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The Impact of Salt Precipitates on the XRF Measurement and the Mitigation Strategy – Case Study of the Wolfcamp Formation, Delaware Basin


Handheld ED-XRF is a powerful and valuable tool, and generates massive, high-quality elemental composition data on conventional cores, plugs and cuttings. In the last decade, the portable XRF has been extensively applied to characterize fine-grained unconventional reservoirs, e.g., Barnett, Eagle Ford, Haynesville, Marcellus, Niobrara, and Woodford. The XRF data can be used to define geochemically and geomechanically distinct zones, which help operators place horizontal wellbores, increase drilling efficiency, and optimize hydraulic fracturing design. However, the impact of salts precipitated on the core surface after core acquisition has never been evaluated, because salt crusts can significantly compromise the XRF data if not removed. In this study, we demonstrate that, in the Wolfcamp Formation of Delaware Basin, salts are common and can be very difficult to be removed by gentle cleaning, a common way that has been carried out in previous studies. Meanwhile, the presence of salts on the core surface has substantial impact on the result of XRF measurement. Therefore, we propose a more aggressive cleaning method to mitigate the issue, and come up with the criteria to monitor the potential existence of the salt contamination problem. Based on the investigation of 12 wells, more than 3800 feet of conventional core across the Delaware basin, it's revealed that salts, including halite and gypsum are common on the slab core surface of the Wolfcamp Formation. The salts tend to be more concentrated in the clay-rich facies relative to carbonate-rich facies. These finely crystalline salt crusts may be visually subtle or evident, and they can reappear on the core surface immediately after cleaning. Compared to the 2 sets of XRF data measured on the same spot before and after cleaning and up to thousands of measurements from several wells, the content of major oxides, especially SiO2 before cleaning is significantly lower, and the concentration of trace elements is measured slightly to negligibly lower. The salt impact on the XRF measurement varies for individual elements, which makes it very difficult, if not impossible, to correct the XRF data. Therefore, it is essential to aggressively scrape, scrub, and water cleaning the core surface as an effective means to remove salt precipitates before the XRF measurement. Meanwhile, the content of S, Fe, Cl, as well as the total Balance of the XRF measurement need be closely monitored to ensure the quality of the XRF data.