Improved Precision Magnetic Resonance Acquisition: Application to Shale Evaluation

Abstract

Abstract Traditional magnetic resonance (MR) acquisition modes tend to focus on the hydrocarbons found in high-porosity permeability rocks. This requires acquisition optimized for longer relaxation times (T1 and T2). Interest in shale resources (gas and oil) has resulted in a reexamination of MR acquisition sequences. Shale hydrocarbon resources are usually in rocks with very fast relaxation times for which traditional application schemes are not optimal. MR acquisition sequences always involve tradeoffs. Modifying current acquisition schemes to improve precision involved increasing the number of short wait-time sequences. Analytical and Monte Carlo modeling were used to evaluate the tradeoffs between different options. The effect of the inversion parameters on the results was also studied to determine the optimal acquisition sequence and processing parameters for shale resources. The improved precision from this acquisition is demonstrated with field data. Case studies demonstrate how the improved precision of the MR measurement, combined with other logs, provides a better estimate of the organic or kerogen content of the formation; this is particularly useful in areas where correlations to density or other logs have not yet been developed. High-precision MR pore size information in pilot wells is used to highlight the best layers for horizontal wellbore placement.