Reservoir formation damage is the impairment of the reservoir rock permeability which causes reduction in the productivity of a well. The quantification of wellbore formation damage using pressure transient analysis (PTA) is vital in the oil and gas industry as it helps to interpret key physical reservoir parameters to improve wellbore and reservoir management strategies for production optimization. In this paper, a single rate build-up test data generated from a vertical producing oil WELL X was analysed. Pressure transient was due to the pressure decline in the high permeability Niger Delta sandstone formation. Reduction in productivity index may have been caused by wellbore or formation damage as a result of pseudoskin due to completion (mechanical skin) or perforation, fines migration, inorganic scale deposition, or organic solids deposition in the near wellbore region. In order to resolve this problem and optimize production from this well, generated build-up pressure time data at a constant production rate was analysed to determine reservoir and wellbore parameters. The damage in the near wellbore zone is simulated using a numerical well test simulator (KAPPA-saphir); and the same build-up test data was analysed using the conventional method alongside with Microsoft Excel Sheet to estimate the reservoir and wellbore parameters like skin, average reservoir permeability, wellbore storage effect, and average reservoir pressure. The results obtained from the numerical well test simulator, and analytical solutions using Microsoft Excel Sheet were compared to ascertain the effectiveness of the numerical simulator in conducting pressure transient analysis on the measured data. Both analysis on the generated build-up test data showed close results for skin and permeability value, hence, the very high skin and permeability value obtained after analysis indicate damage due to mechanical or wellbore skin. This reveals that the vertical oil producing well of this field is a candidate well for workover operations

Keywords : Reservoir Formation Damage, Wellbore, Production, Skin, Permeability, Numerical Simulator, BuildUp Test, Pressure Transient Analysis, Workover Operation