In addition to reducing formation damage and improving drilling performance, underbalanced drilling (UBD) can improve field productivity and recovery via real-time reservoir characterization. A UBD characterization tool to interpret the production associated with the drawdown applied during UBD has been developed using variable rate well-testing theory.

A new well that is drilled in a depleted field is likely to encounter differentially depleted zones. It is important to know what the pressures are so that an underbalanced condition can be maintained. Additionally, the identification of a permeability distribution is relatively easy when the pressures of the different zones are known independently. In classic well testing, only the average permeability and pressure of the whole formation is usually identified; but with UBD, it is possible to identify a permeability distribution, enabling the detection of high-permeability layers or other similar objects, such as fractures and compartments. Other critical aspects—such as production interval length, optimal well length and ways to eliminate/reduce well testing costs—also can be obtained when evaluating the data.

Many of the theories pertaining to this subject seek an integrated, transient model of the formation and the wellbore. In the approach adopted for this program, the problem may be decoupled with independent models for the two parts of the system. This paper will expand the methodology used and will present case studies and results.

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