NMR T2 Cutoff Estimation via Centrifuge Saturation Profiles
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Solution Overview
Problem
Current NMR logging methods for estimating formation properties, particularly in low permeability rocks, face challenges due to saturation gradients caused by centrifugation, leading to overestimation of bulk volume of irreducible water and underestimation of recoverable reserves, and can be destructive to fragile rocks.
Innovation Solution
The method involves determining the T2 cutoff by identifying and measuring the real irreducible water segment using saturation profiles and spatial T2 measurements, reducing operational uncertainty and avoiding high centrifuge speeds that can damage samples, by comparing T2 distributions at different sample orientations and averaging estimates for more accurate results.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If centrifuge spinning is used to obtain irreducible saturation state, then time efficiency is improved, but measurement precision deteriorates due to saturation gradients in low permeability rocks
Solution Approach 1:
The core sample is divided into multiple segments along its length, with each segment having a different saturation state after centrifugation. The low-saturation segment (closer to the centrifuge axis) is identified and selected for T2 measurement, representing the irreducible water state, while excluding the high-saturation segment that would skew results.
Solution Approach 2:
Different regions of the core sample are recognized to have different saturation qualities due to centrifugal forces. The measurement is localized to the specific region (low-saturation segment) that accurately represents irreducible water conditions, rather than averaging across the entire sample.
2Measurement precision
If high centrifuge speed is used to achieve irreducible saturation in low permeability rocks, then saturation gradient is reduced, but sample damage occurs due to grain loosening
Solution Approach 1:
Instead of applying excessive centrifugal force (high speed) to the entire sample to achieve uniform irreducible saturation, the method applies moderate centrifugation and then selectively measures only the portion of the sample (low-saturation segment) that reaches the desired saturation state, avoiding the need for destructive high-speed centrifugation of the whole sample.
3Device complexity
If average saturation across the whole sample is used to represent irreducible water conditions, then measurement complexity is reduced, but manufacturing precision deteriorates due to saturation gradient effects
Solution Approach 1:
Before performing T2 measurements, the sample undergoes centrifugation to establish a saturation gradient, and the saturation profile is measured in advance to identify which segment represents irreducible water conditions. This preliminary segmentation guides the subsequent measurement process.
Solution Approach 2:
The problem transitions from a one-dimensional average saturation value to a spatially-resolved measurement along the length of the core sample. The saturation profile varies along the longitudinal dimension, and measurements are taken at specific positions rather than averaging across the entire dimension.
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 provides more accurate determination of formation properties like T2 cutoff, bulk volume of irreducible water, and permeability, reducing errors associated with uniform saturation assumptions and minimizing sample damage, thus improving the estimation of recoverable reserves.
Implementation Method 1
NMR logging measures the induced magnet moment of hydrogen nuclei contained within the fluid-filled pore space of the formation
Implementation Method 2
The sample is spun in a centrifuge to remove fluid
Data Source
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AI summary
Methods estimating a property of a porous media include: saturating a sample of the porous media; spinning the sample in a centrifuge with a first end of the sample closer to an axis of rotation of the centrifuge than a second end of the sample; obtaining a first estimate of the first property; saturating the sample of the porous media; spinning the sample in a centrifuge with the second end of the sample closer to the axis of rotation of the centrifuge than the first end of the sample; obtaining a second estimate of the first property; and determining the second property of the porous media based at least in part on the first estimate of the first property and the second estimate of the first property. The first property can be a T2 distribution and the second property can be a T2 cutoff.