Prediction Of Near Wellbore Formation Damage:A Case Study From Mnazi Bay Natural Gas Field,Tanzania

Formation damage in oil and gas industries impairs the natural permeability of reservoirs and adversely affects the productivity and injectivity of a well.The key indicators of well formation damage include skin formation near the wellbore region,permeability decline,and the decline of the well performance during production.In this thesis,comprehensive research on formation damage in the Mnazi Bay gas field has been done using mathematical models to predict the magnitude of the formation damage following an adverse decline of gas production(25.34 MMscf)observed in 2019 around all production wells(W-1 to W-5).However,this research has revealed that formation damage is a common occurrence in most field operations or different phases including drilling,completion,production,workover,stimulation,and enhanced oil recovery(EOR).in addition,the causes of formation damage during downhole activities and their mitigations in oil and gas fields have been the subject of numerous researches in oil and gas that is thesis compared different results from various researchers,this is because it is difficult to diagnose formation damage as it is related to several factors and can occur at any time and in any location of the wellbore during any operations.Therefore,different mechanisms have been described that contribute to the formation of near-wellbore damage.The mechanism can be characterized as a fluid-fluid incompatibility mechanism that occurs between mud filtrate and formation fluids.This mechanism has the potential to generate emulsion and slugs,which can block and plug the porous system consequently reducing gas and oil flow capacity.Such mechanism is rock-fluid incompatibility which is a chemical reaction between fluid and formation rock such as clay swelling or clay deflocculation which result from the plugging of the porous media causes formation damage.Another mechanism is solid materials from weighting agents such as bentonite together with the fines from drilling cuttings,can invade and plug the permeable zone of the formations causing significant formation damage leading to low production of oil and gas.The last mechanism is formation of phase trapping within the porous system which can affect the flow of oil or gas to the wellbore during production.Formation damage is contributed by one or more of these mechanisms,hence their effective control and quantification are necessary because attempting to restore formation damage is costly and time-consuming and,in many cases,lead to uncertain successes.However,formation damage depends on several factors,therefore,no single approach can accurately quantify the sort of damageTherefore,this study utilized the mathematical models such as Frick and Economides,and Furui et al.,model.The predicted results are compared to the field data from the W-test analysis.The results have shown that the Furui et al model recorded an average skin factor of 4.3 and flow efficiency of 53.93% which is close to the field data having a skin factor of 4.9 and flow efficiency of 50.56%.In addition,Frick and Economides’ s model has overestimated the formation damage for predicting an average flow efficiency of 63.01 % and underestimated the average skin factor of2.96.However,the study has established a new mathematical model which is the most adaptive model to be utilized in the Mnazi gas field to predict the magnitude of formation damage without conducting a well test analysis or any reference results.Furthermore,this research has detailed the mechanisms that influenced the formation damage to the gas field in a more comprehensive and intelligible manner.