Water Saturation Estimation from Acoustic Logs

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Introduction

Accurate estimation of water saturation (Sw) is crucial for assessing the hydrocarbon potential of a reservoir. Traditionally dominated by resistivity measurements, recent advancements have opened the door to utilizing acoustic logs as a supplementary and, in some environments, a primary tool for Sw estimation. This page delves into the methodology behind using acoustic log data for water saturation estimation, highlighting its significance, and providing real-world examples for a comprehensive understanding.

– Application in Low Resistivity Pay Zones: In zones with low resistivity contrast between saturated and water-filled pores, acoustic logs become particularly valuable, offering an alternative means to estimate Sw.

Rock Physics Basis of New Sonic Log Based Method

In rock physics, it is widely acknowledged that when the fluid filling a reservoir’s pore spaces is primarily gas or light oil, there’s a significant shift in the bulk modulus, while the shear modulus remains relatively unchanged compared to fully brine-saturated reservoirs.
Note how this is evident in the example below of an offshore Gulf of Mexico oil reservoir with a water leg. While there is a significant change in bulk modulus with decreasing water saturation, there is very little to no change in the shear modulus. The bulk modulus in the plot ranges from 11.6 to 19.4 GPa, while the shear modulus ranges from 6 to 8.7 GPa over the same interval.

Note how this is also evident in the following example from the North Slope of Alaska, the Nuiqsut formation, a gas condensate sandstone reservoir. The range in the bulk modulus more than doubles that observed in the shear modulus. In addition, unlike bulk modulus, there is no apparent relation between shear modulus and water saturation, as in the Gulf of Mexico reservoir example.

This is responsible for the lower velocity ratio observed in gas or light oil reservoirs. This principle forms the basis and has been extensively utilized in identifying fluids in conventional siliciclastic reservoirs through seismic interpretation. As well as in qualitatively identifying the presence of hydrocarbons on sonic log data.
Using a proprietary algorithm, we extend this concept to not just identify but also quantify water saturation in gas and oil siliciclastic reservoirs. The new model does not require formation resistivity or brine salinity to estimate water saturation.

Check out the case studies below where we have applied the new model:

Nile Delta example, Egypt
North Malay basin example, Malaysia
Haynesville shale example, Harrison County, Texas, USA
Marcellus shale example, Pennsylvania, USA
Gas sand, offshore GOM, USA
Oil reservoir offshore GOM, USA
Gas condensate reservoir, North slope of Alaska, USA

Conclusion

The integration of acoustic logs in water saturation estimation represents a significant advancement in petrophysical analysis, particularly for challenging reservoirs. By understanding and applying the principles outlined herein, petrophysicists, geoscientists and engineers can enhance their reservoir characterization efforts, leading to more informed decision-making and optimized hydrocarbon recovery.

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Introduction

Rock Physics Basis of New Sonic Log Based Method

Check out the case studies below where we have applied the new model:

Conclusion

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