Preliminary study on imbibition and oil displacement of Chang 7 shale oil in Ordos Basin

  • Zhiyu WU , 1, 2 ,
  • Zhanwu GAO 1, 2 ,
  • Shuwei MA , 2, 3 ,
  • Jiyong ZHAO 1, 2 ,
  • Jianchao SHI 2, 3 ,
  • Zhen LI 2, 3
Expand
  • 1. PetroChina Changqing Oilfield Company,Xi’an 710018,China
  • 2. National Engineering Laboratory for Exploration and Development of Low Permeability Oil and Gas Fields,Xi’an 710018,China
  • 3. Research Institute of Exploration and Development,PetroChina Changqing Oilfield Company,Xi’an 710018,China

Received date: 2021-05-31

  Revised date: 2021-10-21

  Online published: 2021-12-27

Supported by

The China National Science & Technology Major Project(2017ZX05069)

Highlights

Chang 7 Member shale oil reservoir in Ordos Basin, China is continental and shows characteristics of low porosity and permeability with poor reservoir connectivity. Oil displacement efficiency of this type is low, because water breaks through underground during water flooding. Shale oil in the basin has been developed by large-scale fracturing to increase water-oil contact thus to improve oil recovery rate. Oil recovered by water imbibition was proved to be effective, and both development practices and indoor experiments showed that shale oil recovered by water imbibition accounts for 15%-40% of the total, providing a new method for oil displacement in shale oil reservoir. In this study, open-boundary core system was used to quantitatively study the impacts of pore radius, interfacial tension and permeability on oil recovery by water imbibition underground. Indoor experiments showed that shale oil produced from pores with radius less than 10μm accounts for 56%-80% of the total; shale oil recovered by water imbibition peaks when interfacial tension is 1.18 mN/m; core permeability is positively correlated with imbibition recovery when interfacial tension is less than 2 mN/m, while the two are not significantly correlated when interfacial tension is higher than 4mN/m.

Cite this article

Zhiyu WU , Zhanwu GAO , Shuwei MA , Jiyong ZHAO , Jianchao SHI , Zhen LI . Preliminary study on imbibition and oil displacement of Chang 7 shale oil in Ordos Basin[J]. Natural Gas Geoscience, 2021 , 32(12) : 1874 -1879 . DOI: 10.11764/j.issn.1672-1926.2021.10.015

0 引言

鄂尔多斯盆地长7段页岩油储层形成于湖盆最大扩张期,属于深湖—半深湖相沉积,储层物性差,孔隙半径多为2~8 μm,喉道半径多为20~150 nm,渗透率介于(0.13~0.17)×10-3 μm2之间,储层致密、地层压力系数低、天然能量不足1,储层注水驱动的启动压力大,注水开发难见成效2-3。鄂尔多斯盆地长7段页岩油储层为中性、中—弱亲水润湿,地层水矿化度为40~50 g/L,压裂液矿化度为5~8 g/L,地层水矿化度远大于压裂液矿化度4。通过跟踪陇东地区34口页岩油水平井闷井过程中井筒中含水率与矿化度的变化发现,随着井筒中含水率的下降,其矿化度相应呈增大趋势(图1),说明在闷井过程中,井筒中压裂液通过渗吸作用进入储层。盆地页岩油开发实践证明,页岩油开发过程中存在油水置换现象。
图1 研究区34口井含水与含盐离散图

Fig.1 Scatter plots of water saturation of the 34 wells in the study area and their salinity

目前长庆油田页岩油开发主要采用长水平井体积压裂,通过水平井大规模水力压裂改造,储层中脆性基质受拉伸或者挤压形成一定区域的改造体积,使得压裂液与储层基质之间获得较大的接触面,压裂液作为润湿相在毛管力的作用下进入较小孔隙,驱替非润湿相流体至人工裂缝中。在水平井注水、闷井、采油的过程中,裂缝作为导流通道,完成压裂液与原油的置换过程,流体在裂缝网络和基质储层中发生油水渗吸置换和多种渗流,部分非润湿相原油与压裂液返排,从而达到提高采收率的目的5
前期开发实践及渗吸实验表明,页岩油储层渗吸发生的前提条件是岩石的润湿性6-7,即渗吸置换作用发生在亲水—中性储层中,而渗吸量的大小决定于渗吸深度和渗吸接触面的大小。其中,润湿性、孔隙结构、渗透率决定渗吸深度;孔隙结构(微裂缝)、注入压力(决定微裂缝是否开启)决定渗吸面的大小8。前期通过建立的页岩油储层基质和裂缝性2种岩石物理模拟模型,模拟流体在基质与裂缝间的交渗流动,发现裂缝岩样的渗吸作用远大于基质岩样9。本文研究将定量探究储层孔隙半径、界面张力、岩心渗透率等因素对页岩油储层渗吸采收率的影响。

1 实验样品和方法

本文利用鄂尔多斯盆地长7段页岩油储层岩心研究渗吸作用的影响因素。利用静态渗吸仪研究外边界敞开(AFO)时不同渗透率的岩心自吸水排油能力,油水界面张力等对静态渗吸的影响规律,测量累积采油量、渗吸速度、采出程度随时间的变化曲线。
脱气油在地层温度(60 ℃)下黏度高于地层条件下的黏度,为模拟地层条件下渗流规律,本文实验用少量煤油将原油稀释,使模拟油黏度等于地层条件下的黏度。利用煤油/原油质量比与其混合液的平均黏度建立回归公式,当模拟油黏度为1.49 mPa·s时(鄂尔多斯盆地合水地区页岩油地层黏度),原油与煤油质量比为1∶1.19。

1.1 实验条件

(1)实验设备:恒温箱,恒压恒速泵,MicroMR12核磁共振仪。
(2)实验用表面活性剂:表面活性剂TOF-1 浓度及界面张力见表1
表1 表面活性剂体积浓度及相应界面张力数据

Table 1 Volumetric concentration of the surfactant and the corresponding interfacial tension

TOF-1表面活性剂浓度/% 与模拟油界面张力/(mN/m)
5 0.290
0.5 1.183
0.05 3.750
0.005 10.436
(3)实验岩心:岩心基础数据见表2
表2 实验岩心基础数据

Table 2 Basic data of the core used in the experiment

岩心编号 长度/cm 直径/cm 孔隙度/% 渗透率/(10-3 μm2
木21-2 2.360 2.508 9.81 0.159
里21-2 2.306 2.508 8.49 0.139
安25-2 0.081 2.510 8.89 0.081
(4)实验温度:60 ℃。
(5)实验流体:鄂尔多斯盆地长72亚段地层水(矿化度平均为53.9 g/L,水型为CaCl2),模拟油(原油∶煤油=1∶1.19),氟氯平衡液(去氢核油,核磁中屏蔽原油信号)。

1.2 实验方法

渗吸驱替实验流程见图2。为保证结果准确,将饱和水状态、束缚水状态、渗吸至不同时间状态的T 2弛豫时间谱置于同一坐标系,分析不同状态下的渗吸出油量及渗吸采收率。
图2 渗吸驱替实验流程图

Fig.2 Experimental flow chart of imbibition and displacement

2 页岩油储层渗吸置换影响因素

2.1 孔隙半径对渗吸采收率的影响

渗吸初期,由于毛细管力的作用,润湿相首先进入小孔隙将其中的原油通过渗吸置换作用采出,此时采出的原油主要来自小孔隙;在渗吸后期,大孔隙中的原油才被置换采出10。孔隙度对渗吸采收率贡献的探究主要通过依次测试岩心100%饱和地层水状态下核磁共振T 2弛豫时间谱、束缚水状态下T 2弛豫时间谱以及不同渗吸液浓度下不同渗吸时间段T 2弛豫时间谱,分析不同状态下渗吸出油量及渗吸采收率(图3)。
图3 木21-2 岩心在5%助排剂溶液中渗吸T 2弛豫时间谱

Fig.3 Imbibition T 2 relaxation time spectra of core Mu 21-2 under 5% drainage aid solution

根据核磁共振数据,利用姜汉桥等11的方法,将弛豫时间转换为孔隙半径(1)。
r = 0.735 × T 2 C
式中:r为孔隙半径,μm;T 2为核磁共振弛豫时间,ms;C为旋转系数,ms/μm。
本文中C取1.71 ms/μm,根据H原子在流体(油或水)中弛豫时间的不同,可以将孔隙分为大孔隙(>100 ms)、中孔隙(10~100 ms)、小孔隙(<10 ms)共3个大类,其反演计算得到的孔喉半径分别为小于10 μm、10~20 μm和大于20 μm。
由渗吸时间谱可以看出,渗吸初期采出的原油主要来自小孔隙,并且小孔隙内的原油替换很快结束,渗吸后期则主要是大孔隙内原油逐渐被置换采出12。分别做3个不同孔隙半径的岩心在助排剂浓度5%条件下的渗吸弛豫时间谱,计算得到其相对采出程度(表3图4)。
表3 不同孔隙半径岩心渗吸相对采出程度

Table 3 Relative recovery degree of core imbibition with different pore radius

岩心

编号

孔隙度

/%

渗透率

/(10-3 μm2

不同孔隙半径采出程度/%
小于10 μm 10~20 μm 大于20 μm
平均值 9.06 0.13 72.05 16.02 11.92
木21-2 9.81 0.159 74.84 12.58 12.53
里21-2 8.49 0.139 80.94 14.55 4.52
安25-2 8.89 0.081 60.37 20.93 18.71
图4 不同孔隙半径岩心渗吸采出程度

Fig.4 Recovery degree of core imbibition with different pore radius

综合3块岩心核磁共振渗吸试验结果:在渗吸作用下,小于10 μm的小孔隙内采出程度占56%~80%,占总采收率的65%;10~20 μm的中等孔隙采出程度平均占总采出程度的18%;大孔隙内的采出程度平均占总采出程度的17%。渗吸采油时,小孔隙提供主要的采收率13

2.2 界面张力对渗吸采收率的影响

同样,对3块不同物性的岩心在不同助排剂浓度(5%、0.5%、0.05%、0.005%)下的渗吸采收率进行统计(表4图5)。
表4 不同界面张力下渗吸采出程度

Table 4 The degree of imbibition recovery under different interfacial tension

助排剂浓度/% 界面张力/(mN/m) 渗吸采出程度/%
木21-2 里21-2 安25-2
5 0.290 27.169 26.376 25.747
0.5 1.183 32.320 27.483 27.473
0.05 3.750 24.662 26.430 21.641
0.005 10.436 22.495 18.349 20.730
图5 不同岩心渗吸采收率随界面张力的变化

Fig.5 Imbibition recovery efficiency of different cores varies with interfacial tensions

界面张力对渗吸采收率的影响主要通过毛管力和油滴运移难易程度2个方面来实现13。随着界面张力减小,溶液黏附功的降低因子减小,岩心孔隙表面的黏附功降低,脱附油滴尺寸减小且变形能力增强,从而提高渗吸采收率14。然而,在渗吸置换过程中,毛管力为渗吸动力,界面张力的减小势必会降低毛管力15。李浩等16通过研究表面活性剂对化学渗吸采收率的影响发现,界面张力与渗吸采收率存在一定的相关关系,但并不是越低越好,油水界面张力在一定的范围内可使采收率达到最佳。对于低渗的页岩油储层,存在一个最佳的界面张力,使得渗吸采收率为最大。岩心实验显示,当界面张力为1.18 mN/m时,页岩油储层的渗吸采收率最大。

2.3 渗透率对渗吸采收率的影响

渗透率大的岩心其内部孔隙连通性好,孔隙中的油容易被采出,其采收率通常较高16-17。从图5可以看出,当界面张力较低时(<2 mN/m),渗透率与渗吸采收率成正比关系,渗透率较大的岩心渗吸最终采收率较大。而当界面张力较大时(>4 mN/m),渗透率对渗吸采收率的影响不明显。因为当界面张力较大时,渗吸的主要动力(即毛管力)较大,渗吸作用强,岩心孔隙中的油主要靠毛管力被驱出,此时渗透率的影响较小。
同时,从不同渗透率岩心的可动油测试结果来看,随着储层渗透率的增加,总的可动油饱和度呈现增大的趋势,但变化并不显著,而依靠渗吸作用采出的可动油逐渐减少,渗吸作用越来越弱,通过渗吸作用置换的可动油饱和度呈降低趋势(图6表5)。在储层总可动油饱和度大致相等的情况下,储层渗透率介于(0.015~0.1)×10-3 μm2之间时,通过渗吸作用可采出的可动油占比最大。
图6 长7段页岩油气测渗透率与可动油饱和度、渗吸饱和度、驱替饱和度关系

Fig.6 Relationship between gas-measured permeability and movable oil saturation, imbibition saturation and displace-ment saturation of Chang 7 Member shale oil reservoir

表5 合水地区长7段页岩油不同渗透率区间可动油饱和度

Table 5 Movable oil saturation under different permeability intervals of Chang Member 7 shale oil reservoir in Heshui area

渗透率区间

/(10-3 μm2

总可动油饱和度/% 渗吸可动油饱和度/% 驱替可动油饱和度/%
<0.015 24.1 8.35 15.75
0.015~0.100 35.28 9.43 25.85
0.100~0.200 33.67 5.29 28.39

2.4 渗吸作用对驱替采收率的贡献

通过分析岩心渗吸核磁曲线,可以看出渗吸置换作用对驱替采收率的贡献18-19。以最终驱替采收率曲线凹点为界(T 2凹点)。T 2凹点左侧为小孔隙,主要通过渗吸置换作用采出原油20T 2凹点右侧为较大孔隙,其中的原油主要靠驱替作用采出(图7)。定义渗吸对采收率的贡献为小孔隙中的采油量与总采油量之比。通过对不同物性岩心在不同驱替条件下,束缚水饱和度下T 2谱与驱替结束时T 2谱曲线的分析可知,渗吸对采收率的贡献范围为15%~40%,且随着驱替速度的降低,驱替压差减小,渗吸对采收率的贡献减小(表6)。
图7 动态渗吸过程中渗吸及驱替采出量分布

Fig.7 Distribution of imbibition and displacement production during dynamic imbibition

表6 驱替过程中渗吸对采收率的贡献

Table 6 Contribution of imbibition to recovery during displacement

岩心

编号

渗透率 /(10-3 μm2 驱替速度 /(mL/min) 焖井 时间/h 总采收率 /% 渗吸对采收 率的贡献/%
木21-11 0.20 0.10 48 70.75 23.03
木22-11 0.18 0.05 48 69.01 20.04
木22-12 0.18 0.01 48 49.64 14.70
木21-12 0.20 0.10 60 67.80 33.35
木21-12 0.20 0.10 72 70.69 32.87
木27-12 0.98 0.10 48 72.11 31.83
里23-12 0.097 0.1 48 61.125 40.89

3 结论

(1)鄂尔多斯盆地陆相长7段页岩油的开发方式主要是油水渗吸置换。发生油水置换的前提是储层为中性—亲水储层。渗吸置换的采收程度主要受储层孔隙半径、界面张力、岩心渗透率的影响。小孔隙中通过渗吸作用采收率最高;存在一个最佳的界面张力与最佳渗透率范围,使得渗吸采收率最大。
(2)对于长7段页岩油的致密储层,渗吸作用对采收率的贡献范围为15%~40%。在长7段页岩油的开发实践中,应最大化渗吸置换作用,从而提高页岩油采收率。
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Outlines

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