Patent ID: 7830744

Claim:
A method for determining the permeability of a subsurface formation from sonic log data and well log data obtained from a well penetrating the formation, comprising: (a) analyzing the sonic data to extract frequency-dependent Stoneley wave attenuation for a selected sonic log receiver array comprising at least two receivers located at different depths in the well; (b) constructing a mathematical borehole model for the well; (c) programming a computer to solve wave motion equations for acoustic wave propagation from the selected sonic log source location to the receiver location, said wave equations representing a central mud region surrounded by the permeable formation with an annular mud cake region in between where and if mud cake exists; (d) determining boundary conditions from the borehole model; (e) obtaining all constants and parameters for the wave equations from the borehole model and the well log data or by otherwise estimating, except for the formation's permeability; (f) assuming a value for formation permeability κ; (g) solving the wave equations to obtain a solution corresponding to a Stoneley wave; (h) extracting from the solution a theoretical Stoneley wave attenuation as a function of frequency for the assumed value of formation permeability; (i) obtaining experimental Stoneley wave attenuation as a function of frequency from the sonic log data, wherein the experimental Stoneley wave attenuation, α 0 (ω), is given by α 0 ⁡ ( ω ) = 1 Q ST 0 ⁡ ( ω ) = 2 ⁢ α ~ 0 ⁡ ( ω ) ⁢ V ST 0 ⁡ ( ω ) ω where Q ST 0 (ω) is a frequency dependent quality factor, V ST 0 (ω) is experimental Stoneley wave velocity, ω is frequency, and ã 0 (ω) is a Stoneley wave attenuation factor; (j) comparing theoretical Stoneley wave attenuation to experimental Stonely wave attenuation; and (k) adjusting the assumed value of κ and repeating steps (g), (h), (j) and (k) until theoretical and experimental Stoneley wave attenuation values agree according to a predetermined criterion, the corresponding value of κ being a predicted value for formation permeability at a depth range corresponding to the interval covered by the selected receiver positions.