This paper described the result of simulation study in naturally fractured reservoirs, when using underbalanced drilling (UBD). It has been recognized that short period of overbalanced can occur in UBD operation. The intention of this paper to investigate that nature of that damage in short overbalanced conditions during UBD operation. The model intends to study of OBD and time effect during UBD.A PC-based reservoir simulator, which free version is available 6000 cells is used to simulate overbalanced and underbalanced wellbore conditions. The model is characterized by very low matrix permeability (1 mD) and very high fracture permeability (5000 mD) and matrix pore volume. The model is formed by a number of matrix blocks (cubes) intersected by an orthogonal network of fractures. The matrix blocks permanently feed the fractures network. Both OBD and UBD wellbore condition were studied and productivity has been calculated for a various boundary conditions and reservoir parameters. Drilling operation (both UBD and OBD) is simulated in 10 days and followed by 50 days production at constant drawdown pressure. The effects of invading fluid are studied by employing production model with skin and without skin. The skin option represents the permanent damage around wellbore, where non-skin accounts the saturation changes around wellbore region that can be cured by production. For simulation of UBD case, no mud invasion is taking placed, however damage in UBD has arise as capillary forces observed liquid into the formation. The simulations study is summarized as following cases:
UBD with no influx during drilling (without reservoir inflow)
OBD with continuous filtration from the wellbore to the formation (with constant over pressure of 5, 50, 500 psi)
UBD with 72 minutes of over balance condition after the 7th drilling day (10 days for UBD operation).
OBD with change in rock-wettability around wellbore.
UBD with capillary effect.
Finally, the simulation results as productivity gained are discussed for all cases.