This paper describes an experimental investigation of formation damage during underbalanced drilling (UBD) conditions in a fractured carbonate core sample. A major portion of this study has concentrated on problems, which are often associated with UBD and the development of a detailed protocol for proper design and execution of an UBD program. Formation damage effects have been studied, which may occur even if the underbalanced pressure condition is maintained 100% of the time during drilling operation. One major concern to formation damage during UBD operations is the loss of the underbalanced pressure condition. Hence, it becomes vital to evaluate the sensitivity of the formation to the effect of an overbalanced pulse situation. The paper investigates the effect of short pulse overbalance pressure during underbalanced conditions in a fractured chalk core sample. Special core tests using a specially designed core holder are conducted on the subject reservoir core. Both overbalance and underbalanced tests are conducted with four UBD drilling fluids. Core testing is including the initial permeability measurement and return permeability after two different conditions of pressure (underbalanced and overbalance). Then the procedure is followed by applying a differential pressure on the core samples to mimic the drawdown effect to present capacity of the return permeability. In both UBD and short pulse OBP four mud formulation are used which are, lab oil, Brine (3% KCL), Based mud (Bentointe with XC polymer) and fresh water. The return permeability measurements show that a lab oil system performed fairly well during UBD and short OB conditions. The results indicate that short overbalance pressure provides significant reduction in permeability of the fractured carbonate formations. In most tests, even application of a high drawdown pressure during production cannot restore the initial permeability more than 40%.

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