Assessment of inter-cavern containment property of underground water-sealed oil storage caverns combining discrete fracture network analysis with graph theory

The inter-cavern containment property assessment is one of the critical issues of the underground water-sealed oil storage facility and its result determines whether the mixing of different oil products occurs or not. In this study, an inter-cavern containment property assessment method combining discrete fracture network analysis with graph theory was proposed. In the method, the discrete fracture network analysis is adopted to obtain the distribution of the hydraulic head in the rock mass and the graph theory is used to determine the hydraulic connectivity between the two adjacent caverns. The influences of cavern spacing, liquid level differences, water curtain pressures, and geometric parameters on the inter-cavern containment property were investigated based on the assessment method. The results indicated that the inter-cavern containment property could be guaranteed by increasing cavern spacing (d), water curtain pressure (P), or decreasing liquid level differences (Δh), fracture trace length, and linear density. To compare the influence of different factors on the inter-cavern containment property, a parameter p was defined to represent the connectivity of fracture networks between caverns according to the connected relations in graph theory. By comparing changed connectivity parameter p, the influences of cavern spacing, liquid level differences, and water curtain pressures on the inter-cavern containment property were ranked as follows: Δh > d > P. Based on the Huangdao underground oil storage caverns, the inter-cavern containment properties were discussed. The result showed that the cavern spacing should be greater than 40 m, the water curtain pressure should not be greater than 130 kPa and the liquid level difference should be less than 12 m to ensure inter-cavern containment properties, which can offer good guidance for the design and operation of caverns.