Slim Pulsed Neutron Spectroscopy Integrated with Wellbore Imaging to Provide Synthetic Core in Slim Boreholes and Cased-Hole Environment
Authors: Saleh Khaled , Cavalleri Chiara , Espinosa Hugo , Ghanim Mohamed , Galal Mahmoud , Gomes Renata , Morad Aly 
Published: November 2020

Recent advancement in logging technology and data analytics enables measuring a comprehensive set of formation petrophysical properties and rock composition in cased-hole environments. Using state-of-art pulsed neutron logging technology and processing algorithms enables recording capture and inelastic elemental spectroscopy for rock elemental concentrations, including total organic carbon, detailed mineralogy and matrix properties, simultaneously to sigma and other neutron-based outputs. The integration of the interpreted lithology from cased-hole elemental spectroscopy data with electrofacies from high-resolution imaging tools recorded in the open-hole provides the characterization of heterogeneity challenges by building a synthetic core in old wells with limited data gathering from open-hole logging or absence of conventional coring.

An effective way to incorporate those measurements has been developed and adapted to the use of cased-hole spectroscopy logs. The dry weight elemental fractions measured by the advanced pulse neutron technology are corrected for wellbore contribution and converted into dry weight mineralogical outputs. Using an automated processing workflow converts the capture and inelastic gamma-ray yields from the energy spectrum measured behind casing into the dry weight of elements and mineral fractions in the formation. The computed mineralogical outputs are then defined based on a standardized ternary diagram approach to developing dry-weight mineralogy-based lithofacies. This classification is then combined with the calibrated micro-resistivity image data collected during the open-hole logs evaluation to present a high-resolution rock typing (after Kumar & Kear). The resulting log is dry weight mineralogy-based high-resolution lithofacies that contain vital information to support geological and petrophysical reserves modeling adjustments during development and production.

The paper demonstrates the applicability of the method to cased-hole environments in fields with mixed lithology and complex geological background. Once a robust lithofacies classification is achieved, this is applied for detailed stratigraphic analysis, well-to-well correlation, or refined static reservoir modeling. A standardized mineral-based facies scheme guides the selection of higher completion-quality intervals, otherwise difficult to define in old wells with limited original evaluation. Besides, thin beds that were previously bypassed can be detected and characterized for high-resolution net pay calculation leveraging the high-resolution lithofacies output from this approach. The lithofacies classification (synthetic core) provides important input to the study of reservoir connectivity in the development phase and production optimization. Moreover, a synthetic core description would be critical when reassessing mature fields and defining completion and production strategies where core data is not available.

The approach and workflow can be implemented in various cases as a cost-effective solution in multiple scenarios and different formation types.