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Direct measurements can be made with wireline formation testers at isolated points along the well, or on core, but these require additional logging runs and coring costs. Stoneley-wave analysis is a powerful technique that delivers a direct, continuous measurement of mobility along the well.1 7

6 0

The idea of measuring mobility from the Stoneley wave was first expressed in the 1970s, but proved difficult in practice. Many attempts have been made to develop empirical correla- tions

between permeability and attenuation, but these methods

Distance from B orehole Center

Caliper in.

0 1 6

G amma R ay gAP I

1 0 0 of M DT Tool 0 % P osition ft3 0

Compressional Slowness G radient

1 0 0

Distance from B orehole Center


Slowness Differential 1 0


Stoneley required

calibration with other information and neglected several important factors, such as mudcake permeability and the presence of the tool itself. Approaches that simplified the complex behavior of Stoneley waves were seldom successful, but an inversion method that uses a model derived from full Biot poroelastic theory reliably determines pore-fluid mobility from Stoneley waveforms.1 8 For application with Sonic Scanner data, the full Biot inversion technique was extended to incorporate tool response. The full Biot inversion scheme requires several borehole, mudcake and formation parameters to evaluate fluid mobility using Stoneley-wave data. The list includes: hole diameter; mud slowness, attenuation and density; formation P and S slowness, density and porosity; grain modulus; pore-fluid modulus and density; and mudcake density, bulk modulus, shear modulus, thickness and membrane stiffness. The computation outputs fluid mobility and associated error ranges.


This inversion technique has been available for several years, but application has not always been successful because the inversion requires extremely low-frequency Stoneley waves— down to 300 Hz. Data with this frequency content have not been available in the past, because earlier sonic tools interacted negatively with low- frequency signals and required filtering to remove frequencies below 1,5 00 Hz. Now, the wideband sources of the Sonic Scanner tool generate strong Stoneley waves with reliable low- frequency content for mobility calculations. An example from a Statoil well in the Haltenbanken area of the Norwegian Sea shows good correlation between mobilities calculated from Stoneley waves and those measured by MDT pretests. Input values of formation and fluid properties of a zone near the oil/water contact were determined with logs from the Platform Express integrated wireline logging

> A com pressional-w ave radial pro le indicating intervals of successful and risk y  uid sam pling. In interval A, the com pressional-w ave radial pro le

( Track 3 ) show s a sm all differential b etw een near-w ellb ore slow ness and far- eld slow ness. There is little near-w ellb ore alteration in the zone w here the MDT Modular Dy nam ics Form ation Tester successfully collected tw o form ation- uid sam ples. In Track 3 , the am ount of slow ness difference b etw een near and far  eld is indicated b y gold and b row n color intensity ,

w hile depth of alteration is indicated b y the horizontal length of the colored area. In interval B, the com pressional-w ave radial pro le show s dark er colors, indicating a higher degree of near-w ellb ore alteration ex tending farther aw ay from the b orehole. In this zone, the MDT prob e b ecam e plugged and w as not ab le to collect any form ation- uid sam ples. Track 2 display s the slow ness gradient ob tained from the tom ographic reconstruction. The gradient indicates the difference in slow ness from one slow ness- m odel cell to the nex t, m oving aw ay from the b orehole in sm all increm ents.

Spring 2006


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