Improving Well Productivity Post High-Density Mn3O4/BaSO4-Based Workover Fluids Formation Damage in HPHT Naturally Fractured Carbonate Gas Reservoirs

Abstract

High-density mud is typically considered for use in well kill during workover operations. Weighting agents such as barite (BaSO4) and (Mn3O4) are utilized to weight up drilling fluids during workover operations to provide the high-density requirement to balance the reservoir pressure with a safe margin and prevent unplanned and unwanted flow back of reservoir fluid into the wellbore in high pressure–high temperature (HPHT) naturally fractured carbonate gas reservoirs. However, these kill muds can result in formation damage and reduced well productivity post-workover causing the well to require stimulation treatment to restore its productivity. Treatment evaluation at the laboratory level was conducted to determine the effectiveness of two fluid recipes: acid- and chelating-based, to remediate formation damage caused by high-density barite (BaSO4) and manganese tetraoxide (Mn3O4) weighting materials-based workover fluids. A commercially available formation damage simulator was utilized to examine the impact of the workover and treatment fluids on reservoir core samples at downhole conditions. X-ray diffraction/X-ray fluorescence (XRD/XRF) analysis and a baseline computerized tomography (CT) scan of the reservoir core plugs were conducted before core flooding. Differential pressures along core samples were measured at controlled flow rates during nitrogen gas flooding before and after the workover (Mn3O4/BaSO4-based) and treatment fluids (acid- and chelating-based systems) application. Darcy's equation was used to calculate return permeability values and core plug CT scans post-floods were used to describe the productivity improvement mechanisms. Before treatment fluid selection for core-floods, the laboratory rock and fluid compatibility testing were conducted. The laboratory results obtained confirm both the acid- and chelating-based treatment fluids were able to improve the well productivity by fully restoring the core permeability with a negative skin by creating wormholes/pinholes throughout the carbonate core plugs and by dissolving/breaking down the filter cakes. This paper presents a systematic treatment solution formulation and laboratory-based testing of treatment fluid's effectiveness in restoring productivity of Mn3O4 and BaSO4-based workover fluids formation damaged wells.