Anisotropy of Strength and Elastic Properties of Lower Paleozoic Shales from the Baltic Basin, Poland

Przemyslaw Michal Wilczynski,Andrzej Domonik,Pawel Lukaszewski

doi:10.3390/en14112995

Abstract

The paper presents the results of laboratory studies on the strength–strain properties of shales representing four siltstone-claystone lithostratigraphic units occurring in the Baltic Basin. Laboratory studies in a triaxial stress state were conducted as single failure tests on cylindrical samples oriented parallel and perpendicular to lamination within the rocks. Mutually perpendicular samples were cut out from the same drill core sections in order to determine mechanical anisotropy. Samples oriented parallel to lamination were characterised by values of the static Young’s modulus twice as high as from samples oriented perpendicular to lamination. Similar variability was observed in the case of maximum differential stress values and Poisson’s ratio. Samples parallel to lamination registered notably lower axial strains, which influenced increased values of Young’s modulus and Poisson’s ratio. The rocks studied are characterised by VTI type (vertical transverse isotropy) internal anisotropy of the rock matrix, which significantly influences the anisotropy of their geomechanical properties.

Highlights

In the last twenty years, the dynamic development of geomechanics has made a significant contribution to the research field of prospecting and exploitation of unconventional hydrocarbons [1]
The growing demand for hydrocarbons has caused significant intensification of geomechanical studies. These investigations are focused on determining the strength and strain parameters of the rocks building the reservoirs of unconventional gas and oil deposits
The results of geomechanical studies are used mainly for design of the most optimal process of hydraulic fracturing, indispensable for economically viable exploitation of gas from deposits characterised by very low permeability

Summary

Introduction

In the last twenty years, the dynamic development of geomechanics has made a significant contribution to the research field of prospecting and exploitation of unconventional hydrocarbons [1]. Results obtained during geomechanical studies allow for defining crucial parameters (strength, elastic moduli) for determining the optimal orientation of the horizontal section of the drilling [22,23,24], design of the hydraulic fracturing process [25,26], and assessment of borehole stability [27,28,29,30]. All these elements contribute to a better assessment of the effectiveness of making the deposit available and allow for more economically viable exploitation of natural gas accumulated in shale complexes. Understanding anisotropy and its causes is very important for the correct interpretation of seismic studies and microseismic monitoring [49,50,51,52,53]

Materials and Methods

Sample Preparation

Findings

Conclusions