Temperature effect on NMR surface relaxation
an abstract by Jiansheng
The reservoir carbonate sample studied has a porosity of 24% and a water permeability
of 1.5 mD. Electronic microscopy of this rock exhibits the presence of crystals
of CaCO3 varying in size for 1 to 20 mm. From mercury porosimetry, the pore
throat radius distribution is narrow and peak at 0.5 mm. Electron Spin Resonance
(ESR) experiments have shown the presence of Mn2+ paramagnetic ions. The sample
has been cleaned according to IFP’s standard procedure. The USBM wettability
test performed after cleaning indicates a wettability index of +0.50. During
the wettability test, a spontaneous imbibition of water was observed but the
cleaning procedure was not sufficient to remove some absorbed components from
the surface, resulting in a non-strongly water-wet state.
Discussion and application to log calibration
From the various experiments described above, we can observe general trends
describing the temperature dependence of the surface relaxivity (Table 4).
For water- or oil-wet surfaces composed mainly of silica, (SiC grain packing,
Clashash and reservoir sandstones), the water surface relaxivity parameters
increase with temperature according to Eq. 2 to 4 with an observed effective
activation energy in the range [-2; -1.7 kcal/mol], and the oil surface relaxivity
parameters decrease with temperature with an observed effective activation
energy in the range [1.5; 2 kcal/mol]. For water-wet or weakly water-wet surfaces
composed mainly of CaCO3 (calcite grain packing, Lavoux limestone and reservoir
carbonate), the water or oil surface relaxivity decreases with temperature
with an effective activation energy in the range [1.9; 2.5 kcal/mol]. Note
that we used a refined oil (dodecane) as the oil phase to determine the oil
surface relaxivity. More complex fluids (mud oil filtrate or native crude oil)
might complicate the analysis but the general trends should still be valid.
When laboratory and log data are compared in similar saturation conditions
(i.e. at 100% water saturation for a water based drilling mud, or at oil-water
irreducible water saturation for an oil based drilling mud), the relaxation
times might be shifted by a factor equal to the right exponential term in
Eq. 3. For instance, from 25∞ C up to 120∞ C, a shift factor
of 2.1 is possible (taking DE = 2 kcal/mol), either in reducing or increasing
the relaxation times depending on the type of surface. In practice, the temperature
effect might be hindered by pore coupling when considering bimodal pore structures,
or by mixed surface composition (quartz and calcite). There are also further
complications related to the existence of static magnetic filed gradient
present in the logging tools that prevent the measurements of long relaxation
times.
On the basis of temperature dependence measurements of NMR relaxation times
and a theoretical description of the solid-liquid interactions occurring
at the pore surface, we have evidenced two different kinds of temperature
dependence of the relaxation times of liquid in pores. The temperature dependence
of the relaxation times of water confined in pores with silica surface is
anomalous, that is to say, decrease with increasing temperature. On the other
hand, relaxation times of oil in pores with silica surface increase with
temperature. Relaxation times of water and oil in pores with calcium carbonate
surface both increase with increasing temperature. The consideration of these
characteristic temperature behaviors can be used as guidelines to compare
NMR well logging data acquired at elevated temperature and laboratory experiments
performed at room temperature.
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