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.

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 (S_wirr).

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.

Thermal EOR

Thermal recovery techniques principally targets crude oil with very high density  ( < 20 deg API ) which cannot flow on its own. These methods raise the temperature of the reservoir which in turn reduces the viscosity of the crude and breaks the larger crude oil molecules into smaller molecules. The heat also reduces the surface tension and improves the overall mobility of the crude.

Some of the major techniques that are employed in the industry :

• Cyclic Steam Stimulation
• Steam Flood
• In-Situ Combustion

What is MEOR ?

MEOR or Microbial Enhanced Oil Recovery is an enhanced oil recovery method, where a carefully selected microbes are nurtured in the reservoir. The microbes "eat" the heavy components of the crude oil and convert them into higher components so that they can move easily from the reservoir to the well and then up to the surface. The microbes also release chemicals such as surfactants, CO₂ etc that also assist in oil recovery.

Nutrients and oxygen must be supplied to the reservoir for the microbes to flourish and give the desired results. Hence, it is often difficult to implement the process across the reservoir.

Globally, this method seems to have limited success. 

What is Foam Injection?

Foam Injection is a method of Enhanced Oil Recovery, whereby "foam" are either injected or generated in the reservoir to improve the oil recovery. "Foam"s are very small bubbles of gas inside a liquid. 

Foam is as light as gas, but has higher viscosity. This lowers the mobility ratio of the injected phase and give rise to a more favorable mobility ratio. Foam can be injected with an existing gas injection setup.

What is WAG ?

WAG stands for "Water Alternative Gas".

It is an improved oil recovery (IOR) process where water and gas are injected one after another from the same injector wells. For example, water is injected for 3 months followed by gas injected for 3 months and the cycle repeats over and over again. This cycle may be optimized and can vary from weeks to months. 

The process is found to give incremental benefit in terms of ultimate recovery over only water or gas injection.

You may read more details from these sites:

www.statoil.com

What is EOR ?

EOR stands for Enhanced Oil Recovery.

It is a process where oil production is attempted to increase or the ultimate recovery is maximized by injecting an agent not naturally present in the reservoir, such as various kinds of chemicals, microbes or stream etc. Broadly EOR processes can be classified as follows:

1) Thermal Processes

• Steam Injection
• In-Situ Combustion

2) Chemical Processes

• Polymer Injection
• Alkaline Injection
• Surfactant Injection
• Saline Water Injection
• ASP Injection

3) WAG (Water Alternating Gas Injection)

What is Polymer Flooding ?

In simple language, polymer flooding in a reservoir involves injection of water mixed with polymer increase it's viscosity and thereby reduce it's mobility. 

In a heterogeneous reservoir, relatively more mobile water by-passes a viscous oil, creating an unstable water front and leaving behind a huge amount of unswept  oil. Polymer is added to the injected water to increase it viscosity and it helps to establish a stable water front and improve the sweep efficiency of the reservoir. 

Figure-1

Figure-2

In Figure 1, a situation where more mobile water is "finger" towards the producer faster is shown. On the other hand, Figure-2 shows a polymer mixed water injection, where the water front is relatively stable and the fingering effect is less.

What is Artificial Lift ?

When the reservoir pressure is not sufficient to lift hydrocarbon from the bottom of the well to the surface, the lift mechanism employed to lift them to the surface and transport to the nearest collecting or separating facilities is called "Artificial Lift". The following list gives some commonly used artificial lift systems:

• Gas Lift (GL)
• Rod Pump / Sucker Rod Pump
• Electrical Submersible Pump (ESP)
• Hydraulic Pump
• Progressing Cavity Pump (PCP)

What is Water Flooding ?

"Water Flooding" or "Water Injection" is an improved oil recovery method, where water is injected at some carefully selected located parts of  the hydrocarbon reservoir to improve hydrocarbon recovery either by "pressure maintenance" or by improved "sweeping" of the reservoir.