裂缝性低渗油藏周期注水与渗吸效应实验

收稿日期:2007 07 21

基金项目:国家973重大基础研究前期研究专项 低渗透油藏提高采收率基础理论研究(编号:2002CCA 00700)作者简介:王锐(1981 ),男,博士,主要从事低渗透油藏渗流理论及其提高采收率方法研究.

文章编号:1673 064X(2007)06 0056 04

裂缝性低渗油藏周期注水与渗吸效应实验

Cyclic waterf looding and imbibition experiments for fractured low permeability reservoirs

王锐1,岳湘安,尤源1,梁继文2

(1.中国石油大学石油工程教育部重点实验室提高采收率研究中心北京102249;

2.中海油湛江分公司,广东湛江524057)

摘要:结合长岩心周期注水实验与不同压降条件下的渗吸实验,得出了渗吸效应在周期注水中所起

的作用,进行了不同周期下的周期注水实验、无压降条件下的静态渗吸实验、不同压力及压降幅度下的动态渗吸实验,并将三者进行了对比.研究结果表明,周期注水在裂缝性低渗透油藏的开采中效果明显;低渗透基质油藏中毛管力具有较强作用;压力波动幅度对渗吸效应的影响呈先减小后增大的趋势,综合低渗透油藏中较强的压敏性,得出周期间歇性注水实验过程中,压力波动幅度较低才能获得较高采收率.

关键词:周期注水;渗吸;压降幅度;压敏性中图分类号:TE357.46 文献标识码:A 随着勘探开发的深入进行,目前新探明的储量近30.9%属于低渗透油藏,其中,具有经济开采价值的低渗透油藏一般存在着大量的裂缝.基质作为主要的储油空间,裂缝作为主要的渗流通道,常规注水开发由于渗透率存在巨大差异,注入水一般沿着高渗透裂缝窜流,水驱后期高含水饱和度的裂缝系统与高含油饱和度的基质系统交织共存,基质岩块中大量剩余油不能被采出,水驱开发效果很差[1].周期注水是该类油藏开采的一种有效方法,它亦称间歇注水、脉冲注水等,其驱油机理是周期性地改变注入量和采出量,在地层中造成不稳定的压力场,使流体在地层中不断重新分布,从而使注入水在层间压力差的作用下发生层间渗流,促进毛管吸渗作用,增大注水波及体积系数及洗油效率,提高采收率.其实质是发挥低渗透率基质系统的毛管力作用及压力周期涨落时基质与裂缝间流体的质量交换作用,使原油从致密的基质系统排入裂缝系统,即影响周期

注水效果的因素主要是渗吸毛管力作用和压力周期涨落时基质与裂缝之间的压差.前人对于周期注水的研究主要是通过改变注入端注水量以及设定不同的注采周期形成一定的压力波动,并观察和分析该压力波动对驱油效率的影响,进而结合数值模拟研

究了周期注水提高采收率的机理,得出了影响周期注水驱油效果的因素主要是毛管力和压力波动引起的压差[2 5].本文从渗吸实验的角度,通过设定不同的压降幅度和降压速度,来观察其对渗吸作用的影响,从而对周期注水方案的实施进行指导.

1 长岩心周期注水实验

本文选用80cm 45cm 45cm 的自制长岩心进行周期注水实验.岩心基质渗透率5 10

-6

m 2

,

孔隙度24%,为强亲水性岩心.选用长庆原油与煤油1 4配比的模拟油饱和岩心,初始含油饱和度为

2007年11月第22卷第6期西安石油大学学报(自然科学版)

Journal of Xi an Shiy ou U niversity(N atural Science Edition)Nov.2007V ol.22No.6

42.13%,含水饱和度为57.87%,模拟地层水矿化度为25000mg /L.实验温度为60 ,该温度下地层水黏度为0.47377mPa s,密度为0.9905g /cm 3

;模拟油黏度为0.912m Pa s,密度为0.8016g /cm 3.通过人工切割造出人工水平裂缝,并将岩心放置在一起.按相关实验标准抽真空饱和水,然后用模拟油驱替饱和油.在常规水驱结束后,进行周期注入实验.通过对比不同的周期长度,来分析周期注水过程中采油效果.周期注水前,采用常规注水至含水率达到100%时,转周期注水.此时,岩心的含水饱和度为66.85%,含油饱和度为33.15%.实验分4个周期进行,每个周期间隔一段时间,注水水驱至含水率100%,然后关闭岩心两端一段时间,再注水开采至含水率达到100%,再关闭岩心两端,如此反复几个周期,观察并分析每个周期采收率的变化情况.图1为阶段采收率和累积采收率随周期数的变化曲线.在常规水驱含水率接近100%后,常规注水采油效果很不明显,而采用间歇性周期注水后,累积采收率将有所增加.由阶段采收率曲线可知,第一周期的阶段采收率最大,后续周期阶段采收率有所下降.且在高含水饱和度条件下,周期注水阶段采收率均较小,一般在5%以下.这一实验证实了间歇性周期注水在裂缝性低渗透油藏中的存在作用,但是无法说明其作用机理,因此,本文从渗吸实验的角度进行周期注水机理探究

.

图1 长岩心周期注水采收率变化曲线

2 静态渗吸和动态渗吸实验

渗吸效应是指一种润湿相流体在多孔介质中只依靠毛管力作用置换出另一种流体的过程.在低渗透裂缝性砂岩油藏中水驱油的主要机理是渗吸促使裂缝中的水吸入基质而进行采油的.而周期注水过程中,由于压力波动而在裂缝与基质间形成一定的压差,这一压差将会对渗吸效应产生影响.在周期注

水过程中,毛管力渗吸与压力波动所引起的压差这两者之间的相互作用过程将是下文讨论的内容.2.1 无压降条件下的静态渗吸实验

选用与裂缝性长岩心基质渗透率相同的2.5cm 直径的岩心,用模拟油100%饱和,并放入渗吸容器中,然后分别置于30 和60 的恒温箱中进行静态渗吸实验.实验是在常压下进行的,整个过程中,压力保持不变,即渗吸过程是在毛管力下进行的.

图2为2块渗透率接近的岩心在常压和不同温度条件下的静态渗吸实验,从图中可以看到,30 时,渗吸采收率为33.12%;温度为60 时,渗吸采收率为61.61%.通过近似计算,30 时平均渗吸速度为0.322 10-4m/d,60 时的平均渗吸速度为7.22 10-4m/d.即高温下渗吸采收率要明显高于低温下的采收率,且高温下达到渗吸最大采收率的时间要比低温下的短,高温下渗吸速度大于低温下的渗吸速度.另外,在无压降条件下,或处于压力平衡的环境下,其渗吸效应具有较强作用,表明毛管力对提高基质采收率具有较强作用

.

图2 不同温度条件下静态渗吸实验曲线

2.2 不同压降条件下的动态渗吸实验

针对周期注水特点,设计5组不同实验.选用5个高压中间容器,每个容器内放置3块100%饱和

模拟油的2.5cm 直径的岩心,以防止单块岩心由于计量和操作引起实验误差.分别将5个中间容器加压至0.1MPa 、5MPa 、10MPa 、20M Pa 、30MPa,计量渗吸油量周期为24h.即在每24h 时,将每个中间容器压力瞬间降至0.1MPa,不同的瞬间降压幅度对应于不同的降压速度,同时,也将会在岩心内外形成不同的压差,这将会对渗吸作用造成不同的影响.

图3,图4即不同压力及压降条件下的渗吸采收率曲线.从图3中可以看出,压力波动过程中的渗

57 王锐等:裂缝性低渗油藏周期注水与渗吸效应实验

吸采收率增加初期较快,后期逐渐变得平缓.从图4中可以看到,随着瞬时降压幅度的增大,渗吸采收率先减小后增大,即瞬时降压对渗吸作用的影响不是单调变化的.实验周期为24h,降压条件是在瞬间进行的,即在降压的瞬间,可以在岩心内外形成一定的压差,从而对渗吸效应产生影响.当压降从0MPa 变至5MPa 时,采收率变化不大;当压降从5MPa 变化至10MPa 时,采收率降低;当压力从10MPa 升高时,采收率增大.由此可见,不同的压力和不同的降压速度对采收率的影响是先减小后增大的趋势

.

图3

不同压力情况下渗吸曲线

图4 不同瞬时压降条件下渗吸采收率曲线

岩心内外压力变化过程如下所述,即当环境压力处于平衡状态时,岩心内外的压差为油水界面张力引起的毛细管力.在毛管力的作用下,水相逐步渗入到毛细管中,此时的压差由毛管力、水相黏滞力、油相黏滞力组成.当环境压力瞬时改变时,由于在致密基质岩心中压力传播具有延迟性,而导致在岩心内外产生一定的压差,将此压差定义为延迟压差,延迟压差的大小将对渗吸效应产生很大影响.此时压差由毛管力、水相黏滞力、油相黏滞力和延迟压差组成.实验过程中,在温度不变的条件下,油、水两相的黏滞力保持不变.则当压力变化时,油水界面张力和延迟压差对岩心内外的压差产生影响,以及由于压差的存在,岩石和流体物性也会发生相应的变化.当

压力为常压条件,即无压降产生时,岩心渗吸过程是在毛管力作用下进行的.当压降幅度较小时,如实验从0MPa 变为5MPa 时,降压过程中产生的延迟压差较小.此时,岩石物性基本不发生变化,基质内流体膨胀较小,故采收率变化较小.而当压力从10MPa 开始下降时,较高压时油水界面张力减小,降压过程中引起的延迟压差增大,且受力方向与毛管力方向相反.同时,由于延迟压差较小,岩石及流体性质变化较小,则采收率将较5MPa 时有所降低.当压力继续增大时,毛管力继续减小,而此时降压引起的延迟压差大于毛管力的作用,即延迟压差能够克服界面张力的作用,而将原油从岩心中排驱出来.同时,由于较大压差的存在,使得岩心开始膨胀,即孔、渗参数得到改善.基质内原油黏度降低,体积膨胀,也促使原油从基质中渗出来.上述因素均促使渗吸采收率急剧增大,即在较高降压幅度条件下,瞬间延迟压差成为基质内原油的驱动力,将其排驱出来.同时,瞬间压降造成了岩心孔隙结构的部分变形,以及原油降黏膨胀,导致渗吸采收率增大.

由无压降条件下渗吸实验可知,渗吸作用主要是由毛管力和黏滞力引起的,且当温度升高时,流体黏度降低,相应黏滞力减小,即渗吸阻力减小,则渗吸速度和采收率均增大.由不同压降条件下的渗吸实验可知,不同压降及降压速度条件下,在岩心内外可以形成一定的延迟压差,且这种压差将会对渗吸效应产生一定的抑制和促进作用.当压降幅度较小时,延迟压差较小,对渗吸作用影响较小;当压降幅度达到一定的值时,延迟压差将会平衡毛管力及黏滞力的作用,而对渗吸效应产生一定的抑制作用;当

压降较大时,延迟压差将克服毛管力及黏滞力的作用,而将基质中的原油排驱出来.同时,由于基质中流体及岩石弹性能的释放,导致采收率进一步增加.综合两组实验可知,压降幅度必须保持在较小或很大的范围内,才能充分发挥渗吸和弹性效应的作用.而对于实际低渗透油藏开采来说,岩心压力敏感性较强,即当孔隙压力降低较大时,由于上覆岩层压力基本不变,则有效上覆压力增大,从而引起岩心渗透率损失,对岩心物性造成一定的伤害,进而影响原油的产出[6].综上所述,必须保持压降在较低范围内,本实验为5M Pa 以内,渗吸效应较强,且由于压降较小,岩心压敏性较弱,此时可得到最终的综合采收率较大,即周期注水的实施必须综合考虑渗吸效应、弹性作用及岩心的压敏性,压力波动幅度必须保持

58 西安石油大学学报(自然科学版)

较低水平,才能获得较大采收率.

3 结 论

(1)长岩心驱替实验中,当含水率很高时,水驱效果较差,此时采用间歇周期注水,能起到一定的效果.

(2)常压下的静态渗吸实验表明,渗吸效应是由毛管力与黏滞力引起的,当温度升高时,毛管力变化较小,流体黏度急剧降低,黏滞力减小,渗吸速度增加,渗吸采收率增大.

(3)不同的压力及压降速度对采收率的影响是先减小后增大的趋势.即当降压幅度较小时,其降压引起的延迟压差对渗吸作用影响较小;而当降压幅度达到一定值时,延迟压差会一定程度抑制毛管力的作用,导致渗吸采收率减小;当降压幅度较大时,延迟压差会克服毛管力的作用,将基质中的原油排驱出来,渗吸采收率急剧增大.

(4)在低渗透油藏中,周期注水必须保持较低的压降幅度才能得到较高的渗吸采收率,当压降较大时,岩心的压力敏感性会不同程度地抑制驱油效果,导致最终采收率降低.

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(上接第55页)

4 结 论

(1)剩余油饱和度较高区域分布在断层遮挡区、压力平衡区、井网不完善区、沉积微相过渡区,另外在水淹严重的主体区,低效潜力储量所占比例大,有进一步挖潜的物质基础.

(2)针对聚驱后不同类型剩余油,提出了平面液流转向技术、含油污泥深度调剖技术、不均匀井网优化重组技术等高效挖潜对策,这些配套技术方法的核心是通过改变平面压力场分布,实现液流转向,同时通过完善井网,进一步提高油层动用程度,进而提高最终采收率.

(3)通过开展单井交联聚合物注入实验,聚驱后采用交联聚合物驱可进一步提高采收率.

(4)在现有技术及工艺条件下,仍存在部分剩余油无法挖掘,应加大新技术、新方法的研究和应用力度,为后续水驱阶段进一步挖掘剩余油提供保障.

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王锐等:裂缝性低渗油藏周期注水与渗吸效应实验

carbon of t he fourth member distributes in the periphery of the sag,the hy drocarbon of the t hir d member distr ibutes in the central part of the sag,and the mixed hydr ocarbon of them distributes in the intermediate zone of the tw o regions.

Key words:Dongy ing Sag;sour ce rock;lithologic structure;hydrocarbon expulsion;hydrocarbon accumulat ion

CH EN Zhong hong1,L I U Wei2(1.Faculty o f Resources and I nformation,China U niv ersity of Petroleum(East China),Dongy ing257061,Shandong,China;2.M anagement Department of Exploration Project,Shengli Oilfield Co.L td.,Dongying257061,Shan do ng,China)JXSY U2007V.22N.6p.40 43,49

C haracteristics of the passage system s in different sequences in Dongying Depression

Abstract:A ccording to the geolog ical character istics of Dong ying Depression,its hydro carbon passage system is analyzed using the compr ehensive study method of passag e system.It is found that the passage systems of different sequences have difference:(1)T he passage systems of deep depositional sequences(Ek Es4)are characterized by the complex hydrocarbon conduction in t he sides of sand bo dies.In t heir marg inal zone,the essential element of t he passag e systems i s the lateral conduct ion of sand bodies,w hich is also the main passage form in the deep depositional sequences.I n their depression zone,the essent ial element of the passage systems i s t he verti cal conduction of faults,which is the secondary passage form in t he deep deposit ional sequences.(2)In medium shallow depositional sequences(Es3 Ed),sand bodies and faults ar e all very dev eloped,and therefore the t ypes of passage system are div erse,including fault unconformit y type,sand bo dy fault step t ype,net car pet fault type,simple sand body fault type,ladder type and unconformity t ype passage systems.T he passage systems of the medium shallow deposit ional sequences are characterized by the complex hydrocarbon conduction of sand body fault.(3)Fr om the deep depositional sequences to the medium shallow depositional sequences,conducting di rection v ar ies continuously,and the conducting character varies from simple conduction to complex conduct ion.

Key words:Dongy ing Depression;depositional sequence;passage system;complex hy drocar bon conduction

L I Y un z hen1,LI U Zhen1,ZH A O Yang1,ZH A N G Shan wen2,L V X i x ue2(1.Key L aboratory of Education M inistr y for Hy drocarbon Accumulation M echanism,China U niversity of Petro leum(Beijing),Beijing102249,China;2.Shengli Oilfield Co.L td., SI NOP EC,Dongying257000,Shandong,China)JXSY U2007V.22N.6p.44 49

C alculation of the isotherm al running temperature of hot oil pipeline

Abstract:A t present,crude oil is transported by means of heating,and so energ y consumption is hug e.I f the isother mal trans po rtation of crude oil can be realized,the cost of the transportation can greatly be r educed.T he possibility of realizing the tr ansporta tion of crude oil without heating by means of friction heat is analyzed by a case.T he ex pressio n of iso therm running temperature is de r ived based energy equatio ns,and the value of it is calculated by means of prog ramming.T he factors of influencing the isothermal r un ning temper ature are analyzed,and it is held that there are many factors of influencing it,but main factors are flow r ate of cr ude oil, environment temper ature and pipeline diameter.T he calculation result of a case show s that the transportation of crude oil without heating by means of friction heat can co mpletely be realized under the conditions of large diameter pipeline and great flow rate.

Key words:hot oil pipeline;friction heat;non heating transportaton;isot hermal running

L I Feng1,Z H U Jing2,LI Chuan x ian2(1.Guangdong Dapeng Liquefied Natural G as L imited Company,Shenzhen518000, G uang dong,China;2.F aculty of Stor ag e T r ansformation and Building Eng ineering,China U niversity of Petroleum(East China), Dongy ing257061,Shandong,China)JXSYU2007V.22N.6p.50 52

Study on the remaining oil distribution law after polymer flooding and potential tapping measures

Abstract:After poly mer flooding,the remaining oil distr ibut ion is mo re complex and scattered.T he remaining oil distribution o f X iaer men Oilfield is studied by using reservoir numerical simulation and compr ehensive geologic analysis,t he different po tential tapping measur es are put fo rward for differ ent po tential zones:optimization and recombination technology of the irregular w ell pattern,plane liquid flow turning technology,oily sludge profile control technolog y,and so on.T he applications of t he measures gain good r esults.T he r esults in this paper have refer ence to the potential tapping of the remaining o il of other oilfields.

Key words:polymer flooding,remaining oil distribution,potential tapping measur es

S UN Yi li,PEN G G uan yu,ZH EN G Shu w ei,D U Jian ming,H UA N G Yong qiang,CH EN K ai(No.1Production Plant, Henan Oilfield Branch Company,T ong bai474700,Henan,China)JXSY U2007V.22N.6p.53 55,59

C yclic waterflooding and imbibition experiments f or f ractured low permeability reservoirs

Abstract:T he effect o f imbibition on the cyclic w ater floo ding of the fractur ed low permeability r eservo irs is obtained according to t he cyclic w aterflooding experiment of long cores under differ ent cy cles,their static imbibition ex periments w ithout pressure drop and t heir dynamic imbibitio n exper iments at different pressur e drop.T he r esults show that cyclic waterflooding has obvious effect on the r ecovery of the fractured low permeability reservoirs;there is a strong capillary effect is in t he low permeability matr ixes;the effect of

pressure fluctuat ion amplitude on the imbibit ion presents the trend of first decreasing and then increasing;pressure fluctuation ampli tude must be in lower level in o rder to obtain hig her recovery factor due to t he str ong pr essure sensitivity of the low permeability reser voirs.

Key words:cyclic waterflooding;imbibition function;pressure drop amplitude;pressure sensitivity

W AN G R ui1,Y UE X iang an1,Y O U Yuan1,L IA N G J i wen2(1.R esearch Center of Enhanced Oil Recovery,K ey L aboratory of Education M inistry fo r Petroleum Eng ineering,China U niv ersity of Petroleum(Beijing),Beijing102249;2.Zhanjiang Branch Com pany,CN OOC,Zhanjiang524057,G uang dong,China)JXSYU2007V.22N.6p.56 59

Design of well cementing slurry system based on fractal theory

Abstract:T he fractal theory is used in the establishment of t he fractal gr ading model for the proportion design of the well ce menting slur ry materials.T he gr ain size distributio ns of the slurr y materials are measured by a laser grain size test machine,and an ap propriate fractal dimension is scr eened(D=2.561).Suppose that the grain sizes of micro silica,cement and float ing beads distr ibute r espectiv ely in<1.68 m,1.68~56.23 m and>56.23 m,and a ternary low density cement slurr y system of1.40g/cm3is de signed.T hen the mechanical property(co mpression strength,bending strength and interfacial cement ing strength)of the cement stone and the proper ties of t he slurr y in the proposed system are tested.T he results show that the particles in the system closely pile,the me chanical propert y of the cement stone in the ternar y system is obviously hig her than t hat in the binary systems of micro silica or float ing beads w ith the same density.T he performance of the cement slurry of the ter nary system can meet the r equirements of cementing operations due to its low er water loss and syneresis r ate,favor able stability,and desired thickening property and rheolog ical propert y. T hese ex perimental results validate the feasibility of the fractal g rading theory in assisting the design of well cement ing slurr y system.

Key words:w ell cement ing slur ry;fractal theory;gr ain gr ading;ex perimental evaluation

CH EN G Rong chao,WA N G Rui he,B U Yu huan(F aculty of Petroleum Eng ineering,China U niv ersity of Petr oleum(East China),Dongying257061,Shandong,China)JXSY U2007V.22N.6p.60 63,67

potential and charge density on the membrane surf ace of the modif ied polytetrafluoroethylene in the oily sewage from oilfields Abstract:T he streaming potential on the membrane sur face of t he modified polytetrafluo roet hylene of differ ent hole diameters in t he oily sew age fro m oilfields are studied by exper iments.T he potential of the membr ane is estimated on the basis of Helmho ltz Smoluchowski equation,and then its sur face charge density is estimated from the abov e potential based on Gouy Chapmann equa tion.T he potential o f the suspended particles in the oily sewage from o ilfields is measured.T he ex perimental and the calculation re sults show t hat there is negative charg e in the membrane surface,w hose potential maintains at-20mV around.T he potential of t he suspended particles in the oily sew ag e fr om oilfields is also negative.T he membrane has strong er ability to retar d the suspended particles in the oily sewage and to prevent pollutio n because of the charges on the membrane surface.

Key words:o ily sewag e;membrane;str eaming potential; po tential;surface charg e density

L IN A i guo(Faculty of Chemistr y and Chemical Engineer ing,China U niversity of Petroleum(East China),Dong ying257061, Shando ng,China)JXSY U2007V.22N.6p.64 67

Studies on the ef fect of polyacrylate(high carbon alcohol)on pour point of diesel

Abstract:Poly(tetradecy l acr ylate)(PA 14)has a good pour po int depressing performance to the diesel o il whose carbon num ber is near to14because the side chain leng th of its alkyl matches w ith the carbon chain length of the nor mal alkane in the diesel oil. Polymerizat ion conditions dir ectly influence the relat ive molecular mass distr ibut ion of polymer and therefo re influence its pour point depressing performance to the diesel oil.T he polymerization conditions are optimized by ort hogonal experiments,and the PA 14pre par ed under the best conditions can make the solidifying point depr ession(SP)and cold filter plug ging point(CFPP)of the diesel oil decrease18 and6 respectively.T he optical microscope observ at ion results of wax crystallite mor phology show that the pour point depressing behavio r of the pour point depressant may be related to the thinning of the wax crystal by the polymer as cr ystal nu cleus.

Key words:pour point depressant;poly(alkyl acrylates);diesel oil;wax crystal

W AN G Le q i,H U D ao dao(K ey Laboratory of Shaanxi Pro vince for M acromolecular Science,School of Chemistry and M ateri als Science,Shaanx i N ormal U niversity,Xi an710062,Shaanxi,China)JXSY U2007V.22N.6p.68 73

Preparation and evaluation of a new thickener for acidif ication treatment

Abstract:T he sy nthesis conditions of acrylamide(A M)and dimethyl diallyl ammonium chloride(DM DAA C)copoly mer are op timized.Based on this,a new thickeneer is prepared by adding a chain extender to the copolymer.T he effects of the concentration and adding time of the chain ex tender on its t hickening performance are studied.T he molecular structure of the copolymer is character ized