Oil-based drilling fluids using a micronized ilmenite (5 µm) as weighting materials are proved to be suitable for HP/HT drilling operations with good rheological properties, low sag tendency, and low filtration volumes, even after heating at 400°F for 16 hours. The objective of this study is to assess the efficiency of hydrochloric acid to remove ilmenite oil-based filter cake, and therefore remediate any damage that might occur during its use in drilling operations.
An oil-based drilling fluid containing micronized ilmenite with 1.9 S.G. was used for filter cake formation. A HP/HT filter press was used for the removal process of ilmenite oil-based filter cake. Prior the acid removal, a mutual solvent (10 vol%) was used to break the oil-based filter cake for 2 hours. The purpose of the use of a mutual solvent is to remove the oil-soluble components and change the wettability of oil-based filter cake. Berea sandstone cores with an average porosity of 18 vol% and an average permeability of 50 md were used. CT scan, XRD, SEM, and chemical analysis were used to examine the reaction characteristics of different concentrations of acid with ilmenite oil-based filter cake. Coreflood tests with different concentrations of HCl were also conducted to provide recommendations on how to remove the internal damage caused by oil-based drilling fluid invasion during drilling operations.
Results showed that ilmenite oil-based filter cake can be successfully removed at 300°F after soaking with 5 wt% hydrochloric acid for 4 hours, even without causing much damage to the core permeability. The filter press test showed that the retained permeability of Berea sandstone cores was more than 90%. In the core flood tests, with the increment of the HCl concentration from 5 to 15 wt%, more internal damage could be removed. There was even some stimulation effect with 15 wt% HCl. This study will provide a complete evaluation of the efficiency of hydrochloric acid in removing ilmenite oil-based filter cake for HP/HT applications.