Analytical calculation of velocity-dependent relative permeability from well test data in gas-condensate reservoirs, and its applications
Alabdulwahab, Ibrahim Mohammed
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Well testing analysis is a valuable tool that has evolved to become a tool that can be used to estimate formation permeability, skin, wellbore storage and reservoir pressure. The aim of the study presented in this thesis is to propose a set of techniques that utilise well test pressure buildup data to better characterise and manage gas-condensate (GC) reservoirs. The thesis starts by investigating the ability to calculate the pressure versus radius profile from the pressure versus time well test data under various condensate richness conditions using the probe radius concept proposed previously for single-phase oil conditions. First, the technique is validated in single-phase oil and single-phase gas systems before applying it to a more complex two-phase GC system. The results show how the pressure profile can be used to estimate the radial extent of the condensate bank. Then, the study proposes a method to analytically calculate the relative permeabilities (kr) affected by pressure and velocity due to coupling and inertia in single-rate and multi-rate tests using Darcy’s law and the aforementioned pressure profile. The results highlight the importance of velocity effects in the calculated kr data and show that ignoring such effects could drastically distort the estimation of reservoir parameters. The validity of these calculations is verified by comparing the results with the output from a numerical simulator based on a single-layer single-well radial reservoir model. Moreover, the importance of data treatment, such as smoothing, in order to obtain a reasonable and usable results is discussed. The study then explores other applications of the analytically obtained velocity-dependent kr data, more specifically for production data analysis based on the type-curve technique. In addition, the thesis suggests a new method to estimate the formation permeability of GC reservoirs by combining both methods, the kr calculation from well test data with the two-phase analysis of the production data, through an iterative process. The iterative method produced excellent permeability estimation within 5% margin of error. This can be beneficial in the case where reservoir permeability is unknown.