Perturbed Pressure Decay Method for Ultra-Low Permeability Rock Measurement
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Solution Overview
Problem
Current methods for measuring the permeability of ultra-low permeability rocks are impractical due to excessive pressure drops and lengthy establishment of steady-states, leading to inaccurate results, as they fail to account for gas non-ideality and dead-volumes in the measurement hardware.
Innovation Solution
A perturbed pressure decay method is developed, accounting for gas non-ideality and dead-volumes, which involves measuring the decay of pressure differences across chambers connected through a porous medium, allowing for precise determination of permeability by calibrating volumes and using pressure sensors to monitor pressure changes over time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If steady-state permeability measurement is used for ultra-low permeability rocks, then measurement accuracy is improved, but experimental time becomes excessively long and pressure drop becomes unmanageable
Solution Approach 1:
The patent applies transient pressure decay measurement instead of steady-state measurement. The system periodically introduces gas into the upstream chamber and measures the pressure decay over time, allowing permeability determination without waiting for steady-state conditions. This reduces experimental time from potentially hours to minutes while maintaining accuracy through proper mathematical modeling of the transient process.
Solution Approach 2:
The patent changes the measurement parameter from steady-state flow rate to transient pressure decay rate. By measuring how pressure decreases over time during a controlled gas introduction and shutdown sequence, the system determines permeability without requiring sustained steady-state flow, thus avoiding excessive pressure drops and reducing measurement time.
2Adaptability or versatility
If gas is used for permeability measurement in ultra-low permeability rocks, then measurement is feasible, but gas non-ideality causes measurement errors
Solution Approach 1:
The patent replaces the assumption of ideal gas behavior with a more accurate real gas model. The system uses the compressibility factor Z and its derivative dZ/dP to account for gas non-ideality in the permeability calculation, substituting the simple ideal gas law with a corrected equation of state that accurately reflects real gas behavior at the pressures used in ultra-low permeability measurements.
3Device complexity
If dead-volumes in measurement hardware are ignored, then device complexity is reduced, but measurement accuracy deteriorates
Solution Approach 1:
The patent applies preliminary calibration to determine the dead-volumes of the upstream and downstream chambers before actual permeability measurements. By characterizing these volumes in advance using known reference materials or calibration procedures, the system incorporates dead-volume corrections into the permeability calculation without adding complexity to the measurement procedure itself, thus maintaining both simplicity and accuracy.
Data Source
AI summary
Apparatus and calibration methods are disclosed for measuring the permeability and/or porosity of ultra-low permeable rock samples. The apparatus uses a gas source, a sample chamber, chambers of calibrated volume, and pressure measurement devices.