated by micro-heterogeneous residual oil.Oil displacement tests and plugging matching experiments in low-permeability cores with various ranges were carried out using strong emulsifying capability agents of obviously contrasting SI value.It is found that decreasing SI can substantially increase the flow resistance in the middle and deep position of the model.Under the experimental conditions of this paper,the flow resistance coefficient in the middle-deep position increases noticeably as permeability decreases,indicating that the effect of in-depth conformance control in low permeability reservoirs is clearly superior to that in medium-high permeability reservoirs.The mechanism of in-situ emulsion dissection was confirmed by the experiments as the"Jamin"effect of emulsion droplets at the pore throat.Then,it is concluded that the matching relationship between emulsion droplet size and pore throat size is one of the key factors for in-situ emulsification conformance control.Accordingly,the paramater,R_d,of the emulsion droplet diameter formed by in situ emulsification to the average pore throat diameter of the reservoir,was used to characterize the relative value.The results show its plugging ability to block water channel grows monotonically with R_dcreated by the in-situ emulsions.This result can be used to explain why in-situ emulsification is more effective in conformance control in low permeability reservoirs than middle-high permeability reservoirs,as well as to obtain the fundamental requirement that in-situ emulsification requires a sufficiently large emulsion particle size for oil displacement.Besides,combined with the findings of heterogeneous cores flooding experiments,the weak-stabilized emulsions created by in-situ emulsification improves the oil recovery more than the strong-stabilized emulsions.In summary,it is clear that the main-control behavoirs of oil displacement agent required to improve sweep efficiency in low-permeability reservoirs are high emulsification ability,weak emulsion stability,and appropriately big emulsion particle size.The effects of oil displacement agent slug,injection time and reservoir heterogeneity on in-situ emulsification displacement and conformance control in low-permeability reservoirs were investigated.Besides,a chemical flooding combination consisted of deep profile control agent(CSA particles)and in-situ emulsification oil displacement agent(TIECC)was further proposed and were evalutaed.In addition,the compositions of chemical slugs were optimized to acieve the benefitial synergistic effect of in-depth conformance control and oil displacement in the low-permeability reservoir models. |
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