Aging Time Control by NMR Relaxation

摘要

It may become possible to simultaneously determine the full water-oil capillary pressure bounding curve cycle in the laboratory by two methods: (1) directly, by traditional fluid displacement through oil- and water-wet membranes, and (2) indirectly, by NMR relaxation. The coreholder has to be made of nonmagnetic, e.g., peek, material and placed inside the static magnetic field of the spectrometer. It may be possible then to estimate the branch of the capillary pressure curve from the relaxation data at each equilibrium point of the membrane method. Or, if the capillary pressure curve from the membrane data is fitted to a correlation, the parameters of the correlation could possibly be estimated from the NMR data. If crude oil displaces formation water in a primary drainage process, the wettability may gradually change as the fluid-core system ages during the equilibrium wait periods, even at laboratory conditions, unless the system has been pre-aged. The standard way of doing this is to flood a water saturated core with crude down to a selected water saturation and then age for an appropriate time interval. In this paper we report the results of an investigation into the possihility of using the NMR T_2-data to determine the appropriate aging time for a crude oil/brine/rock system. We have used Berea cores and crude from the Snorre field. The main results are that the T_2-distribution of continuously aged cores at 90 ℃ seems to be rather insensitive to wettability changes for water-wet systems, as determined by spontaneous water imbibition, but becomes a good indicator as the fluid-rock system approaches intermediate wetting conditions. A procedure with aging intervals at 90 ℃ interupted by cooling and NMR relaxation measurements at 35 ℃ exhibits a gradual change in the geometric mean T_2-value, right from the start. Also, it is suggested that the aging procedure should perhaps be a slow, quasi-equilibrium process down to the selected water saturation to have a well-defined, equilibrated rock-fluid system similar to that in the reservoir, before the capillary pressure bounding loop measurements are initiated.