Page 211 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Application of Gas Wettability CHAPTER 6 195

the compatibility of reservoir protective agent with drilling fluidis is
imperfect. In addition, the special damaging effects of low-permeability and
extra-low-permeability reservoirs and the difficulty in accurately predicting the
pore diameter of reservoirs are also the factors that can’t be ignored.
This section studies two kinds of acrylic polymers that are used as oil and gas res-
ervoir protective agents, based on the basic theory of gas wettability: fluorinated
acrylate polymer waterproof agent FCS-08 and (fluoride-free) acrylate amphiphi-
lic polymer filming agent LCM-8. By improving the wettability of rock surfaces
and reducing the surface tension of filtrate and that of oil and water interface,
avoiding the damaging effects on reservoirs caused by the filtrate, especially the
transient filtrate, in combination with “filming” theory, “Zero” damage of low
and extra-low-permeability reservoirs by drilling fluid can be achieved. That is,
new methods are proposed for protecting reservoirs combining wettability alter-
ation technology, water-block resistance technology, and new filming technology.
New technology involving drilling and completion of low and extra-low-
permeability reservoirs mainly feature “improving properties of rock surfaces”
by adding FCS-08 and LCM-8 in the drilling fluid, on top of the reservoirs. The
indoor evaluation and field test shows that the core permeability plugging rate
in the drilling completion fluid system and the recovery rate both are greater
than 90%, thereby realizing “zero damage” to reservoirs around wellbore.
Compared to other drilling completion fluid technology to protect reservoirs, in
use currently, the reservoir protection effect is better, and the daily oil produc-
tion of a single well indicates more than 20% improvement. A combination of
wettability alteration technology, water-block resistance technology, and filming
technology provide new ways to mitigate damage to low-permeability and
extra-low-permeability reservoirs, caused by the drilling completion fluid.

6.1.1 Development of Drilling Fluid Technology for Low
and Extra-Low-Permeability Reservoirs
Normally, low- and extra-low-permeability reservoirs feature high argillaceous
cement, high capillary pressure, fine pore-throats, complex structure, severe
heterogeneity, and greater resistance to oil and gas flow. Damages including
hydration swelling and water-blocks are easy to be generated due to invasion
by external fluids during the drilling process. The rate of damage is up to
70%B90%.
Currently, as drilling fluid technology has been used for a long time in explor-
ing reservoirs in China and overseas, several methods to protect these reser-
voirs, such as temporary shielding method [1,2], fractal geometry temporary
plugging method [3], broad-spectrum temporary plugging method [4], alkali
soluble micron-grade cellulose temporary plugging method [5],D 50 temporary
plugging method [6],D 90 ideal temporary plugging method [7], film-forming
method [8,9], oil-film method [10 14], etc., are being studied and developed,
respectively. However, the protection offered by these methods to low- and
extra-low-permeability reservoirs is unsatisfactory. The reason is that all these