• Sari, Mustafa
• Nguyen, David
• Mandal, Partha Pratim
• Esteban, Lionel
• Frery, Ema
• Sarout, Joel

Abstract

We evaluate the potential suitability of a rock salt formation in Western Australia for permanent underground waste disposal, or energy storage, e.g., natural/town gas, compressed air, or hydrogen. A suite of cores from a Devonian rock salt formation in the Canning basin are available for this research program, ranging from 600 to 1200m in depth (FRDDH001 well). Two contrasting rock salt facies were identified by visual inspection of the available core sections. Samples from various depths within these facies were successfully extracted to conduct a preliminary mineralogical, microstructural and petrophysical characterisation in the laboratory. Up to 14 MPa effective confining pressure, the deeper virtually pure and homogeneous halite salt (c.a. 1100m) exhibits a porosity of 0.2 to 0.3%, and a gas permeability of 1 to 20 mD; while the shallower dolomite/quartz-rich and heterogeneous rock salt (c.a. 802m) exhibits a porosity of 0.8 to 1.1%, and a gas permeability of 50 to 500 mD.In addition, a cylindrical sample plug from each salt facies (802m and 1100m depth) was subjected in the laboratory to four different confining pressures in a so-called multi-stage triaxial (MTXL) test in order to simulate the impact of depth, including the sample’s native/in-situ recovery depth (third stage of the MTXL), and that of anthropic stress perturbations at a given depth, e.g., drilling, mining, excavation. Considering a lithostatic stress gradient of 23 MPa/km, typical in sedimentary basins, and the ductile deformation of salt formations at depth over geological time scales, we anticipate the native stress state at depth to be approximately isotropic within the salt dome, with a native effective confining pressure of 18 and 25 MPa for the plug samples recovered at a depth of 802 and 1100m, respectively.At each stage of an MTXL test an increasing vertical differential stress is superimposed to the prevailing (constant) confining pressure in order to estimate the quasi-static Young’s modulus and Poisson’s ratio; the vertical stress is further increased, slightly beyond the elastic domain to evaluate the stress at which mechanical yield initiates. The yield envelope, Unconfined Compressive Strength (UCS), cohesion, and friction coefficient could also be estimated. For each stage of an MTXL test, after unloading to approximately 75% of the estimated yield stress value, the test is halted and the stress conditions are maintained constant, while monitoring (i) axial and radial deformation (creep) for up to 42 days to evaluate its impact on the long-term stability of the disposal/storage site and access/monitoring wellbores at depth; (ii) gas transmission rates for up to 400 hours to assess the impact of triaxial stress and creep on the sealing capacity of the rock salt formation at depth; and (iii) ultrasonic P- and S-wave velocities (dynamic Young’s modulus and Poisson’s ratio) to evaluate rock salt damage, and help interpret more robustly sonic logs or seismic surveys for site monitoring purposes.Finally, the MTXL creep and gas transmission data recorded for these two rock salt facies are also used to (i) model and extrapolate to the long-term creep rates for geomechanical modelling purposes; and (ii) estimate the relative change in gas permeability (in mD) with creep or triaxial stress conditions. Note that the ultrasonic wave velocity data can also be used to evaluate the contribution of brittle damage (in the form of intergranular gaps and/or micro-cracks) to rock salt creep and gas permeability.The data reported here are pivotal for robustly predicting the evolution of a potential disposal/storage site in this Devonian rock salt formation, considering (i) the contrasting properties of the two constituting salt facies; and (ii) some of the key time scales involved in designing, executing and operating such a site. The approach and methodology presented here can be applied to other potential sites where a feasibility analysis is sought.

Topics

• impedance spectroscopy
• molecular dynamics
• crack
• strength
• Hydrogen
• ultrasonic
• permeability
• isotropic
• porosity
• creep