Experimental study of the gas permeability and bulk modulus of tight sandstone and changes in its pore structure

Tight sandstones samples from an Ordovician gas field in Algeria studied in this work are characterized by low connected porosity (below 10%) and low gas permeability (below 0.1 mD under ambient condition). Under confining pressure (up to 40 MPa) the permeability has decreased by more than 80% while the porosity goes down between 10% and 25%. Regarding the porous structure which is constituted of large angular pores connected by micro-cracks, the high stress-sensitivity of permeability is mainly the result of micro-crack closure. In addition, the decrease of porosity potentially involves porosity trapping up to a confining pressure of 20 MPa caused by the closure of certain cracks. This hypothesis is further supported by the pore volume variation test and poro-mechanical test. The resulting improvement in our understanding of these physical phenomena will be very useful in the forthcoming analysis of the combined impact of water saturation and confinement on the effective gas permeability of this type of sandstone.