Evaluation of Shale Oil Extraction by Supercritical CO2 and Analysis of Influencing Factors
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摘要:
为明确裂缝及压力对超临界CO2萃取页岩油的影响机理,在获取试验用页岩岩样孔径分布、比表面积和孔体积的基础上,进行了超临界CO2岩样萃取试验,采用改进的磁悬浮天平高压吸附仪,实时测定了高温高压下页岩岩样质量的变化;并结合页岩核磁共振T2谱,精确测定了超临界CO2对页岩油的萃取效率,明确了萃取过程中页岩孔隙动用特征及动用孔径下限。试验结果表明,目标储层页岩中介孔(孔径2~50 nm)发育程度最高,占总孔隙体积和总比表面积的69.72%和73.47%;而大孔(孔径>50 nm)发育程度最差,仅占总孔隙体积和总比表面积的4.45%和10.77%。原油主要赋存于孔径1.4~120.0 nm的小孔径孔隙中,CO2对大孔径(>86 nm)孔隙中原油的萃取效果高于小孔径(≤86 nm)孔隙;裂缝能够增大CO2与基质中页岩油的接触面积,加快油气传质速度,提高基质动用深度,降低页岩油渗流阻力和孔隙动用下限。然而,CO2萃取效率除与裂缝数量相关外,还受基质渗透率及裂缝−基质连通特征的影响。CO2动用孔隙孔径下限随注入压力升高而降低,由8 MPa时的6.54 nm减小至18 MPa的3.27 nm。研究成果可为注CO2提高页岩油采收率提供借鉴。
Abstract:To define the influence mechanism of fractures and pressure on the extraction of shale oil by supercritical CO2, the core extraction experiment by supercritical CO2 was conducted on the basis of identifying the pore size distribution, specific surface area, and pore volume of experimental shales. The improved magnetic suspension balance high pressure adsorption instrument was used to measure the shale mass change under high temperature and pressure in real time. Combined with the nuclear magnetic resonance (NMR) T2 spectrum of shale, the extraction efficiency of shale oil by supercritical CO2 was accurately measured, and the producing characteristics of shale pores and the lower limit of producing pore size in the extraction process were defined. The experimental results show that the target reservoir shale mesopore (pore size of 2~50 nm) is the most developed, accounting for 69.72% and 73.47% of the total pore volume and total specific surface area. However, macropores (>50 nm) are the least developed, accounting for only 4.45% and 10.77% of the total pore volume and total specific surface area. The crude oil mainly exists in the pores with a small pore size of 1.4~120 nm. The extraction effect of CO2 on the crude oil in the pores with large pore size (>86 nm) is better than that in the pores with small pore size (≤86 nm). Fractures can increase the contact area between CO2 and shale oil in the matrix, accelerate the mass transfer rate of oil and gas, improve the depth of matrix production, and reduce the shale oil seepage resistance and the lower limit of pore production. However, the CO2 extraction efficiency is not only related to the number of fractures but also affected by matrix permeability and fracture-matrix connectivity. The lower limit of pore size for CO2 production decreases with the increase in injection pressure from 6.54 nm at 8 MPa to 3.27 nm at 18 MPa. The research findings provide a reference for enhancing the recovery rate of shale oil by injecting CO2.
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Keywords:
- shale oil /
- fractures /
- CO2 /
- injection pressure /
- NMR /
- extraction efficiency /
- weighing method
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表 1 试验用页岩岩样的基本物性参数
Table 1 Basic physical parameters of experimental shale samples
岩样编号 孔隙度,% 渗透率/mD 有机碳含量,% 热成熟度,% 矿物组分及含量,% 石英 钠长石 方解石 白云石 黄铁矿 黏土矿物 1# 5.62 0.004 47 3.52 2.31 26.3 4.5 9.4 3.4 2.7 53.7 2# 6.38 0.006 56 3.18 2.04 41.7 4.4 15.1 3.1 4.8 30.9 3# 6.15 0.007 52 4.22 2.27 37.2 4.1 11.4 2.7 3.5 41.1 4# 5.62 0.003 81 3.06 2.14 29.7 5.7 8.6 4.2 3.3 48.5 表 2 低温氮气吸附试验测得页岩岩样的孔隙结构参数
Table 2 Measurement of pore structure parameters of experimental shale samples by low temperature nitrogen adsorption experiment
样品编号 BJH孔体积/(μL·g−1) 平均
孔径/nm不同孔径孔隙体积占
总孔隙体积的比例,%BET比表面积/
(m2·g−1)不同孔径孔隙比表面积占
总孔隙比表面积的比例,%<2 nm 2~50 nm >50 nm <2 nm 2~50 nm >50 nm 1 22.47 6.26 30.92 63.42 5.67 1.44 18.81 69.38 11.81 2 26.98 6.34 20.54 74.48 4.98 2.05 14.92 72.03 13.05 3 29.79 7.28 24.34 72.64 3.03 2.93 12.64 79.97 7.39 4 24.64 6.05 27.52 68.35 4.13 1.76 16.71 72.48 10.81 均值 25.97 6.48 25.83 69.72 4.45 2.05 15.77 73.47 10.77 表 3 采用实时称重法和NMR法计算CO2萃取效率的对比
Table 3 Calculation of CO2 extraction efficiency by real-time weighing method and NMR method
岩心编号 岩心质量/g 饱和原油量/mL CO2萃取效率,% 萃取前 萃取后 实时称重法 NMR法 1# 61.86 61.65 0.57 37.00 39.89 2# 56.59 56.20 0.66 58.68 60.43 3# 60.10 59.58 0.71 73.04 74.29 4# 58.46 58.09 0.61 60.43 61.65 -
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