NMR Log Calibration for Permeability Prediction with Multimodal Carbonates: A Case Study from a Silurian-Aged Pinnacle Reef in Northern Michigan
Nuclear magnetic resonance logging (NMR) is a powerful logging technique that utilizes the magnetic moment of hydrogen protons within formation fluids to yield continuous estimates of downhole permeability, producibility, total and effective porosity, pore-body distributions, and fluid profiles. The implication of these logs within multimodal pore systems (i.e. vuggy carbonates) is problematic, creating the need for calibration, typically achieved through expensive NMR laboratory analyses. This study attempts to mitigate these issues by providing a workflow to calibrate open hole wireline NMR logs for quick-look permeability estimation within multimodal carbonates, in the absence of laboratory NMR analyses. NMR interpretation is derived from the distribution of T2 relaxation times. T2 distributions are affected by numerous factors, but when saturated with 100% brine, may be interpreted as the distribution of pore-body sizes. The cumulative T2 distribution is partitioned into clay-bound, capillary-bound and free fluid volumes. A T2 cutoff is applied, dividing the cumulative T2 distribution into bulk volumes of irreducible fluid saturations (BVI) and movable fluid saturations (BVM), each being main parameters used to estimate permeability, total and effective porosity, and fluid saturation values. The application of industry standard T2 cut-offs (~92 milliseconds) leads to erroneous BVI and BVM volumes within multimodal carbonates. This study combines data from thin-section pore typing data and MICP experiments to determine pore system governance throughout 16 intervals of the Brown Niagaran formation, a carbonate zone in Northern Michigan used for CO2-enhanced oil recovery, within one well that features an NMR log. Pore-type-specific T2 cut-offs were then applied to generate more representative BVI and BVM values. Permeability values were calculated using the Timur-Coates permeability prediction model with the updated BVI and BVM values and were compared to laboratory permeability measurements from core samples. Results indicate that a cutoff of 512 milliseconds was the optimal T2 cutoff to use in the Brown Niagaran, yielding permeability estimates ranging from 1-to-3.5 millidarcies, and were in good agreement with core measurements. This workflow for NMR calibration may be applied to any multimodal carbonate formation, eliminating the need for costly laboratory measurements. The study is part of the Midwestern Regional Carbon Sequestration Partnership (MRCSP) Michigan Basin Large-Scale Injection Project under DOE/NETL Cooperative Agreement # DE-FC26-0NT42589 with co-funding by Core Energy, LLC, and several other partners.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019