Influence of the Viscoelastic Properties of the Overlying Rocks on the Behaviour of the Deep Aquifers

The pressure variations in the aquifers, consequent on the production, of an oil or gas field or on gas injection for the production, of an oil or gas field or on gas injection for the underground storage, causes variations of the tensions being both in the permeable medium and in the overlying strata. This fact, as we know from the literature about this argument, causes both permeability and porosity variations of the medium, influencing therefore the water influx, and remarcable deformations in the enclosing rocks which might cause subsidence phenomena too. In the note, we are going to present, the phenomenon is studied in all these aspects through the Finite Elements Method, taking in consideration the results obtained in the Mining Science Institute of the Bologna University. The behaviour of the water-bearing stratum and of the overlying rocks was namely studied unitedly. After dividing properly, the medium into finite elements, both the pore properly, the medium into finite elements, both the pore pressure distribution and the consequent tension variations pressure distribution and the consequent tension variations as well as the deformations through the time, were studied at the same time through an iterative process; the viscoelastic medium was represented according to the Kelvin-Voigt generalized model. Introduction 1. The developing of the reservoir engineering compelled the researchers to the requirement to always know more thoroughly the characteristics of fluids and reservoir rocks. As far as reservoir rocks are concerned, it has been thought to be necessary, already since many years, to take into account their compressibility and the variations of their physical characteristics when reservoir pressure changes, especially in the case of liquid one phase expansion; on this subject several studies have been performed. From these researches the importance aroused to wholy study the phenomenon, by taking into account not only reservoir rocks or the aquifer, but also all surrounding rocks. As regards the above said, the important conclusions reached by Geertsma, Van der Knaap and Van Vlis, and Evangelisti and Poggi are mainly aimed to detect the subsidence phenomena. Under this note, the attention is particularly called upon the influence of a variation of the reservoir pressure both on the whole medium strain and on the fluid influx in case of overlying rocks with a viscoelastic behaviour. To this purpose, we took as a basis the results obtained from the research in course at the Instituto di Scienze Minerariein Bologna, performed by means of funds from Consiglio Nazionale delle Ricerche.