Page 29 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Concept of Gas Wettability and Research Status CHAPTER 1 13

core is intermediate-wet. It decreases greatly when the surface is water-
wet. It is lowest when the surface is oil-wet, but has a slight drop when
compared to the recovery when the surface is water-wet. They believe
oil-wet surface is one of the reasons for high water injection pressure of
reservoirs with low permeability. Changing the wettability of the reser-
voir into intermediate-wet can enhance the injection capability of water
(and chemical oil-displacement agents).
Song Xinwang et al. [27], did further experimental studies on the effects of
wettability on water-displacement recovery. Study results indicate that during
the actual exploration of reservoirs, water-displacement recovery of reservoirs
with intermediate-wet surface is the highest. The reason is that water-wet reser-
voirs are also beneficial for improving water-displacement recovery, but the
extent is not as great as that of an intermediate-wet reservoir. Capillary force is
the driving force of displacement in pores of water-wet rocks. Due to the com-
plexity of pore structure and wettability alternation during displacement pro-
cedure, oil in a porous medium is divided into oil droplets or separated in
different sizes in the process of dynamic water flooding. Jamin effect is gener-
ated when the discontinuous oil droplets or separated oil flows through the
pore throat, so that percolation resistance arises. Furthermore, the smaller the
pore throat is, the more complicated the pore structure and the greater the per-
colation resistance. In strong water-wet cores with low permeability, though
the capillary force allows injected water to access small pores easily, clay swell-
ing makes the pore structure more complicated. The efficiency of water dis-
placement is the lowest for materials like clay. For strong oil-wet cores, the
recovery is lower than that of strong water-wet cores, as the capillary force
makes the percolation resistance direction quite chaotic. This is generated by
extensively dispersed oil/water interfaces in complicated pore structure and the
influence of oil film on final recovery. When exploring strong oil-wet reser-
voirs, the water displacement method is not used too often, since oil wettabil-
ity is not contributed to enhancing recovery. However, in the pores of
intermediate-wet cores, oil columns of different sizes are present in the station-
ary state for the dispersed residual oil, caused by different sized pores. Though
wettability hysteresis is caused in the flow process, the Jamin effect is smaller
compared to the core with strong water-wet or strong oil-wet surfaces. Hence
sweep efficiency of water and macroscopic displacement efficiency are pro-
moted, thereby improving final recovery.
In the same year, Yao Changyu et al. [28], conducted an experimental study
on decompression and augmented injections with intermediate-wet ability
agent. Intermediate-wet augmented injection is a new type of decompression
and augmented injection agent, which increases water phase permeability and
reduces the injection pressure by changing reservoir wettability and decreasing
oil-water interfacial tension. They examined the effect of the agent through
oil/water interfacial tension, wetting CA and decompression, and augmented
injection displacement experiments. The strata water used in experiments is
3
CaCl 2 type with a density of 1.03 g/cm and total salinity of 54.2 g/L. The