Rigless Mineralogy Logging- A Reality for Improved Evaluation of Complex Formations

Abstract A new slim pulsed neutron (nPN) logging technology has recently been introduced in the industry. Although the technology's main deliverables aim at reservoir saturation monitoring, the technology has seen substantial improvements in quantitative mineralogy logging. It is now capable of providing mineralogy data quality on par with openhole measurements, thus filling the gap where mineralogy data are required but not acquired during openhole logging. The main objective of this paper is to assess this nPN technology for formation mineralogy, with discussions of advantages, sensitivities, uncertainties, and limitations. Best practices in obtaining high-quality mineralogy data with this technology in open hole or cased hole wells, riglessly, are also recommended. The primary application of pulsed neutron (PN) logs is for reservoir saturation monitoring, be it with time-domain capture data (sigma) or with energy-domain spectral data (carbon-oxygen). With the current slim PN technology, mineralogy derived from the capture spectroscopy (featuring direct measurement of the main elements of silicon Si, calcium Ca, sulfur S, iron Fe, and gadolinium Ga) has always been the last resort and has been used only if no other lithology information is available, because it suffers from low precision and the difficulty to evaluate elements like magnesium Mg in carbonate reservoirs. With the new nPN, the quality of the mineralogy log has improved dramatically, making it equivalent to the mineralogy obtained with openhole logging. It benefits from the latest technological advancements, including high output minitron source, fast electronics, and new detector material with high spectral resolution. Methodologically, it combines information from both capture and inelastic spectroscopy to deliver more precisely the elements of Si, Ca, S, Fe, Gd and magnesium Mg, aluminum Al, titanium Ti, and potassium K. The combination of capture and inelastic spectroscopy confers an advantage in increasing the sensitivity to magnesium, which allows separating calcite from dolomite. Results show that the rigless nPN mineralogy data quality is close to the latest and most advanced openhole spectral mineralogy log quality, complementing the lithology logs interpreted from traditional openhole logs. It allows for a more precise evaluation of the mineralogy in complex evaluation situations, where mineralogy data are required but were not acquired during openhole logging, such as those wells drilled before elemental mineralogy logs were available just two decades ago. As with any other technologies, nPN has limitations. Proper job planning, recommended in this study, is the key to ensure data quality. New nPN provides an opportunity to gather reservoir mineralogy data in any wells, including rigless, in small holes, and through small restrictions, thereby filling the gap where mineralogy data is required, but not acquired during primary openhole logging.