The most prominent factor in using UBD is to eliminate or minimize formation damage provided that underbalanced condition maintained 100% of the time during drilling operation. However, during a UBD operation, it is difficult to guarantee that the drilling fluid pressure is always maintained in underbalanced condition. A series of dynamic filtration and return permeability tests have been conducted using reservoir sensitivity core flood apparatus to investigate the permeability alteration due to transition of UBD condition to overbalance one during UBD operation. The results evinced significant permeability reduction when diesel was used as a drilling fluid without any extra additives, due to lack of mud cake and consequently invasion of fine particles existing in the diesel used as drilling fluid for UBD operation into the core samples. Results were plotted in terms of permeability versus diesel volume lost into the formation per square inch of wellbore wall and a linear pattern was found for permeability reduction. So, better plan of UBD operations is necessary for the industry to avoid undesired permeability reductions and it is crucial to develop ways to have an underbalanced condition all the time while drilling and to use filtered diesel or diesel with low solid particle content as a UBD drilling fluid, knowing that usually we do not use loss control materials in UBD drilling fluids. In addition, completion should be planned to be conducted without exposing the formation to overbalanced pressure.

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