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Sunday, May 27, 2018

ASP Flooding

ASP flooding is a Chemical Enhanced Oil Recovery method where Alkali, Surfactant and Polymers are injected either in a given sequence or all together. The three chemicals have different effects. Often they interact with reservoir fluids (oil & water), rock minerals and among themselves to enhance oil recovery. In this article, we will explore these effects.

Effects of Alkali:
Alkali such as Caustic Soda (Na2CO3) are added to the chemical cocktail. It has two primary impact:
(a) Alkali reacts with the in-situ "Petro-Acids" and produces "Soap", a kind of surfactant. It compliments the surfactant requirement.
(b) Alkali competes for the absorption sites along with polymer and surfactant.
(c) Alkali causes 'emulsification' which improves sweep efficiency.
(d) Oil Entrainment Effect
(e) Bubble Entrapment Effect
(f) Wettability Reserval

Effects of Surfactant:
Surfactant primarily works and IFT reducer. It loosen up the oil particles attached to the rock surfaces and thus assist move them closer to  the producing well. The effect is usually quantified using "Capillary Number", defined by:

          Nc = uμ / σ

          u = displacing fluid velocity
          μ = displacing fluid viscosity
          σ = Inter-Facial Tension (IFT)

As Capillary No. increases, more oil droplets get detached from the rock surfaces and move towards the producers. In other words, it reduces the Irreducible Water Saturation (Swirr).

Effects of Polymer:
  • Polymer helps in improving the sweep efficiency of the injected chemical by mobility control.
  • Due to "ViscoElastic" property, polymer flooding helps push/pull oil droplets from dead-end pores.
  • Another, often ignored positive impact is the favorable economics compared to water injection. For the same effect, less amount of pore volume need to be injected.
The Synergies and Interactions of ASP:
When the three types of chemicals, namely Alkali, Surfactant and Polymer are injected "together", an increased oil recovery is observed. It is probably due to the advantage obtained due to the synergies and interactions between the said chemicals. These effects can be summarized as follows:
1. Alkali competes with absorption sites and hence reduces polymer and surfactant absorption, hence their requirements.
2. Alkali reacts with acidic crude oil and generate soap, called "petro-soap". This soap has low optimum salinity whereas injected synthetic surfactant has higher optimum salinity. As a result, the mixture of petro-soap and synthetic surfactant has a wider range of optimum salinity where IFT is low.
3. Emulsion improves sweep efficiency as the small bubble blocks the wider pore throats and forces the displacing fluid to enter the narrower pore throats. Petro-soap and surfactant make emulsion stable due to reduced IFT. Polymer may also help to stabilize emulsions owning to its high viscosity to reduced coalescence.
4. Addition of polymer improves sweep efficiency of the entire chemical slug, enabling more oil bearing zone exposed to surfactant and alkali.

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