Water flooding is a widely-used process
for oil recovery throughout the world. However, during the water
infection phase the injected water can quickly break through to
the producing wells along high-permeability zones or fractures due
to heterogeneity of the oil formation and the mobility difference
between oil and water, thus causing quick rise of water cut and
quick drop of oil production. In such case, production well water
shut-off and injection well profile modification treatments can be
effective measures to lower water cut and increase oil production.
In the above
operation, the most effective one usually is to modify the injectivity
profile through injection of a chemical profile modifier into the
injection well. This process can effectively improve the volumetric
coverage of injected water and minimize the water cut of producing
wells, increasing oil yield. Based on this concept, our company
has developed several kinds of new profile modifiers from PAM compositions.
The following table shows types and compositions of such products
developed by our company.
and Compositions of Water Shut-off and Profile Modification Products
(or sodium chloride)
formation profile modification
the above table, all the products are compounded. PAM is the main
composition, associated with other chemicals to form a crosslinking
system applicable to water shut-off and profile modification. A
KTP type is for near-well bore treatments and STP is for deep-into-formation
treatment. Our company can make these products, conduct field service,
and help to design and implement the treatment process. Below is
a brief discussion about the above products:
is a nonionic PAM from copolymerization. Thanks to its special nonionic
structure, it has good stability at elevated temperatures and in
high salinity formations. PAM is combined with lignosulfonate, potassium
perchromate, calcium chloride (or sodium chloride) in certain proportions.
Its gelling mechanism is based on the hexavalent chromium ion converting to
a very active trivalent chromium ion through a redox reaction, which
them brings about a series of crosslinking reactions which are: (1)
crosslinking between PAM molecules; (2) crosslinking between lignosulfonate
molecules; (3) crosslinking PAM with lignosulfonate. Through these
three kinds of crosslinking reaction, gel is formed to block this
zone. Addition of calcium chloride can reduce the dosage of perchromate
in the formulation, and also can adjust the gel viscosity and gel-forming
time. This system shares the advantages of both crosslinked PAM
gel and crosslinked lignosulfonate gel. It is tolerant to high temperature,
so it can be used as water shut-off and profile modification chemicals
in steam flooding or in oilfields with high formation temperature.
is a compound profile modifier, composed of mainly swellable PAM
(SPAM) and hydrolyzed PAM (PHPAM-5), associated with a redox system.
The swellable PAM is a new product of our company. It is characterized
by swelling from absorbing water. It can absorb water 10 to 300
times its original weight. After absorbing water, the swelled grains
maintain high strength without releasing any water. As the swellable
PAM is combined with PHPAM, a gel with three-dimensional structure
is formed which has higher viscosity and strength. The formaldehyde
contained in the system will react with PHPAM, to further increase
the viscosity and strength of the resulted gel. Since PHPAM crosslinking
time can be adjusted over 24 hours, the chemical system has quite
low viscosity when injection, which means good pumping ability.
If the system is injected into a water well without any isolation
between the zones, the system will preferentially get into the high-permeability
zone. So it has good selectivity when injected. The swellable SPAM
in the system is added at the wellhead during injection. Its initial
swelling time is over 5 hours, so it will not affect pumping operation.
Along with the time going on, the profile modifier will get deeper
and deeper in the formation. Crosslinking of PHPAM is proceeding
simultaneously with the swelling of SPAM. Finally, a crosslinked,
swelled, three-dimensional structure is obtained, which can act
as profile modifier in the water injection wells.
Based on the results of oilfield trials, a conclusion can be drawn
that this profile-modification system has good performance in large
pore, high permeability zones, and also in fractured sandstone oil
reservoirs. Success rate can be as high as 100%.
is a newly-developed deep-penetration profile modification system.
KTP-1 and KTP-2 are chemical profile modification systems used
for near well bore treatments. The injected amount of those two
systems is relatively small, so their treated radius and covered
area is quite limited. After such a profile modification treatment,
the later injected water or EOR polymer can bypass the treatment
in a short time, then channel along the high permeability zone behind
the treatment. Thus, the incremental sweeping area will not be large,
and the increased oil output and decreased water production will
be limited. The effective period is also short. For this reason,
near-well bore mechanical and/or chemical treatment cannot always
meet the goal when the oilfield is in high water-cut development
stage. The oilfields urgently need a practical technology for deep-penetration
water blocking and profile modification. Such technology is of great
importance in the oilfields in high water-cut developing stage to
improve water drive or polymer drive efficiency. STP-1 is an efficient
profile modification system specially formulated for deep penetration
into the oil formation. STP-1 is composed of amphoteric PAM, crosslinker,
stabilizer and gel promoter. The amphoteric PAM employed is a tri-polymer
with cationic, anionic and nonionic groups. The amimo and carboxyl
groups on its molecule can adsorb on the surface of rock which has
been exposed to water flooding. In particular, the cationic groups
on its molecule can electrically neutralize the negative charge
of sandstone surface, thus let the polymer firmly adsorb on the
rock surface. The non-adsorbed part of the polymer molecule will
extend into the flooding water, creating resistance to the water
flow. Thus, the permeability of the high-perm zone is lowered and
the adsorbed polymer is protected from being washed off by the water.
The effective period of the chemical system is extended. The amphoteric
PAM firmly adsorbed on the rock surface is combined with crosslinker,
stabilizer, and gel promoter, to form a light strength, gel-like
water blocking agent. It contains both polymer gel and resin gel.
Such kind of gel is a non-selective blocking agent. It can plug
the pores and fractures in the formation through several mechanisms,
such as adsorption, dynamic trapping, and physical plugging. By
adjusting the proportions of the components in the SPT-1 chemical
system, the gel forming time and gel viscosity can be effectively
controlled to fit different geological conditions and realize deep
penetration profile modification.
the molecular weight of amphoteric PAM is 10 to 12 million and the
formation temperature is 35 to 50, the crosslinking time of SPT-1
can be delayed up to 30 days, and the resulting gel viscosity can
reach over 20,000 mpa.s.
Since STP-1 crosslinking time is adjustable, in application it can
form several slugs in the high-perm zone, the crosslinking time
of each slug can be designed as per requirement. The deepest slug
should be designed to have the longest gel-forming time. This time
is determined as per geological conditions, well spacings, customer
economic requirement, required penetration depth, etc. After the
gel-forming time of the deepest slug is determined, the gel-forming
time of the following slugs will be shortened one after another.
The quickest gel-forming time will be designed for the slug closest
to the well bore. After all the injection is completed, the well
is shut in for one week. STP-1 can also be used in near-wellbore
profile modification. Either in near-wellbore or in deep-penetration
profile modification, the amphoteric profile modification system
has been proven far more cost effective than conventional ones both
in its dosage and results.