In the Daqing oilfield, acid stimulation has become a main method used to increase water volume injected into the low permeability sandstone reservoirs, but there are still three problems. First, much of the acid flows into the high permeability layer leaving the low permeability layer untreated due to the permeability difference. Second, clay particles migrate in the low permeability layer, which results in high-speed acid reflux and causes new damage to the formations. Finally, operation costs increase because the spent acid needs to be flushed out of the hole and safely treated.
This paper analyzes the properties of lithology, block-up, and interlayer differences for the thin interbeds, thus showing the newly developed separate zone and zero discharge acidizing techniques.
Through separate zone mathematical models and solid and liquid diverting agents, acid can more effectively enter the low permeability layer achieving separate zone acidizing. The two kinds of diverting agents can be solely or jointly applied in terms of permeability differences. When an agent (solid agent or liquid agent) plugs up a layer on the maximum degree, the injection pressure is at or above 2900 psi, and the plugging ratio exceeds 90%. Diverting agents will also dissolve completely in 24 hours without any damage to the formations.
In addition, a new complex acid system has been developed. The HF-HNO3 acid system mixed with good-performance complexing agents can more effectively inhibit precipitation. Therefore, acid reflux is not necessary after acidizing and "zero discharge" is achieved. Laboratory experiments showed that natural core permeability increased by 240%, and the resulting acid reaction did not cause secondary precipitation until pH values were above 7.
In the Daqing oilfield, tapping the potential of the thin and poor layers in a reservoir has become an important method in the stabilization of oil production. In order to continue developing the Daqing oilfield, the acidizing technique needs to be applied to numerous water injection wells because of low permeability, high clay mineral content, and severe pollution in the thin and poor layers.
