Page 54 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

38 Gas Wettability of Reservoir Rock Surfaces with Porous Media

FIGURE 2.5
Diagram of wettability evaluated by capillarimetric method.

If water can wet the glass tube, the driving energy is positive. However, if oil
can wet the glass tube, it is negative. If either one of the two liquids can
completely wet a glass tube, the contact angle is zero, cosθ is 1, and the driving
energy is equal to the interface tension.

2.1.8 Imbibition Rates
The imbibition rate method is very effective for measuring wettability. When
Amott wetting index I W is fairly high for a system, wettability can be measured
with imbibition rate method. Imbibition is controlled by capillary pressure to
a large extent [20]. The capillary force and surface tension between two fluids
are in direct proportion to the cosine value of contact angle. So the imbibition
effect featuring total fluid rate (especially the initial rate) and displacement
depends on wettability.
Imbibition rate is simple and characterized by short cycles, but it only applies
to rock samples from strongly water-wet to intermediate-wet. It requires a
strongly water-wet reference sample and is sensitive to it. Moreover, the imbi-
bition is related to not only wettability, but also relative permeability, viscos-
ity, surfacetension, pore structure, and initial saturation. Meanwhile, it is not
sensitive to intermediate-wetting.

When the rock sample is strongly water-wet, the specific values (see Eq. 2.9)of
imbibition rate (m) and reference imbibition rate (m r ) are taken as the index
for wettability evaluation to offset the effect of other factors on imbibition.
Strongly water-wet can be obtained by heating cores for 24 hours at 400 C.

:
m
R 5 : (2.9)
m r
In the equation, R—is relative imbibition rate.
:
m is initial imbibition rate when the core is soaked at the beginning.
:
m r is initial imbibition rate of clean and strong water-wet cores.