Temperature effect on mode Ⅰ fracture behavior of tight sandstone
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摘要: 致密砂岩裂缝复杂程度对于提升致密气储层压裂改造效率、增大资源动用程度具有实际意义。本文利用三点弯曲加载手段结合SEM分析对致密砂岩半圆弯曲(SCB)试样在室温、液氮低温条件下的力学特性、断裂行为、断裂路径规律及断裂能特征进行表征，讨论液氮低温作用下的致密砂岩预裂机制。研究结果表明：①液氮低温处理后致密砂岩试样断裂荷载、断裂韧度较室温状态均明显降低，而层理平行加载方位试样上参数略有增大，推测与液氮低温对层理面增强有关；②液氮低温处理后试样裂缝扩展路径复杂度提升，裂缝条数增多并出现裂缝分叉汇聚等现象，液氮预裂对致密砂岩断裂复杂度提升效果显著；③液氮预裂后致密砂岩断裂能显著降低，最大降幅达46.98 %；④室温下致密砂岩断裂路径主要沿矿物晶界发展，形成沿晶裂缝，岩石改造程度低；液氮低温处理后砂岩试样以沿晶断裂、穿晶断裂及颗粒断裂形成复杂裂缝形式，促进了穿晶裂缝与复杂缝网形成。综上，致密砂岩储层压裂中低温预处理能有效降低储层破裂压力，实现裂缝端部岩石韧-脆转换，提升压裂裂缝复杂程度。Abstract: The fracture complexity of tight sandstone could work to improve the efficiency of tight gas reservoir fracturing.In this paper, three-point bending loading tests was used to characterize the fracture behavior, mechanical properties, fracture path and fracture energy characteristics of tight sandstone at room temperature(RT)and low temperature(LT)of liquid nitrogen(LN2), and to interpret the pre-fracture mechanism of tight sandstone at LN2.The results show that: 1)The fracture load and fracture toughness of the tight sandstone treated with LN2 are significantly lower than those at RT, while the parameters of the samples under parallel bedding slightly increased, which may be related to the enhancement of the bedding plane by LN2.2)After low temperature treatment with LN2, the complexity of fracture propagation paths increases, the number of fracture increases, and the fracture bifurcation and convergence occurs.The fracture complexity of sandstone is low at RT, and the effect of LN2 precracking on the fracture complexity of tight sandstone is significant.3)After LN2 pre-splitting, the fracture energy of tight sandstone decreases significantly, with a maximum decrease of 46.98 %.4)It is found that the fracture paths of tight sandstone mainly develops along the mineral grain boundary and forms intergranular fracture at RT, and the rock transformation degree is low.However, the complex fracture forms are formed by intergranular fracture, transgranular fracture and intergranular fracture, and the formation of transgranular fracture and complex fracture network are promoted under low temperature.The results of this study suggest that low-temperature during fracturing of tight sandstone reservoir can effectively reduce the fracturing pressure, realize the toughening-brittle transformation of rock and improve fracture complexity.
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图 4 不同层理方向致密砂岩半圆弯曲试样加载位移-荷载曲线
Figure 4. Displacement-load curves of tight sandstone SCB samples with different bedding directions
图 5 不同层理方向致密砂岩试样断裂韧度与断裂速率特征
Figure 5. Characteristics of fracture toughness and fracture propagation rates of tight sandstone samples with different bedding orientation
图 6 不同层理方位致密砂岩试样断裂特征与破坏模式
Figure 6. Fracture characteristics and failure modes of tight sandstone samples with different bedding orientation
图 7 试样压缩过程断裂能计算原理
Figure 7. Calculation principle of fracture energy during compression of sample
图 8 不同层理倾角致密砂岩试样断裂能量计算结果
Figure 8. Fracture energy of tight sandstone samples with different bedding orientation
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