Calcium carbonate scale formation is a major drawback in oil or gas production wells, in geothermal processing equipments and in industrial cooling water installations. In most cases it reduces the overall fluid flow rate, induces formation or production damage and may lead to a shutdown of exploiting operations.
Various chemical inhibitors have been proposed to inhibit scale deposits. A major challenge is the evaluation of their action in near real operating conditions. Usually effectiveness is evaluated by the analysis of residual scale inhibitor in brines.
A novel approach is here proposed which uses an electrochemical technique to accelerate scale formation. Measurements on an electrode immersed in the tested water include as a function of time the decrease of the incident current (chronoamperometry) and the simultaneous increase of the weight recorded by a microbalance (chronoelectro-gravimetry) as a consequence of the progressive electrode surface coverage by scale deposit. By using a specific experimental device it is possible to follow and visualize in situ scale deposition or inhibition on a metallic surface under near-real hydrodynamic flow conditions.
Introduction
The precipitation of insoluble mineral deposits is a common cause of production damage in oil or gas production wells. Calcium carbonate scale formation is usually generated through up-hole degassing and loss of CO2 in producers exhibiting more than 70% water production. The natural solubility of Ca++ and HCO3- ion pairs is thus reduced by a pressure or temperature change in the flowing fluids. Calcium carbonate precipitation, contrary to other types of mineral scales such as sulfates of Ba, Ca or Sr, generally appears in the upper section of the production tubing and normally does not pose a threat to the actual formation. Major primary damages thus occur through filling of the production string and downstream through the inability of valves and safety equipments to function properly.
Various remedial actions were investigated. Originally chemical HCl acid wash and mechanical scrapping were used to alleviate the damage. In very severe cases, tubing replacement was used. These methods were labor intensive, economically unacceptable and did not address the long term mitigation of the problem. Furthermore in the case of acid dissolvers many drawbacks were observed such as corrosion of carbon steel equipments, formation wettability inversion and physical plugging by fines generated by dissolution of cementing minerals, by clay swelling and by water blocks1.
Since more than a decade threshold organic inhibitor treatments to prevent carbonate scale precipitation were proposed instead of concentrating on removal the scale after the fact. The major incentive was long term economic feasibility. Two methods of implementation of the inhibitor were considered : down-hole injection using high pressure low volume pumps and capillary tubing or inhibitor squeeze into the reservoir2. Although less risky down-hole continuous injection of inhibitor is found in most cases to be economically undesirable. The inhibitor precipitation squeeze is usually performed with a calcium overflush to cause precipitation inside of the formation of a 1% concentrated inhibitor solution. Slow release of the inhibitor in the production system is a function of its solubility. The risks of damaging irreversibly the formation are great. On the contrary the squeeze adsorption technique requires no calcium overflush, seems cheaper to perform and theoretically minimizes the chances of formation damage.
