Using Transient Inflow Performance Relationships to Model the Dynamic Interaction Between Reservoir and Wellbore During Pressure Testing

Abstract

The fundamental understanding of the dynamic interactions between multiphase flow in the reservoir and that in the wellbore remains surprisingly weak. The classical way of dealing with these interactions is via inflow performance relationships (IPR’s), where the inflow from the reservoir is related to the pressure at the bottom of the well, which is a function of the multiphase flow behaviour in the well. Steady-state IPR’s are normally adopted, but their use may be erroneous when transient multiphase flow conditions occur. Transient multiphase flow in the wellbore causes problems in well test interpretation when the well is shut-in at surface and the bottomhole pressure is measured. Pressure build-up (PBU) data recorded during a test can be dominated by transient wellbore effects (e.g. phase change, flow reversal and re-entry of the denser phase into the producing zone), making it difficult to distinguish between true reservoir features and transient wellbore artefacts. This paper introduces a method to derive the transient IPR’s at bottomhole conditions in order to link the wellbore to the reservoir during PBU. A commercial numerical simulator was used to build a simplified reservoir model (single well, radial co-ordinates, homogeneous rock properties) using published data from a gas condensate field in the North Sea. In order to exclude wellbore effects from the investigation of the transient inflow from the reservoir, the simulation of the wellbore was omitted from the model. Rather than the traditional flow rate at surface conditions, bottomhole pressure was imposed to constrain the simulation. This procedure allowed the flow rate at the sand face to be different from zero during the early times of the PBU, even if the surface flow rate is equal to zero. As a result, a transient IPR at bottomhole conditions was obtained for the given field case and for a specific set of time intervals, time steps and bottomhole pressure. In order to validate the above simulation approach, a preliminary evaluation of the required experimental set-up was carried out. The set-up would allow the investigation of the dynamic interaction between the reservoir, the near-wellbore region and the well, represented by a pressured vessel, a cylindrical porous medium and a vertical pipe, respectively.