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Reservoir inflow in underbalanced drilling
In underbalanced drilling (UBD), as soon as the bit penetrates the reservoir, reservoir fluids start to flow into the wellbore. At this stage, the stabilized multiphase flow regime in the well prior to reservoir fluid entry must be adjusted to account for inflow and minimize the impact to the circulation system or moving out of the UBD window already established.
Factors influencing reservoir inflow
The rate of reservoir fluid inflow depends, in part, on the drawdown and reservoir rock properties (the differential pressure between circulating bottomhole pressure (BHP) and reservoir pressure). There are a number of models that can be used to estimate the reservoir fluid inflow based on the rock and fluid parameters. However, the reservoir rock properties are fixed, and the only variable is the drawdown to control reservoir fluid inflow.
Reservoir inflow performance
The inflow performance of a well represents the ability of the reservoir to produce fluids under a given condition of drawdown. The reservoir fluid inflow performance is the most important parameter in UBD, operationally and economically, because of its impact on well production and the safety operating envelope.
The intent of drilling any well underbalanced is to:
- Create conditions that induce the flow of reservoir fluid into the well while drilling
- Minimize reservoir damage
- Optimize production of reservoir fluid from the well
- Characterize the reservoir
Therefore, the relationship between the BHP and reservoir inflow is one of the most important parameters in UBD design and management. It is important that the BHP and reservoir inflow rate are managed and maintained within the defined operating envelope and flow control matrix as defined by the well control strategy. Where the surface pressure, production rate, or BHP cannot be maintained within appropriate well control levels or underbalanced, drilling operations must cease immediately and pre-defined well control actions are to be taken.