Phase behavior characteristics of near-critical oil and gas reservoirs in Tarim Basin

  • Li-ming ZHANG , 1 ,
  • Xiao-qiang WANG 1 ,
  • Da-li HOU 2, 3 ,
  • Ke XIAO 1 ,
  • Chen-guang JIANG 1 ,
  • Xiao-mei ZOU 1
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  • 1. Research Institute of Experiment and Detection of PetroChina Tarim Oilfield Company, Korla 841000, China
  • 2. The State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
  • 3. College of Energy, Chengdu University of Technology, Chengdu 610059, China

Received date: 2019-07-09

  Revised date: 2019-10-31

  Online published: 2020-03-26

Supported by

The Youth Fund of National Natural Science Foundation of China(51604043)

Highlights

The phase behavior of near critical fluid is very complex, and the similar properties of volatile oil or condensate gas result in the difficulties to judge whether it is an oil reservoir or a gas reservoir by conventional methods. Two types of near critical formation fluids, Zhonggu 43 well area and Well Jinyue 201, Tarim Basin, are studied. By observing the near-critical "emulsion phenomenon", different methods such as gas-oil ratio, phase diagram, density and composition are used to judge the types of oil and gas reservoirs. By comparing different methods, the near-critical fluid types can be judged accurately by combining the fluid image and dimensionless pressure curve method during testing. The Zhonggu 43 well area of Tazhong 1 Gas Field is characterized by the condensate gas phase with very high condensate content, while Well Jinyue 201 is characterized by the phase of near critical volatile oil. Through gas injection experiments, the near-critical reservoirs have three-phase coexistence of dry gas, condensate gas and condensate liquid, resulting in large differences in gas-oil ratio of production wells at different locations.

Cite this article

Li-ming ZHANG , Xiao-qiang WANG , Da-li HOU , Ke XIAO , Chen-guang JIANG , Xiao-mei ZOU . Phase behavior characteristics of near-critical oil and gas reservoirs in Tarim Basin[J]. Natural Gas Geoscience, 2020 , 31(3) : 370 -374 . DOI: 10.11764/j.issn.1672-1926.2019.10.015

0 引言

近临界油气藏一般是指油气藏地层温度(T r)介于0.95T c~1.05T cT c为临界温度)之间的油气藏流体[1]。近年来,在国内外深部地层勘探中发现了相当数量的近临界油气藏。表现为气液性质差异减小、一种流体在温度压力变化较小的范围内可能具有易挥发油或凝析气的共同特征[2]。在塔里木盆地,如塔中、富源、金跃等区域,奥陶系油气藏多为气油比高、液烃密度低、体积系数较大的轻质油藏,它们都具备易挥发油或凝析气的相近性质,常规方法难以判断它是油藏还是气藏,必须经过合格的取样,严格的实验,多方面的分析才能正确判断其流体性质和变化规律。
前人对油气藏分类已经做了很多研究,目前根据流体对油气藏划分的方法已有很多。沈平平等[3]按组成参数、气油比和油罐油密度等参数对油气藏类型进行划分。这些方法对普通的油藏、气藏划分效果比较好,然而对特殊油气藏进行划分时,由于各种参数有时会相互冲突,造成划分油气藏类型比较困难。侯大力等[4]、罗凯等[5]、张利明等[6]对近临界流体在高温高压下的流体特征进行了描述,但并未进一步论述如何区分近临界挥发油藏与近临界凝析气藏。目前,对近临界流体进行更细的类型划分的研究较少[7,8,9,10,11,12,13,14,15,16,17,18,19,20],本文以塔里木盆地新近勘探开发的近临界油气藏为例,得到塔里木盆地近临界挥发油藏和近临界凝析气藏相态特征,优选出划分近临界流体类型参数,以期对塔里木盆地特殊油气藏开发提供基础数据。

1 近临界凝析气藏相态特征

近临界流体在近临界区内一般处于高压状态,处于近临界区的轻质挥发油和富凝析气的开采过程中, 压力的微小降低便可能导致大量气体或凝析液的生成。对于富凝析气藏,最有价值的重质馏分、凝析油由于反凝析而大量存留于地层中;对于挥发油藏,压力降低后体积收缩性比较大,会造成原油低于临界流动饱和度后损失在地层中[3,4,5,6]。本文以2种典型近临界流体为研究对象,分析近临界凝析气与近临界挥发油的区别。

1.1 实验流体参数

中古45-H2井地层流体属于近临界凝析气藏流体,位于塔中1号气田中古43井区,气藏压力为64.126 MPa,气藏温度为129.28 ℃,生产气油比为652 m3/m3,油罐油密度为0.786 7 g/cm3;金跃201井地层流体属于近临界挥发油藏地层流体,位于哈拉哈塘鼻状构造南翼,生产气油比为487 m3/m3,油罐油密度为0.813 5 g/cm3。根据气油比和罐油密度分类标准,2个油气藏位于挥发性油藏与凝析气藏过渡带,判断这类油气藏比较复杂,用气油比这类参数难以判断。

1.2 实验装置及流程

实验运用斯伦贝谢高温高压PVT实验装置开展,该装置带有一个100 mL可视高温高压PVT室,温度范围为0~200 ℃,压力范围为0.1~103.0 MPa,通过计算机系统控制PVT室压力与温度,并实时观测PVT室现象并记录样品体积,满足对高温、高压油气藏地层流体分析要求,测试系统如图1所示。
图1 斯伦贝谢PVT测试系统

(a)地层流体相态实验装置;(b)可视PVT筒玻璃筒及金属活塞

Fig.1 Schlumberger PVT test system

2 近临界流体相态特征

2.1 近临界凝析气藏流体相态特征

取得中古45-H2井地层流体,进行PVT实验,根据样品井流物组成、露点、含液量等进行拟合计算,得到烃类流体相图(图2),图中临界压力为45.22 MPa,临界温度为67.70 ℃。图中地层温度不同压力点对应流体相态变化图像如图3所示,不同压力点与图像通过字母代码对应。当压力从对应于图3(a)的压力点逐渐降低到对应于图3(b)的饱和压力点时,PVT实验装置中的流体由金黄色变为不透光的乳白色,当压力降低至图3(c)对应压力点时,流体颜色由乳白色变为底部红色上部灰褐色,随着压力进一步降低,PVT实验装置中的地层流体由底部红色变为金黄色,并具有清晰的气液界面。液体体积也迅速增加到最大值后缓慢下降。需要注意的是,近临界流体在低于饱和压力后,会出现“乳光现象”,流体变为乳白,透光性变差。
图2 中古45-H2井地层流体相态

Fig.2 Phase state diagram of formation fluid of Well ZG 45-H2

图3 近临界凝析气随压力变化图像

(a)对应压力64.13 MPa;(b)对应压力43.94 MPa;(c)对应压力40.00 MPa;(d)对应压力38.00 MPa;(e)对应压力28.00 MPa

Fig.3 Image of near critical condensate gas changing with pressure

2.2 近临界挥发油藏相态特征

取得金跃201井地层流体,进行PVT实验,在饱和压力附近时与近临界凝析气比较相似,图4为金跃201井地层流体相态图,图中地层温度不同压力点对应流体相态变化图像如图5所示,图5(a)为地层条件下流体图像,呈黄色,透光性好。随着压力降低,当压力降至饱和压力时如图5(b)所示,流体变为黑色,几乎不透光,这些现象与图3近临界凝析气相似,依靠饱和压力以上地层流体状态很难判定流体性质。图5(c)至图5(e)为压力低于饱和压力后继续降压时流体变化,流体呈气、液两相,液体体积随压力降低迅速收缩,体积减小。这一段压力内流体变化与近临界凝析气[图3(c)—图3(e)]有明显区别,近临界凝析气地层流体收缩性特征为液相体积低于饱和压力后先迅速增加,随压力降低液相体积达到最大值后略有减小,而近临界挥发油液相体积则迅速减少。
图4 金跃201井地层流体相态

Fig.4 Phase state diagram of formation fluid of Well JY201

图5 近临界挥发油藏流体随压力变化图像

(a)对应压力56.40 MPa;(b)对应压力40.02 MPa;(c)对应压力36.00 MPa;(d)对应压力32.00 MPa;(e)对应压力25.00 MPa

Fig.5 Image of fluid pressure in near critical reservoir

3 地层流体收缩性特征

以中古45-H2井和金跃201井为例,模拟油气藏开采过程,经实验测定,得到地层流体液相相对体积与相对压力变化规律(图6)。与不同类型的油气藏液相相对体积对比,中古45-H2井地层流体位于高含凝析油凝析气区间,金跃201井液相体积压力低于饱和压力后液相体积迅速收缩,处于近临界挥发油区间,与观测到的结果(图3图5)一致,应用地层流体液相体积与无因次压力关系曲线判断比较准确。因此,综合相图、流体随压力变化图像及地层液相体积变化,中古45-H2井所在塔中1号气田中古43井区属于特高含凝析油凝析气相态特征,金跃201井所在井区属于近临界挥发油相态特征。开采此类近临界油气藏均需注意压力保持饱和压力以上开采,最大程度采出液相,提高液态烃采收率[20]
图6 地层流体液相相对体积与无因次压力关系曲线(图版据文献[21]) Fig.6 Relationship between relative volume and dimensionless pressure of formation fluid (Plate from Ref.[21])

4 近临界油气藏注干气相态行为特征

牙哈凝析气藏自开发初期采用回注干气开发,10余年来有效遏制了地层压力下降,降低反凝析油损失。伴随着开发进行,地层压力已低于露点,出现生产气油比升高、干气气窜等现象。为研究产生这种现象的原因,采用牙哈23-1-24H井MDT井下近临界流体样品开展注干气实验,注气温度为135 ℃,注气压力为49 MPa。在恒温恒压条件下,从PVT筒上部持续向PVT筒内注入干气模拟气藏顶部注干气驱替过程。注气方式采用恒定注气速度,保持压力的方式注气,速度为5 mL/30min,注气过程如图7所示,图7(a)为原始流体,体积为60 mL,图7(a)—图7(g)每个记录图像间隔30 min,累积注气30 mL。随着注气进行,呈现出上部干气相、中部凝析气相、下部凝析液相三相流体共存状态,说明如果近临界油气藏实施顶部注气或者底部注气气窜至顶部,近临界油气藏在不同深度相态特征相差较大。这也解释了采用注气开采的近临界油气藏不同井生产气油比相差较大的原因。
图7 近临界油气藏注干气非平衡相态特征

Fig.7 Non-equilibrium phase behavior of dry gas injection in near-critical oil and gas reservoirs

5 结论

(1)近临界凝析气藏/挥发油藏在饱和压力附近呈现出“临界乳光”现象,流体变为乳白,透光性变差,综合不同方法对比,近临界油气藏利用测试时流体图像(临界乳光现象)与无因次压力关系曲线法相结合判断流体类型比较准确。
(2)近临界挥发油在饱和压力附近时与近临界凝析气比较相似,低于饱和压力后,流体变化与近临界凝析气有明显区别:近临界凝析气地层流体收缩性特征为液相体积低于饱和压力后先迅速增加,随压力降低液相体积达到最大值后略有减小,而近临界挥发油液相体积则迅速减少,在小压差下游离气大量产生。
(3)中古45-H2井所在塔中1号气田中古43井区属于特高含凝析油凝析气相态特征,金跃201井区属于近临界挥发油相态特征。开采此类近临界油气藏时均需注意压力保持饱和压力以上开采,最大程度采出液相,提高液态烃采收率。
(4)近临界油气藏实施注气开发,会出现干气、凝析气、凝析液三相共存的流体特征,造成不同位置的生产井气油比差异较大。
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