Porosity form sonic log in gas-bearing shaly sandstones: field data versus empirical equations

Abstract

The effect of clay content on elastic wave velocity is an important factor which should be considered in porosity estimation from sonic log data. Over the past ten years several investigators have reported experimental results of ultrasonic P-wave velocity measurements in clastic rocks and a number of empirical relationships have been proposed for velocity as a function of porosity and clay content for sandstones. These studies provide a better understanding of the complex nature of elastic wave velocities in reservoir rocks. However the practical aspects of applying the empirical equations derived from these studies have not been examined. This paper briefly reviews some of the most often-cited empirical velocity/porosity equations and compares them with commonly used sonic porosity methods for a series of gas-bearing shaly sandstones from the Cooper Basin in South Australia. None of these empirical equations present a globally acceptable method for porosity estimation from the sonic log. The shortcomings of these equations are more noticeable when they are used for gas-bearing reservoirs in shaly sandstones with low to moderate porosities. The application of some of these equations to field data in the Cooper Basin implies that their reliability when used on other datasets should be considered suspect until a good calibration can be achieved.