Formation Damage part 2

A Formation can be damaged mainly while one of following activities (ordered in timely manner).

1) During Drilling and Completions (click to see)

2) During and after production and workover operations

3) During Injection 

Formation Damage Mechanisms:

2) During  and after production and workover operations.

This kind of formation damage is divided into following types.

1) Due to increase in relative permeability of fluid other than desired fluid.

2) Due to viscosity increase

3) Due to precipitations.

1) Due to increase in relative permeability of fluid other than desired fluid:

Imagine, you are left with 1 GB of internet with your plan and you are sharing it with your friend with help of wifi. You are surfing at your normal speed, but your new friend is watching movie online

What will happen ? You will be not able to surf more time ? 

Same thing happened in reservoir also. We want more oil flow for More time, but other fluids (water, gas) block path of oil flow and results into less oil production.

As production from reservoir is going on, pressure of reservoir depletes, which increases pressure forces or inertial forces and results into water conning or gas conning. Both results into water blocking or gas breakouts respectively.

This kind of problem of conning where water comes out of reservoir, can increase the interfacial tension between oil and water and increases water saturation near well bore region, which further results into Water blockage. 

Water blockage can also occur due to invasion of drilling or work over fluid also.

The water blocks causes increase in water relative permeability and decrease in oil and gas relative permeability near wellbore region (As saturation increases, relative permeability of that phase increase). It proves to be more troublesome in following conditions: 

1) Low permeability, 

2) Low pressure gradients(gas wells),

3) High surface tensions (gas wells).

Tight gas reservoirs are more susceptible to this kind of damage. In fracture fluids also water blocking may be effective and lead to damage.

Gas with more permeability.

Another reason of formation damage which is mechanism wise same as water blocking is, Gas breakout, The gas breakout is caused by increase in gas saturation near wellbore or downward coning (Gas conning) of gas cap present above oil zone (if any). This leads to gas breakout and increase in gas saturation and gas permeability and decrease in oil permeability and hence oil production. 

In gas wells, Condensate banking is also mechanism wise similar problem, in which condensed liquid block the oil passage of flow. This kind of problem occurs in gas reservoirs with significant condensate yield and relatively high dew point pressure. High dew point pressure allows it to form gas condensate earlier inside well bore. As the near wellbore pressure reduces during production, and dew point pressure of gas is reached. Dropout of liquid hydrocarbons in near wellbore zone increases the condensate's relative permeability and reduces relative permeability of gas. Once formed, condensate bank itself causes additional pressure drop, and is self-aggravating. 

Water conning in reservoir

2) Formation damage Due to viscosity increase :

Just think, what will easily flow ? Water or honey?

Definitely water. Because it is less viscous. 

Similarly, emulsions have higher viscosity than either constituent fluid, and if they are present or formed inside reservoir, then it will be difficult to remove or even move oil with thick emulsion.(Darcy's law: higher the viscosity, lower the production rate.) This problem of formation damaged is known as Emulsion blocking near wellbore.

Emulsion formation results from the incompatibility of different types of fluids (introduced during Work over and production related operations) in the formation or near the formation.

Eg. Operator is trying to wash perforations with water with some additives and due to incompatibility of washing fluid,resulted into emulsion formation downhole or inside reservoir rock).

Emulsion formation may be enhanced by presence of surfactants and fine particles such as clays or asphaltene.

  

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