Acoustic P-Wave And S-Wave Logging for Shaly Sand Saturation
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Solution Overview
Problem
Conventional methods for determining hydrocarbon saturation in subsurface formations, such as the Archie saturation model and NMR logging, are ineffective in unconventional reservoirs like shale gas and tight gas sandstone due to low porosity and permeability, and assumptions about formation resistivity, leading to inaccurate measurements and inability to quantify fluid saturations.
Innovation Solution
Utilizing acoustic logging tools to measure compressional and shear wave velocities, and employing equations to calculate water saturation by comparing these velocities to known or theoretical trends, allowing for accurate quantification of hydrocarbon saturation in low resistivity environments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Archie saturation model is used, then water saturation can be estimated in conventional reservoirs, but the model becomes inaccurate in shaly sandstone and gas shale reservoirs with high brine conductivity
Solution Approach 1:
The patent changes the fundamental parameter used for saturation estimation from electrical resistivity to acoustic wave velocities (P-wave and S-wave). By using the ratio of P-wave to S-wave velocities, the method can accurately estimate water saturation in shaly sandstone and gas shale reservoirs where the Archie model fails due to high brine conductivity and clay content.
2Measurement precision
If NMR logging is used, then fluid saturation can be quantified without requiring formation resistivity knowledge, but the method is limited by lower porosity and permeability in unconventional reservoirs
Solution Approach 1:
The patent replaces NMR logging with acoustic logging that measures P-wave and S-wave velocities. This substitution allows the method to penetrate deeper into the formation and provide accurate saturation estimates in low porosity and permeability reservoirs where NMR is limited, while also being effective in both conventional and unconventional reservoir types.
3Adaptability or versatility
If NMR logging is used, then electrical resistivity knowledge is not required, but the lateral depth of investigation is shallow compared to acoustic logging tools
Solution Approach 1:
The patent uses acoustic wave propagation to measure formation properties, replacing NMR's electromagnetic field-based approach. Acoustic logging tools have a significantly larger lateral depth of investigation, allowing them to penetrate through mud invasion zones and measure saturation in the uninvaded formation zone, while still providing independence from electrical resistivity measurements.
4Measurement precision
If acoustic logging is used to measure P-wave and S-wave velocities, then accurate water saturation estimation is achieved in shaly sand reservoirs, but the method requires new equations and approaches compared to conventional models
Solution Approach 1:
The patent transforms the saturation estimation problem by changing from resistivity-based parameters to acoustic velocity-based parameters. By using the ratio of P-wave to S-wave velocities and their relationship to bulk and shear moduli, the method derives new equations that accurately estimate water saturation in shaly sand reservoirs, providing a more robust approach that works across different reservoir types.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Provides accurate estimation of water and hydrocarbon saturation in shaly sand reservoirs, overcoming limitations of conventional methods by using acoustic logs to determine fluid saturations without relying on electrical resistivity measurements.
Implementation Method 1
measuring the compressional waves and shear waves
Implementation Method 2
measuring the compressional waves and shear waves
Implementation Method 3
a formation's bulk modulus is significantly affected but the shear modulus stays relatively constant
Implementation Method 4
the shear modulus stays relatively constant
Data Source
AI summary
The present application relates to new systems and methods of quantifying hydrocarbon saturation using measured acoustic logs. The methods and systems herein can accurately quantify the water saturation and hydrocarbon percentage in a low resistivity low contrast shaly sand reservoir where previous methods would indicate the reservoir was wet. In an embodiment, the disclosed system utilizes acoustic logging equipment or tools and techniques to transmit and/or record acoustic signals. Acoustic logging equipment operates by generating acoustic signals and detecting the acoustic signals after the acoustic signals pass through one or more geologic formations.