New Nuclear Magnetic Resonance Log T2 Cut-off Interpret Parameters for the Unconventional Tight Oil of the Bakken Petroleum System Using 2-D NMR Core Laboratory Measurements on Native State and Post-Cleaned Core Samples

摘要

Operators have embraced the use of nuclear magnetic resonance (NMR) well-logging measurements to evaluate the hydrocarbon-producing potential of unconventional reservoirs such as the tight-oil Bakken Petroleum System (BPS) in North Dakota. With the use of NMR logging in these reservoirs, the widespread use of conventional NMR T2 interpretation parameters has been incorporated into the analysis. Typical T2 cutoff interpretation parameters in use are ≤ 3 ms clay bound water, 3 to 33 ms for capillarity-bound fluids, and >33 ms for free fluids, which is the traditional value used for carbonate reservoirs. While it appeared that these conventional parameters gave a satisfactory interpretation on NMR logging data, a set of laboratory NMR core measurements has led to a re-evaluation of the T2 cutoff parameters of NMR data traditionally used for the BPS.The set of NMR laboratory data came from a comparative study among three commercial core laboratories on core saturation measurement techniques made on samples from the BPS. One part of the study was to confirm that core plugs from the Middle Bakken/Three Forks section of the BPS cleaned using the Dean-Stark core-cleaning protocols established in the API RP-40 standards, did not completely clean all fluids from these core samples.To confirm this assumption, the laboratories made native state 2D NMR measurements on core plugs before the plugs were subjected to a Dean-Stark cleaning process. The measurements were repeated at the end of the completed cleaning process to evaluate the presence of any remaining fluids left in the core plug. Examination of the NMR data sets confirmed the incomplete cleaning assumptions. NMR core samples which were measured post-cleaned, revealed water, oil, and bitumen remained in the cleaned core plugs. After extensive study of the dataset it became apparent that the traditional T2 interpretation parameter cutoffs were incorrect, and thus yielded incorrect saturation values on the core and log data. The data revealed that NMR T2 cutoff for free fluids ranged between 0.7 and 1.0 ms, and not the traditional 33-ms value. Secondly, the dataset suggested that the clays in the core sample were likely oil-wet and that clay-bound water T2 cutoff component was ≤0.4 ms. The capillary-bound water portion was observed between the clay-bound T2 cutoff and the free-water T2 cutoff value. The data also seemed to indicate the possibility of dual wettability for reservoir samples.Lastly, this work identified the need to make hydrogen index corrections on both NMR core and log porosity measurements. These corrections allow the appropriate computation of formation waters volume due to the hyper salinity that exists in the BPS (300,000 to 400,000 ppm NaCl). These corrections are not typically applied to NMR logging or core data in the BPS.This work has identified new NMR T2 cutoffs parameters that give more accurate formation saturation analysis and indicate methods for correcting the measurement for salinity that needs to be applied to NMR logs and core data to achieve more accurate results.