Single-Phase Simulation of an Aquifer Gas Storage Field

Abstract

ABSTRACT In an aquifer gas storage field, the gas/water boundary or transition zone expands and contracts cyclically due to the cyclic injection and withdrawal of gas in the field. The physical pore volume occupied by the gas phase thus changes with time. Although this change is cyclic, it is usually not in phase with respect to the gas injection and withdrawal cycle due to relatively slow response of the aquifer. The two phase (gas-water) system in aquifer gas storage field can be simulated using a single phase simulator, provided accurate predictions of the gas phase pore volume can be made as a function of gas injection/withdrawal rate and schedule. The established procedure for predicting gas/water boundary movement based on influence functions and the principal of superposition is tedious, time consuming and requires field history data from its early development stages. This paper presents an alternative approach based on an equilibrium pore volume concept. An equilibrium pore volume at a given time may be defined as the pore volume that will be occupied by the gas in storage at that time if brought to the equilibrium aquifer pressure. At any given time, the actual pore volume occupied by the gas phase may be different than the corresponding equilibrium pore volume; and the difference provides the driving force for the water movement. This approach was successfully used in the analysis of data from an existing aquifer storage field. This concept for predicting gas/water boundary movement was incorporated in a reservoir simulation model to carry out a single gas phase simulation of an existing aquifer gas storage field. The field history was accurately predicted by this simulation procedure. This approach is simple, easy to incorporate in a simulator code, and can utilize current operating history data to obtain necessary model parameters.