地质力学学报  2021, Vol. 27 Issue (1): 1-9
 引用本文

YU Xin, LI Gao, CHEN Ze, ZHANG Yi. Experimental study on physical and mechanical characteristics of tight sandstones in the Xujiahe Formation in western Sichuan after high-temperature exposure[J]. Journal of Geomechanics, 2021, 27(1): 1-9.

1. 西南石油大学油气藏地质及开发工程国家重点试验室, 四川 成都 610500;
2. 中海石油 (中国) 有限公司湛江分公司, 广东 湛江 524057

DOI10.12090/j.issn.1006-6616.2021.27.01.001     文章编号：1006-6616(2021)01-0001-09
Experimental study on physical and mechanical characteristics of tight sandstones in the Xujiahe Formation in western Sichuan after high-temperature exposure
YU Xin1,2, LI Gao1, CHEN Ze1, ZHANG Yi1
1. State Key Laboratory of Oil&Gas Reservoir Geology and Exploitation, Chengdu 610500, Sichuan, China;
2. Zhanjiang Branch of China National Offshore Oil Corporation Limited, Zhanjiang 524057, Guangdong, China
Abstract: This study aims to boost the seepage capacity in the near-well area by the downhole heating so as to improve the production efficiency of low-permeability reservoir while ensuring the sidewall stability. Taking the second member of the Xujiahe Formation in Longchang as the subject, the effect of high temperature on the microstructure, mechanical property and permeability of tight sandstones were studied. The samples underwent thermogravimetric analysis, scanning electron microscopy (SEM), acoustic wave test, physical parameter measurement, uniaxial compression test and permeability test after high-temperature exposure in the range of 26 ℃ to 1000 ℃, and temperature's relevance to the composition, microstructure, mechanical parameter and permeability were analyzed. The test results showed that the internal moisture of the samples was removed continuously with the increase of temperature, and the content of clay minerals decreased by stages in the range of 26 ℃ to 1000 ℃, which led to the decrease of sample mass and apparent density. There was a threshold temperature at about 400 ℃ for the performance of tight sandstone. When the temperature was higher than 400 ℃, the compressive strength and deformation resistance of the samples decreased sharply. With the increase of temperature, more internal fractures emerged and the network size was enlarged, leading to the continuous growing increase of permeability. Therefore, it is considered that keeping the downhole heating temperature above 400 ℃ and expanding the heating range as well are conducive to improving the productivity of single well. The findings of this study are of value for evaluating the wellbore stability and stimulation effect of single well while applying the electric heating technology in tight sandstone reservoirs.
Key words: high-temperature heating    tight sandstone    wave velocity    permeability    mechanical properties
0 引言

1 试验方法 1.1 试验设备 1.1.1 加热装置

1.1.2 微组构试验仪器

1.1.3 单轴压缩试验仪器

1.1.4 波速测试仪器

1.1.5 气体渗透率测试仪器

1.2 样品选择与制备

1.3 高温处理

 图 1 经历不同温度后试样表观颜色变化 Fig. 1 Colors of the samples after exposing to high temperatures
2 试验结果分析 2.1 试样组分变化

 图 2 高温后岩样矿物含量变化图 Fig. 2 Variation diagram of mineral contents after exposing to high temperatures
2.2 试样微观结构变化

 图 3 岩样热重分析结果 Fig. 3 Thermogravimetric analysis results for the samples

 图 4 高温后致密砂岩微观结构特征 Fig. 4 Microstructure characteristics of tight sandstones after exposing to high temperatures
2.3 试样物理参数变化

 图 5 试样质量与温度的关系 Fig. 5 Effect of temperature on sample quality

 图 6 试样体积与温度的关系 Fig. 6 Effect of temperature on sample volume

 图 7 试样视密度与温度的关系 Fig. 7 Effect of temperature on apparent density
2.4 试样力学性质变化 2.4.1 试样纵波波速

 图 8 试样波速与温度的关系 Fig. 8 Effect of temperature on wave velocity
2.4.2 应力-应变曲线

 图 9 高温后试样单轴应力-应变关系曲线 Fig. 9 Diagram showing the uniaxial stress-strain curves of the samples after exposing to high temperatures
2.4.3 单轴抗压强度及弹性模量

 图 10 试样单轴抗压强度与温度的关系 Fig. 10 Effect of temperature on uniaxial compressive strength

 图 11 试样弹性模量与温度的关系 Fig. 11 Effect of temperature on elastic modulus
2.5 试样渗透性变化

 图 12 高温后致密砂岩渗透率变化曲线 Fig. 12 Diagram showing the permeability curves of the tight sandstones after exposing to high temperatures

 $\begin{array}{l} K = 0.001 \times T - 4.92 \times {10^{ - 6}} \times {T^2} + 6.18 \times \\ \;\;\;\;\;\;\;\;{10^{ - 9}} \times {T^3} - 0.00389\;{R^2} = 0.99162 \end{array}$ (1)

1000 ℃高温后试样渗透率从初始0.044×10-3 μm2增至2.31×10-3 μm2，渗透性增大近52倍。结合高温后致密砂岩微组构的试验结果进行分析：在温度处于26~400 ℃阶段，虽然试样内部岩石矿物在温度作用下发生热膨胀导致少量微裂缝生成，但对岩样渗透率影响较小，因此400 ℃以下岩样渗透性未发生明显改变；当温度处于400~1000 ℃阶段，由于岩石矿物热膨胀程度增加导致岩石内部不断发生热破裂，因此高温后试样次生裂缝得以发育，且内部裂缝网络随着温度的增高，裂缝数量不断增多，裂缝网络规模不断增大，导致高温后岩样渗透率不断增加，且斜率随温度增大。

2.6 讨论

3 结论

(1) 当热处理温度在1000 ℃范围内，四川隆昌须家河组须二段致密砂岩随温度升高不断脱去内部水分，粘土矿物含量分阶段减少，进而造成试样质量减小、视密度降低。致密砂岩在高温作用后组分的变化将对岩石的宏观物理参数产生影响。

(2) 热处理温度在26~1000 ℃内时，高温后致密砂岩试样纵波波速在300 ℃后随温度升高不断降低，微裂缝随温度升高不断发育成网络；26~300 ℃范围内试样单轴抗压强度、弹性模量在一定范围内波动、整体上有所增加，而在400~1000 ℃范围内急剧降低，说明400 ℃左右存在一个致密砂岩强弱转化的阈值温度。因此在油气井中根据岩石力学性能的变化规律，控制好近井筒处及地层内各处岩石的温度有利于提高井壁稳定性。

(3) 当热处理温度低于400 ℃时，试样内部在温度作用下所生成的微裂缝较少，因此400 ℃内岩样渗透性未发生明显改变。当热处理温度高于400 ℃，试样内部裂缝网络随着温度的增高，裂缝数量不断增多，裂缝网络规模不断增大，导致高温后岩样渗透率不断增加，且增速随温度不断增大。400 ℃左右是致密砂岩渗透性随温度变化的一个阈值温度，因此油气井将加热温度控制在400 ℃以上并增大加热范围有利于提高单井产能。