Localization of underground hydrocarbon gas storages in respect to performance criteria

Abstract

Underground storages for hydrocarbon gases are often found to be more feasible than the above ground option. For the design of the underground openings, the geological investigations and subsequent hydro-geological conditions reveal, are of most importance. The information obtained is a requisite in order to design and optimize, at a given location, the shape and orientation of the caverns, suitable excavation method, adequate support for long term stability etc, with reference to the economical, environmental and safety requirements of the project. In the design process for underground hydrocarbon storages, additional design criteria's compared to other underground openings, must be considered due to the products properties and their storage conditions. Depending on the characteristics of the hydrocarbon product to be stored, different design criteria's must be fulfilled to ensure secure containment and to protect both the environment as well as the stored product from contamination. Crude oil and some refined products may be stored at atmospheric pressure in unlined caverns since the containment is secured by the ground water pressure, which blocks the product from escaping the cavern and penetrating the rock mass. Petroleum gases are stored in liquid state as liquefied petroleum gas (LPG). The LPG's are stored, either in a compressed or refrigerated storage. In the compressed storages, the containment is secured by the groundwater and groundwater pressure, while in the refrigerated storages; the containment relies on the impermeable ice ring formed around the frozen cavern. The design criteria commonly used is that the groundwater pressure shall always exceed the cavern liquid pressure, and the ground water pressure shall always increase with distance from the cavern perimeter. The latter means, in principals that a ground water inflow is required in all rock joint which are in contact with both the cavern opening and the surrounding groundwater aquifer. For a LPG cavern with compressed gas, the location depth will, in principle, be a function of the product vapor pressures, i.e. the crown will be located at depth where the groundwater pressure has equivalent pressure. However, due to pressure losses in water flow, safety factors and geometric reasons, the caverns are located at greater depth. In cases of refrigerated LPG storages, the location depth will be a function of the thermal conditions of the rock mass, since a design criteria is the presence of free water above the frozen zone i.e. the zero-isotherm must be below the groundwater level. Commonly, the characteristic of propane is used for depth location since it has most adverse storing conditions. When storing natural gas in man made underground openings, the gas may be stored in gas phase under very high pressures or as liquefied natural gas (LNG) at very low temperatures (-162 degrees C). With references to the economical, environmental and safety requirements of these types of projects, a lining system is required. For the compressed storage alternative, the weight of the overburden rock and deformability characteristic of the rock mass and the interaction with the lining system will be most critical for the location depth. A 200-bars lined pilot cavern has been built and is in operation in Sweden. In case of LNG storages, the insulted lining will be needed both to ensure containment and for controlling heat flux and thermo-mechanical loads for mechanical stability reasons. When attempting to store LNG in underground caverns, the thermo-mechanical interaction between the rock mass and the lining system is most crucial. The cavern must be located at depth that ensures that the zero-isotherm is below the groundwater level at all times. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.