W
ater 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.

Type and Compositions of Water Shut-off and Profile Modification Products

Trade name
PAM
0
0
0
Type
Molecular Weight
Cationicity
Anionicity
Auxiliary Chemicals
Application
KTP-1
PAM-2
8-10x106
s-
3%
Lignin,potassium
perchromate
calcium chloride
(or sodium chloride)
Near-well
bore profile
modification
KTP-2
PHPAM-5
13-15x106
-
8-15%
Potassium perchromate,
sodium thoisulfate,
formaldehyde,
MPAM
Near-well
bore profile
modification
STP-1
LHPAM-1
10-12x106
5-10%
8-12%
Crosslinker,
stabilizer,
gel promoter
Deep-into formation profile modification

In 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:

1. KTP-1 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.

2. KTP-2 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%.

3. STP-1 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.

When 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.

 

 
 

 


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