Page 56 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

40 Gas Wettability of Reservoir Rock Surfaces with Porous Media

Normally, the spin-lattice vertical relaxation time T 1 of neutron in water is the
basis for measurement. Brown and Fatt [23] were the first to put forward the
idea of measuring subwettability of porous medium using the NMR relaxation
method. They found that in the sand mold mixed with oil-wet and water-wet
sand, a linear correlation exists between the fractions of relaxation speed and
an oil-wet surface in a certain proportion. After measuring the spin-lattice ver-
tical relaxation time T 1 of neutron in water in each system, it can be concluded
that the relaxation speed of a water-wet system is faster than that of an oil-wet
system.
The porosity and pore size distribution of various cores is different, even in an
water-wet or oil-wet core, and the NMR value may be different. When the core
is completely water-wet or oil-wet, the NMR value must be measured for mea-
suring the wettability of cores as a reference point of the original core. Thus
the accuracy of this method greatly depends on how the surface is treated, and
the measuring procedure is complex, so it should be used carefully.

2.1.10 Dye Adsorption
This method measures wettability of a sand-packed model with water-wet and
oil-wet sand. It cannot be applied for measuring wettability of reservoir rock
core containing clay [24].
After reviewing the wettability evaluation methods for a liquid-liquid-solid
system, it can be observed that there are three states of wettability: water-wet,
oil-wet, and intermediate-wet. All the methods mentioned above are com-
monly used evaluation methods of wettability for an oil-water-rock system,
and every method has its own advantages and disadvantages as well as scope
of application. Methods involving a gas-liquid-rock system are: contact angle
method, relative permeability method, flotation method, and spontaneous
imbibition. Due to the large difference inviscosity and density of gas com-
pared to liquids and its strong compressibility, it is necessary to further explore
other methods that can be used for evaluating gas wettability.

2.2 EVALUATING GAS WETTABILITY USING
TRADITIONAL METHODS
As gas wettability is different from liquid wettability, theories and methods
applicable for studying liquid wettability do not necessarily apply to research-
ing gas wettability. Qualitative evaluation of gas wettability conducted by ear-
lier researchers was processed using the methods for studying liquid
wettability without taking into account its advantages, disadvantages, and
applicability. So the frequently used capillary force rise method, contact angle
method, and Washburn method were adopted to evaluate gas wettability. The
applicability and features [25] were concluded by verifying and comparing
results.