Analysis of Stress Dependent Permeability and Velocity: A Siltstone Core Analogue of a Tight-Gas Reservoir

Unconventional reservoirs are becoming important resources for the future as a result of the drop in production from conventional reservoirs. Tight gas reservoirs are an example of unconventional reservoirs which have very low porosity and permeability, and often they typically have large lateral extent. We analyse several Mam Tor siltstone samples as a means to characterise the stress dependence of tight gas reservoirs. Mainly, we study permeability, through loading and unloading cycles using triaxial core holder against very high effective stresses up to 8000 psi. Such loading tests provide ideal conditions for characterising tight gas reservoirs at great depths. We also study the stress sensitivity of partially brine saturated samples whereby stress sensitivity increases as brine saturation increases. We take this analysis further by measuring ultrasonic velocities against effective stress illustrating permeability-velocity relationship. We then substitute dry ultrasonic velocity measurements to get Gassmann predictions and compare it with ultrasonic velocities at various brine saturations. There is no significant increase in P-wave velocity at low brine saturation (less than 50%) compared to the increase in P-wave velocity at high brine saturations (greater than 50%). Consequently, Gassmann predictions fit lab data better at high brine saturations and over estimate velocities at low saturations.