Two-phase oil/water seepage in fractured granite rock mass: Insight from seepage visualization experiment and numerical simulation

doi:10.13745/j.esf.sf.2022.12.54

Abstract

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

CLC Number:

TE822

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.

Figures/Tables 14

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

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

Fig.2 Objet30 printer system

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

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

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

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

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

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

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

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

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

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

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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