Arsenic Mobilization in Shallow Aquifers Due to CO2 and Brine Intrusion From CCUS Reservoirs

Abstract

CO₂ enhanced oil recovery (CO₂-EOR) is a proven technology for improved oil production. In addition to economic benefits, ancillary associated CO₂ storage is causing recent renewed interest in the technology. Most CO₂ –EOR targets exhibit not only excellent storage reservoirs bounded by effective seal layers, but also Underground Sources of Drinking Water (USDWs). Although risks to USDWs are definitively low, the potential leakage of CO₂, saline water and/or oil from the reservoir is a concern of commercial CO₂ -EOR projects. Understanding of CO₂-water-sediment interaction mechanisms in USDWs and effective uncertainty assessments provide maximum protection for USDWs is critical for current policy as well as likely future USDW-specific regulatory protections. The primary focus of this research was the potential for release and migration of heavy metals in USDWs. Specifically, we developed an integrated framework of combined batch experiments and reactive transport simulations to quantify CO₂-water-sediment interactions and arsenic (As) mobilization responses to CO₂ and/or saline water leakage into USDWs. Experimental and simulation results suggest that when CO₂ is introduced, pH drops immediately and initiates release of As from clay minerals. Calcite dissolution can increase pH slightly and cause As re-adsorption. Thus, the mineralogy of the USDW is ultimately a determining factor of the fate and transport of As and other such risks. Salient results of this research include: (1) Arsenic desorption/adsorption from/onto clay minerals is the major reaction controlling its mobilization, and clay minerals could mitigate As mobilization with surface complexation reactions; (2) dissolution of available calcite plays a critical role in buffering pH; (3) high salinity in general hinders As release from minerals; and (4) the magnitude and quantitative uncertainty of As mobilization are predicated on the values of reaction rates and surface area of calcite, adsorption surface areas and equilibrium constants of clay minerals, and cation exchange capacity. These results are intended to improve ability to quantify risks associated with potential leakage of reservoir fluids into shallow aquifers, in particular possible environmental impacts of As mobilization at CO₂ –EOR and other types of carbon sequestration sites.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018