This paper describes the results of an investigation of productivity and recovery improvement in naturally fractured reservoirs when using underbalanced drilling (UBD) technology in horizontal wells. A numerical simulator is used to simulate the results in terms of local changes in permeability and saturation of overbalanced and underbalanced wellbore conditions during the drilling operation. A single horizontal well model has been established to predict production rates for both UBD and OBD cases. The naturally fractured reservoir is simulated with a dual porosity, dual permeability model. Fine grids are used to capture dynamic effects of pressure and saturation in the near-wellbore region. To model the formation damage in the near wellbore region, an integrated approach is introduced that combines dynamic reservoir simulation and local formation damage based on dynamic core flood test data. The test results are interpreted to obtain an estimate of filter cake effect on filtration loss and impact of solid invasion on return permeability. The formation damage model is then used to generate a profile of saturation and pressure as initial conditions in the numerical simulator to model the production. The study is summarized as follows

  • UBD case with influx during drilling

  • OBD case with continuous filtration from the wellbore to the formation

  • UBD case with short time of overbalance condition.

The study results provide a set of criteria, which can be used as a reference and guidelines to consider UBD technology for horizontal well in naturally fractured reservoirs.

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