The stress sensitivity of shaley sandstones

The link between the stress sensitivity of shaley sandstones and their porosity and clay content is investigated. This is achieved by firstly fitting a compliance-based stress-sensitivity law to laboratory measurements of ultrasonic velocity taken from four sets of reservoir sandstones, extracted from a variety of depositional settings. Correlations are then sought between the independent parameters of this law and the porosity or clay fraction of the rocks, which are then subsequently interpreted in terms of framework or pore-space-related microstructural clay models. The general conclusion drawn from the results is that both of the parameters defining the stress-sensitivity law (the asymptotic modulus and the stress-dependent excess compliance) clearly vary with porosity. However, only the asymptotic modulus shows a convincing trend with clay and there is little observed variation of the stress-dependent compliance with clay. There is therefore a resultant variation of stress sensitivity with clay, but it is controlled only by the asymptotic modulus. The analysis also concludes that all four data sets fall into a framework-related category of clay model. © 2007 European Association of Geoscientists & Engineers.