Formation Damage : part 3

Porous Formation

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

1) During Drilling and Completions

2) During and after production and workover operations

3) During Injection 

Formation Damage Mechanisms:

2) During  and after production and workover operations.

(This is part 2 of this topic, for part one of this topic click here.)

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.

3) Formation Damage Due to precipitations:

Main precipitation problems on field during production:

1) Inorganic scales,

2) Precipitation related to acidizing In stimulation jobs (Sludge formation),

3) Paraffin

4) Asphaltene

We have aqueous formation water is already present inside reservoir rock pores along with oil, and during Workover or other activities, when other fluid enter into formation, which is not compatible with our reservoir fluid, then it creates problems.

If, in any circumstances, if any of invading waters (such as filtrates, completion fluids & injection water) contain anions that are potential scale generators invades and get into contact with reservoir water (if formation water contains significant percentages of Ba, Sr or Ca ions), then this mix up, under certain conditions of temperature, pressure, pH and concentration, can precipitate compounds such as BaSO4, SrSO4 and CaCO3 (In-organic scales), which can block pore spaces. and leads to formation damage. 

Sometimes,  it might happen to everyone of us that, we want to help someone, but by mistake we make him or her more trouble. Same thing can also happen in our production operations also.

We try to help reservoir by clearing its choked pores by acidizing, but if we do not take care of compatibility of acid which we are going to use, than it can also be a trouble maker for our reservoir.

Eg. If our reservoir is having more calcium (or carbonate) and we use direct hydrofluoric acid (HF) During acid treatment, then it can develop white powder precipitation of calcium fluoride (CaF2). And it can choke pores or damage our formation.

(So, we try to remove Carbonate present inside, before final acidizing with mud acid (HCl + HF) with help of HCl acid.)

Another example is when formations are rich in minerals like chlorite and ferric dolomite, then acid can solubilizes the iron, and may lead to of iron hydroxide Fe(OH)3.

As acid dissolves matrix and matrix cementing materials, mineral grains may be liberated and can migrate to plug pores. (Fines migration)

Another problem which can be developed due to acidization job is,

 Sludge formation.

 (Sludge is semi-solid compound and its formation takes place due to reaction between acid and crude oil.)  It usually occurs with heavy crude.  It may also occur in lighter crude, if invading acid is iron contaminated. 

Above two precipitations occurs while mixing of two incompatible fluids.

Another kind of precipitation can occur while changing its pressure, temperature or compositions like, 

Paraffin deposition and

Asphaltene deposition.

As fluid move through reservoir to surface, it undergo various pressure temperature modifications.

When fluid pass through reservoir to wellbore, it experiences majority of pressure changes, and while coming from well bore to surface, it experiences temperature reduction.  

1) Due to pressure changes.

When we produce crude oil from reservoir, part of crude oil (asphaltene) which is difficult to remove from pores of reservoir is left trapped inside reservoir rock (Due to pressure reduction with production) and choke the pores (Damage the reservoir).

So, as production continues, reservoir pressure depletes, hence asphaltene will be precipitated and deposited on rock grains.

**(Asphaltene is black carbonaceous compound of petroleum and are insoluble in high paraffinic hydrocarbon solvents. These compounds have relatively high molecular weights and are considered as polar materials due to the presence of S, O, N and complex metals.) 

Asphaltenes occur in many crude oils in the form of colloidal suspended solid particles. Asphaltene precipitation takes place when the crude oil loses its ability to disperse the particle due to alteration of certain equilibrium condition.  It occurs mainly when pressure drops.

Remember, Asphaltene deposition is pressure related phenomena & it occurs mostly in reservoir.

2) Next part after reservoir to wellbore, where fluid flows happen is, inside tubing.

While having journey from wellbore bottom to surface, crude oil mixture undergone temperature reduction which can lead to paraffin or wax precipitation and deposition inside tubing.

(Another spot where temperature reduction can happen is downstream side of choke. (Joule Thomson effect).)

 If sufficient temperature reduction is happening in reservoir itself, then paraffins can also precipitate and deposits in pores of reservoir also.

And it can lead to formation damage.

Paraffins are the  hydrocarbon fractions with a carbon chain length starting of C18-20 up to C70 or higher. 

Temperature at which the first solid paraffin crystal forms from an all-liquid solution is called the cloud point. 

Remember,  Asphaltene deposition is more over pressure related phenomena while Wax or paraffin deposition is more  over temperature related phenomena.

Other problems can also lead to formation damage like, 

Bacteria, it can be a serious problem in production operations because of what they consume and their byproducts. Bacteria can grow in many different environments and conditions: temperatures ranging from 12°F to greater than 250°F [–11° to >120°C], pH values ranging from 1 to 11, salinities to 30% and pressures upto 25,000 psi. If the invading fluid contains bacteria then formation pore spaces may be blocked by accumulation of bacteria. 

  

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