Analysis of Pressure and Pressure Derivatives Without Type-Curve Matching: I—Skin and Wellbore Storage

Abstract

ABSTRACT The current type-curve matching technique is essentially a trial-and-error procedure without any independent and accurate way of checking the validity of the end results. This paper introduces a new technique for interpreting pressure tests. This technique uses log-log plots of the pressure and pressure derivative versus time to calculate reservoir and well parameters WITHOUT type-curve matching. This paper concentrates on the interpretation of pressure tests in which wellbore storage and skin are present. The technique essentially consists of obtaining characteristic points 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 undoubtedly its accuracy because it uses exact analytical solutions to calculate permeability, skin, and wellbore storage. The second most important feature of this new technique is that it is verifiable. Any two parameters calculated from two independent equations corresponding to two different portions of the pressure derivative curve are verified by a third equation which corresponds to a known and unique point relating the two parameters. The proposed technique is applicable to the interpretation of pressure buildup and drawdown tests. This technique is illustrated by several numerical examples.