After polymer flooding in Daqing Oil Field, the injection pressure in the flooded areas has increased. The injection pressure of one third of the polymer-injection wells is near to its fracturing pressure, and many polymer injection wells have to lower its injection rate. Since the effective period of fracturing in polymer injection wells is less than 3 months, a new polymer injection well stimulation technology was in great need to be developed.
The system does not need alkali, and under formation conditions, the interfacial tension remains ultra-low for at least 28 days. A series of lab tests shows that to first remove the polymer adsorbed on the rock, a slug containing an oxidant needs to be injected before the surfactant slug. The surfactant slug lowers the residual oil concentration, increases the relative permeability to water and adsorbs on the rock near the well bore, preventing polymer to adsorb on the rock in the subsequent stage of polymer flooding. Lab tests shows that adopting a flooding process of subsequently injecting an oxidant slug, then a surfactant slug and last followed by polymer flooding obtains the best results (the permeability to polymer fluid is increased about 120%).
At present, the starting pressure (e.g., the pressure at which water starts to be injected into the well) of polymer-injection wells is commonly high and injection rate is relative insufficient in Daqing Oil Field. The high starting pressure and injection pressure of the polymer injection wells greatly influence the development results of the oil field, these are mainly reflected in the following aspects:
The starting pressure is too high so that an effective pressure differential cannot be set up between production and injection wells, resulting in poor developing result.
The high injection pressure causes an increase in the number of casing damage wells.
The injection pressure is too high; therefore the injection solution cannot advance evenly into the wells.
In order to maintain the injection rate of medium and low permeability layers developed by polymer flooding, the well spacing needs to be reduced and the development cost is increased.
Therefore, it is very important to study technology that can effectively reduce the starting pressure and injection pressure of polymer-injection wells.