Impact of Stress and Equilibration Time on the Petrophysical Properties of Tight Gas Sandstones

Petrophysical properties (permeability, ultrasonic velocity and electrical properties) of tight gas sandstones are found to be more stress dependent at ambient conditions than at in-situ stresses. Analyses of the petrophysical properties measured at wide ranges of net stresses coupled with scanning electron microscopy (SEM) images indicate the presence of microfractures formed during or after drilling might play a major role on the stress dependency of the properties. Yet, the stress dependencies of the properties diminish as the net stress increases. This could be explained by partial or complete closure of the microfractures. Drawdown experiments were conducted to further analyse the stress dependency of permeability at reservoir conditions and to mimic the gas production from a tight gas sandstone reservoir. Results showed all the petrophysical properties were less stress dependent at higher net stress (>3000 psi) which was consistent with the theory of the microfracture closure. Drawdown tests (5000-7000 psi net stress) showed the least stress dependent of permeability.
Net stress is often defined as the difference between the confining pressure (Pcon) and the pore pressure (Pp) with the assumption that changes in pore pressure and confining pressure have an equal effect on the permeability. The introduction of an effective stress coefficient χ into the simple net stress (Pcon – χ Pp) provides better understanding and quantitative analyses of which pressure has more effect on the gas permeability. This was investigated by measuring the gas permeability under 12 different combinations of confining stress and pore pressure. Biot’s coefficients that are dependent on the elastic properties of the samples were calculated and correlated with the determined coefficient χ. The determined effective stress coefficients for permeability were less than unity indicating less effect of the change in the pore pressure compared to the change of the confining stress.
Comparison between capillary pressure data obtained from porous plate and vapour desorption methods using air-brine system and capillary pressure obtained from mercury injection (MICP) technique has shown discrepancies. Several factors identified as the key causes of the discrepancies are the equilibration time required for the porous plate and vapour desorption, the impact of injecting mercury on the delicate clay in the samples, the conversion factor of MICP to air-brine capillary pressure. The impact of stress on MICP measurements were analysed by conducting MICP under stress using a custom-built equipment. Results indicated capillary pressure was stress dependent similar to the other petrophysical properties. Also, permeabilities estimated from MICP models showed a better correlation with permeabilities measured at stresses equivalent to the displacement pressures of the samples, which confirmed the effect of stress on MICP measurements of the tight sandstones.