A Study of the Optimum Injection Rate of CO2-Saturated Brine in Limestone Aquifers and Its Impact on Rock Geomechanics

Abstract

Deep saline aquifers are promising locations for carbon sequestration, but the impact of the CO₂ injection rate on rock properties requires careful investigation. This study examines the effects of CO₂-saturated brine injection on limestone, specifically analyzing how the injection rate influences wormhole formation and subsequent changes in petrophysical and geomechanical properties. Various injection rates were employed to investigate the impact of the injection rate, ranging from 0.25 to 5 cm³/min, for the coreflooding experiments. Before and after coreflooding, the Young’s modulus (YM) and Poisson’s ratio of the samples were measured at various confining pressures. Additionally, the porosity and the permeability of the treated samples were assessed. A micro-CT scan was employed to visualize the generated wormholes and quantify their volumes to calculate the Damkohler number. Our study revealed that among the five tested injection rates 1 cm³/min resulted in the lowest pore volumes to breakthrough (PVBT). We employed the Buijse and Glasbergen model, curve-fitting our data to determine the optimum velocity for CO₂-saturated brine, which was found to be 0.4 cm/min (equivalent to an injection rate of 0.75 cm³/min). All samples showed a noticeable change in rock YM, with the rock exposed to a 5 cm³/min injection rate showing the least reduction. Furthermore, our experiments revealed that conventional methods for determining the optimum conditions for wormhole formation, such as the Buijse and Glasbergen model and the generalized curve using the Damkohler number, are insufficient for accurately predicting wormhole formation in CO₂-saturated brine. The experiments demonstrated a significant deviation from the expected behavior in terms of the generalized curve. Additionally, the Buijse and Glasbergen model overestimated PVBT at interstitial velocities lower than the predicted optimum. These findings underscore the need for alternative approaches to accurately predict wormholing phenomena, specifically in CO₂ storage applications, where the brine is saturated with CO₂.