Manganese tetraoxide (Mn3O4) has been recently used as a weighting material for water-based drilling fluids (Al-Yami et al., 2007). Mn3O4 particles are spherical, 1-2 µm in diameter, and have a specific gravity of 4.8 g/cm3. A mud (102 ± 5 pcf) was developed to drill deep gas wells. The filter cake formed by this fluid contained polymers (starch, XC-polymer, and polyanionic cellulose polymers), Mn3O4 and a small amount of CaCO3. Unlike CaCO3, Mn3O4 is a strong oxidizer and, as a result, HCl not recommended to be used to remove the filter cake. The objective of this work is to develop a new cleaning fluid to effectively and safely remove the filter cake that contains large amounts of Mn3O4.

Various organic acids, chelating agents, enzymes, and a combination of these chemicals were tested up to 300 °F. Characterization of filter cake before and after soaking in several cleaning fluids were conducted using XRD/XRF/SEM techniques. Solubility of Mn3O4 was conducted using various acids and chelating agents at different temperatures up to 284 °F and 200 psi. The concentration of manganese in spent chemicals was measured using Inductivity Coupled Plasma (ICP).

Extensive lab testing indicated that Mn3O4 particles in the filter cake were coated by polymeric materials that acted as a barrier. The coating material reduced the ability of cleaning fluids to remove filter cake. The efficiency of cleaning fluid was improved by soaking filter cake in a starch specific enzyme, and then applying the cleaning fluids. HCl, citric, and in-situ lactic acid were found to be the most effective fluids in dissolving Mn3O4 particles. This paper will discuss the characteristics of filter cake and the effectiveness of various cleaning fluids. New cleaning fluids were designed to remove Mn3O4 filter cake, while maintaining the integrity of the formation and well tubulars.

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