Role of thermal transfer on the corrosion of the concentrics used in wells producing Albien water
The Haoud-Berkaoui region comprises three producing oil fields: Haoud-Berkaoui, Benkahla and Guellala, all located 100 km west of Hassi-Messaoud (Algéria). Oil is recovered from this heterogeneous medium sized layer by injecting water under pressure into the geological formations; the water is extracted from the Albien aquifer by 15 producing wells. To improve exploitation conditions and to increase injection capacities of this water, concentrics were installed within the producing wells to introduce anti-corrosion products. The life time of these facilities, initially predicted to be five years, has been reduced to only two years; important water leaks as well as deterioration of the wellheads have been noticed. The concentrics in the producing wells were in a very advanced state of corrosion, with apparent holes. Chemical analyses of water samples taken from several wells showed that their ion concentrations varied from 107 to 258 mg•L‐1 for calcium, 73 to 139 mg•L‐1 for magnesium, 24 to 36 mg•L‐1 for potassium, 282 to 304 mg•L‐1 for sodium, 459 to 648 mg•L‐1 for chlorides, while bicarbonate concentrations varied from 116 to 176 mg•L‐1. The water from these wells is generally hard, rich in sulphates, but poor in dissolved CO2 gas and lacking H2S and oxygen. The pH was neutral or slightly basic with values ranging between 7.02 and 7.58. A temperature gradient (∆T) was measured along the depth of the well, variable with depth, with a maximum gradient of 16 °C. The temperature gradient profile showed that ∆T was small at the surface of the well and then increased considerably at 584 m depth. For depths between 584 m and 984 m, ∆T was somewhat attenuated but remained high. It is in this zone that the concentric was most affected by corrosion. The corrosion of the concentric in an Albien medium under heat transfer was examined using electro-chemical techniques. The rate of corrosion increased sharply with increasing temperature, regardless of the water quality (i.e., treated with anti-bacterial or anti-corrosion agents, or not). These thermal effects are attributed to the acceleration of kinetic exchange reactions between the metal and the solution and are mainly linked to the energetic contribution associated with the heat flux crossing the concentric by conduction and convection. The efficiency of the anti-corrosion treatments was evaluated using the equation % E = 100 (Vcorr-V’corr)/Vcorr, where Vcorr and V’corr are the rates of corrosion with and without treatment, respectively. The best treatment efficiency of 30 % was obtained at a zero temperature gradient. The total absence of FeS eliminated the possibility of bacterial corrosion; the sulfato-reducing bacterium effect was eliminated by the bactericidal treatment.