The presence of smectitic shales can cause problems for drillers in the form of wellbore stability problems resulting from the presence of swelling clays and overpressures due to low permeability. Prediction of overpressures in such sediments is hampered by interlayer water in the mineral structure that is not accounted for in porosity determinations whether by wireline methods or laboratory drying techniques. This study looks at the composition and microstructural response to stress of acoustic properties of the Muderong Shale, a regional seal in the Carnarvon Basin on the Northwest Shelf of Australia. Physical properties and composition are intimately tied to the acoustic response of the Muderong Shale at both sonic and ultrasonic frequencies. Velocity variations from downhole shales in wells from the Carnarvon Basin can mainly be tied to smectite content but also to overpressure. The presence of smectite results in anomalously low velocities in the Muderong Shale and the development of both compaction fabrics and induced microfracture generation result in significant acoustic anisotropy. The high degree of anisotropy of the Muderong Shale at ambient pressure can be related to the presence of both a sedimentary compaction fabric and high aspect ratio compliant microfractures. Variations in velocity and elastic constants can be related to the microstructural response to imposed stresses. Determination of the relative contributions of these two causes of anisotropy in shales may provide a method by which to distinguish overpressure-generating mechanisms from acoustic data.
Knowledge of the interrelationships between composition, microstructure, physical and acoustic properties of shales are vital to understanding propagation of seismic waves through thick shale sequences in sedimentary basins. Prediction of overpressure pre-drill in such lithologies is of obvious importance to the petroleum industry, although current methods of doing so are fraught with uncertainty. A problem yet to be addressed concerns the response of smectite-rich sediments to the passage of acoustic waves, as their behaviour appears to be anomalous in this and many other respects. The Muderong Shale, from the Carnarvon Basin on the Northwest Shelf of Australia, is often associated with severe drilling problems which have been related to both overpressure and borehole instability.
Shale microstructure affects acoustic properties such as velocity, amplitude, anisotropy, attenuation and dispersion. Few investigations of shale ultrasonic properties have included detailed microstructural investigations of the rocks in question and this omission increases the risk of incorrectly evaluating factors governing acoustic response. Some critical aspects of clay/shale microstructure in relation to these properties include1,2:
preferential alignment of both clay and silt sized particles;
the nature of the sediment framework and associated grain contacts;
the presence and scale of macroscopic and microscopic laminations in relation to wavelength;
the absolute abundance of high density mineral phases present; and
the nature, distribution and orientation of microfractures.