New perspectives in underground storage in depleted fields

The underground storage of natural gas is a procedure compatible with the optimal management of the transportation system. The amount of gas that exceeds consumption is stored underground in order to be extracted during high demand periods. Due to the structure of the gas market, greatly affected by heating gas consumption, this translates into a yearly storage cycle whereby gas is injected underground during the warmest periods of the year and retrieved in the coldest months. The fundamental parameters for storage are: • working gas, or maximum gas quantity that can be received during the ream cycle; • cushion gas, or the difference between total gas volume in place in a situation of maximum flood and working gas (although it represents a fixed quantity, cushion gas is necessary to keep reservoir pressure and to allow adequate flows during the ream cycle); • maximum gas flows allowed by the characteristics of the reservoir and the production units. In Italy, where storage is presently carried out only in semi-depleted gas reservoirs, it is not permitted, during the injection cycle, to exceed in reservoir the original static pressure recorded at the time of discovery. This practice, however, is beginning to be successfully adopted abroad. The storage pressure of the gas in reservoir is limited, from a technical point of view, by the ability of the cover rock to oppose the passage of the hydrocarbons and by its resistance to fracture. Particular attention is then paid to the aptitude of the storage cover rock in preventing the passage of gas. Such property depends, on the sample scale, on the rock's chemico-physical characteristics, particularly the capillarity properties. From an experimental point of view, this study was carried out on the basis of tests run on samples of formations of the Santerno Clay type, a cover rock common to many Italian storage fields. The samples belong to four fields located at depths between 1000 and 1650 metres sub sea level. The threshold displacement pressure of these samples was measured; that is the difference in pressure that must exist at the extremities of the sample initially saturated with water in order for it to let the gas through. This test is very burdensome, given the length of time necessary (up to several months) to perform it. The cost of the test and the need to extend the values obtained on the field scale make the possibility of correlating this parameter with the values of porous media, otherwise measurable, particularly attractive. Thus, the change of the threshold displacement pressure vis-à-vis the petrophysical (permeability and porosity), mineralogical, granulometrical and sedimentation characteristics of different samples was studied. Given the number and the varied nature of the values under consideration, the task, complex in itself, could not generate the possibility of correlating these values in reliable manner. The values of the threshold displacement pressure seem to be influenced by the content in clay minerals of the samples (Figure 1).