Geochemical Characterization of Borehole Production Water in Southern Tunisia: Relationship with the Nature of the Reservoir and Environmental Quality of the Water Discharge

Petroleum production is constantly coupled to the production of large quantities of water, called formation waters, which are originally associated to hydrocarbons from a given oil reservoir. In this work, we propose to chemically characterize and environmentally assess the waters produced from boreholes in the regions of Jeffara and the Ghadames basin in southern Tunisia. For that, water samples from the TAGI reservoir, Triassic in age, the Acacus reservoir, Upper Silurian in age, and the Bir Ben Tartar reservoir, Ordovician in age, which are major siliciclastic reservoirs producing oil in southern Tunisia, were the subject of a thorough sampling. A total of twenty-eight borehole water samples were collected from the wellheads (17 from the TAGI reservoir, 7 from the Acacus reservoir and 4 from Bir Ben Tartar reservoir) and two samples were recovered from the Waha treatment centre. All samples were analyzed in the Petroleum Research and Development Centre (CRDP) ETAP using the ICP-AES method. The geochemical results and the chemical facies evaluation show a tight similarity between all waters recovered from different reservoirs with a predominance of the chloride-sodium facies. This is despite that the three reservoirs are not connected. This resemblance is probably due to the common marine origin of these formation waters. Nevertheless, waters recovered from the Acacus reservoir are characterized by a strong acidity and a very high salinity, which can reach 290 g/l. Produced waters are treated and discharged of traces of hydrocarbons (gas and liquid) and their solid loads in order to be reinjected into injection wells and the excess is injected into discharge wells. The quality of this water does not comply with the standards for discharge into the water environment. No produced water is currently discharged at the surface. Should the case arise in the future, the NT106.03 standards must be strictly obeyed. Potentially, excess production water from oil drilling can be recovered and adequately treated for use in agricultural irrigation networks.