Conventional buildup analysis techniques are based on analytical solutions to the diffusivity equation that are subject to well-known assumptions. Single-fluid flow in a homogeneous reservoir is among them. During actual well tests multi-phase flow is often observed in several layers with different properties.

We investigated the applicability of the conventional pressure buildup analysis technique to the interpretation of complex cases involving flow of oil and gas in two layers communicating through the wellbore only. In this study we estimated the degree of error we should expect when conventional techniques are used. Our research shows that calculated values of average effective oil permeability and skin factor depend on a wide range of parameters, such as contrast in layer properties, production mode, production rate and production time prior to shut-in. We used a commercial black oil simulator with variable bubble-point to generate pressure and production data.

Several production forecasts were made using calculated values of permeability and skin in a single-layer reservoir model. A single-layer analogy produces acceptable results if the contrast in layer properties is not too great. If the contrast is high the forecasts generated by single-layer and two-layer reservoir models can differ significantly.

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