To solve the problem of poor effect of single profile modification agent for multilayered commingled reservoir in offshore oilfield at high water-cut stage, a new classified combination system of in-depth profile modification system was designed based on reservoir characteristics.
Pressure index method was studied for profile modification decision-making, laboratory experiments was studied for screening classified combination system for profile modification, water-swelling property of particle in dispersed phase were tested, porous flow theory was studied for matching relationship between reservoir pore-throat and particle size, a model for calculating particle size which matched the original reservoir was built by using Weibull distribution function, and the model was modified by considering pore structure change after longtime waterflooding and formation heterogeneity, and particle diameter range which matched the post-reservoir formation was obtained with taking particle water-swelling property into account.
Laboratory experiments showed that injection of high-intensity(final frozen strength 0.07 MPa) profile modification agent in continuous phase after injection of low-intensity(final frozen strength 0.05 MPa) agents provided a recovery efficiency which was 3.1% higher than that by reverse injection sequence, and injection of profile modification agent with small particle size(initial particle size 0.5-3.0 µm) in dispersed phase after that with large particle size(initial particle size 50-500 nm) contributed a recovery efficiency which was 4.1% higher than that by reverse injection sequence. Highest plugging rate was obtained when the ratio of dispersed phase particle size after expansion to that of pore throat inside formation was 1, and it was required that initial particle diameter was less than 1/7 of pore throat diameter, and minimum particle diameter after expansion was greater than 1/3 of pore throat diameter. Proposed technology was applied in SZ oilfield. Results from two well patterns which conducted proposed technology in 2014 indicated that water cut reduction ranged from 5.0% to 13.0%, accumulative oil production increased by 3,4000 m3 and oil recovery of the pilot zone was enhanced by about 0.5%, which suggests that new classified combination system of in-depth profile modification had good prospect in conventional heavy oil reservoirs of China offshore oilfield.
If appropriate profile modification agent is selected according to the reservoir property and optimized combination of profile modification system is designed, the best effect of profile modification at high water-cut stage can be achieved. Classified combination system of in-depth profile modification based on reservoir characteristics is a new idea and it is an effective way for developing multilayered commingled reservoir at high water-cut stage, it is also an important trend to adjust the water injection profile for high water-cut stage reservoir.