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Technical Paper: Permeability Determination from Stoneley Waves in Gas Saturated Sands, India: A Case Study

Society: SPWLA
Paper Number: 2008 SPWLA Stoneley Waves India
Presentation Date: 2008
 

Abstract

Determination of permeability using empirical core-log correlations has been applied in reservoir characterization of an Indian offshore field having complex lithological environment. With the advent of latest logging techniques, it has also become possible to infer continuous in-situ values of permeability to reduce the uncertainty. One such method uses Stoneley waveform inversion through full Biot Theory. This study demonstrates the application of Stoneley waveforms acquired by advanced sonic technology with high fidelity over a wide frequency range to estimate permeability in gas-saturated sands in an OBM well of Western Offshore, India. As conventional transforms were not enough to capture permeability distribution in the field, the Stoneley permeability estimation played a decisive role in optimizing the selection of zones for completion.

Permeability influences both Stoneley wave slowness and attenuation as a function of frequency, a phenomenon well described by Biot theory. Increased permeability leads to increased dispersion and attenuation. In the method used, wide band Stoneley waveforms were back propagated in the frequency domain with generated dispersion curves. The fluid mobility was determined using a least mean square error method in the inversion technique. The process uses the complex phase slowness over a wide frequency range for optimum results. The membrane impedance, one of the important factors in mobility computation, was characterized in detail. The effect of the sonic tool presence in the borehole is negligible using the advanced sonic tool design.

Based on the estimated permeability in pre-drill development well from conventional method and core data, it was planned to complete three gas sands in development well with Cased Hole Gravel Pack (CHGP) for optimized production. However, the Stoneley wave analysis indicated higher permeability in the gas sands showing great improvements in well deliverability. As a result, only two intervals were taken up for completion saving the equipment cost and time while attaining the desired well deliverability. This study validated the reliability of the Stoneley method using advanced sonic technology to evaluate the in-situ formation permeability in both high and low permeable gas saturated sands.

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