Contamination risk and geophysical signature of abandoned wells in the gulf coast aquifer system, Texas

Over the years, many oil and gas wells have been abandoned, without proper plugging, around homes, farms, industrial sites, and urban areas. These abandoned wells are potentially becoming pathways for groundwater contamination through short-circuiting between aquifer units. The primary goal of this research is to create a risk map for the Texas Gulf Coast Aquifer (GCA, area: 108 x 103 km2) showing locations that are prone to groundwater contamination due to the presence of abandoned oil and gas wells. For this purpose, a Generalized Linear Regression model (GLR) was constructed and calibrated using the Geographic Information Systems (GIS) environment. Model inputs included well locations, surface lithology, locations of petroleum storage tanks, superfund points, wastewater outfalls, landfill sites, surface reservoirs, slope, stream density, and rainfall rates. Some of the abandoned wells might be buried underground over the years, identifying these wells is important to plug and eliminate their risk of contamination. To help identifying the buried wells, different geophysical datasets (magnetic, electromagnetic) were acquired and processed to explore their geophysical signature. The geophysical data was collected at the TAMU-CC's geophysical test site, where multiple well covers and steel drums were installed at various depths. Modelling results indicated that 11.53% of the total area of the GCA region is under high risk of groundwater contamination, whereas 43.79% of the area is at moderate risk, 31.05% of the area is at low risk, and 13.61% of the area is at no risk. The high-risk zones are mostly concentrated in the central part of the GCA region (Liberty, Jefferson, Jackson, Live Oak, Zapata, Washington, Calhoun, Chamber, and McMullen counties) and the risk-free zones in the southern part (Duval, Jim Hogg, Webb, Kenedy, Brooks, Lavaca, and Polk counties). Geophysical investigations indicated that both magnetic and electromagnetic anomalies were mainly generated, and correlated with the locations of, the buried drums (45,450–45,700 nT for magnetic, 226–254 mS/m for electromagnetic). Relatively lower magnetic and electromagnetic anomalies are associated with the locations of the well covers (45,200–45,400 nT for magnetic, 210 –222 mS/m for electromagnetic). Our research results could be utilized by decision makers to develop enhanced mitigation scenarios for risks associated with abandoned wells in Texas and around the globe.