Mechanisms of Cold Shock during Coalbed Fracturing Assisted with Cryogenic Gases
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摘要: 液N2等气体辅助煤层气压裂的常规机理已较为清楚，但其低温特征对煤层物性的影响以及对压裂效果的改善机理尚未引起重视，缺乏理论认识.归纳总结了低温气体对煤岩的冷冲击作用机制，评价了地层水的结冰条件和低温气体的热物性，通过室内实验和数值模拟等手段验证和预测了液N2对煤岩及近井煤层的冷冲击效果，并进行了冷冲击机理应用潜力分析.研究结果表明，煤层气压裂过程中注入N2等低温气体对煤层进行冷冲击，可引起煤岩基质收缩和地层水结冰膨胀，使煤岩产生大量微裂缝和力学强度降低(10%~30%)，有利于实现冰晶暂堵和改善煤层气压裂效果.以沁水盆地樊庄区块3#煤层为例，当液N2注入总量为30~120 m3时，可在近井周围3~5 m内形成低温区(＜-20 ℃)，煤层孔隙度将平均增大约1.5倍，渗透率增大4倍，还可造成煤岩的拉伸和挤压破坏.利用低温气体的冷冲击机理及其产生的冰晶暂堵可以作为改进煤层气和页岩气压裂工艺的新方向.Abstract: The common mechanisms of coalbed fracturing assisted with cryogenic gases, such as liquid nitrogen, have been relatively clear, but the impact of low-temperature characteristics of cryogenic gases on coalbed physical properties and its mechanism of improving the fracturing performance have not yet attracted wide attentions, and lack of theoretical understanding. The mechanisms of cold shock of cryogenic gases to the coal rock are summarized first. Then based on the freezing condition of formation water and the thermal properties of cryogenic gases, the cold shock performance of liquid nitrogen to the coal rock was verified by laboratory experiments, and the behavoirs of the cooled coalbed around the wellbore after liquid nitrogen injection were predicted by numerical simulation method. Then the application potential of cold shock was analyzed. The results show that injecting cryogenic gases, such as liquid nitrogen, during coalbed fracturing, can cause a cold shock to the coal rock around wellbore, which will induce the coal matrix shrinking and the formation water freezing. A large number of new microcracks will generate in the coal rock. The mechanical strength of the coal rock will decrease by 10%-30%. These phenomena are favorable for the ice temporary blocking and the improvement of coalbed fracturing. Taking the coalbed of layer 3 in the Fanzhuang block in the Qinshui basin as example, when a amount of liquid nitrogen with a volume of 30-120 m3 is injected underground, a good cooling and shock performance in the coalbed will be obtained within 3-5 m around wellbore. The porosity of the nearby coalbed will increase about 1.5 times and the permeability will increase 4 times. A stretching and squeezing damage may also occur in the coal rock around wellbore. The cold shock of cryogenic gases and its induced ice temporary blocking can be used for the improvement of hydraulic fracturing techniques in coalbed methane and shale gas wells.
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Key words:
- coalbed methane /
- liquid nitrogen /
- CO2 hydrate /
- cold shock /
- microcrack /
- coal rock strength /
- coal mine
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图 3 低温气体的质量热容和导热系数
a.N2质量热容；b.CO2质量热容；c.CH4质量热容；d.N2导热系数；e.CO2导热系数；f.CH4导热系数；据Aspen Hysys(2006)
Fig. 3. Heat capacity and thermal conductivity of cryogenic gases
图 5 冷冲击前后试样A1纵波波幅变化
Fig. 5. Amplitude change of P-wave in coal sample A1 before and after cold shock
图 6 试样B1(a)和B3(b)冷冲击前后效果对比
Fig. 6. Comparison of cold shock performance between samples B1 (a) and B3 (b)
图 7 近井煤层温度场分布
a.不同模拟时间(40 min后停注)；b.不同注入速度(40 min时)；c.不同注入温度(40 min时)；d.不同割理渗透率(40 min时)
Fig. 7. Temperature distribution in coalbed around wellbore after cold shock
图 10 不同深度煤层冷冲击后产生的净拉应力
Fig. 10. σnet-pull at different depths with coal rock temperature decreasing to certain level
表 1 低温气体热力学性质
Table 1. Summary of thermal properties of cryogenic gases
低温气体 N2 CO2 CH4 分子量 28.013 44.010 16.040 临界温度(℃) -146.9 31.1 -82.6 临界压力(MPa) 3.396 7.372 4.599 三相点温度(℃) -210.00 -56.50 -182.46 三相点压力(MPa) 0.012 5 0.518 0 0.011 7 沸点(℃) -195.8(0.101 MPa) -78.4(0.101 MPa)a -161.5(0.101 MPa) 冰点(℃) -210.00(0.012 5 MPa) -56.50(0.518 0 MPa) -182.46(0.011 7 MPa) 液态密度(kg/m3) 810(-196 ℃，0.10 MPa) 1 154(-50 ℃，0.68 MPa) 423(-162 ℃，0.10 MPa) 质量热容(kJ/(kg·℃)) 1.03b ~1.72c 1.09b ~2.02c 2.21b ~3.92c 导热系数(W/(m·℃)) 0.025 5b~0.056 9c 0.016 9b ~0.108 1c 0.033 7b ~0.092 5c 气化潜热(kJ/kg) 198.6(5.56 kJ/mol) 230.5~339.6(0~-50 ℃) 510.9(0.10 MPa) 低温气体用量d(kg/m3) 193.22~351.40 265.49~935.74 90.18~196.25 主要优缺点 超低温、安全可控、低温区范围较大 置换甲烷、易形成水合物、低温区范围较小 超低温、可形成水合物、易燃爆、低温区范围较大 注：a.CO2的沸点是指干冰升华为气体的温度；b.0.1~1.0 MPa下平均质量热容和导热系数；c.16 MPa下平均质量热容和导热系数；d.前者为考虑气化潜热计算得到的结果，后者为不考虑气化潜热计算得到的结果；据张家荣，1987；黄建彬，2002. 
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表 2 冷冲击前后煤样纵波波速变化
Table 2. Velocity change of P-wave in coal samples before and after cold shock
煤样编号 干燥煤样声速(m/s) 饱和水煤样声速(m/s) 声速衰减率(%) 抗压强度下降幅度(%) 弹性模量下降幅度(%) 煤样取心方向 冷冲击前 冷冲击后 冷冲击前 冷冲击后 干燥处理 A1 1 739 1 545 2 509 1 741 1 580 9.14 14.70 10.53 平行于割理 A2 2 017 1 804 2 312 1 888 1 833 9.12 16.81 12.09 平行于割理 A3 1 886 1 773 2 328 1 662 1 693 10.25 17.58 12.66 垂直于割理 B1 1 364 1 184 1 471 1 164 953 30.13 33.70 25.00 垂直于割理 B2 1 173 980 1 301 998 874 25.49 25.84 18.88 平行于割理 B3 1 466 1 289 1 572 1 298 1 089 25.72 31.41 23.19 平行于割理
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表 3 液N2冷冲击煤岩数值模拟模型参数设置
Table 3. Parameter setting of numerical simulation model for cold shock by liquid nitrogen
模拟参数 取值 模拟参数 取值 煤层埋深(m) 550 煤岩热容(J/(m3·℃)) 1.75×106d 煤层厚度(m) 5 煤岩导热系数(J/(m·min·℃)) 16.8e 煤层温度(℃) 30 盖底层热容(J/(m3·℃)) 2.26×106f 煤层原始压力(MPa) 4 盖底层导热系数(J/(m·min·℃)) 180g 基质孔隙度(%) 2.66a N2热容(J/(mol·℃)) CN2=36.302-0.047 6t+0.002 8t2+1.68×10-5t3，其中t为温度，℃ 割理孔隙度(%) 0.84a N2导热系数(J/(m·min·℃)) 3.414 基质渗透率(10-15 m2) 0.001 模拟地层半径(m) 186 割理渗透率(10-15 m2) 0.5，1*，5 井眼半径(m) 0.108h 割理间距(m) 2.5×10-3 注入速度(Sm3/min)j 500，1 000*，2 000 煤岩密度(kg/m3) 1 550b 注入温度(℃) -60，-80*，-100 煤岩孔隙压缩系数(kPa-1) 6×10-5c 网格划分 120×1×1 注：a.设煤岩总孔隙度3.5%，割理压缩比率β(割理孔隙度/总孔隙度)一般为0.11~0.37，取平均值为0.24；b.煤岩密度一般为1.136~1.783 t/m3；c.煤岩压缩系数一般为1.8×10-4~2.2×10-3 MPa-1，设孔隙度为3.5%，则孔隙压缩系数为5.1×10-3~ 63.0×10-3 MPa-1，取60.0×10-3 MPa-1；d.煤岩质量热容一般随温度增大而增大，为1.00~1.26 kJ/(kg·℃)，取1.13 kJ/(kg·℃)；e.煤岩导热率为0.173~1.335 W/(m·℃)，平均值为0.220~0.330 W/(m·℃)，取0.280 W/(m·℃)；f.砂岩质量热容一般为0.837~1.315 kJ/(kg·℃)，砂岩密度为1.2~3.0 t/m3，取平均值得到2.26×106 J/(m3·℃)；g.砂岩导热系数为1.852~4.133 W/(m·℃)，取平均值为3.000 W/(m·℃)；h.3-1/2″油管，5-1/2″套管，8-1/2″井眼；j.标况下N2密度为1.25 kg/m3，根据现场经验，国外单井液N2注入速度一般为0.5~2.0 m3/min(=324~1 296 Sm3/min)，注入总量一般为20~55 m3( McDaniel et al., 1997 ；Grundmann et al., 1998 )，国内液N2注入总量为73 m3(http://www.zgsyb.com.cn； http://www.oilhb.com)，液态CO2为77 m3(焦中华等，2011 ).带*数值为基本模型参数设置.
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