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Limitations of Clay Speciation via Bulk Chemical Analysis


The presence of expandable clay minerals in a shale play poses a unique problem for well drilling and completion operations and may ultimately hamper well performance. These issues can be mitigated by treating the wells with clay stabilizers if the presence of swelling clays is first documented. We document a case in which bulk-rock chemical data is insufficient for clay speciation and careful mineralogical analysis is required to adequately characterize the clay content of a portion of the Eagle Ford source/reservoir system. Spectral gamma ray (SGR) data collected from the lateral portions of recent APC-operated Eagle Ford wells and wellsite X-ray fluoresce (XRF) analyses of cuttings have high Th/K, suggesting a high modal abundance (60-90% by volume) of swelling-type clay minerals in the targeted zone of the wells. Compatibility of the XRF and SGR datasets confirms the robustness of the new dataset. However, this result contrasts with regional X-ray diffraction (XRD) data for Eagle Ford cores and cuttings that indicate a low modal proportion of swelling clays. Additionally, a temperature-composition binary phase diagram suggests that any smectite initially present in the clay should have converted to illite given reservoir maturation temperatures and potassium concentrations appropriate for the targeted zone of the Eagle Ford. To test the conclusions of the SGR/XRF study, oriented-clay fraction XRD analyses were performed on the cuttings from the suspect wells. For comparison, we selected 30 intervals of cuttings to match those analyzed via XRF. The oriented-clay XRD technique reveals much lower expandable clay content (1-5% by volume) than the SGR/SRF bulk chemistry dataset. Based on experience and the XRD dataset, we elected not to use clay stabilizers and no issues were encountered during completion of the suspect wells. K and Th concentrations measured via SGR and XRF have proven to adequately characterize the clay contents of other shale plays [eg. Marcellus]. However, the Th/K technique grossly overestimated the swelling clay content of our Eagle Ford samples. Therefore, either the calibration of the Th/K technique is inadequate for this part of the Eagle Ford, or the SGR/XRF methodology is measuring K and Th concentrations of non-clay minerals. Regardless, careful consideration of the sample context and mineralogy (via oriented-clay XRD) should be used to compliment and confirm the results of clay speciation via bulk rock chemistry.