The paper reports the results of an experimental study concerning the influence of confining pressure on gas permeability of sandstones having different mineralogical composition, with permeability values ranging from 0.0009 md to 200 md and taken from a maximum depth of 4400 m. Correlations among permeability variation, initial permeability, porosity and other properties of the samples are shown.
Increasing demand for natural gas along with depletion of gas reserves has raised considerable interest in the possibility of exploiting gas-bearing formations of low permeability (see Haas et al.(1987)). As a result, greater attention has been given in recent years to tight sandstone technology since it has become obvious that methods of exploiting tight formations must be improved. Of particular importance is better understanding of the factors that modify the permeability of these formations both so as to avoid using exploitation techniques that excessively reduce permeability and choose the most indicated laboratory methods that will provide reliable data on the effective permeability of a formation. In recent years numerous interesting experimental and theoretical studies have been carried out on the permeability of tight sandstones. The aim of these studies was to investigate why decrease of permeability due to a decrease in pore pressure during exploitation are much greater than in normal sandstones (see Jones (1975),Jones and Owens (1980), Gangi (1978), Walls et al.(1982), Ostensen (1983), Freeman and Bush (1983), Wei et al. (1986), Kilmer et al. (19871, etc.). The phenomenon is still far from being understood. This is why the author is presenting the preliminary results of research carried out at the Istituto di Scienze Minerarie into the extent to which changes in permeability can be the effect of variations in overburden pressure and temperature, the characteristics of the reservoir fluids as well as the mechanical, physical and mineralogical properties of the rock itself. This paper examines in particular the decrease in sandstones' gas permeability caused by changes in confining stresses. The permeability of the rocks examined varied under laboratory conditions within the range of 0.0009 - 200 md and the sandstones presented differing mineralogical characteristics. Tests were performed on oven-dried cores at 110°C and cores dried in a humidity controlled oven. Only isostatic stresses were applied to the cores. The effect of artificially induced fractures was also examined.
The apparatus used is suitable for measuring permeability to both gas and liquids with changing confining pressure (range 5 - 50 MPa) and temperature (range 10 - 150°C). Fig. 1 gives a simplified configuration of the equipment used to measure the permeability to gas. The core plugs are 2.5 - 5 cm long. They are cleaned in a solvent extractor, flushed with demineralized water to remove residual salt and then oven-dried at 110°C. The cores are then geometrically measured and weighted and their porosity determined under laboratory conditions. Tests on the oven-dried cores are carried out after each plug is wrapped with an overlapping layer of TEFLON.
