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Upscaling the high resolution measurements conducted on plugs and using these for the calibration of empirical relationships with properties obtained from low resolution wireline logs can be achieved correctly providing the following conditions are met:
- Plug samples must not be taken in local heterogeneous zones which are smaller than the resolution of the wireline (mostly between 0.5m to 1m);
Mix plugs measurements originating from different rock facies.
Continuous High Resolution Core Based Measurements offers the many advantages among which are the possibilities to :
- Identify facies on the basis of patterns shared by data groups in multidimensional data spaces;
- Perform adequate signal processing to match the resolution of the wireline logs and reduce considerably the influence of outliers in the calibration process. The outliers are often created by local heterogeneities measured on plugs but smoothed out by the wireline resolution.
Numerous case studies conducted by Epslog have demonstrated the importance of acquiring Continuous High Resolution Core Based Measurements to bridge the resolution gap between plug samples and wireline logs. The state of the art Continuous High Resolution Core Based Measurements developed by Epslog share the following key features:
- Compatibility with a highly transportable device, the “Wombat”;
- Rapidity of measurements to fit in standard core analysis workflows (6 feet/hour/device) without compromising the preservation of the cores;
- Low requirements: Core slabbing is not required prior to these measurements
- Low impact :Quasi non-destructive tests that do not hinder the subsequent acquisition of plugs ;
- Versatility: Accommodate variety of sample geometries and sizes (full cores, slabs, plugs, sidewall cores);
- Real Time analysis of the results;
The Scratch Test
The Scratch Test provides a direct, independent quantitative and true continuous measure of rock strength (UCS) (i.e. not a combination of several elastic and hardness indexes as obtained from rebound tests such as Schmidt hammer or others) at the centimetric scale. It uses a PDC cutter to trace a groove at the surface of the tested sample. The shear failure induced in the rock mass ahead of the cutting tool is equivalent to shear bands produced during UCS tests. The high resolution rock strength profile is derived from the forces acting on the instrumented PDC cutter.
Sonic Vp and Vs
In addition, the groove left by the cutter provides a flat and unaltered surface on the full core, which is ideal to run ultrasonic measurements. Epslog has developed state of the art miniaturised ultrasonic probes adapted to the groove width in order to measure ultrasonic velocities at the groove bottom. This technology enables the logging of continuous profile of Vp and Vs on core samples, from which dynamic Elastic moduli can be derived. The resolution is similar to the plug size, i.e. 4cm (2 inches).
Digital Core Photography
High resolution pictures of the groove document the core state upon testing and record the depth reference for latter tests. The superimposition of the different measurements on the image allows for an easy identification of the intervals to select for plugging.
The groove left by the scratch test also opens a window for sedimentological, lithological & stratigraphical description of the core before plugging and slabbing.
Epslog recently added a very high resolution imaging capability revealing rock features of few microns. Those very high resolution images are used for early sedimentological studies and grain size distribution analysis.
Rock Powder Collection
The scratch test creates naturally powder that can be collected sample per sample or foot per foot. The volume of powder being uniformly distributed along the entire sample or a subsection of the sample, its analysis with XRD, XRF or FTIR techniques provides a truly reprenstative elements or mineralogy content of the sample or its subsection.
Results obtained from standard sampling technics, although quantitative, are usually representing a very small portion of the core and might vary greatly with the position of the sample, making its upscaling impossible.
Epslog offers the highest quality in on-site core testing and core data interpretation services.
Our services include:
- On-site testing package. Fully dedicated engineers mobilized at our customer's location to operate our reliable hardware and software solutions;
- Advanced interpretation of core and well data including but not limited to, scratch test and ultrasonic data, with our proprietary software. Depending on the problem at end, the analysis may include:
- Quality control of well logs and core data;
- Identification of best well logs for strength proxies;
- Identification of facies with clustering algorithms;
- Derivation of empirical laws;
- Recommendations for plug sample selections and geomechanical testing programs;
- Continuous profiles of UCS, Young Modulus and Poisson’s ratio.
Our base-line deliverables set a new industry standard for rock strength testing: continuous high-resolution profiles of rock strength and elastic moduli directly measured on core samples.
Core Analysis Workflow
Plan the Scratch Test early in your core analysis program (before plugging and slabbing) and unlock the full value of your core data:
- Facies recognition - Scratch test results (strength and ultrasonic wave velocities), combined with other core data profiles (Gamma-ray, CT scans) and observations or measurements on the surface exposed by the scratch test (micro-permeability, texture, description of sedimentological features,…) and wirelines are available at the first stage of the core analysis workflow: sedimentologists and petrophysicists can take a preliminary step towards rock typing even prior to core slabbing;
- Mapping rock heterogeneity – The Scratch Test captures rock heterogeneity at the centimetric scale. High resolution strength and elastic profiles and statistically significant correlations with plug sample measurements and well logs are combined to quantify not only rock strength variations but also variations of petrophysical properties such as porosity, density and sonic velocity;
- Selection of plug samples – The accurate description of the centimetric-scale heterogeneity along core samples is a pre-requisite to the adequate selection of plug samples for conventional testing. This is particularly valuable when the number of conventional laboratory tests is restricted by the availability of plug samples (rock mechanics, SCAL);
- Core shift – Scratch test results (when averaged at the resolution of well logging tools) can be matched against well logs in order to estimate core shift values, even in the absence of clay marker;
- Establishing robust correlations with well logs – The shear volume of scratch data increases the statistical significance of correlations between core data profiles and well logs.
- Core aging – Time lapse scratch test campaigns on the same core reveals the effect of aging or weathering on rock mechanical properties.
The groove left by the scratch test provides a flat and unaltered surface on the full core fit for the testing of permeability with a probe permeameter before slabbing and plugging. This technology is available with most of rock testing laboratories and can be easily combined with services provided by Epslog.