Using surface-downhole ERT for detecting contaminants in deep aquifers due to exploitation of oil reservoirs
The exploitation of a hydrocarbon reservoir, whether conventional or unconventional, may cause environmental damages. Even though sink wells used by oil companies are located much deeper than the aquifers, they may eventually contaminate the water with brine leakage, oil spill or other polluting agents, such as chemicals used in hydraulic fracturing. This makes vulnerability studies extremely important. In this work we aim to detect and characterize contamination of an aquifer that lies above an oil reservoir, at approximately 250 m depth, using electric current injected in the ground surface and voltage sensors installed in the well. At that depth, it is not usual to use downhole measurements to monitor the contaminated aquifer. This gave us the motivation to check if it was possible to identify the contamination of a part of the aquifer using surface-downhole measurements. In order to achieve this we have designed an electrical model of the reservoir including the aquifer with either a conductive or resistive contaminant such as saline brine, oil spill or CO2, and have numerically simulated the forward and inverse geoelectrical responses. Numerical results show that the detection of contamination is possible only with a surface-downhole configuration. Our results illustrate the advantage of using the electrical resistivity tomography configuration in order to detect and characterize the contaminated deep aquifers.