CALCULATION OF COMBINED T_1 AND T_2 SPECTRA FROM NMR LOGGING DATA

摘要

Bulk and surface relaxation phenomena are commonly described in terms of longitudinal relaxation times (TO and transverse relaxation times (T_2). Of these, T_1 information is not easily obtained by current nuclear magnetic resonance (NMR) logging tools for two reasons. First, T_1 measurements are time intensive, resulting in low logging speeds. Second, tool motion destroys the depth coherence within a T_1 measurement.rnConsequently, efforts have been concentrated on extracting as much information as possible from T_2 spectra. This approach lacks resolution power for slow relaxation processes because long pulse-echo trains are required to follow slow T_2 decay. Formation evaluation of carbonate reservoirs provides an example for the need for resolving slow relaxation processes. Both surface relaxation (which mostly affects the water phase) and bulk relaxation arise from weak molecular interactions causing slow relaxation.rnThe newest generation of NMR logging tools can directly capture slow relaxation processes by observing the T_1 polarization buildup. Multiple individual measurement volumes can be excited in rapid succession, with polarization sequences specifically designed to obtain T_1 information. In this way, depth coherence of the measurement is maintained, and correct T_1 spectra are obtained even when the tool is in motion.rnThis paper shows how the appropriate acquisition sequences are implemented and how the resultant two-dimensional data sets are processed to yield composite T_1/T_2 relaxation information. We use forward modeling to show that improved resolving power is obtained with the combined T_1/T_2 measurement.