Gas recovery potential of sandstones from tight gas reservoirs

• At 63–68% water saturation Sw, hydraulic cut-off is obtained under confinement Pc.
• At intermediate Sw (40%), gas relative permeability depends on Pc, possibly due to pore trapping.
• Increase in solid matrix bulk modulus Ks when Pc increases relative to micro-crack closure.
• Decrease in solid matrix bulk modulus Ks when Pc increases relative to pore trapping.
• Pore trapping is observed when pore volume variations are significantly higher than volumetric strains.

The aim of our experimental study is to characterize experimentally the petro-physical properties of a set of sandstones originating from different depths from a single tight gas field, in order to improve our knowledge on their gas recovery potential. The initial characterization of these sandstones is performed in the dry state, and in the “as received” states: porosity, initial water saturation level, and gas permeability at a confining pressure of 5 MPa. For two different samples under increasing confining pressure, we have evaluated the water saturation threshold, above which no more gas passes through the porous network, owing to hydraulic cut-off, to be about 63–68%. Then, at intermediate saturation level (on the order of 40%), two different sample types were identified, depending on whether their relative gas permeability is affected, or not, by confining pressure. For one sample of each type, poro-elastic experiments show that the variation in normalized matrix bulk modulus (due to confining pressure increase) may be attributed to the closure of portions of the connected pore network. Finally, to ascertain the above, a dedicated test was designed to measure the pore volume changes under confinement, simultaneously to volumetric strains. Whenever pore volume variation is significantly higher than volumetric strains, pore trapping is identified; otherwise, microstructure changes are mainly attributed to crack closure.