Shale properties impact significantly on exploration, development and production costs through the effect of seismic anisotropy on imaging and depth conversion and the role of shales in 4D seismic response, in addition to associated issues such as wellbore stability, seal integrity and pore pressure prediction. Shales are often not cored or preserved properly, rendering geomechanical testing all but impossible. The goals of the research reported here is to look to predict geomechanical properties of shales from more easily measured physical and petrophysical properties and to record the rock physics response of shales to the imposition of anisotropic stress fields. Ultrasonic tests were carried out to evaluate the full elastic tensor and its variation with stress. Ultrasonic tests evaluating the full elastic tensor on single shale core plugs show smooth responses in terms of velocity, elastic coefficients and anisotropy over a large stress range and are interpretable in terms of composition and the orientation of microfabric anisotropy with respect to stress anisotropy. Dielectric properties appear to be well correlated to both static and dynamic properties of shales. Some reasonable correlations were found between physical and geomechanical properties of shales, given their geologic variability in time and space.
1. INTRODUCTION
Shale properties are important from a petroleum industry perspective as inputs for basin models, seal evaluation, pore pressure prediction, for interpretation of 4D seismic response and with regard to wellbore stability and drillability of rocks. However, shale cores are rarely taken due to cost of acquisition and the perception that little value can be gained from knowledge of their properties. However, a recent study by [1] indicated savings of ~US$2.5M on one well through knowledge of shale properties and given that the field had a further 50 wells to drill, total savings would be in excess of US$100M! In recent years, some data have become available detailing shale physical properties (e.g. [2]) and other studies have evaluated the influence of diagenesis, compaction and lithology on geomechanical properties of shales [3-7]. Shale properties have been shown to be sensitive to factors such as composition, organic content, pore pressure and stress history. However, shale cores are rarely recovered and even when they are, rarely are they preserved during storage to ensure no loss of pore fluid which can alter rock properties through desiccation and fracturing. As such, sparse information is available concerning physical properties and geomechanical behaviour of these rocks. Some of the aforementioned shortcomings (lack of core and/or preservation) have been overcome through preservation of selected shale cores for this project, which have allowed laboratory evaluation of the static and dynamic properties of these shales. Brief objectives of the study can be summarised as follows:
Characterization of preserved shales with regard to composition, cation exchange capacity, pore size distribution, porosity, grain size and geomechanical properties.
A series of geomechanical tests on preserved shale cores to determine the failure behaviour of a range of caprock lithologies.
Correlate laboratory-measured geomechanical properties of well-characterized shales with petrophysical and physical properties.
