【作者】 姜海峰；

【导师】 王德民；

【作者基本信息】 大庆石油学院 ， 油气田开发工程， 2008， 博士

【摘要】 关于多孔介质中聚合物驱的驱油机理,得到广泛认可的理论是对于一定的液体-岩石系统,驱油效率由驱替液的粘滞压力梯度和残余油上滞留力的比值决定,前者与驱替液的粘度和流速成正比,后者主要取决于界面张力,二者的比值定义为毛管数。根据该理论,聚合物溶液与原油间的界面张力与水驱相近,宏观压力梯度相同,两种驱替液的毛管数相近,因此聚合物驱的驱油效率与水驱相同,这与聚合物驱的室内和现场实验数据不符。此时,无法用宏观力解释聚合物溶液粘弹性对驱油效率的影响,聚合物驱时驱油效率的提高只能是不增加宏观压力梯度的微观力引起的。本文通过对不同种类聚合物溶液粘弹性的测试,系统的对比了不同聚合物溶液的粘弹特性,找出了适合本实验的定量表征聚合物溶液粘度和弹性的方法;从残余油的微观受力分析出发,提出了驱替液的弹性性质会改变孔隙中的微观流线,从而增加作用于残余油团突出部位的微观作用力,使突出部位移动这一微观驱油机理;通过宏观岩心驱替实验验证了微观力对不同类型残余油的作用,及粘弹性和毛管数对驱油效率的影响。本研究中采用的聚合物包括聚丙烯酰胺类聚合物、梳形聚合物和缔合聚合物三种。通过动态力学实验和稳态剪切实验分别测试了三种聚合物溶液的粘弹特性,分析了聚合物相对分子质量、质量浓度等对溶液粘弹性的影响。通过测试结果的对比分析,揭示了三种不同分子结构的聚合物溶液的粘弹性特点。结果显示,聚丙烯酰胺类聚合物溶液的粘度和弹性随分子量和聚合物质量浓度的增加而增加,在动态力学试验和稳态剪切实验中都观察到了相同的结果,粘度对质量浓度更为敏感而弹性对分子量更为敏感;与同分子量的聚丙烯酰胺类聚合物相比,梳形聚合物溶液在质量浓度相同时具有更高的粘度,但弹性较低,当聚合物种类不同时,动态力学实验的结果在粘弹性的对比中往往不如稳态剪切实验的结果准确,利用稳态剪切实验中第一法向应力差随剪切速率变化直线的斜率可以定量表征不同聚合物体系的弹性大小;缔合聚合物在低浓度时体现出的粘弹性质与普通聚丙烯酰胺相同,浓度达到一定程度后溶液的粘度急剧上升,该浓度为缔合聚合物溶液的临界缔合浓度,实验结果显示,高于临界缔合浓度后的缔合聚合物溶液具有比高分子量聚丙烯酰胺溶液更高的粘度,缔合作用对弹性贡献不大,弹性仍然取决于聚合物的分子量。分析了不同孔隙介质条件下微观残余油的状态及分步,对稳定状态下残余油团的应力分步进行了分析,研究发现,残余油团在综合驱动力的作用下必然会在沿流动方向的后段形成一个突起部位以提供足够的毛管力平衡前方的驱动力,使残余油团平衡。因此驱替液的流线在突起部位变化最大,微观流速的改变会产生微观的惯性力,对残余油团的突出部位产生推动力。粘弹性流体由于具有法向应力效应,流线改变的幅度大于牛顿流体;另外,粘弹性流体在毛细管中的速度剖面更均匀,靠近残余油团处的流速更大,所以粘弹性流体可以产生更大的微观驱动力,并且弹性越高,微观力越大。微观力是粘弹性流体提高驱油效率的主要原因。在亲油和亲水人造均质岩心上研究了不同聚合物溶液驱时粘弹性及毛管数对驱油效率和残余油饱和度的影响。利用驱替过程中岩心两端的压力梯度计算毛管数值,有效的避免了驱替液粘度和渗透率等因素的影响。对于聚丙烯酰胺类聚合物、梳形聚合物和缔合聚合物等不同类型的聚合物驱,增加体系的毛管数和增加溶液的弹性都可以有效的提高驱油效率、降低残余油饱和度。对具有不同粘弹性特点的聚合物溶液的驱油效果进行对比,发现驱油效率与聚合物溶液弹性的对应关系具有很好的规律性,证明了弹性对驱油效率的作用。梳形聚合物和缔合型聚合物由于弹性低于同分子量的聚丙烯酰胺溶液,所以驱油效率较低,粘度对驱油效率没有影响。合理的提高驱替液的弹性,可以获得与超低界面张力同样的驱油效果。高浓度聚合物驱的现场试验结果显示,提高驱油效率的幅度达到20%OOIP以上,为普通聚合物驱的二倍,且聚合物溶液前缘更加平缓,波及效率更高。综上所述,聚合物溶液的弹性性质,在不增加驱替压力梯度的情况下,可以提高多孔介质中的驱油效率。粘弹性驱替液提高驱油效率的主要机理是:驱替液的弹性性质会改变孔隙中的微观流线,从而增加作用于残余油团突出部位的微观作用力,使突出部位移动。这一机理可以比较好地解释聚合物驱在微观实验和宏观试验中所见到的现象,有助于化学驱化学剂的设计、合成与筛选,有助于化学驱驱油方案的设计和优选、化学驱数模的发展和微观渗流力学的发展。更多还原

【Abstract】 About the researches in polymer flooding mechanism, the most accepted mechanism is: for certain fluid-reservoir system, the displacement efficiency is determined by the viscous pressure gradient of the driving fluid and the restrain force of the residual oil, the former is proportional to the viscosity and velocity of the fluid, the latter is proportional to the interfacial tension, the ratio of them is defined as capillary number. According to this theory, the interfacial tension between polymer solution and oil is equal to that of water, and the macro pressure gradient is equal either, then the capillary numbers of polymer flooding and water flooding are near, the displacement efficiency of them should be equal, this is not agree with the results of polymer flooding lab tests and field tests. Then the macro forces can not explain the increase in displacement efficiency in polymer flooding, the increase in displacement efficiency of polymer flooding can only be caused by micro forces which do not increase the macro pressure gradient. In this paper, the rheological properties of different kinds of polymer solutions are tested, the viscoelasticity of different polymer solutions are compared systemically, methods in characterizing the viscosity and elasticity of polymer solutions are found; the analyzes of forces on residual oil show that the elasticity of driving fluid can change the micro flow lines in porous media, then increase the micro force which acting on the protruding part of residual oil blob and mobilize the protruding part; through flooding experiments on artificial cores, the influences of micro force on different types of residual oil, and the influences of capillary number and viscoelasticity on displacement efficiency are verified.Three kinds of polymers are studied in this research, they are polyacrylamide, comb polymer and associative polymer. Through dynamic mechanics test and steady state shear test, the viscoelasticity of different polymer solutions are measured, and the viscoelastic features of different polymer solutions are studied through comparison of the test results. The results show that, the viscosity and elasticity of polyacrylamide solutions increase with molecular weight and concentration both in dynamic mechanics test and steady state shear test, the viscosity is more sensitive to concentration while the elasticity is more sensitive to molecular weight; compared with polyacrylamide with equal molecular weight, comb polymer solution has higher viscosity but lower elasticity, when comparing different kinds of polymers, the results from dynamic mechanics test are not as precise as the results from steady state shear test, the slope of first normal stress difference vs. shear rate can characterize the elasticity which influences the displacement efficiency of different polymer solutions; when the concentration is low, the viscoelasticity of associative polymer solution is the same as that of polyacrylamide solution, with the increase of concentration, the viscosity of associative polymer solution starts to increase sharply, the concentration when this increase occurs is the critical associative concentration (CAC), the test results show that, when the concentration is higher than CAC, the viscosity of associative polymer solution can be greater than polyacrylamide with larger molecular weight, the association has no contribute to elasticity, which is still mainly determined by the molecular weight of polymer.The micro states and distributions of residual oil in different porous media are studied in this research, and the stress distribution of residual oil blobs under steady state are analyzed, the results show that, under the driving forces, there will be a protruding part along the flow direction which provide enough capillary force to counter the driving forces, the changes in micro velocity will influence the micro force acting on the protruding part of residual oil blob. The normal stress effects of viscoelastic fluids is stronger than that of Newtonian fluid, which make the change in flow lines stronger too; otherwise, the velocity profile of viscoelastic fluid in porous media is more uniform, the velocity of fluid near the residual oil blobs is greater, so the micro force of viscoelastic fluid is much more bigger than that of Newtonian fluid, and the higher the elasticity, the higher the micro force. The micro force is the main reason which makes the displacement efficiency of viscoelastic fluid flooding greater.Through polymer flooding experimtnes on both oil wet and water wet heterogeneous artificial cores, the influences of viscoelasticity and capillary number on displacement efficiency and residual oil saturation are studied. The pressure gradient of core is used in calculating the capillary number, which avoids the influences of viscosity of driving fluid and permeability of porous media. For polyacrylamide, comb polymer and associative polymer flooding, the increases in capillary number and elasticity of solutions are all effective in increasing the displacement efficiency and decreasing the residual oil saturation. The comparison of flooding results with different polymer solutions which have specific viscoelastic features show that, the changes in displacement efficiency and the changes in elasticity of polymer solutions match regularly. The elasticity of comb polymer and associative polymer is lower than that of polyacrilymide solution, and the displacement efficiency is lower, the viscosities of polymer solutions do not influence the displacement efficiency. Rationally increase the elasticity of the driving fluid can achieve the same effect of ultra low interfacial tension. The field test results of high concentration polymer flooding show that, the increase in recovery ratio is more than 20%OOIP, which is nearly two times than conventional polymer flooding, and the front of polymer solution is more uniform, the sweep efficiency is greater.In conclusion, the elasticity of polymer solution can increase the displacement efficiency in porous media and maintain the macro pressure gradient constant. The mechanism of viscoelastic fluid increases the displacement efficiency is: the elastic fearures of the fluid can change the flow line in pores, then increase the micro force on the protruding part of the residual oil blob and make it mobile. This mechanism can explain the phenomena of polymer flooding in micro and macro scale, it is helpful in designing, synthesizing and screening the chemical agents, in designing and optimizing the plans, and in developing the micro fluid mechanics of chemical flooding,更多还原