引用本文
Zhang Chong,Zhang Chaomo,Zhang Zhansong,et al.Comparative experimental study of the core irreducible water saturation of tight gas reservoir[J].Natural Gas Geoscience,2016,27(2):352-358.[张冲,张超谟,张占松,等.致密气储层岩心束缚水饱和度实验对比[J].天然气地球科学,2016,27(2):352-358.]
doi:10.11764/j.issn.1672-1926.2016.02.0352
致密气储层岩心束缚水饱和度实验对比
关键词: 半渗透隔板 核磁共振 压汞 束缚水饱和度 致密气储层
中图分类号:TE155 文献标志码:A 文章编号:1672-1926(2016)02-0352-07
Comparative experimental study of the core irreducible water saturation of tight gas reservoir
Key words: Semi-permeable plate; Nuclear magnetic resonance (NMR); Mercury injection; Irreducible water saturation; Tight gas reservoir ;
引言
目前致密砂岩气藏已经成为全球非常规天然气勘探的重点领域[1-8],致密气藏是指储层须经过大型水力压裂或采用水平井或多分支井技术等措施才能获得经济产量、绝对渗透率一般小于1×10-3μm2的烃源岩外气藏[9,10]。与常规储层相比,致密气储层具有非均值性强、孔隙结构复杂、孔隙连通性差等特点,且致密气储层主要以纳米尺度的孔—喉连通体系为主[11-13]。 束缚水饱和度是进行致密气储层油气评价、产能预测及储量计算的关键参数。目前,实验确定束缚水饱和度的方法有半渗透隔板法、压汞法及核磁共振法等[14-21],这些实验方法的行业标准多是针对常规储层制定,对于更为复杂的致密气储层,实验确定的结果往往会导致较大的误差。基于此,笔者选取大量具有代表性的致密气储层砂岩岩样,设计不同标准的半渗透隔板、核磁共振及压汞实验并进行对比研究,以期遴选出能准确确定致密岩样束缚水饱和度的实验方法。
1 半渗透隔板法
半渗透隔板法获取束缚水饱和度的基本原理是:将饱含地层水的岩样放在隔板上,利用注加湿氮气方法在岩样两端建立驱替压差,驱替过程中毛细管压力(等于将水从岩样孔隙中驱替出来所需的压力)平衡时可以得到岩样中相应的含水饱和度,用一系列毛细管压力和含水饱和度值作图就可以得到半渗透隔板法毛细管压力曲线,其中,半渗透隔板所能测定的最大毛细管压力对应得到的含水饱和度即为测量束缚水饱和度。 半渗透隔板所能测定的最大毛细管压力主要取决于隔板的半渗透性,即隔板的阈压值。隔板的孔隙越小,阈压值越高,测试范围就越大,同时测量的时间也越长。按目前中华人民共和国石油天然气行业标准(SY/T 5346-2005)[22],隔板法设定的阈压值为1.5MPa,这种标准是针对常规砂岩储层岩样制定的,对于致密岩样并不一定适用。按照邹才能等[10]制定的烃类储集空间孔隙标准(图1),致密砂岩的孔喉下限为50nm,即需要克服的最大毛细管压力为Pc=2σcosθ/r=2×72×0.001/0.05μm=2.88MPa,远超过目前标准的阈压值1.5MPa。因此,为了考察最合适于致密砂岩的阈压值,以获取准确的束缚水饱和度,本文研究利用Autopore Ⅳ9505孔隙结构测定仪,设计了2组实验,一组是采用阈压值为1.5MPa的冲压烧结陶瓷隔板进行半渗透隔板实验测量;另一组采用阈压值为3MPa的冲压烧结紫砂隔板进行测量。
2 核磁共振法
岩心核磁共振实验确定束缚水饱和度的方法[23]是:先对饱和水的岩心进行核磁共振测量,得到横向弛豫时间T2分布谱(饱和T2谱),然后在离心力0.690MPa下将饱和水的岩心在离心机上甩出其中的自由水,只剩下束缚水后,再对岩心进行核磁共振测量得到离心后的横向弛豫时间T2分布谱(离心T2谱),结合饱和T2谱和离心T2谱得到T2截止值之后,将饱和T2谱中小于T2截止值的不可动峰包络面积除以整个T2谱下包络面积就等于核磁束缚水饱和度。 离心力0.690MPa下致密砂岩岩样在离心机上甩出的自由水并不充分[24-27],李海波[28]选取了一定数量有代表性的致密砂岩,对每块岩样均进行了0.345MPa、0.690MPa、1.034MPa、1.379MPa、1.724MPa及2.069MPa等6个不同离心力离心后的核磁共振测量(等待时间TW=5s,回波间隔TE=0.2ms),得出致密砂岩岩样最佳离心力为1.379MPa的结论(图3),即离心力为1.379MPa下测量的核磁束缚水更接近实际致密砂岩岩样束缚水。
岩样编号 | 岩性 | 孔隙度/% | 渗透率/(×10-3μm2) | 模拟水型 | 模拟气 | 水矿化度/(g/L) | 束缚水饱和度/% |
No.1 | 岩屑石英砂岩 | 8.500 | 0.447 | 5%CaCl2 | N2 | 46.280 | 53.560 |
No.2 | 岩屑石英砂岩 | 6.300 | 0.138 | 5%CaCl2 | N2 | 46.280 | 70.840 |
No.3 | 岩屑砂岩 | 6.600 | 0.085 | 5%CaCl2 | N2 | 46.280 | 87.200 |
No.4 | 岩屑砂岩 | 9.500 | 0.226 | 5%CaCl2 | N2 | 46.280 | 78.360 |
No.5 | 岩屑石英砂岩 | 9.400 | 0.846 | 5%CaCl2 | N2 | 46.280 | 69.540 |
No.6 | 岩屑砂岩 | 4.000 | 0.099 | 5%CaCl2 | N2 | 46.280 | 83.220 |
No.7 | 岩屑石英砂岩 | 8.800 | 0.315 | 5%CaCl2 | N2 | 46.280 | 79.380 |
No.8 | 岩屑砂岩 | 5.100 | 0.157 | 5%CaCl2 | N2 | 46.280 | 82.500 |
No.9 | 岩屑砂岩 | 12.100 | 0.370 | 5%CaCl2 | N2 | 46.280 | 71.910 |
No.10 | 岩屑石英砂岩 | 10.800 | 0.682 | 5%CaCl2 | N2 | 46.280 | 89.360 |
No.11 | 岩屑砂岩 | 8.410 | 0.290 | 5%CaCl2 | N2 | 46.280 | 40.300 |
No.12 | 岩屑石英砂岩 | 6.430 | 0.660 | 5%CaCl2 | N2 | 46.280 | 57.300 |
No.13 | 岩屑石英砂岩 | 8.120 | 0.790 | 5%CaCl2 | N2 | 46.280 | 39.100 |
No.14 | 岩屑砂岩 | 9.240 | 0.174 | 5%CaCl2 | N2 | 46.280 | 42.900 |
No.15 | 岩屑砂岩 | 6.880 | 0.148 | 5%CaCl2 | N2 | 46.280 | 47.900 |
No.16 | 岩屑砂岩 | 7.380 | 0.367 | 5%CaCl2 | N2 | 46.280 | 32.700 |
No.17 | 岩屑砂岩 | 4.470 | 0.168 | 5%CaCl2 | N2 | 46.280 | 63.500 |
No.18 | 岩屑石英砂岩 | 8.170 | 0.311 | 5%CaCl2 | N2 | 46.280 | 34.200 |
No.19 | 岩屑石英砂岩 | 7.150 | 0.458 | 5%CaCl2 | N2 | 46.280 | 36.500 |
No.20 | 岩屑石英砂岩 | 9.760 | 0.693 | 5%CaCl2 | N2 | 46.280 | 38.600 |
图2 半渗透隔板毛细管压力曲线
Fig.2 Capillary pressure curve obtained with the high pressure semi permeable membrane method
3 压汞法
恒压压汞法是以汞作为驱替流体的一种测量毛细管压力曲线的常规方法,其确定束缚水饱和度的常用做法[29]是:当岩样受离心力作用使得岩石所受的毛细管压力达到0.690MPa(转换为气—水系统)时,岩样剩下的水即为束缚水。显然,根据以上认识,对于致密砂岩岩样,当毛细管压力达到1.379MPa时,岩样剩下的水才为束缚水。 为了考察压汞法确定束缚水饱和度的准确性,本文研究选取了3块致密砂岩岩样,岩样形状为柱塞状,直径为2.54cm,长为5cm,先后进行了半渗透隔板(阈压值:3MPa)和压汞实验测量,实验结果如图5所示。从图5中可以明显看出:压汞与半渗透隔板毛细管压力曲线在形态上有明显差异,造成这种差异的原因主要有2个方面:①压汞与半渗透隔板毛细管压力曲线在反映泥质砂岩孔隙空间黏土束缚水体积部分存在差异,半渗透隔板毛细管压力曲线反映除黏土束缚水部分以外的孔隙结构,与实际气藏情况相符,而压汞毛细管压力曲线反映整个孔隙空间的孔隙结构;②压汞法是在真空条件下将汞压入岩样,汞是一种界面张力很大、压缩性微弱的流体,而气藏是气驱水的过程。
图4 致密砂岩岩样核磁共振实验T2谱(离心谱的离心力:1.379MPa)
Fig.4 NMR T2 distribution spectrum of tight sandstone cores
岩样编号 | 岩性 | 孔隙度/% | 渗透率/(×10-3μm2) | 半渗透隔板束缚水饱和度/% | 核磁束缚水饱和度/% |
No.11 | 岩屑砂岩 | 8.410 | 0.290 | 40.3 | 41.39 |
No.14 | 岩屑砂岩 | 9.240 | 0.174 | 42.9 | 44.34 |
图5 压汞与半渗透隔板毛细管压力曲线对比
Fig.5 The capillary pressure curve comparison by mercury injection method and the high pressure semi permeable membrane method
岩样 编号 | 岩性 | 孔隙度 /% | 渗透率 /(×10-3μm2) | 0.690MPa时 压汞法束缚水 饱和度/% | 1.379MPa时 压汞法束缚水 饱和度/% | 半渗透隔板法 束缚水饱和度 /% |
No.13 | 岩屑石英砂岩 | 8.12 | 0.79 | 49.44 | 36.3 | 39.1 |
No.21 | 岩屑石英砂岩 | 11.39 | 3.72 | 50.27 | 35.631 | 33.1 |
No.22 | 岩屑石英砂岩 | 16.5 | 1.13 | 35.09 | 27.275 | 29.1 |
4 结论
(1)隔板阈压值为1.5MPa的半渗透隔板法不能准确获取致密砂岩岩样的束缚水饱和度,应选用阈压值为3MPa的隔板;致密砂岩岩样半渗透隔板毛细管压力曲线与压汞毛细管压力曲线在形态上存在差异,在缺乏半渗透隔板实验数据的情况下,可以用毛细管压力为1.379MPa时所对应的压汞束缚水饱和度替代。 (2)结合核磁共振饱和T2谱与1.379MPa离心力下测量的离心T2谱可以有效确定致密岩样的束缚水饱和度。
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