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.


Minimizing formation damage that occurs during conventional drilling is a critical point for optimizing an oil field development, especially in fractured carbonate reservoirs that often exhibit low matrix permeability. Drilling fluid invasion into the fractured formation can create severe formation damage around wellbore and reduce the productivity of the well and ultimate recovery of the field. Therefore minimizing of fluid invasion is very important in this type of reservoirs. The productivity benefits of underbalanced drilling are well known in the industry. When UBD is implemented correctly, it may considerably reduce or eliminate mud invasion into the fracture systems. Moreover, these field experiences have shown that UBD minimizes mud losses into the pay zone rock matrix. Even though UBD has many advantages over OBD, quantification of possible formation damage effects by comprehensive reservoir characterization and feasibility studies is central to judge the feasibility of UBD.

Different approaches to analyse the feasibility of UBD is reported in the literature. Ding et al.1 presented a model for evaluation of well performance by taking into account near-wellbore formation damage. This model applies laboratory data related to filter cake properties in analyzing formation damage for horizontal wells, using relative permeabilities to represent effects of formation damage due to polymer adsorption/retention, water blocking, etc. The model can be used to evaluate formation damage in heterogeneous media.

In the next work, Ding et al.2 presented a model of near wellbore formation damage in anisotropic media, which is particularly important for horizontal wells.

Leising and Rike3 have used an analytical model to estimate the productivity index (PI) for UBD and OBD cases, respectively. Different skin factors are applied, however without a feature to include the dynamic filtration process.

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