凝析油气低界面张力对凝析油流动的影响罗 凯(1) 方义生(1) 宋文杰(1) 蒲 建(2) (1)中国石油天然气集团公司石油勘探开发科学研究院 (2)大港油田集团有限责任公司地质勘探开发研究院
前 言
由于流体渗流规律的复杂性，尽管对影响油藏原油采收率的因素很早就进行过广泛研究，但至今仍未清楚地认识某些因素的影响。 开采凝析气藏最令人关注的问题是凝析油的采收率。随着气藏压力衰减，当地层压力低于流体的露点压力后，必然伴随着凝析油的析出。特别是近井地带凝析油饱和度较高，常常降低井的产能[1]，油的流动性就成为问题的焦点。随着研究的深入，人们发现渗流速度、多相流体在储集层中的微观空间分布会影响凝析气的流动特征[2,3]。
储集层流体与多孔介质作为一个整体系统，在开采过程中必然会相互影响。因此，采出流体导致地层压力下降对储集层状态的影响，以及储集层岩石孔隙空间状态变化对流体流动的影响成为当今凝析气藏开发研究的难点。新近的研究表明，由于多孔介质的压缩性，在井筒周围乃至地层中因地层压力降低导致储集层渗流空间缩小，以及储集层非均质性使流体流动阻力增加[4]等因素，也会导致凝析气井产能降低。
模拟凝析油分布状态影响气相渗流的实验发现，液相铺展于岩石孔隙壁面或水相表面会改善气相流动，从而使气相流动性更好[5]。这表明液相析出并不是影响气井产能唯一的原因[3,4]，确切理解凝析油的影响，关键在于对流体物理特性和储集层流体微观分布的全面了解。
凝析油气的界面张力特征
对界面形成的力学分析表明，界面张力实际上是由于界面区域分子受到不同于流体相分子的分子力场作用的结果。当凝析气析出后，气液界面分子由于受到分子力场作用，必然会产生过剩自由能，并表现为界面张力。尽管人们已普遍认识到凝析气的反转凝析特性，但对许多其它的性质还未完全了解。过去普遍接受的某些结论(如凝析油临界流动饱和度很高，大致为30%～50%)不断被新的研究所完善。若按照这一通常论断，绝大多数凝析油几乎都会滞留地下，因为只有极少数高含凝析油凝析气藏和常规凝析气藏近井地带的凝析油饱和度能达到上述临界流动饱和度范围，众多凝析气藏的凝析油含量几乎都低于上述标准。上述
结论实际上是从极少量研究得到的结果，忽视了凝析油气界面张力低的特点。
事实上，由于凝析流体高温高压特点对测试技术的限制，直到近几年人们才真正清楚地认识凝析油气界面张力低的特点。图1为采用激光技术测定实际凝析油气在压力衰竭过程中的两相界面张力随压力
降低的变化关系[6]。从中可明显看出界面张力随压力降低而有所增加，但却在很小的范围变化；当压力从41MPa 降到30MPa时，界面张力仍小于1mN/m。相似的结论亦被其它采用不同测定方法或不同流体组分的研究所证实[2,7]。这些研究有力地说明凝析油气界面张力低具有普遍性。
图1 凝析油气界面张力随压力变化关系图
凝析油气界面张力低以及流体在储集层内的空间分布状态使人们意识到，由常规实验方式获得的相对渗透率曲线用于预测凝析气藏有缺陷；并促使人们更加深入地研究在低界面张力作用下，重力对凝析油渗流的影响[2]，以及凝析油的临界流动饱和度。研究表明，凝析油的最小流动饱和度可以低于10%，甚至更低。正如在提高常规油藏原油采收率的许多措施中，能够通过降低相间界面张力来改善原油的流动性一样，凝析油气的低界面张力也会明显改善凝析油的可流动性。 77
石 油 勘 探 与 开 发
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亚星奔驰
凝析油气的相对渗透率特征
描述凝析油气在多孔介质中渗流的主要参数是油气的相对渗透率曲线，通常由经验公式和相对渗透率实验获得。实际上，这两种实验方法最初是在认识油藏流体流动特征的基础上发展起来的，对凝析流体微观分布及界面张力考虑甚少。
在凝析气藏的开采过程中，其变化的主要特征是凝析油的析出、分布、流动性及其对凝析气渗流动态的影响。因此，凝析气藏与油藏的区别在于凝析油的析出是从无到有、由少到多的变化过程，在多孔介质中凝析油与凝析气的接触面积很大，气相的流动起着剪切油相的作用。在常规油藏原油的气驱过程中是气相对油相的整体驱替，其两相接触区通常存在前缘过渡带。由此可见，流体空间相互分布的差异必然会导致不同的流动特征。
凝析油、
气之间的低界面张力会产生许多有意义的结果。众多研究表明，随着流体界面张力的变化，凝析油气的流动特征要发生相应变化[7～9]。由图2可
见，随着界面张力(σ
)降低，凝析气藏油气相对渗透率曲线呈现“直线化”趋势，即流体的临界流动饱和度和残余饱和度不断减小，此曲线直观地反映了流体流动性随界面张力的动态变化特征。这些新的认识促使人们对凝析油的流动特征有了更加深入的理解。
图2 气液相对渗透率随界面张力变化示意图
目前，获得凝析油气相对渗透率曲线的方法仍是常规的实验手段。模拟流体典型的界面张力值约为30mN/m ，这与凝析油、气之间的界面张力范围相比要高出数十倍。鉴于常规的凝析油气相对渗透率实验曲线几乎都没考虑界面张力的影响，在应用实验曲线时，须采用一些经验的校正方法。本文拟采用如下线性化方法校正相对渗透率的实验曲线[10]:
K ro =αK ro1+(1-
α)K ro2(1)K rg =αK rg1+(1-α
)K rg2(2)
K ro2,K rg2采用如下公式计算:
K ro2=K rh S o /(1-S w )(3)K rg2=K rh S g /(1-S w )(4)
K rh =
K ro S o
lec=1 +K rg S o =0
/2
(5)α=
(σ/0.2)0.2
σ<0.2mN/m 1
σ≥0.2mN/m (6)
式中 K ro1,K rg1———相对渗透率实验值;K ro2,K rg2———
界面张力为零时的相对渗透率;S ———流体饱和度。下标:o ,g ,w ———油相、气相、水相。
实例分析
本文单井模型中的地质参数和流体组分数据均取自一实际储集层，其平均孔隙度为0.1073，平均渗透率
为0.0527μm 2，束缚水饱和度为0.24。表1为凝析气流
体各组分组成的摩尔百分数，其露点压力接近原始地层压力，最大反凝析液量约为22%，这意味着在实际储集层的开采过程中，会有大量液相析出。
表1 凝析气组分的摩尔百分数表
组 分N 2CO 2C 1C 2C 3摩尔百分数 3.280.6378.678.77 2.2组 分iC 4nC 4iC 5nC 5C 6+摩尔百分数
0.46
0.68
0.25
0.26
4.8
根据前面的分析可知，因凝析油气的界面张力低,
保持冷静 继续前行凝析油(特别是近井的凝析油带)本身应有一定的可流动性。为便于研究凝析油的流动性对单井动态特征的影响，本文用(1)式至(6)式对相对渗透率曲线进行了校正，并与实验曲线对比(见图3)，可见，校正后油气相对渗透率明显增大。
图3 凝析油气相对渗透率曲线校正对比图
图4为近井区域凝析油饱和度、日产油量、单井累
积产油量随生产时间的变化关系。从图中可知，校正
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前的凝析油饱和度明显高于校正后的饱和度；校正前油饱和度在开采初期高达0.50，而校正后油饱和度大大降低，在初期降到0.25左右，凝析油的滞留量大大
减少。随着生产时间的延长，凝析油饱和度逐渐减小
。
图4 相对渗透率校正前、后对生产动态的影响图
从日产油量与生产时间的变化关系可知，在生产初期，校正前日产油236.7m3，校正后日产油249.7m3 (多13m3);1a后校正前日产油154.4m3，校正后为168.5m3(多14.1m3)。可见，由于校正后油相相对渗透率曲线明显上升，凝析油滞留量减少，因而校正后日产油量高于校正前日产油量。
根据图4中单井累积产油量随生产时间的变化可知，当生产10d后，校正前累积产油0.24×104m3，校正后累积产油0.25×104m3，校正后比校正前增加0.01×104m3。第二年末时，校正前累积产油量11.70×104m3，校正后累积产油12.68×104m3，相差0.98×104m3。随着井继续生产，校正后累积产油量比校正前越来越高。
结 论
1，凝析油气界面张力低，即使压力降到远低于流体露点压力时，界面张力数值仍然很小。界面张力低会改善凝析油的流动性。
2，凝析油气相对渗透率曲线随界面张力减小呈现的“直线化”趋势表明，凝析油临界流动饱和度可能很小，并随界面张力降低有明显减小的趋势。
3，常规相对渗透率实验往往忽视凝析油气界面张力的影响，应用时须进行校正。本文研究结果表明，相对渗透率曲线显著影响近井区域凝析油饱和度。因此，进行凝析油气相对渗透率实验时需注意凝析油气的低界面张力特点，才可能用数值模拟方法更可靠地预测凝析气藏动态。
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第一作者简介 罗凯，男,27岁，博士研究生，现从事流体相态及凝析气开发研究工作。地址：北京市910信箱油气田开发研究所，100083。
收稿日期 1998207230
(编辑 郭海莉)
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1999年8月 罗凯等：凝析油气低界面张力对凝析油流动的影响
extend its alg orithm,in this paper PR2E OR,LHSS2E OR and their derivative matrix are computed with Newton2Rabins on method,and furtherm ore,tw o actual cases have been studied as well and their phase diagrams have als o been drawn out.In addition,in this paper a comparis on am ong and evaluation on Newton2Rabins on method,iterative method and Broyden method are als o made.The results show that the Newton2Rabins on method is better.Subject heading:C ondensate field, Phase equilibrium,Calculation,Application
A Theoretical study about effect of direct electrical current on relative permeability of sandstones.G uan,Jiteng;et al.(University of Petroleum,Shandong257062,P.R.China).Shiyou Kantan Yu Kaifa1999,26(4),72276.According to the theory of electrical double2layer and electroosm osic theory of porous media,a electrokinetic2waterflooding m odel was developed.The effect of direct electrical current on relative permeability at steady state or non2steady state was discussed.The results indicate that the relative permeability to water is decreased and the relative permeability to oil is increased with the application of electrical potential gradient.The effect grows up with increasing imposed potential gradient.Subject heading:Direct electric current,T w o phase flow,E ffective permeability
E ffect of low interfacial tension on mobility of condensate.Luo, K ai;et al.(Research Institute of Petroleum Exploration and Development,C NPC,Beijing100083,P.R.China).Shiyou Kantan Yu Kaifa1999,26(4),77279.This paper is dedicated to study the effect of low interfacial tension on the m obility of condensate near wellbore through the compositional simulator mimicking constant v olume depletion of a true gas condensate reserv oir.The average porosity and permeability of reserv oir are0.1073and0.0527μm2respectively.The fluid is a rich gas condensate with maximum liquid dropout up to22%. Although the interfacial tension of synthetic oil and gas is usually several tens higher than 江苏公路信息网
that of gas condensate,often are the regular relative permeabilities to synthetic oil and gas used to simulate the gas condensate reserv oir.Low interfacial tension may change the relative permeabilities to gas and condensate,s o the regular relative permeabilities are corrected in the simulation according to an empirical formula.A fter correction the relative permeabilities become greater, especially for condensate.The dynamic performance of condensate is analyzed through three cases such as condensate saturation near wellbore,production rate per day and cumulative condensate production.The results show that at early stage condensate builds near wellbore rapidly up to a maximum saturation,and then become less gradually with time.The comparis on of performance made with and without correction show that the production rate per day and cumulative production of condensate are greater after correction and much less condensate builds up near wellbore,because of the improvement of low interfacial tension on m obility of condensate.The results als o clearly dem onstrate that the proper description of relative permeabilities to gas and condensate is vital for prediction of performance at gas condensate reserv oir condition Subject heading:Interfacial tension,Relative permeability,C orrection,M obility of condensate,Production, Performance
N ew T echnology study for the development of a supper2heavy oil reservoir in Liaohe oil field.Liu,Sh
angqi;et al.(Research Institute of Petroleum Exploration and Development,C NPC,Beijing100083,P. R.China).Shiyou Kantan Yu Kaifa1999,26(4),80281.X inglongtai F ormation in Du84block,Liaohe oil field is a super2heavy oil reserv oir.Because of high viscosity and low m obility of oil in the reserv oir,the conventional steam stimulation performance with vertical wells was not g ood,with average oil2steam ratio per cycle only of0.35. In order to improve steam stimulation efficiency,a series of technical measures have been taken in the field,including using high performance vacuum heat2insulating tubing,deepening steam injection string and employing large capacity pump etc..By adopting these measures,the steam stimulation efficiency was improved,leading the oil2steam ratio increase to0.559.Meanwhile,according to the reserv oir and its fluid properties,s ome new technologies and methods have been studied and tested,such as steam injection recovery with horizontal wells,S team Assisted G ravity Drainage(S AG D),Fracturing Assisted S team Drive (FAST)and C ombustion Override S plit2production H orizontal well (C OSH)process etc..The results indicated that the combination of horizontal well technology and gravity drainage will be the major recovery methods for enhancing recovery effectiveness of super2heavy oil reserv oir.Subject heading:Viscous crude oil,Recovery method, G ravity drainage,H orizontal well
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V ol.26 N o.4 ABSTRACT
