Comparative Experimental Study of High-Density Drilling and Workover Fluids

Abstract

Abstract Comparative assessment experimental study was conducted for high density NaBr and CaCl2/CaBr2 brines and water-drilling fluids based on manganese tetraoxide and barite as weighting agents. The main objective of this study were to evaluate the core damage caused by such drilling/workover fluids. XRD/XRF were used to determine the composition of the core plug samples. CT scan was utilized to assess the effect of workover fluids, workover brines, and acid recipe on fluid invasion profile. The mudloop system was exploited to expose core plugs to workover fluids and assess the effectiveness of treatment recipes in restoring core plug permeability at 280°F. Filter press was used to generate filter cake and assess fluid filtration rates. Solubility testing was conducted using HCl/formic and DTPA-based recipes with mud cakes generated at 280°F The solubility of the filter cake generated by water-based drilling fluids weighed with BaSO4, mixture of BaSO4\Mn3O4, and Mn3O4 were 40, 65, and 85 wt%, respectively in 5 wt% HCl/9 wt% formic acid mixture. DTPA-based recipe increased the barite-based filter cake solubility from 40 to 60 wt%. The observed reduction in dolomite core plug permeability was a stronger function of core plug characterization than drilling fluid type. The core damage was higher in heterogeneous core plugs samples. The core damage increased with drilling fluid filtrate volume, regardless of the drilling fluid type. Coreflood and CT scan testing indicated the examined manganese tetroxide water-based drilling fluid was less damaging to core plug samples than barite water-based drilling fluid. The factor controlling acid propagation into core plug was not reaction limited. Acid propagated in the flow direction where higher permeable steaks exist. Acid propagation and drilling fluid invasion profiles were a function of core plug characteristics. The core damage caused by CaCl2/CaBr2 brines were significantly higher than NaBr brine. The injection of low salinity brine removed the core damage caused by heavy brines (i.e., precipitated salts).