Modeling of borehole Stoneley waves in the presence of skin effects

Abstract

An important aspect of oil production from a geological formation is the process of flow into a well penetrating the formation. Recent laboratory experiments indicate that well-inflow can be significantly reduced by a skin effect caused by the presence of a thin layer of low permeability around the wellbore. This impairment layer is drilling induced and differs from the mudcake that is usually encountered when logging a well. Whereas a mudcake may have built up on the borehole wall during drilling, the drawdown pressure often removes it when oil is produced from the well. In contrast with such a temporary mudcake, the impairment layer is believed to form a permanent flow impedance throughout the lifetime of a well unless extensive cleaning measures are taken. A simple poro-elastic model is constructed to investigate the propagation of Stoneley waves in a borehole with a thin impairment layer. If the impairment layer affects Stoneley wave propagation, it may be possible to acoustically detect this layer with sonic logging tools. Using realistic values for the impairment flow impedance, model calculations for a typical sandstone reservoir of 20% porosity indicate that the impairment layer changes the dispersion characteristics of borehole Stoneley waves. The magnitude of these changes increases with increasing mobility of the reservoir pore fluid.