Spectral analysis of natural and stimulated gamma rays is a well-established open-hole technology that enables accurate mineral characterization and petrophysical evaluation of conventional and unconventional reservoirs. The determination of detailed mineralogy in the cased-hole environment, however, has been a challenge because of the significantly increased uncertainties caused by the additional attenuation and contribution effects of casing and cement that are observed in the gamma ray spectra. The acquired spectral gamma ray data is processed with proprietary algorithms that are based on a combination of lab experiments and modeled tool response standards. The resulting elemental composition, corrected for the cased-hole environment, is further processed in an expert interpretation system to determine lithology and detailed mineralogy of the target formation. Candidates for this technology include older and newly cased wells where lithology and detailed mineralogy from open-hole logs are not available.

This work discusses some aspects of the corrections needed for an accurate quantification of chemical elements from measurements through casing. The impact of casing collars and presence of cement on the spectral data are also discussed. Finally, we report a case study that uses this technology and illustrates a successful mineral characterization of a complex reservoir rock in the United States. Results show good agreement with an x-ray difraction (XRD) analysis of ditch cuttings. The resulting logs were productively used by the operator to understand the siliciclastic influx as well as the distribution of carbonates in the Big Lime formation. The results also show the ability of the methodology to identify organic carbon directly from measurements of the inelastic spectrum in the cased-hole environment.

The application of pulsed neutron technology (PNT) in cased holes has, so far, been limited to basic lithology identification. This methodology expands the applicability of the PNT, previously mostly confined to open-hole cases, to the cased-hole environment and enables operators to take full advantage of the valuable information contained in high-resolution inelastic, capture and natural spectra. This enables characterization of hydrocarbon-bearing formations to a level of detail previously possible only from open-hole data.

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