Calculation of Cementation Exponent and Multiplier Using P-Wave Velocity and Porosity for Sandstone Reservoirs

Abstract

The formation resistivity factor is important as reservoir property because it is required to calculate volume of hydrocarbon in reservoir rocks. It depends on Winsauer’s multiplier (a) which is a function of fluid tortuosity and cementation exponent (m). Cementation exponent is a major source of uncertainty in the calculation of hydrocarbon saturation for a given reservoir condition. A small error in the cementation exponent and/or multiplier could give a huge error in the formation factor and have serious consequences. The empirical relationships between reservoir parameters such as P-wave velocity and formation factor depend on local conditions such as environment of deposition, rock type, mineralogy, and diagenesis. For promoting hydrocarbon reservoir development; numerous petrophysical parameters are required. Some of them are measured but the others are sometimes assumed such as cementation exponent (m) and the multiplier (a) of Archie’s general equation. The present work offers some empirical models and charts to be used for resolving the cementation exponent and the Winsauer’s multiplier. Thirty-nine core samples obtained from the Upper Cretaceous reservoir zones; Gebel Nazzazat, Western central Sinai (Raha, Wata and Matulla formations) were laboratory analyzed for porosity, density, electrical resistivity, and ultrasonic wave velocity. They are composed mainly of clean sandstone, with some limestone and dolomitic limestone. This paper introduces seven innovative charts to estimate formation factor parameters using electrical quality index concept (EQI). They are classified according to the value of multiplier (a = 0.5 – 4.0) which is a function of electric tortuosity. The first chart estimates the multiplier (a) from P-wave velocity. The other six charts estimate the cementation exponent (from m = 1.0 up to 3.0) using fractional porosity and P-wave velocity at the previously determined multiplier (a). The present charts are based on a reliable empirical relationship (R2 = 0.82) connecting elastic-electrical parameters as formation resistivity factor and P-wave velocity. The model reliability verification is accomplished using 82 laboratory measurements of porosity and P-wave velocity for sandstone core samples from the Bahariya Formation side by side with 51 empirical estimated multiplier and cementation exponent for different lithofacies (sandstones and carbonates) and obtained from several geographic locations.