Abstract
Some features of the behavior of underground storage of natural gas in aquifers have shown the important influence of the modifications occurring in the distribution of fluids inside the pores of the reservoir space on the evolution of the gas bubble during the successive cycles of production. We have performed experimental work to study systematically the effects of alternate displacements of water and gas on the hydrodynamic characteristics of rock. We have measured the distribution of the liquid and gas phases in the porous medium and the corresponding permeabilities in relationship with the available capillary pressures. These parameters were examined during various combined parameters were examined during various combined cycles of drainage and imbibition. A schematic representation of the behavior of the porous medium, based on experimental results, is porous medium, based on experimental results, is presented It allows a satisfactory extrapolation for presented It allows a satisfactory extrapolation for the prediction of the evolution of the hydrodynamic rock characteristics in the case of a given history of alternated fluid displacements. The integration of this previsional scheme into the mathematical models used in reservoir engineering should be preceded by a preliminary study of the sensitivity of the reservoirs to certain parameters of the scheme in order to perform parameters of the scheme in order to perform eventual simplifications.
Introduction
Important developments in the study of underground gas storage facilities have shown a series of problems that, although not specific to gas storages, problems that, although not specific to gas storages, have acquired a renewed importance. Particularly important is the concept involving the evolution of the hydrodynamic characteristics of a rock submitted to multicycle two-phase displacements. Indeed it appears that a reliable prediction of the behavior of an underground gas storage in aquifer, submitted to a cyclical mode of exploitation, may be obtained only if it is recognized that, at every point in the storage, the relationships among point in the storage, the relationships among permeability, saturation and capillary pressure are permeability, saturation and capillary pressure are a function of the past history of fluid movements at this point. Based on results from experimental study, we attempt to define a theoretical experimental model that can predict the hydrodynamic characteristics of a porous formation. The model is designed to use a minimum number of measurements and to take into account all circumstances surrounding the evolution of the formation's hydrodynamic characteristics. This work is a continuation of an experimental study undertaken by Gaz de France and represents a further attempt to develop a greatly simplified model that can account for the above mentioned behavior. The result is the introduction in the model of a schematic representation, based on an interpretation of physical measurements that confirms the importance of hysteresis in the main hydrodynamic properties of a rock. The physical model thus developed will allow a satisfactory prediction of these characteristics. prediction of these characteristics.
EXPERIMENTAL METHOD
APPARATUS AND EXPERIMENTAL TECHNIQUE
Various combinations of drainage and imbibition cycles aimed at creating a series of situations resulting from the exploitation of a gas storage have been studied on two large-dimensional sandstone samples:a Vosges sandstone anda well consolidated Hassi R'Mel sandstone.
The experimental apparatus used is similar to that used by Rapoport and Leas. The pressure pw of be wetting phase initially saturating the sample is set by means of a constant level device located close to the upper end of the core and connected to the sample by semipermeable partitions:
(1) pw2 = Pa (atmospheric pressure) on the upperend of the core (output of the gas) .
SPEJ
P. 28
Finite Element Optimised Back Analysis of In Situ Stress Field and Stability Analysis of Shaft Wall in the Underground Gas Storage