The objective of this paper is to evaluate the formation damage mechanisms on carbonate porous media due to the effect of solids (high-density Mn3O4 and BaSO4 weighting materials) and other particles dispersed in workover fluids. Barite (BaSO4) and Manganese Tetroxide (Mn3O4) are both highly dense compounds (4.5 – 4.8 g/cm3) with particle size of approximately 50 and 5 microns (μm) respectively, which can significantly impact permeability due to solids invasion.
A formation damage lab simulator was utilized to take selected core samples up to reservoir conditions. Initially, XRD/XRF and a baseline CT scan of the reservoir core plugs were conducted prior to core flooding. Differential pressures along core samples were measured at controlled flowrates during nitrogen gas flooding carried out before and after the workover fluid application and also, after removal of the filter cake formed by the workover fluid. Darcy's equation was used to calculate permeability values, and core plug CT scans post-floods were used to assist with the interpretation of the associated formation damage mechanism.
This study shows that the presence of solids inside the porous media physically plugging fluid pathways and a thick external filter cake due to high fluid filtration are the main mechanisms that contributed to the reduced return permeability observed on the core plug samples. The internal filter cake associated with the penetration of filtrate and solid particles into the pores had a greater effect on the observed permeability reduction compared to that of the external filter cake. The characteristics of the filter cake is strongly controlled by the mud particle type, size, and concentration. The combination of Manganese Tetroxide (Mn3O4) based workover fluid filter cake and the carbonate rock sample's face had low permeability causing a larger pressure drop and a lower productivity compared to the Barite (BaSO4) based filter cake in interaction with the same rock type. The permeability of the filter cake was lowered with decreased filtration.
