Analysis of pressure and pressure derivative without type-curve matching — Skin and wellbore storage

Abstract

The current type-curve matching technique is essentially a trial-and-error procedure. A new technique for interpreting pressure tests using log-log plots of the pressure and pressure derivative versus time to calculate reservoir and well parameters without type-curve matching is presented. This paper concentrates on the interpretation of pressure tests in which wellbore storage and skin are present. Characteristic points are obtained of intersection of various straight line portions of the pressure and pressure derivative curve, slopes and starting times of these straight lines. These points, slopes and times are then used with appropriate equations to solve directly for permeability, wellbore storage and skin. A step-by-step procedure for calculating these parameters without type-curve matching for five different cases is included in the paper. The most important aspect of this new technique is its accuracy because it uses exact analytical solutions to calculate permeability, skin, and wellbore storage. The proposed technique is applicable to the interpretation of pressure buildup and drawdown tests and is illustrated by several numerical examples.