Modeling of borehole Stoneley waves in the presence of skin effects

An important aspect of oil production from a geological formation is the pr ocess of flow into a well penetrating the formation. Recent laboratory expe riments indicate that well-inflow can be significantly reduced by a skin ef fect caused by the presence of a thin layer of low permeability around the wellbore. This impairment layer is drilling induced and differs from the mu dcake that is usually encountered when logging a well. Whereas a mudcake ma y 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 how impedance throughout the lifetime of a well unless extensive cleaning m easures are taken. A simple pore-elastic model is constructed to investigat e the propagation of Stoneley waves in a borehole with a thin impairment la yer. If the impairment layer affects Stoneley wave propagation, it may be p ossible to acoustically detect this layer with sonic logging tools. Using r ealistic values for the impairment flow impedance, model calculations for a typical sandstone reservoir of 20% porosity indicate that the impairment l ayer changes the dispersion characteristics of borehole Stoneley waves. The magnitude of these changes increases with increasing mobility of the reser voir pore fluid. (C) 1999 Acoustical Society of America. [S0001-4966(99)001 01-0].