High-Quality 3-D Microct Imaging Of Source Rocks – Novel Methodology To Measure And Correct For X-Ray Scatter

Abstract

Micro Computed Tomography (microCT) of cores yields valuable information about rock and fluid properties at pore-scale for conventional rock and at rock heterogeneity scale for unconventionals. High levels of uncorrected X-ray scatter in CT data leads to strong image artifacts and erroneous Hounsfield Unit (HU) values making reconstructed images unsuitable for accurate digital rock characterization (e.g., segmentation, material decomposition, and others). MicroCT scanners typically do not include scatter correction techniques. To fill this gap, we developed a new methodology to measure and remove the scatter component from raw projection microCT data collected during rock core scans, and to ultimately improving image quality of scanned cores. Widely used approaches for scatter estimation, based on Monte-Carlo simulation and simplified analytical models, are time-consuming and may lose accuracy when imaging complex unconventional shale cores. In this paper, we propose a more practical approach to perform scatter correction from direct scatter measurements, which is based on the Beam-Stop Array (BSA) method. The BSA method works as follows. The radiation scattered by the core sample is emitted in random directions. By placing an array of small, highly-absorbing beads between the source and the core, the primary X-ray signal through the beads is blocked, but the overall object scatter signal is not affected. The observed values in the beads’ shadows on the detector are assumed to be scatter signal. Performing interpolation of the scatter signal between the observed pixels on the detector gives an estimate of the scatter signal at every pixel on the detector. Subtracting scatter from projection data yields corrected data used for 3D CT core image reconstruction. To develop the core scatter correction methodology, we (1) performed modeling of primary and scatter signals to optimize the BSA design (beads layout, size, scan parameters); (2) developed and implemented an accurate scatter correction algorithm into our 3D microCT image reconstruction workflow, and (3) tested the proposed methodology using four shale core samples from the United States and the Middle East. To better assess the impact of scatter, all experiments with shale core plugs presented in this paper were conducted using a source energy of 160 kVp. Our results demonstrated that in many cases, especially with higher attenuating cores, scatter cannot be ignored due to its significant impact reducing accuracy of image reconstruction. We also showed that the developed methodology allows for accurate estimation and removal of scatter from the raw (projection) CT data, enabling reconstruction of high-quality core images required for performing digital rock analysis. The presented scatter correction methodology is general and can be utilized with any microCT scanner employed by the petroleum industry to improve image quality and derive accurate HU values. This is of significant importance for quantitative characterization of highly-heterogeneous rock with fine structural changes as is the case for shale. Ultimately, this methodology should expand the operational envelope and value of microCT imaging in the Exploration & Production (E&P) workflows.

Publication Date

1-1-2018

Publication Title

SPE/AAPG/SEG Unconventional Resources Technology Conference 2018, URTC 2018

Document Type

Article; Proceedings Paper

Personal Identifier

scopus

DOI Link

https://doi.org/10.15530/urtec-2018-2902457

Socpus ID

85053608207 (Scopus)

Source API URL

https://api.elsevier.com/content/abstract/scopus_id/85053608207

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