A Novel Methodology for the Analysis of Well Test Responses in Gas Condensate Reservoirs

Single-phase analysis based on pseudopressure has been extensively used in the oil industry to analyse well test data from gas condensate reservoirs. However, the presence of the two-fluid system which forms when the pressure drops below the dew-point requires advanced techniques to derive accurate skin values. This quantity contains contributions from the mechanical, non-Darcy and liquid drop-out components. Many remedial well treatments (acidification, hydraulic fractures, etc) have been designed on such incorrect analysis, the resulting lack of treatment success leading to an unnecessary increase in operational cost. The aim of this study is to develop a full understanding of multiphase flow effects, to establish a systematic methodology using the two-phase pseudopressure function and to obtain accurate results from gas condensate well test analysis. This work uses the steady-state, two-phase pseudopressure in the same manner as the familiar single-phase pseudopressure analogue. A 1D compositional simulation model was used to generate the input data for the well test interpretation, special attention being paid to the time step and grid size distribution to avoid numerical truncation errors. The mechanical skin used varies from negative values (wells with fractures) to positive values (wells with formation damage). A correlation between the skin value obtained from the two-phase pseudopressure analysis and the mechanical skin was derived. This allows the reservoir engineers to make the key discriminations between mechanical skin and liquid drop-out skin. The approach used here was also extended to generate a general understanding of the production behaviour of condensate production wells e.g. fluid, velocity and relative permeability effects on the resulting pressure drops created by condensate accumulation near to the wellbore. This will find application to well design e.g. the conditions under which horizontal wells can eliminate a condensate bank near to the wellbore can be confidently predicted.