Effective porosity determination in clean/shaly formations from acoustic logs with applications

Reliable evaluation of porosity in clean and/or shaly formations requires knowledge of matrix (type and nature), fluid, shale volume and shale transit time affecting the determination of such parameter. If all are linked together in a compatible and quantitative way, good results for porosity are obtained. However, the objective assigned to this paper is to introduce an equation for estimating effective porosity in clean/shaly formations from sonic logs. This is considered as an acoustic-Vsh model that represents a few advantages over the other equations. In addition to being a simpler expression, it treats the shale effect as a correction term subtracted from the clean–sand term (i.e., the equation clearly points out the practical aspect of the shale effect and its correction.). Also, it gives reliable and acceptable values for porosity with Δt ranging between 70 to 110 μs/ft and Δtsh from 70 to 95 μs/ft (i.e., good results could be achieved with variable shale transit time). After being tested among a wide variety of samples representing different lithologies, the equation gives acceptable values of porosity with reasonable accuracy specifically where no porosity tools other than sonic logs are available. Successful application to real field data in the Gulf of Suez Basin of Egypt and Williston Basin in the USA, is also demonstrated.