Rapid nondestructive screening using high-definition (HD) imaging techniques provides a viable solution for superior visualization and detailed quantitative core assessment. These detailed deliverables are available before the core is removed from the inner core barrel and other measurements are commenced. X-ray scanners, popularly known as computed tomography (CT) scanners, are used for such qualitative and quantitative description of cores. Similar to medical applications, CT scanners are used to capture the entire 3-D aspect of rocks when they arrive from the well site and while cores are still in their barrels.

The affordability and speed of modern computing systems has led to the conversion of billions of pixels of acquired CT-scan imaging data into simple viewable formats in a quick turnaround. UltraScan, developed internally by Core Laboratories (Core Lab), provides such capability for high-resolution core visualization and assessment.

A suite of important applications comes into perspective once these high-resolution images are captured. Quantification of fracture volume, fracture orientation, high-density mineralogy, vug-porosity assessment, etc., is possible. In conglomeritic sands segmenting out larger size nonreservoir sands results in refinement of net-to-gross calculations. Tests also have been performed to capture images to quantify subsequent fracture closure because of increasing overburden pressure.

Digital preservation

Conventionally, after subsurface acquisition cores are handled for quality assessment, description, sampling and preservation. UltraScan is performed early on, before cores are extruded and handled, making it indispensable from a reservoir state preservation perspective. This digital preservation helps in visualization and assessment of cores at any point in time months or years after the cores are acquired.

Dual-energy CT

Newer and advanced CT scanners have capabilities to quickly scan objects at more than one energy level. Technology to scan reservoir rocks at two energy levels was developed decades ago, but faster computers make it possible to compute and process thousands of feet of core data quickly. Dual-energy CT scanning of cores provides information on relative density and atomic number at a millimeter scale vertically. When combined with extensive laboratory physical measurements, high-resolution bulk density and photoelectric factors are quantified. Unique cluster analysis of these data combined with core databases encompassing global lithotypes results in a solid interpretation of core mineralogy and, therefore, a mineralogy log for the cored intervals. These HD, high-resolution data also provide information complementary to borehole images, which makes lithological bedding interpretation advanced and accurate.

Applications of dual-energy CT include detailed fracture description; orientation of cores without physically scribing them; assessment of slabbing angle for visualization of maximum bedding dip on slabbed core surfaces; enhanced sub-sample selection; and prediction of high-resolution bulk density, photoelectric factor, porosity and strength index. Strength index at such high resolution helps petrophysicists quickly assess and refine lateral landing zones in a well. Additionally, this technology aids in sample selection decisions for geological, mechanical and advanced rock testing, leading to a higher confidence in reservoir upscaling. It also is used to correct downhole logs since resolution is typically close to a foot while properties from CT-scanning are determined at half a millimeter resolution and are calibrated to laboratory measurements.

The early capture of data and quick interpretation of properties via these methods make dual-energy CT scanning of cores a vital part of core analysis programs.

Micro-CT scanners

Micro-CT scanners are able to perform X-ray imaging of core sub-samples from 10 to 500 times higher resolution than macro-CT scanners. Scan resolution is generally guided by the size of the sample, so the smaller the sample, the higher the resolution.

A 25-micrometer resolution sub-sample scan (a standard test plug) is being used in the sample selection process for sophisticated reservoir-condition flow studies. MicroScan is a tool developed by Core Lab for visualizing high-resolution 3-D images of core plug samples. Operators can better understand how variations in the pore system properties will impact both laboratory test results and reservoir performance. Small features (e.g., micro fractures, small vugs, anomalies, etc.) are hard to visualize with macro-CT imaging but are captured well with micro-CT scanners. Primarily, these scans are used as an advanced screening tool for high-value testing. However, scan data bring extraordinary value for quantifying varying phases in plug samples (e.g., salt crystal locations, location of asphaltene, distribution of fluids, etc.) for assessment of their impacts on porosity, permeability and flow behavior. Fracture propagation as a result of increasing vertical stress on rocks also can be visualized.

Micro-CT scans at higher pore-scale resolutions help resolve features as small as 0.5 micrometer. Image acquisition and segmentation at such resolutions aid in grain and pore system characterization. This analystindependent image segmentation and characterization results in a host of image-derived data with the help of industry-standard segmentation software. When consolidated with appropriate and extensive physical measurements, petrophysical data such as permeability, capillary pressure and electrical properties are quantified. Core Lab’s digital rocks group has developed global models constrained by these laboratory physical measurements to quantify these properties. Confidence in modeling petrophysical data is improved using models calibrated to laboratory measurements. Imaging and quantification of these properties also are ideal for friable to unconsolidated samples and large-sized drill cuttings or when core recovery is poor. The rock-based digital physically constrained models provide data that correlate well to conventional core analyses.

Extracting sub-volumes

Another application of this technology is being able to digitally extract sub-volumes that represent optimal reservoir rock from full bulk samples in heavily shale-laminated reservoirs. This type of analysis helps operators focus on properties of producing rocks as opposed to bulk volume analysis that may contain nonproducing zones, leading to better fine-tuning of reservoir estimates.

Grains can be segmented and well separated on consolidated to unconsolidated clastic rocks at these higher resolutions. Sophisticated measurement techniques enable three-axes measurement of individual grains as small as medium silt size, resulting in unique particle size distributions that were not possible before. This technique provides a true 3-D grain distribution and has significant advantages over some conventional methods to help understand source and provenance of geologic depositions. It also can provide information for sand/gravel-pack designs and quantification of flow zone indicators.

Acquisition and segmentation of a single set of images is needed for sample analysis. Overall turnaround time is reduced for quantification of properties given the efficiency of these rock-based physically constrained models on moderately powerful computers. Rapid access to key petrophysical properties helps operators make time-dependent critical decisions.

These new image acquisition and processing techniques, ranging from macro scale on whole cores to micro, pore-level scans on sub-samples, not only provide tools to characterize reservoirs but also deliver rapid and accurate petrophysical measurements, helping operators make critical early-time decisions.

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