Using depleted oil or gas fields and aquifer structures
Reservoir engineering is the discipline which concerns itself with describing the flow properties and possibilities of gases and liquids when producing or storing hydrocarbons in porous underground rock formations. One of the most important aspects is determining the spatial extent and the recoverable content of a natural reservoir. Key roles are played here by the properties of the reservoir and the associated conditions (e.g. pressure and temperature at depth), which also need to be determined as well. Reservoir engineers do this evaluation by applying the basic principles of physics and chemistry to the behaviour of liquids and gases in porous and permeable underground rock formations.
Whilst the goal of oil and gas production is to estimate the amount of recoverable reserves as accurately as possible, and to achieve maximum production rates, the main objective in natural gas storage projects is optimising the utilisation of the available capacities. Technologies originally developed for oil and gas fields, can also be applied to designing underground storages in depleted oil and gas fields as well as in aquifer formations (water-bearing horizons).
In the case of depleted oil and gas fields, the reservoir engineering properties are already very well known because of all the previous exploration and production work which has been carried out. Moreover, the tightness of these fields has already been proven over geological time scales so that the work involved when converting a depleted field into a gas storage is mainly limited to purpose-specific computer simulations. The situation when utilising an aquifer is quite different because comprehensive investigations are still required. The main objectives in the evaluation of aquifers in terms of reservoir engineering are confirming the tightness of the surrounding rock formations (see Fig. ), determining the storage capacity of the aquifer, and assessing the potential injection and production rates. This requires wells to be drilled and cores to be taken to verify the tightness of the cover rock, the permeability of the aquifer formation, and the pore volume of the storage rock. These initial exploration wells can also be subsequently used for injection and production tests to determine the properties of the potential storage. Reservoir engineering also uses numerical methods to simulate the storage capacity, and dynamic performance figures for the final evaluation of porous rock formations to determine their suitability for acting as storages.