Seeing the effects of rock heterogeneity on CO2 movement

July 12, 2011 By Linda Morton
Seeing the effects of rock heterogeneity on CO2 movement
Dry sample scan of the CAS sandstone core using NETL’s medical, industrial, and micro-CT scanners.

All three DOE's National Energy Technology Laboratory X-ray CT scanners were recently used to characterize flow patterns during CO2 flooding of a sandstone sample from China. This work was part of a U.S.-China Energy Partnership that involves the Chinese Academy of Sciences (CAS), NETL, and PNNL. A delegation of scientists from the CAS brought a core sample from a sandstone formation in the Ordos Basin, China, that is of interest for potential CO2 storage.

NETL’s medical and industrial CT scanner were initially used to determine macro-variations in the sample structure, which included visually distinguishable bedding planes. Petrographic analysis of this core revealed that the lighter-colored bedding planes contained a high level of calcite. The figure above left shows the results from the medical CT scanner and industrial CT scanner in the two left-most images. Note the higher resolution that is capable with the new industrial CT scanner.

A small sub-core was analyzed with NETL’s micro-CT scanner to determine the pore scale features of these bedding planes. A region was scanned that contained both a calcite-rich bedding plane and a calcite-lean region next to it. This scan is shown in the right-most image in the figure above. The resolution of this image is 2 microns/pixel and shows that some of the calcite (lighter gray) is not bonded to the sand grains (darker gray) and that there is some porosity (black), even in the calcite-rich region.

Seeing the effects of rock heterogeneity on CO2 movement
Sequence of 3D reconstructions of CO2 within the core preferentially filling the open bedding planes during the flow experiment; flow is from left to right.

A CO2 flood of a brine-saturated core was then performed in NETL’s medical scanner and the motion of the fluids was captured by multiple scans. The results, shown at left, illustrate that the CO2preferentially flowed through the bedding planes that contained less of the calcite in-fill. This multi-scale tomography clearly showed that preferential flow paths existed in the sandstone sample that would impact its use for storage of CO2. Such information is vital for deciding on the applicability of a formation for storage and helps to guide estimates of what percentage of the pore space could be filled if sequestration were to occur. Combining NETL’s imaging capabilities with traditional core-scale petrographic analysis enables researchers at NETL to characterize samples and understand how they will behave in a CO2-rich environment.

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