Thermal recovery processes can generate substantial porosity, permeability and mineralogical changes to carbonate rocks. Understanding these changes is critical to evaluate the success and safety of the process. Tests to evaluate these changes under steam soak conditions have been done on a carbonate bitumen reservoir rock. Thermal testing done in the presence of water at 265°C and 5.1 MPa (740 psi) under unstressed conditions for ~60 days showed considerable fracturing, dolomite dissolution, calcite precipitation, magnesium-clay formation, and carbon dioxide (CO2) generation above 240°C. Calcite precipitation in the matrix porosity and newly formed fractures was extensive. However, reservoir steaming conditions will be at lower temperature (180°C), pore pressure (1.0 MPa [145 psi], with primarily steam rather than water), and under confining stress (5.8 MPa [845 psi] horizontal, 6.7 MPa [970 psi] vertical). A 17-day steam-soak test in a triaxial pressure vessel under these reservoir conditions was done on an uncleaned core sample (~80% bitumen saturation) for comparison. Water/steam flowed through the sample at intervals indicated a relative permeability of 0.3 mD. At the end of the test the accessible (i.e., water filled) pore space was filled with epoxy under stressed conditions. The remaining bitumen was removed post-test before thin section manufacture. Thin sections from the post test sample were compared to thin sections made from untested end trims and adjacent core material, and to ones from the prior unconfined, higher temperature and fluid pressure testing. The sample tested under confined reservoir conditions also shows dolomite dissolution and neoformation of magnesium-clays. The amount of calcite precipitated is minor. However, calcium sulfate (CaSO4) (not observed in the pretested equivalent sample) appears as a by-product of the reaction. Newly formed open fractures are not observed in the test under lower temperature, lower pore pressure, confined conditions. This work highlights the importance of mimicking expected reservoir conditions (temperature, pressure, stress & steam) for evaluating thermal decomposition effects on reservoir rocks. Future tests will help to evaluate whether longer duration heating and flow at the lower expected temperature will produce similar reaction products and physical damage (including fracturing) as more bitumen is mobilized and larger surface areas are in contact with steam.


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