花岗岩裂隙岩体油水两相渗流可视化试验及数值模拟研究

doi:10.13745/j.esf.sf.2022.12.54

摘要/Abstract

摘要：

地下水封油库利用“隙存水封”的原理通过地下洞室进行石油地下存储,并多建于结晶岩体中,岩体裂隙渗流特性直接影响地下油库储油安全。地下洞库水封机理可简化为围岩裂隙中的油水两相渗流问题,需同时考虑油驱水和水驱油过程。基于分形理论定量刻画了天然花岗岩裂隙面的粗糙度,通过3D打印技术制备了不同粗糙度的透明裂隙,利用研制的单裂隙多相渗流可视化试验装置开展了模型试验,并结合数值模拟对花岗岩粗糙裂隙中的油-水驱替规律进行了研究。研究表明:水油流度比的差异导致花岗岩裂隙两相驱替呈现出不同效果,油驱水前缘呈活塞式整体推进,而水驱油前缘呈指进式推进;压力差或隙宽的增加更加显著地提升油驱水效果,但裂隙粗糙度的增加则对裂隙中流体的流动起到了阻碍作用,使得驱替难以进行;在考虑裂隙粗糙度和油相黏度的情况下,仅通过围岩裂隙水压力大于洞库内部油压来判别水封性能的“水封准则”存在缺陷;由于“贾敏效应”的作用,水驱油时裂隙中的油将难以被完全驱替,当油库发生油品泄漏时,难以将油品完全驱回洞库内部。研究成果可为粗糙裂隙岩体两相渗流规律及地下洞库水封机理的研究提供参考依据。

关键词: 花岗岩, 粗糙裂隙, 油水驱替, 多相渗流装置, 3D打印技术, 可视化试验, 数值模拟

Abstract:

Hard rock caverns used for underground oil storage are mostly constructed in crystalline rock mass and water sealed, thus the seepage characteristics of the rock mass directly affect the safety of oil storage. The challenge of water sealing in underground caverns can be simplified to the problem of two-phase oil/water seepage through fractures in the surrounding rock mass, so it is necessary to consider both oil and water displacements to addressing this challenge. In this study, seepage visualization experiment and numerical simulation were carried out on fractured natural granite. The fracture roughness was quantified by fractal analysis, and transparent fractures with different roughness values were prepared by 3D printing. Physical model test was carried out by using the home-built single-fracture multiphase-seepage visualization device, and fluid displacement in rough fractures was studied by numerical simulation. Difference in the mobility ratio (water to oil) leads to different two-phase (oil-water) flow dynamics, where the oil-flooding front moves as piston flow while water-flooding front as fingered flow. In oil displacement, the increase of fluid pressure or fracture width can significantly improve oil displacement efficiency, whereas increasing fracture roughness hinders oil flow and impedes oil displacement. Considering the relevance of fracture roughness and oil viscosity, safety requirements for water sealing are not adequate if only fluid pressure is considered. Due to Jamin effect, it is difficult to remove oil completely from rock fractures by water flooding, or to recover leaked oil. The research results provide a reference for understanding two-phase seepage in rough fractures in rocks and water sealing mechanism in underground caverns.

Key words: granite, rough fracture, oil-water displacement, multiphase flow device, 3D printing technology, visualization experiment, numerical simulation

中图分类号:

TE822

孙哲, 张彬, 陈大伟, 李玉涛, 王汉勋. 花岗岩裂隙岩体油水两相渗流可视化试验及数值模拟研究[J]. 地学前缘, 2023, 30(3): 465-475.

SUN Zhe, ZHANG Bin, CHEN Dawei, LI Yutao, WANG Hanxun. Two-phase oil/water seepage in fractured granite rock mass: Insight from seepage visualization experiment and numerical simulation[J]. Earth Science Frontiers, 2023, 30(3): 465-475.

图/表 14

图1 花岗岩结构面三维表面形貌图 a和b为每组圆柱样的两个断裂面。

Fig.1 Collection of 3D laser scans of natural granite with different surface features

表1 分形参数计算结果

Table 1 Calculated fractal parameters

组别	D			R²				δ/mm
组别	a面	b面		a面		b面		a面		b面
1	2.687	2.696	0.999 6		0.999 8		0.975 6		0.740 6
2	2.620	2.661	0.999 7		0.999 7		0.565 7		0.634 3
3	2.663	2.612	0.999 7		0.999 7		0.888 1		0.647 5
4	2.730	2.640	0.999 7		0.999 7		0.634 3		0.585 8
5	2.619	2.583	0.999 7		0.999 8		0.888 1		0.467 1
6	2.742	2.695	0.999 8		0.999 6		0.922 5		1.128 4

图2 Objet30打印机系统

Fig.2 Objet30 printer system

表2 所需材料物理力学参数对比表

Table 2 Comparison of physical and mechanical properties of resin materials and granite

材料	抗压强度/MPa	弹性模量/MPa	渗透系数/(m·s^-1)
透明光敏树脂	79	2 000~3 000	10^-12~10^-10
花岗岩	75	35 000	10^-8~10^-7

图3 透明粗糙裂隙模型打印结果

Fig.3 Transparent rough fracture model prepared by 3D printing

图4 水-油两相驱替试验装置图

Fig.4 Instrumentation setup in two-phase water/oil displacement experiments

图5 油驱水不同时刻界面形态图

Fig.5 Change of fluid morphology with time during two-phase water replacement

图6 水驱油不同时刻界面形态图 a—第一阶段;b—第二阶段。

Fig.6 Change of fluid morphology with time during two-phase oil displacement

表3 数值模拟计算参数

Table 3 Parameters used in numerical simulation

参数	数值	单位
水相黏度	0.001	Pa·s
油相黏度	0.003 4	Pa·s
水相密度	998	kg/m³
油相密度	920	kg/m³
渗流压力	50,100,150	kPa
隙宽	0.3,0.7,1.0	mm
分形维数	2.3,2.5,2.8
标准差	0.5,0.7,1.0	mm
相界面张力系数	0.031	N/m
相界面厚度	0.000 5	m

图7 油-水两相渗流过程对比图 a—油驱水过程;b—水驱油过程。

Fig.7 Comparison of observed and simulated oil-flooding (a)/water-flooding (b) fronts at different time points resulting from piston-flow/fingered-flow movements

图8 油-水两相渗流流量时程图 a—油驱水过程;b—水驱油过程。

Fig.8 Time-history plots of flow rate and total flow under oil (a)/water (b) flooding

图9 不同条件下的水驱前缘图(水驱油过程) a—压力差50 kPa;b—压力差150 kPa;c—隙宽0.3 mm;d—隙宽1.0 mm。

Fig.9 Comparison of water-flooding front under different fluid pressures (a, b) and fracture widths (c, d)

图10 不同条件下的油驱水时间

Fig.10 Variability of water displacement-time under oil flooding as functions of (from left) fluid pressure, fracture width, fractal dimension and fracture roughness

图11 油-水驱替纵截面压力分布图

Fig.11 Longitudinal pressure profile within rock fracture at different time points under water (lines with dots)/oil (lines with squares) flooding

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