Abstract
The petrophysical interpretation of 2D diffusion-relaxation NMR responses is
a long-standing problem complicated by the complexity of sedimentary rocks as well
as the properties of saturating fluids including crude oils and the ill-posedness of the
underlying mathematical problem. We aim to improve the quality of petrophysical
properties estimates, such as permeability, capillary pressure and relative permeability
curves and fluid typing with the aid of NMR techniques using a combination
of numerically simulated low-field NMR and μ-CT imaging techniques.
In first instance we study in detail the NMR responses of complex fluids and
the influence of environmental factors like oxygen saturation. A simulation strategy
for the fluids themselves using a multi-component fluid consisting of independent
components is developed.
The distribution of fluids in sedimentary rocks depends on capillary pressure
history. We simulate primary drainage on μ-CT digitized rock representations using
morphological transformations. The approach is used for different potential functions
to implement common capillary pressure techniques (porous plate, mercury
porosimetry (MICP) and centrifuge multi-speed). We evaluate the influence of specific
boundary conditions, the effect of image resolution, and compare to saturation
profiles measured by NMR methods. We show good agreement for three sandstone
rocks.
Spatially-resolved NMR is used to estimate two-phase air-brine relative permeability.
Using a simulated centrifuge (primary) drainage to set a gradient of partial saturations,
a set of longitudinal relaxation time distributions is obtained on linearly-spaced
subsets each emulating a volumetric point of MRI experiment. Simulated
NMR-based relative permeability curves show reasonable agreement with experiment
and to standard steady- and unsteady-state special core analysis (SCAL)
techniques.
Quantitative interpretation of NMR response from rocks containing crude is
complicated. We model the NMR response of crude oil as a weighted mixture of
distinctive components. Environmental effects like oxygen paramagnetic relaxation
enhancement are corrected using experimentally established correlation to a carbon
number. We show agreement between experiment and simulation for two sandstone
rocks for fully and partially saturated systems, potentially paving the way for extending
NMR-based relaxivity permeability techniques to samples saturated with
crude.