Permeability and Acoustic Velocity Evolution of Ultratight Jurassic Anhydrite Caprock (Saudi Arabia) in Response to CO2 Exposure Under Reservoir Conditions: Implications to CO2 Storage

Saudi Arabia's Jurassic reservoir-caprock sequences play an important role for CO2 sequestration as they have the potential to store and seal large volumes of CO2 in saline aquifers. Containment longevity relies on caprock integrity, which is composed mainly of anhydrite layers with a thickness of several 10s of meters. To investigate some of the complex chemo-thermo-hydro-mechanical coupled processes associated with injection and assess risk for caprock failure, we conducted an experimental study that tests several Hith anhydrite core samples retrieved from a shallow, near outcrop well drilled close to the capital Riyadh. The goal is to observe how permeability evolves under in situ reservoir conditions as well as the presence of supercritical CO2. Initial gas filled anhydrite porosities are around 2% or less. Five specimens were tested either dry or brined saturated by flowing supercritical CO2 at two constant mean effective stress (σm) values. These varied from ambient to presumed in situ reservoir conditions (i.e., σm from 1,000 psi to 4,000 psi and constant temperature of 50°C). Each test lasted between 3 and 6 days while monitoring upstream/downstream flowrates, fluid pressures, differential pressure (AP), acoustic p-wave velocities and amplitudes, and calculated upstream/downstream permeability to ensure a steady state condition is achieved. The following summarizes our experimental observations: - initially permeabilities (in the nD to low mD range) change up to two orders of magnitudes due to CO2-mineral and CO2-fluid interactions; - rehydration reduces permeability; - compressional velocities and associated amplitudes provide a real time indication of CO2 saturation changes and its impact on rock moduli: velocities vary up to ±50 m/s (or ±1%); these relatively small variations suggest that saturation as well rock frame (matrix) changes within a week's testing period due to CO2 injection do not have a marked impact on the rock's elastic constitutive behavior; - given the small anhydrite porosity saturating its available pore space with brine accelerates acoustic energy transmission.