Effect of CO 2 ${\rm CO}_2$ -Enhanced Oil Recovery on Wave Velocities in Upper Assam Basin

Abstract

This study examines the impact of ${\mathrm{CO}}_2$ flooding on $V_p$ and $V_s$ wave velocities in the Upper Assam Basin. Laboratory experiments were conducted on 15 consolidated sandstone cores from the Naharkatiya and Rudrasagar reservoirs ($>3000$ m depth) and one unconsolidated sand pack (lightweight proppant, Houston, USA). Samples, with porosities ranging from 8.03% to 47.00%, were saturated with ${n-C}_{16}{H}_{34}$ before ${\mathrm{CO}}_2$ injection.

Results indicate ${\mathrm{CO}}_2$ flooding reduces $V_p$ (compressional velocity, m/s) by 4–11% in consolidated samples and over 25% in the unconsolidated pack, with less pronounced but significant reductions in $V_s$ (shear velocity, m/s). These changes are influenced by pore pressure, porosity, and temperature. Higher pore pressure amplifies velocity reductions by increasing fluid density and altering the bulk modulus. In consolidated sandstones, velocity reductions diminish with increasing porosity, whereas unconsolidated sands exhibit greater sensitivity due to their lower bulk modulus.

Theoretical analyses confirm that $V_p$ reductions observed in laboratory settings are representative of in situ conditions. These findings highlight the value of time-lapse seismic monitoring in tracking ${\mathrm{CO}}_2$ plumes and optimizing enhanced oil recovery strategies. The integration of seismic techniques improves reservoir management while supporting carbon sequestration initiatives in the Upper Assam Basin and similar geological settings.