NMR Logging Depth Segmentation for Porosity Accuracy
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Solution Overview
Problem
NMR logging for determining porosity and pore type in subterranean formations is challenged by fluid infiltration from drilling mud, which introduces errors due to changing fluid composition and interference from drilling mud solids, especially in gas-bearing formations where hydrogen index of gas is low, leading to uncertainty in porosity and pore type measurements.
Innovation Solution
The method involves measuring apparent porosity in both shallow and deep sensitive volumes using NMR tools, comparing these measurements to account for fluid infiltration and solid interference, and using characteristic NMR signals from mud filtrate and whole mud to determine the accurate porosity and pore type, with additional magnetic field gradient-based diffusion analysis for ambiguous cases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If NMR measurements are performed close to the wellbore to obtain formation properties, then the measurements can be performed during drilling operations, but the measurements are affected by fluid infiltration from drilling mud which increases error
Solution Approach 1:
The patent divides the measurement process into two distinct depth zones: shallow sensitive volume measurements and deep sensitive volume measurements. By segmenting the measurement depths, the method can differentiate between mud-filtrate-influenced zones and formation zones, allowing correction of infiltration effects while maintaining rapid logging capability
Solution Approach 2:
The patent uses the deep sensitive volume measurement as an intermediary reference that is less affected by mud infiltration. This deep measurement serves as a mediator to correct the shallow measurement data, enabling accurate porosity determination despite the presence of fluid infiltration
2Measurement precision
If multiple depth of investigation measurements are performed to account for fluid infiltration, then measurement accuracy improves, but the complexity of the NMR tool and measurement process increases
Solution Approach 1:
The NMR tool is designed with multi-functional capability to perform both shallow and deep sensitive volume measurements using the same basic tool configuration. This universality allows the tool to gather multiple depth measurements without requiring multiple specialized tools, thereby improving accuracy while limiting complexity increase
3Loss of information
If NMR measurements are performed in gas-bearing formations, then pore type information can be obtained, but the low hydrogen index of gas leads to uncertainty in porosity measurements
Solution Approach 1:
The patent replaces reliance on hydrogen index (which is weak for gas) with diffusion-based measurements using magnetic field gradients. This substitution allows accurate porosity determination in gas-bearing formations by using a different physical mechanism that is not dependent on hydrogen content
4Reliability
If drilling mud solids infiltrate the formation, then mud cake forms to stabilize the wellbore, but the solids interfere with NMR signal detection
Solution Approach 1:
The patent extracts and separates the solid particle signal from the overall NMR measurement by using diffusion-based techniques. This extraction allows the NMR tool to identify and exclude signals from drilling mud solids, thereby eliminating their interference with porosity and pore type measurements while the mud cake continues to provide wellbore stability
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
This approach reduces uncertainty in porosity and pore type determination by differentiating between fluid and solid contributions, providing more accurate measurements of porosity and pore characteristics, even in gas-bearing formations, by using multiple sensitive volumes and NMR signal analysis.
Implementation Method 1
nuclear magnetic resonance (NMR) logging for determining the porosity and the pore type of subterranean formations
Implementation Method 2
additional magnetic field gradient-based diffusion analysis for ambiguous cases
Data Source
AI summary
Nuclear magnetic resonance methods may be used to determine the porosity and the pore type of subterranean formations while accounting for fluid infiltration from the drilling mud or mud filtrate thereof into the formation. For example, the apparent porosity of (1) a shallow sensitive volume (ϕsh) and (2) a deep sensitive volume (ϕdeep) may be measured. Then, a comparison of the ϕsh and the ϕdeep may be performed, and the porosity (ϕ) of a portion of the subterranean formation may be calculated based on the comparison of the ϕsh and the ϕdeep.