Acoustic anisotropy in rock formation

Introduction

For petroleum engineers to understand the environment that they work in, they need to comprehend rocks' physical and mechanical properties perfectly. Physical properties of rocks can include but are not limited to porosity, permeability, density, where the mechanical properties of rocks are elastic modulus, Poisson's ratio, rock strength, among many others. These properties can be obtained through lab experiments on rock core samples or through well logging and geophysical surveys.

Acoustic anisotropy

Rocks' mechanical properties affect a fundamental characteristic of rocks which is acoustic anisotropy. As defined in a research paper on "Untangling Acoustic Anisotropy," this is a directional variation in velocities with mainly two geological factors causing this phenomenon to occur: depositions and tectonic effects (Market. et al. 2015). Depositions, or layering, are usually referred to as intrinsic anisotropy, while tectonic effects, such as faults and fractures, are referred to as stress anisotropy (Market. et al. 2015).

History of acoustic anisotropy

For a long time, acoustic anisotropy has been described as 'black art' (Market & Tudge, 2017), which means there is so much mystery behind it. Even though the technology has been advancing by the day, this topic has not been getting the emphasis it needs from the industry, research, as well as the academia sectors, especially the latter. For a very long time, petroleum engineers have been assuming that the waves in the formation are propagating equally in all directions, meaning that they have an isotropic behavior (Armstrong, et al. 1995). However, it was noticed that waves travel through the rocks in different velocities as well as in different directions. Acoustic anisotropy has been largely ignored by exploration and production geophysicists as they were under the impression that the effect of the presence of acoustic anisotropy could be neglected. And this is precisely what describes the problem statement this research paper is mainly discussing. Acoustic anisotropy has variant applications in the industry such as fracture characterization, wellbore stability, production enhancement and optimization, seismic integration, and geosteering (Market, et al. 2015). The complexity behind the physics of acquiring and processing the data obtained for acoustic anisotropy is very high and challenging. Thus, it is vital to understand the theory behind it, which in return will aid petroleum engineers to apply and use the data gathered in different applications that will enhance the quality of the final outcome of well characterization and production optimization.