Experimental Study on Improving Permeability of Shale by CO2 Huff and Puff Assisted by Ultra-Low Temperature Liquid Nitrogen
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摘要:
页岩储层岩石致密、孔喉细小、渗透率极低，明确页岩渗流机理是提高页岩油气采收率的关键。为此，进行了页岩岩心液氮（LN2）低温处理试验和循环注CO2吞吐试验，分析了页岩经液氮低温处理后不同注气压力循环注气吞吐对页岩油采收率、岩心物性和油气两相相对渗透率的影响，明确了处理前后微观孔隙结构的变化特征。试验结果表明，LN2注入页岩后可产生313.5 MPa热应力，诱导形成微裂缝。LN2气化体积膨胀作用和循环注CO2吞吐能够在形成的微裂缝中形成再加压机制，扩展诱导裂缝，提高渗流能力。CO2吞吐采收率与注入压力成正比，超临界态CO2首轮吞吐采收率和3轮吞吐累计采收率比亚临界态CO2高32.4%和34.9%，提高幅度达154.6%和101.7%。高压注CO2所需的吞吐次数减少，产出油量主要来源于前2轮吞吐。与初始页岩岩心相比，超临界态CO2循环吞吐后岩心平均孔径增大176%，最大油、气相对渗透率分别提高了1.8倍和2.3倍。研究结果对页岩油气增产具有一定的参考价值。
Abstract:The shale reservoir has tight rock, small pore throat, and very low permeability The knowledge of flow mechanism in shale is the key to improving the recovery of shale oil and gas. By conducting low-temperature liquid nitrogen (LN2) treatment experiments on shale cores and experiments of cyclic injection of CO2, the effects of cyclic gas injection at different injection pressures after low-temperature LN2 treatment on the recovery of shale oil, physical properties of cores, and relative permeability of oil and gas were studied, and the changes in the microscopic pore structure before and after the treatment were identified. The experimental results show that after LN2 injection, the shale can generate a thermal stress of 313.5 MPa, inducing the formation of micro-fractures. The volume expansion effect of LN2 vaporization and cyclic injection for CO2 can form a re-pressurization mechanism in the core after the formation of micro-fractures, expanding the induced fractures and improving the permeability. The recovery by CO2 huff and puff is proportional to the injection pressure. The cumulative recovery by the first and third rounds of supercritical CO2 huff and puff are 32.4% and 34.9% higher than those of subcritical CO2 , with an increase of 154.6% and 101.7%, respectively. The number of required huff and puff cycles for high-pressure CO2 injection is reduced, and the produced oil mainly comes from the first two rounds of huff and puff. Compared with that of the initial shale core, after supercritical cyclic CO2 huff and puff, the average pore size increases by 176%, and the maximum relative permeability of oil and gas increase by 1.8 and 2.3 times, respectively. The research results provide a reference for the production increase of shale oil and gas.
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Keywords:
- shale /
- CO2 huff-puff /
- liquid nitrogen /
- pore structure /
- micro-fracture
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图 2 页岩岩心矿物组成及含量
Figure 2. Mineral composition and content of experimental shale samples
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图 3 LN2低温处理过程中岩心温度随时间的变化
Figure 3. Changes in core temperature over time during low-temperature LN2 treatment
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图 4 未处理岩心和LN2低温处理后岩心CO2吞吐采收率对比
Figure 4. Comparison of recovery by CO2 huff and puff of core with and without low-temperature LN2 treatment
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图 5 LN2低温处理后岩心在不同注气压力下吞吐的采收率对比
Figure 5. Comparison of recovery by huff and puff of core with low-temperature LN2 treatment under different gas injection pressures
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图 6 初始岩心与LN2低温处理和CO2吞吐后的岩心孔隙度和渗透率变化
Figure 6. Changes in porosity and permeability of initial core and core after low-temperature LN2 treatment and CO2 huff and puff
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图 7 3种不同注入压力吞吐后页岩渗透率随上覆压力的变化
Figure 7. Changes in shale permeability with overlying pressure after huff and puff at three different injection pressures
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图 8 初始页岩与LN2低温处理和循环注气后T2谱分布
Figure 8. T2 spectrum distribution of initial shale and shale after low-temperature LN2 treatment and cyclic gas injection
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图 9 初始岩心和经LN2低温处理及循环注气后岩心3端面CT扫描图像
Figure 9. CT scan images of three end faces of initial core and core after low-temperature LN2 treatment and cyclic gas injection
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图 10 处理前后页岩油气两相相对渗透率曲线的变化
Figure 10. Changes in relative permeability curve of shale oil and gas before and after treatment
表 1 页岩岩心基本参数
Table 1 Basic parameters of experimental shale core
岩心编号 直径/mm 长度/mm 孔隙度，% 渗透率/10−3mD 总有机碳含量，% 镜质体反射率，% 试验设计 1 38.25 51.75 7.32 0.45 2.11 1.15 LN2低温处理+4 MPa循环注CO2吞吐 2 38.14 54.21 7.47 0.37 2.83 1.21 LN2低温处理+7 MPa循环注CO2吞吐 3 38.16 52.42 6.74 0.24 3.03 1.18 LN2低温处理+10 MPa循环注CO2吞吐 4 38.11 51.56 7.21 0.33 2.81 1.07 10 MPa循环注CO2吞吐（对比试验） 
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表 2 岩心在低温处理中形成的热应力及计算参数
Table 2 Thermal stress and calculation parameters formed during low-temperature treatment
岩心编号 弹性模量/103 MPa 泊松比 热膨胀系数/10−6 ℃−1 低温LN2处理前温度/℃ 低温LN2处理后温度/℃ 热应力/MPa 1 61.0 0.296 15 78.5 −177.3 303.3 2 56.4 0.284 15 78.5 −181.1 281.9 3 70.3 0.311 15 78.5 −178.6 355.5
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表 3 不同注气压力下页岩处理前后孔径参数变化
Table 3 Changes in pore size parameters before and after shale treatment under different gas injection pressures
岩心 平均弛豫
时间/ms平均孔径/
nm最长弛豫
时间/ms最大孔径/
nm岩心1处理前 0.63 2.7 76 326 岩心1处理后 0.96 4.1 564 2425 岩心3处理前 0.48 2.1 151 649 岩心3处理后 1.36 5.8 982 4222
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