This study introduces a novel approach in the use of two-phase pseudo-pressures for the interpretation of gas condensate well test data in naturally fractured reservoirs.In a gas condensate reservoir, where the pressure falls below the dew point in the vicinity of a well, calculation of pseudo-pressures requires knowledge of the relationship between condensate saturation and pressure in all regions of the reservoir. An equation that relates condensate saturation to pressure is developed in this study for the region away from the well where condensation occurs but the condensate is immobile until its critical saturation is reached.A different equation, based on steady state flow, is presented for the region near the wellbore where the condensate is mobile. A single-phase, pseudo-pressure equation is used for the region of the reservoir further away from the well, where the pressure is still above the dew point pressure. This approach is verified using well test data generated with a compositional simulator, and real PVT data from a gas condensate field. Analysis of the computer-generated data yields dual porosity reservoir parameters and condensate saturation profile that are similar to the ones used in, or produced by simulation.It is also shown that the majority of the condensate dropout occurs in the fractures and not in the matrix in a dual porosity system and that the saturation profile in fracture media governs the build-up derivative curve.The amount of condensate in matrix has no effect on the derivative curve.
Introduction
Analyzing well test data is one of the most common methods for characterizing dual porosity reservoir parameters.But interpretation of the well test data in gas condensate reservoir, particulary in complicated geological reservoir (e.g. dual porosity reservoir) is not straightforward. When the bottomhole pressure of a gas condensate reservoir is reduced below the dew point, a bank of condensate is formed around the wellbore. In the early stages of this condensation, the condensate saturation is below its critical saturation, but after a relatively short period of time, at some point in the vicinity of the wellbore, the condensate becomes mobile and may move simultaneously with the gas.The presence of this condensate region around the wellbore decreases the mobility of gas and makes the pseudo-pressure derivative curve of a radial homogenous system behave like a "radial composite" one[1,2,3,4]. By using dry gas pseudo-pressures, some parameters in a gas condensate reservoir, for example the effective permeability, skin and radius of the condensate bank can be calculated approximately[5,6]. These parameters are important in the formulation of the production strategy and reservoir management of a gas condensate field. However, as the use of dry gas pseudo pressure diffusivity equations for condensate and gas are not linear, and the results of radial composite models are not unique[7], the aforementioned parameters calculated by this method are not reliable. Also, in some complicated geological reservoir behaviour; e.g., dual porosity reservoirs, the dry gas pseudo pressure method can not be used for interpretation of condensate bank effect. As far as authors are aware, there is no relevant publicated paper about analyzing condensate bank effect on the well test data in a dual porosity reservoir using dry gas pseudo pressure method. In the almost of the publicated papers, the effect of condensate bank due to the lack of PVT data or remaining bottomhole pressure in drawdown above dew point, have not been analysed or not observed in the build up well test data[8,9,10]. Aguilera[11] used two phase pseudo pressure for analyzing a set of well test data, which were generated using a simulator in a gas condensate naturally fractured reservoir. In this paper we have tried to address some points that authors felt were lacking in the Aguilera paper. These points are:Discussion about the effect of condensate bank on the buildup derivative curveUsing the relative permeability of the fracture instead of the matrix relative permeability in the application of two phase pseudo pressureSensitivity analysis on the effect of capillary pressure and inter porosity exchange between the matrix and the fracture.
