True triaxial mechanical characteristics of tight sandstone with different oil and water staturation
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摘要:
致密砂岩储层中水、石油或天然气等流体赋存可能会对致密砂岩的力学特征产生影响。选取鄂尔多斯盆地蟠龙地区延长组致密砂岩，利用YSZS-2000型多功能电液伺服岩石真三轴试验机对不同含油和含水饱和度致密砂岩力学特征进行了研究。结果表明：①含油性对砂岩的峰值应力和弹性模量产生弱化作用，表现为峰值应力和弹性模量随着含油饱和度的增大而减小，“准饱和”时峰值应力可下降5.35%，弹性模量可下降11.38%；②含水砂岩根据其吸水特性曲线分为Pa型与Pb型，含水性对砂岩的峰值应力产生弱化作用，饱和时Pa型砂岩峰值应力可下降11.58%，Pb型砂岩峰值应力可下降15.95%；③观察实验后浸油砂岩内部结构发现，油集中分布于砂岩矿物间裂隙，对破裂面产生润滑作用从而降低了砂岩的承载力。本研究成果可以对致密油气的开发提供可靠依据。
Abstract:The occurrence of fluids such as water, oil or natural gas in tight sandstone reservoirs may affect the mechanical characteristics of tight sandstone. This paper selects tight sandstones of Yanchang Formation in Panlong area of Ordos Basin, and uses YSZS-2000 multifunctional electro-hydraulic servo rock true triaxial testing machine to study the mechanical characteristics of tight sandstones with different oil and water saturation degrees. Results show that: ①The oil contents have a weakening effect on the peak stress and elastic modulus of sandstones. Both the peak stress and elastic modulus decrease with the increase of oil saturation. The peak stress can be reduced by 5.35% and the elastic modulus can be reduced by 11.38% when it is "quasi-saturated". ②Sandstones containing water can be divided into the Pa-type and the Pb-type according to the water absorption characteristic curve. Water content weakens the peak stress of sandstones. When saturated, the peak stresses of a Pa-type and a Pb-type sandstone can be reduced by 11.58% and 15.95%, respectively. ③After observing the internal structure of the oil-soaked sandstone, it is found that the oil is concentrated in the fractures between the sandstone minerals, which lubricates the fractured surface and reduces the bearing capacity of the sandstone. The research outcomes of this article can provide a reliable basis for the exploitation of tight oil and gas.
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Key words:
- true triaxial test /
- tight sandstone /
- mechanical characteristics /
- peak stress /
- elastic modulus
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图 1 YSZS-2000多功能电液伺服岩石真三轴试验装置图
Figure 1. YSZS-2000 multi-functional electro-hydraulic servo rock true triaxial test device diagram
图 4 砂岩不同含油饱和度轴向应力σ1-轴向应变ε1关系曲线
Figure 4. Relationship curve between axial stress σ1-axial strain ε1 of sandstone with different oil saturations
图 5 Pa型与Pb型砂岩不同含水饱和度轴向应力σ1-轴向应变ε1关系曲线
Figure 5. Relationship curves of axial stress σ1-axial strain ε1 for Pa and Pb type sandstones with different water saturation degrees
图 6 峰值应力σ1、弹性模量与砂岩/含水饱和度S0关系曲线(S0=63.9%)
a.光学显微镜下未压裂砂岩内部结构; b.光学显微镜下压裂后砂岩内部结构; c.荧光显微镜下未压裂砂岩内部结构; d.荧光显微镜下压裂后砂岩内部结构
Figure 6. Relationship curves of peak stress σ1, elastic modulus and sandstone oil water saturation S0
图 7 光学显微镜下砂岩浸油后内部结构图
Figure 7. Internal structure of sandstone after oil soaking under an optical microscope
图 8 含油砂岩内部结构示意图
a.未经压裂浸油砂岩内部结构示意图; b.经压裂后浸油砂岩内部结构示意图
Figure 8. Schematic diagram of the internal structure of oil-bearing sandstone
表 1 砂岩样品基本参数
Table 1. Basic parameters of sandstone samples
样品编号 密度/(g·cm-3) 面孔率/% 单轴抗压强度/MPa P1 2.259 1 2.3 35.480 7 P2 2.264 9 2.5 35.206 9 P3 2.282 2 2.7 34.987 7 平均值 2.268 7 2.5 35.225 1 
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表 2 天然砂岩烘干质量变化
Table 2. Drying quality change table of natural sandstone
样品编号 天然质量/g 烘干后质量/g 质量/g 天然含水率/% P1 2 234.8 2 224.8 10.0 0.45 P2 2 233.4 2 223.2 10.2 0.46 P3 2 237.9 2 228.0 9.9 0.44 平均值 2 235.4 2 225.3 10.0 0.45
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