High Resolution Acoustic Analysis for Improved Formation Evaluation of Carbonate and Clastic Reservoirs

Abstract

Developing new interpretation methods in line with new technology measurements to improve reservoir characterization becomes a must to overcome the challenges that petrophysicists are facing on a daily basis, among others, thinly bedded reservoirs. The standard acoustic logs vertical resolution (VR) is oftentimes insufficient to resolve the Formation features. In the case of laminated thin beds (LTB), conventional petrophysical characterization has often delivered results, such as hydrocarbon saturation, affected by uncertainties. The main objective of this work is to develop a methodology, and to reliably integrate high-resolution acoustic processing outputs into the Formation evaluation (FE) routines, including the quality controls to identify and remove artifacts and ultimately enhance the accuracy of reservoir characterization. The approach used in this study is based on the high-resolution acoustic slowness analysis. The measurement standard VR of the acoustic array measurement is determined by the array aperture (the length of the receiver’s array section; generally 3.5 ft), which tends to obscure features that are thinner than the aperture length. The configuration of the highest resolution is 0.5 ft aperture subarray combination, which considers only two receiver levels in the subarray, i.e., single Rx-Rx spacing. The processing has limitations, inherent to physics; one of them is the "Validity condition". The high-resolution acoustic slowness methodology cannot be applied to acoustic waveform arrivals with very long wavelengths. The subarray aperture should be greater than a quarter wavelength of the wave being used. In addition, as increasing the resolution goes hand in hand with decreasing the number of waveforms, the signal-to-noise ratio decreases. There are several quality control and consistency check workflows to be introduced in this study, which will help in differentiating between the real high-resolution acoustic slowness data and any encountered processing artifacts. These analyses will enable the right evaluation strategy that will enhance the accuracy of FE. Examples highlighting the value of high-resolution acoustic processing/analysis for both carbonate and clastic reservoirs evaluation are provided in the paper. Finally, this approach will evaluate the potential added value to the FE of the heterogeneous reservoirs. We propose workflows in various Formations and borehole settings to demonstrate that with the aid of enhanced VR acoustic data, the log interpretation becomes more representative of the actual subsurface. The resulting sonic profiles show better consistency with other log responses of similar resolution. These workflows then can be used on a more regular basis, especially in complex situations.