Gas Storage Reservoir Performance Optimization Through The Application Of Drainage And Imbibition Relative Permeability Data
Authors: Bietz R.F. , Bennion D.B. , Patterson J. 
Published: March 2010

Abstract
Gas storage reservoirs are utilized in many locations to seasonally store gas in summer months for periods of high demand in the winter. This generally involves the cyclic pressurization and depressurization of the reservoir on an annual basis. If the storage reservoir overlies an aquifer or residual oil leg, this may involve the cyclic motion of the gas-liquid contact and gas zone. This paper documents an extensive series of reservoir condition relative permeability experiments conducted on core material from a gas storage reservoir in southern Ontario to investigate the effects of cyclical pressurization and depressurization on reservoir performance. The tests illustrate how the cycling of applied net overburden pressure and the presence of trapped gas and water saturations in the dynamic gas-liquid transition zone can affect the production and injection of gas in the storage reservoir. Applications of the data, and extensions to field performance are also presented.

Introduction
For many years, natural underground geologic traps have been utilized for storing gas and liquid hydrocarbons; these reservoirs are usually developed from known hydrocarbon reservoirs which have since been abandoned. The primary application of storage reservoirs is in the accumulation and storage of natural gas during periods of low demand in the summer months so that a large localized source of gas is readily available for periods of increased demand in the winter. For this reason, the reservoir is subjected to annual cyclic pressurization and depressurization as the gas is stored and removed from the reservoir.
When an active aquifer or residual oil leg is present in the reservoir, these cyclic pressure fluctuations can result in an annual transgression and regression of the gas liquid contact. Encroachment of the liquid phase into the previously uninvaded portion of the reservoir is of interest to the operators of these fields because the residual saturation of the encroached fluid can have a profound effect on the deliverability and general performance of the contacted pore space. Experimentation and investigation of this phenomenon is possible through imbibition and drainage simulations to quantify the effects caused by two phase relative permeability relationships.
An additional phenomenon associated with reservoir pressure cycling is that of rock compressibility effects. The gross overburden of the field remains constant as the internal pore pressure of the system oscillates. This yields a variation in the magnitude of the rock compressibility while the net overburden pressure changes as a function of pore pressure. The complexity of this scenario is further complicated by the compression and expansion of the trapped gas phase in the liquid filled pores.
Hycal Energy Research Laboratories Ltd., in conjunction with Union Gas Limited, applied recent advancements in reservoir condition relative permeability techniques to conduct an extensive series of experiments to investigate the effects of these phenomena in the gas storage environment. Applications of the experimental data to field scenarios is also discussed.