ACOUSTIC PROPERTIES OF MUDERONG SHALE

Abstract

The presence of smectitic shales often causes serious problems for drillers in the form of both overpressures due to low permeability and wellbore stability problems resulting from the presence of swelling clays. 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 detailed physical properties and microstructure in relation to acoustic response of the Muderong Shale, a smectitic shale from the Carnarvon Basin on the Northwest Shelf of Australia. Porosity determined from acoustic or standard drying techniques is far in excess of that determined by both helium and mercury porosimetry, but correcting for interlayer water results in the values determined by the latter techniques. Physical properties are intimately tied to the acoustic response of the Muderong Shale at both sonic and ultrasonic frequencies. 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. Velocity variations from downhole shales in wells from the Carnarvon Basin can mainly be tied to smectite content but also to overpressure. 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. 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.