GEOCHEMICAL AND MECHANICAL EVOLUTION OF BLUFF AND ENTRADA SANDSTONE DUE TO CO2-FLUID-ROCK INTERACTION

The Bluff and Entrada Sandstone of the San Juan Basin in New Mexico are being considered as potential reservoirs for a large commercial CO₂ sequestration project. During injection, acidified brines enhance many fluid-rock interactions, causing mineralogical and structural changes that are responsible for modifications to hydromechanical properties and may affect injectivity and reservoir integrity. In siliciclastic reservoirs, dissolution of carbonate and chlorite cement and lithic framework grains are often responsible for these changes. To better understand the chemical, mechanical, and hydraulic sensitivity of the Bluff and Entrada Sandstone in the San Juan Basin to CO₂ storage, we conducted flow-through experiments on outcrop samples using both a CO₂-enriched or unenriched synthetic reservoir fluid, combined with forensic petrographic characterization, X-ray computed tomography, petrophysical analysis, and ultrasonic velocity measurements. The Bluff and Entrada Sandstone both contain disseminated sections of poikilotopic carbonate, quartz, and iron oxide cement, while the Bluff is also cemented with grain rimming illite/smectite. Results of flow-through experiments revealed increases in Ca, Mg, Fe, K, and Si in the downstream fluid of all experiments from the dissolution of carbonate and clay cement and silicate framework. Geochemical alterations modified porosity and caused permeability changes of 1 to >6 mD. Mineral saturations were modeled in Geochemist’s Workbench, aligning downstream fluid chemistry with petrographic observations and indicating potential precipitation of quartz, feldspars, carbonates, and clays on geologic time scales. Ultrasonic velocities decreased from 8 – 23% in samples BlfOC1 and EntOC1, which informs the extent of geochemically-induced mechanical degradation within the rock matrix. Together with future analysis of core, characterization performed in this study provides insight on injectivity, premature compaction, and wellbore instability in the Bluff and Entrada Sandstones in the San Juan Basin, and it extends our understanding of the degree to which siliciclastic reservoirs are chemically and mechanically robust during geologic carbon capture and storage (GCCS) operations.