In order to effectively analyze reservoir inflow data from a well drilled underbalanced, a true multilayer reservoir model is required. In a previous paper,1  a single layer analytical model was developed to handle reservoir inflow resulting from the changing wellbore geometry during the underbalanced drilling process. A method to analyze data acquired during underbalanced drilling for reservoir permeability was developed and presented. The resulting reservoir model was shown to be unique in the industry. However, it was recognized that a multilayer model was needed to fully analyze the data and generate a more complete reservoir evaluation during underbalanced drilling.

This paper presents a true multilayer model that describes inflow from multiple reservoir layers encountered during underbalanced drilling. The model is fully transient and is based on the single layer model presented in previous work. The model accounts for the lengthening of the wellbore through any number of reservoir layers having similar or different properties and the resulting flow into the wellbore. Results from validation of the model are presented. The model enables formation evaluation during the underbalanced drilling process.

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