Removal of filter cake generated by drilling fluids weighted with Mn3O4 particles may require strong acids such as HCl. Organic acids are not efficient with high density drilling fluids (> 100-Ib/ft3) that contain large amounts of weighting materials such as Mn3O4. A single stage of HCl treatment in a high temperature and corrosive environment cannot be used. In certain cases, a two-stage treatment to degrade polymers first, then solid particles in the filter cake is recommended.
The reaction of acids with solid particles in the drilling fluids was investigated as a function of time up to a reaction temperature up to 250°F. The solubility of filter cake was examined using an HP/HT filter press. A gas chromatography/mass spectrometry (GC/MS) technique was used to analyze produced gases. Atomic absorption was used to measure ion concentrations in spent acids, and X-ray diffraction was used to analyze solids.
A safe, efficient, and economic technique that involves mixing strong and weak acids was developed to dissolve the filter cake in one stage treatment. The results showed higher removal efficiency (> 85%). The solubility of Mn3O4-based filter cake using 10-wt% HCl at 250°F was 80-wt%, but the reaction released chlorine gas, a toxic and corrosive gas. However, at HCl concentrations below 5-wt%, the release of chlorine gas was eliminated but filter cake solubility was low. Lactic acid alone did not dissolve the filter cake. However, a combination of 4-wt% lactic acid and 1-wt% HCl dissolved Mn3O4 particles completely at 190ºF. At a temperature of 250ºF, nearly 85-wt% of the filter cake was dissolved.
This technique uses low concentrations of a strong and a weak acid, which will reduce the overall cost of the acids compared to using a two-stage treatment of enzyme/organic acid system. Using HCl alone is an inexpensive approach but HCl can not be used at concentrations above 5-wt% with Mn3O4-based filter cake.