Transient pressure analysis of polymer flooding fractured wells with oil-water two-phase flow

The oil-water two-phase flow pressure-transient analysis model for polymer flooding fractured well is established by considering the comprehensive effects of polymer shear thinning,shear thickening,convection,diffusion,adsorption retention,inaccessible pore volume and effective permeability reduction.The finite volume difference and Newton iteration methods are applied to solve the model,and the effects of fracture conductivity coefficient,injected polymer mass concentration,initial polymer mass concentration and water saturation on the well-test type curves of polymer flooding fractured wells are discussed.The results show that with the increase of fracture conductivity coefficient,the pressure conduction becomes faster and the pressure drop becomes smaller,so the pressure curve of transitional flow goes downward,the duration of bilinear flow becomes shorter,and the linear flow appears earlier and lasts longer.As the injected polymer mass concentration increases,the effective water phase viscosity increases,and the pressure loss increases,so the pressure and pressure derivative curves go upward,and the bilinear flow segment becomes shorter.As the initial polymer mass concentration increases,the effective water phase viscosity increases,so the pressure curve after the wellbore storage segment moves upward as a whole.As the water saturation increases,the relative permeability of water increases,the relative permeability of oil decreases,the total oil-water two-phase mobility becomes larger,and the pressure loss is reduced,so the pressure curve after the wellbore storage segment moves downward as a whole.The reliability and practicability of this new model are verified by the comparison of the results from simplified model and commercial well test software,and the actual well test data.

Polymer flooding: Current status and future directions

This review presents our perspective on the factors that have brought polymer flooding to its current state. Insights are provided on why HPAM is the dominant polymer used as well as what is needed to make alternative polymers and mobility-control methods viable. Explanation is given for why large polymer banks are needed for polymer flooding, and design of the injected polymer viscosity is detailed for cases with/without crossflow. The role of fractures and horizontal wells are discussed for improving injectivity and extending polymer flooding to recover oils with viscosities as high as 10,000 cP. Operational improvements are described to minimize mechanical and oxidative stability to allow HPAM polymers to be viable to 70 °C and ATBS polymers to 120 °C. Key factors affecting polymer retention are summarized. The paper points out unresolved issues and future directions for polymer flooding.

Establishment of Unstable Flow Model and Well Testing Analysis for Viscoelastic Polymer Flooding

At present, the polymer solution is usually assumed to be Newtonian fluid or pseudoplastic fluid, and its elasticity is not considered on the study of polymer flooding well testing model. A large number of experiments have shown that polymer solutions have viscoelasticity, and disregarding the elasticity will cause certain errors in the analysis of polymer solution seepage law. Based on the percolation theory, this paper describes the polymer flooding mechanism from the two aspects of viscous effect and elastic effect, the mathematical model of oil water two-phase three components unsteady flow in viscoelastic polymer flooding was established, and solved by finite difference method, and the well-test curve was drawn to analyze the rule of well test curve in polymer flooding. The results show that, the degree of upward warping in the radial flow section of the pressure recovery curve when considering polymer elasticity is greater than the curve which not considering polymer elasticity. The relaxation time, power-law index, polymer injection concentration mainly affect the radial flow stage of the well testing curve. The relaxation time, power-law index, polymer injection concentration and other polymer flooding parameters mainly affect the radial flow stage of the well testing curve. The larger the polymer flooding parameters, the greater the degree of upwarping of the radial flow derivative curve. This model has important reference significance for well-testing research in polymer flooding oilfields.

Numerical Simulation of High Concentration Polymer Flooding in Oilfield Development

The field test of high concentration polymer flooding has the characteristics of high cost, long cycle and irreversibility of the reservoir development process. In order to ensure the best development effect of the development block, this paper simulated and calculated the high concentration polymer flooding development case of the polymer flooding pilot test area through numerical simulation research, and selected the best case through the comparison of various development indicators. The simulation results showed that the larger the polymer dosage and the higher the concentration, the better the oil displacement effect. The best injection method in the construction process was the overall injection of high concentration polymer. The test area should implement high concentration polymer oil displacement as soon as possible. The research results provided theoretical guidance for the future development and management of the pilot area.

Enhanced oil recovery mechanisms of polymer flooding in a heterogeneous oil reservoir

Taking reservoir rocks and fluids of the Daqing,Dagang and Changqing oilfields as research objects,the EOR mechanisms and technical approach of polymer flooding were discussed.By comparing the displacement performances of ordinary polymer,glycerol,polymer in"sheet-net"structure and heterogeneous weak gel at the same viscosity and concentration,the relationship between the viscosity of polymer displacement agents and displacement performance was demonstrated,and the method of improving polymer flooding effect was worked out.The main mechanism of polymer flooding to increase oil recovery is the swept volume expansion of water injection due to polymer retention in porous media.The viscosity of polymer agents has no positive correlation with polymer flooding effect.Although polymer of"sheet-net"structure has strong capacity in increasing viscosity,it has poor compatibility with pore throat structure of reservoir rock,low injectivity and low shear resistance.Heterogeneous weak gel system has higher adsorption and capture capacity in porous media,which is easy to retain in porous media,and can effectively establish seepage resistance in high permeability layers(zones).Compared with polymer solutions with the same viscosity or concentration,it has stronger ability to expand swept volume.Long term injection of polymer flooding agents will inevitably lead to fluid entry profile reversal,and thus worsening of polymer flooding effect.Alternate injection of high retention and low or non-retention displacement agents can further improve the displacement effect of polymer flooding agents.

CURRENT COLLOIDAL DISPERSION GELS ARE NOT SUPERIOR TO POLYMER FLOODING

The suggestion that the colloidal-dispersion-gel (CDG) process is superior to normal polymer flooding is misleading and generally incorrect. Colloidal dispersion gels, in their present state of technological development, should not be advocated as an improvement to, or substitute for, polymer flooding. Gels made from aluminum-citrate crosslinked polyacrylamides can act as conventional gels and provide effective conformance improvement in treating some types of excess water production problems if sound scientific and engineering principles are respected.

Experimental Study and Numerical Simulation of Polymer Flooding

The numerical simulation of polymer flooding is a complex task as this process involves complex physical and chemical reactions,and multiple sets of characteristic parameters are required to properly set the simulation.At present,such characteristic parameters are mainly obtained by empirical methods,which typically result in relatively large errors.By analyzing experimentally polymer adsorption,permeability decline,inaccessible pore volume,viscosity-concentration relationship,and rheology,in this study,a conversion equation is provided to convert the experimental data into the parameters needed for the numerical simulation.Some examples are provided to demonstrate the reliability of the proposed approach.

Study of Polymer Flooding in Heterogeneous Porous Media by Using Nuclear Magnetic Resonance

Polymer has been successfully used to enhance crude oil recovery at high water cut stage.However,the application of polymer flooding is limited by the heterogeneity of reservoir.In this work,the role of polymer flooding in heterogeneous reservoir was explored by nuclear magnetic resonance(NMR)spectroscopy.Parallel core displacement experiments were carried out to study polymer flooding in heterogeneous formation.The results showed that the polymer flooding area was related to permeability and pore connectivity.At the end of the water flooding stage,the residual oil was not evenly distributed in porous media.The percent crude oil recovery increased with the increase of pore diameter.Crude oil recovery from cores with larger pores was higher,and water broke through the highly permeable core first.After 0.3 PV polymer injection,the water mobility of the high permeability core decreased.Polymer injection showed a dual effect,an oil displacement effect and a traction effect.The oil displacement effect was responsible for driving out the crude oil in large pores,whereas the traction effect was responsible for driving out residual oil from small pores.After 1 PV polymer injection,the polymer solution channeled through the highly permeable core rapidly.The crude oil in large pores was washed out completely.The NMR water-phase spectrum line of the highly permeable core was much higher than that of the low permeable core.Under different core permeability and porosity,the NMR peak of the crude oil-phase spectrum line indicated that the polymer flooding is mainly effective on pores>40μm.

A Polymer Flooding Pilot in Thin Layers of Non-essential Reservorirswith Medium-low Permeability and Heterogeneity in Daqing

DaqingOilfieldisalargeinlandfluvialdelta-lacustrinesedimentationreservoirwithmulti-layersandhighreservoirheterogeneity.Fromnorthtosouththereare7localstructuresinDaqingOilfield,andverticallythereare3setsofoil-bearingformations-Saertu,PutaohuaandGaotai...

Pressure transient characteristics of non-uniform conductivity fractured wells in viscoelasticity polymer flooding based on oil-water two-phase flow

Polymer flooding in fractured wells has been extensively applied in oilfields to enhance oil recovery.In contrast to water,polymer solution exhibits non-Newtonian and nonlinear behavior such as effects of shear thinning and shear thickening,polymer convection,diffusion,adsorption retention,inaccessible pore volume and reduced effective permeability.Meanwhile,the flux density and fracture conductivity along the hydraulic fracture are generally non-uniform due to the effects of pressure distribution,formation damage,and proppant breakage.In this paper,we present an oil-water two-phase flow model that captures these complex non-Newtonian and nonlinear behavior,and non-uniform fracture characteristics in fractured polymer flooding.The hydraulic fracture is firstly divided into two parts:high-conductivity fracture near the wellbore and low-conductivity fracture in the far-wellbore section.A hybrid grid system,including perpendicular bisection(PEBI)and Cartesian grid,is applied to discrete the partial differential flow equations,and the local grid refinement method is applied in the near-wellbore region to accurately calculate the pressure distribution and shear rate of polymer solution.The combination of polymer behavior characterizations and numerical flow simulations are applied,resulting in the calculation for the distribution of water saturation,polymer concentration and reservoir pressure.Compared with the polymer flooding well with uniform fracture conductivity,this non-uniform fracture conductivity model exhibits the larger pressure difference,and the shorter bilinear flow period due to the decrease of fracture flow ability in the far-wellbore section.The field case of the fall-off test demonstrates that the proposed method characterizes fracture characteristics more accurately,and yields fracture half-lengths that better match engineering reality,enabling a quantitative segmented characterization of the near-wellbore section with high fracture conductivity and the far-wellbore section with low fracture conductivity.The novelty of this paper is the analysis of pressure performances caused by the fracture dynamics and polymer rheology,as well as an analysis method that derives formation and fracture parameters based on the pressure and its derivative curves.

Review on chemical enhanced oil recovery using polymer flooding: Fundamentals, experimental and numerical simulation

The EOR techniques are employed to recover more oil from mature reservoirs after the primary and secondary oil production stages.Polymer flooding as a chemical EOR method involves adding polymer molecules in order to increase water viscosity.Increasing water viscosity will improve the mobility ratio of injected fluid to reservoir fluid toward a more favorable value.Therefore,vertical and areal sweep efficiencies are increased compared to typical water flooding.Polymer flooding will be most effective if applied in the early stages of a water flood while the mobile oil saturation is still high.Polymer is also a critical component when considering other chemical EOR technologies such as alkaline-polymer or alkaline-surfactant-polymer.The present study covers two main parts.In the first part,fundamental of polymer flooding as it related to experimental work and simulation are discussed.The challenge of polymer flooding applications in high temperature and high salinity(HTHS)environment is also discussed in this part.In the second part,the synergy of polymer with surfactant and alkaline as chemical enhancing oil recovery(CEOR)methods is discussed.

Optimized polymer flooding projects via combination of experimental design and reservoir simulation

The conventional approach for an EOR process is to compare the reservoir properties with those of successful worldwide projects.However,some proper cases may be neglected due to the lack of reliable data.A combination of experimental design and reservoir simulation is an alternative approach.In this work,the fractional factorial design suggests some numerical experiments which their results are analyzed by statistical inference.After determination of the main effects and interactions,the most important parameters of polymer flooding are studied by ANOVA method and Pareto and Tornado charts.Analysis of main effects shows that the oil viscosity,connate water saturation and the horizontal permeability are the 3 deciding factors in oil production.The proposed methodology can help to select the good candidate reservoirs for polymer flooding.

Polymer efficiency and sulfate concentration for hybrid EOR application to an acidic carbonate reservoir

Polymers play an important role in hybrid enhanced oil recovery (EOR), which involves both a polymer and low-salinity water. Because the polymer commonly used for low-salinity polymer flooding (LSPF) is strongly sensitive to brine pH, its efficiency can deteriorate in carbonate reservoirs containing highly acidic formation water. In this study, polymer efficiency in an acidic carbonate reservoir was investigated experimentally for different salinity levels and SO42− concentrations. Results indicated that lowering salinity improved polymer stability, resulting in less polymer adsorption, greater wettability alteration, and ultimately, higher oil recovery. However, low salinity may not be desirable for LSPF if the injected fluid does not contain a sufficient number of sulfate (SO42−) ions. Analysis of polymer efficiency showed that more oil can be produced with the same polymer concentration by adjusting the SO42− content. Therefore, when river water, which is relatively easily available in onshore fields, is designed to be injected into an acidic carbonate reservoir, the LSPF method proposed in this study can be a reliable and environmentally friendly method with addition of a sufficient number of SO42− ions to river water.

Visualization of adaptive polymer flow and displacement in medium-permeable 3D core-on-a-chip

Polymer flooding has been witnessed an effective technology for enhancing oil recovery from medium-to low-permeability reservoirs;however, direct visualization of polymer solution flow in such reservoir condition is still lacking. In this work, a three-dimensional (3D) core-on-a-chip device with a permeability of around 200 mD was prepared and employed to visualize the pore-scale flow and displacement of a self-adaptive polymer (SAP, 8.7 × 106 g·mol−1)−whose microscopic association structure and macroscopic viscosity can reversibly change in response to shear action−versus partially hydrolyzed polyacrylamide (HPAM), by recording their flow curves, monitoring dynamic transportation process via particle imaging velocimetry, and building 3D structure of remaining oil. The results show that, in single-phase flow, all polymer solutions exhibit flow thinning and then thickening regions as flow rate increases, but the transition between two regimes occurs at a small Weissenberg number (10−3−10−1) in this medium-permeable condition. In contrast to HPAM-1 with close weight-average molecular weight (Mw), the adaptive character not only extends SAP's shear-govern region, allowing SAP to propagate piece by piece and achieve higher accessible pore volume, but it also enhances the elastic resistibility of polymer in the extension-dominated regime, increasing the microscopic displacement efficiency. These two effects result in 1.5–3 times more oil recovery factor for SAP than for HPAM-1. Regarding ultra-high-Mw HPAM-2 (25 × 106 g·mol−1), plugging and chain degradation do occur, thus producing lower oil recovery than SAP. This work provides a direct approach for in-situ assessment of polymer-based displacing system under a more authentic condition of practical reservoirs.

An overview of chemical enhanced oil recovery and its status in India

India is currently producing crude oil from matured fields because of insufficient discoveries of new fields.Therefore,in order to control the energy crisis in India,enhanced oil recovery(EOR)techniques are required to reduce the import of crude from the OPEC(Organization of the Petroleum Exporting Countries).This review mentions chemical EOR techniques(polymers,surfactants,alkali,nanoparticles,and combined alkali-surfactant-polymer flooding)and operations in India.Chemical EOR methods are one of the most efficient methods for oil displacement.The efficiency is enhanced by interfacial tension(IFT)reduction using surfactants and alkali,and mobility control of injected water is done by adding a polymer to increase the volumetric sweep efficiency.This paper also reviews the current trend of chemical EOR,prospects of chemical EOR in Indian oilfields,the development of chemical EOR in India with their challenges raising with economics,and screening criteria for chemical EOR implementation on the field scale.Furthermore,the review gives a brief idea about chemical EOR implementation in Indian oilfields in future prospects to increase the additional oil recovery from existing depleted fields to reduce the import of crude oil.The outcome of this review depicts all chemical EOR operations and recovery rates both at the laboratory scale and field scale around the country.The additional recovery rates are compared from various chemical EOR methods like conventional chemical flooding methods and conventional chemicals combined with nanoparticles on a laboratory scale.The development of chemical EOR in the past few decades and the EOR policy given by the government of India has been mentioned in this review.The analysis provides an idea about enhanced recovery screening and implementation of chemical EOR methods in existing fields will significantly reduce the energy crisis in India.

绝热反应合成AM/AMPS共聚物及其性能

选用过硫酸铵〔(NH4)2S2O8〕和亚硫酸氢钠(NaHSO3)为氧化-还原引发体系,以N,N,N',N'-四甲基乙二胺(TMEDA)、乙二胺四乙酸二钠(EDTA-2Na)和尿素为助剂,采用水溶液绝热反应合成丙烯酰胺(AM)/2-丙烯酰胺基-2-甲基丙磺酸(AMPS)共聚物,考察了AM与AMPS绝热共聚合的放热过程,并对共聚物的结构和主要性能进行了考察。结果表明,降低引发温度和减小引发剂用量,有利于聚合物分子链的增长,合成的共聚物溶解性好,相对分子质量达2 500万;在95℃、采用新鲜污水配制共聚物溶液,60 d的黏度保留率达84.2%,模拟油藏条件进行驱油效果评价,可提高原油采收率达17.38%。

聚合物驱后进一步提高采收率方法及其技术经济效果评价

依据矿场实际需求,本文通过物理模拟驱油实验对聚合物驱后采用高浓度聚合物溶液(平均相对分子量M(下同)=2500×104,聚合物浓度CP=2.5g/L)、"碱/表面活性剂/聚合物"ASP(CP=2g/L,表面活性剂HAPS浓度CS=3g/L,碱浓度(CA=12g/L)、"表面活性剂/聚合物"SP(M=2500×104,CP=2g/L,非离子表面活性剂浓度CS=3g/L)、聚表剂溶液(CSp=2g/L)驱油效果进行了对比与评价。结果表明,在化学剂用量相同条件下,4种驱油体系均可以进一步提高原油采收率,采收率增幅在10.1%～22.9%之间。在化学剂费用相同条件下,采用单一高浓度聚合物段塞增油降水效果最好,最终采收率均高于4种驱油剂多段塞组合方式。多段塞组合方式中,"低黏度驱油体系+高黏度驱油体系"方式的增油效果好于"高黏度驱油体系+低黏度驱油体系"的增油效果。综合技术条件和经济效果两方面考虑,推荐聚合物驱后采用单一高浓度聚合物驱,采收率增幅预计为15%～18%。

非离子型聚丙烯酰胺絮凝剂的制备及其絮凝性能

以丙烯酰胺(AM)和烷基酚聚醚(APAP)为单体,采用反相乳液聚合法制备了流动性好的非离子型聚丙烯酰胺絮凝剂(PAM-APAP)。考察了乳化剂和引发剂的种类、单体总质量分数、单体配比和反应温度等因素对聚合反应的影响。实验结果表明,制备PAM-APAP的最佳工艺条件为:单体总质量分数35%、m(AM)∶m(APAP)=9.0∶1.0、Span 80+Tween 80复配乳化剂加入量为油水总质量的7%、m(Span 80)∶m(Tween 80)=7.0∶1.5、V50加入量为单体总质量的1.5‰、反应温度50℃。PAM-APAP具有较好的絮凝效果,与其他絮凝剂相比用量较少,在加入量为100 mg/L时,对聚合物驱油中产生的含聚合物污水的浊度去除率为87.9%,去油率为89.5%,且絮体不黏壁。

聚表剂驱原油破乳剂的研究

油田常用聚驱破乳剂已经不能满足聚表剂驱采出液破乳的需要,通过对聚表剂驱采出液油水分离特性测试,用瓶试法对比了8种常用且效果较好的聚驱破乳剂对聚表剂驱采出液的破乳效果,实验发现HHY601酚醛树脂型破乳剂能够达到较好的破乳效果,并进行了应用条件的优化实验,得出在40℃下,最佳用量为40mg.L-1,破乳时间为10min时,HHY-601型破乳剂即可达到良好的破乳效果。