Shale Surface Relaxation Rate Calculation via NMR and Mercury Injection
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Solution Overview
Problem
Current methods for calculating the surface relaxation rate of shale reservoirs are inaccurate due to the use of approximate values, leading to significant errors in characterizing the effective pore throat structure distribution, as the surface relaxation rate varies significantly with different shale lithofacies.
Innovation Solution
A method involving NMR and high-pressure mercury injection experiments to obtain accurate relaxation time and pore throat radius distributions, followed by data processing to calculate precise surface relaxation rates, which are then averaged and weighted to account for different lithofacies, allowing for non-destructive detection of complete effective pore throats.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an approximate surface relaxation value of 10 is used for all shale lithofacies, then the calculation process is simple, but the measurement precision of surface relaxation rate deteriorates significantly
Solution Approach 1:
The patent transforms the surface relaxation rate calculation from using a fixed approximate value to dynamically calculating it through parameter transformation. By establishing a transformation relationship between NMR relaxation time parameters and mercury injection pore throat radius parameters, the method calculates surface relaxation rate as ρ = b/(α×T2), where α and b are transformation parameters derived from matching the two measurement systems. This resolves the contradiction by making the calculation process more complex but achieving precise, lithofacies-specific surface relaxation rates.
2Ease of operation
If a fixed surface relaxation value is applied across different shale lithofacies, then the operation is convenient, but the characterization accuracy of pore throat structure distribution deteriorates
Solution Approach 1:
The patent applies local quality by making the surface relaxation rate specific to each lithofacies rather than using a universal value. Different shale lithofacies (e.g., siliceous shale, carbonate shale, mudstone) have different mineral compositions and pore structures, which result in different surface relaxation rates. The method calculates separate surface relaxation rates for each lithofacies based on their specific NMR and mercury injection data, thereby improving pore throat structure characterization accuracy while maintaining operational convenience through automated calculation.
3Device complexity
If approximate surface relaxation values are used, then the experimental process is simple, but the reliability of reservoir evaluation deteriorates
Solution Approach 1:
The patent introduces an intermediary transformation relationship between NMR relaxation time and mercury injection pore throat radius. Instead of directly using approximate surface relaxation values, the method uses the transformation parameters α and b as intermediaries to bridge the two measurement systems. This intermediary approach allows the calculation of accurate surface relaxation rates ρ = b/(α×T2) by matching the distribution curves from both experiments, thereby improving reservoir evaluation reliability while keeping the experimental process manageable through established measurement techniques.
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 method provides high accuracy and universality in calculating surface relaxation rates, improving the characterization of pore throat structures and enabling more precise determination of shale oil and gas contents and favorable reservoir intervals.
Implementation Method 1
selecting a rock sample from a target shale reservoir to perform a nuclear magnetic resonance (NMR) experiment under saturated formation water conditions to obtain a relaxation time T distribution curve
Implementation Method 2
selecting a target rock sample from the rock sample to perform a constant-rate high-pressure mercury injection experiment to obtain a pore throat radius r distribution curve
Data Source
AI summary
A method for calculating a surface relaxation rate of a shale includes: a relaxation time T distribution curve and a pore throat radius r distribution curve are obtained through experiments; abscissas of the two distribution curves are standardized, and the abscissa of the relaxation time T distribution curve is expanded or shrunk to ensure an abscissa value corresponding to a maximum ordinate value in the transformed relaxation time T distribution curve is same as an abscissa value corresponding to a maximum ordinate value in the pore throat radius r distribution curve; straight lines with a number of N parallel to a y-axis of a combined curve graph including the two distribution curves are drawn and a ρ value corresponding to each straight line is calculated; and ρ value with the number of N are processed to obtain a final surface relaxation rate ρ′.