Multiphase Pressure Buildup Analysis: Field Examples

Abstract

Abstract Multiphase pressure buildup tests from a West Coast Field were analyzed using a variable bubble point. black oil simulator. By history matching the observed pressure data, possible explanations were found for unusual buildup behavior, for which conventional analysis methods were ambiguous. Introduction It is a common practice to conduct pressure buildup tests to obtain reliable information about the producing formation and the condition of the wellbore. Analysis of transient well tests is usually based on analytical solutions of the radial diffusivity equation, where a major assumption is the flow of a single fluid with small compressibility. Perrine proposed a simple method to analyze pressure buildup tests when more than one phase is involved. Martin' later justified Perrine's method on a theoretical basis assuming the products of pressure and saturation gradients to be negligible. This method gained wide acceptance in the Petroleum Industry, since it allows the engineer to use single phase techniques after replacing mobility and compressibility by total mobility and compressibility of the reservoir fluids. The Perrine-Martin approach, although simple, has been investigated extensively and has been shown to yield misleading results under some circumstances. These circumstances are the consequences of the assumptions inherent in the method and usually include non-uniform saturation (and thus compressibility) distributions within the drainage area. Fetkovich used a pseudopressure function given by ................(1) which is similar to the real gas pseudopressure proposed by Al-Hussainy et al. With this function he demonstrated the similarities between isochronal tests conducted in oil and in gas wells. Assuming a linear relationship with pressure for the function .........(2) Fetkovich showed that the performance curve for an oil well follows the equation .............(3) which is frequently used for gas wells. Raghavan used this pseudopressure function to analyze multiphase drawdown and buildup tests. To calculate the pseudopressure function for drawdown, he used the instantaneous producing gas-oil ratio at the sandface to relate the pressure and saturation at the wellbore. This approach assumes a constant gas-oil ratio everywhere within the drainage radius of the well. The pseudopressure function for pressure buildup assumes that the gas-oil ratio is constant during the buildup and is equal to the producing gas-oil ratio at the instant of shut-in. Boe et al. proposed a method to predict the saturation change at the sandface from flowing pressure change. This method can be used to calculate the pseudopressure function, eliminating the use of producing gas-oil ratio at the instant of shut-in. P. 127^