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Tuesday, May 29, 2018

ASP Flooding - Various Lab Tests Required

Polymer
- Aqueous stability test
- Filtration test
- Viscosity @ various shear rate

Surfactant
- Salinity scan test
- IFT (may be Solubility Test) - done with O/W=1:1 ratio
- Oil Scan Test ( IFT test donet at different O/W ratio)

Core Flood Test
- To see if significant oil recovery could be achieved.
- To see the reaction of rock mineralogy with the proposed chemical slug.

ASP Flooding - Reservoir Selection Criteria

A] Formation / Rock Mineralogy Sandstone Reservoirs are preferred and "Anionic Surfactant" can be used. In Carbonate Reservoirs, "Anionic Surfactant" are highly absorbed, hence cannot be used. On the other hand, "Cationic Surfactant", which might be used in Carbonate reservoirs are expensive and economically not viable. Moreover, there is also a rock of Anhydrite formation in such reservoirs. Clay also absorbs surfactant. If the reservoir has high clay contents, it might not be suitable for ASP flooding. 

B] Oil Composition and Acid Number/Acidity These are important specially for alkali & surfactant. The Acid Number of Crude should be high (min. 0.3mg KOH/g of oil or higher) so that Alkali reacts with it and produce enough soap. 

C] Oil Viscosity This is particularly important for polymer component of the flooding. It determines the required Mobility Control and hence the design Polymer concentration, slug size etc. Usually crude oil of lower viscosity of 35cP or less is preferred. 

D] The Reservoir Temperature is expected to be less than 90 C, and preferably between 40-50C. Higher temperature degrades polymer and impacts the optimal salinity of surfactant.

E] Formation Permeability is expected to be high from 500mD to Darcies for easy movement of high molecular weight chemicals, especially polymers.

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.
PLUS
(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

where,
          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.

Tuesday, February 20, 2018

Abbreviations

OAPL = Open Acreage License Policy HELP = Hydrocarbon Exploration and Licensing Policy NELP = New Exploration Licensing Policy NDR = National Data Repository

Sunday, December 13, 2015

Black Oil vs Compositional Modeling

We often hear the Reservoir Engineers talking about Black-Oil model and Compositional Model. What are these? How do they compare? 

All these are different fluid model or the part of numerical simulation model that handles how the fluids, that is, oil, water, gas and any other substances would behave at different pressure and temperature.

Black Oil model is relatively simpler model with only three fluids - oil, water and gas. 

An extended version of Black-Oil model, called Extended Black Oil model takes are of volatile oil in gas and dissolved gas in oil.

A Compositional Model  on the other, tracks any number of components the fluid might actually has - like different hydrocarbons (C1, C2, C3, C4. C5, C6, C7,... etc), H2S, CO2, Water etc. These models are complex and takes lot more computing time than Black-Oil models.