Formation Damage part 1

Formation Damage Part 1

Definition: Formation damage is impairment or resistance to fluid flow from or to Reservoir (Formation) near the wellbore area.(vicinity of wellbore).

So, it’s a process.

If formation is damaged, we cannot produce or inject fluid at predicted or desired rate.

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:

1) During Drilling and Completions

We start from drilling activities which result into formation damage. First is Selection of Drilling fluids and its effects.

Drilling fluids are part and heart system of Drilling activities. 

When drilling fluids come into contact with Formation, many changes occurs to reservoir rock and to fluid also (here two different things are going to get into contact, so its natural to happen something).

Whole drilling fluid is unable to go into formation when it is subjected to pressure, as pore throat size of formation rock is so less. So, part of drilling fluid pass through it like filtration with help of sieves.

Simple analogy is shown what happens down hole there.

You may connect now, why rate of tea/fluid is decreasing after some accumulation of tea leaves on filtration surface.

Here in our case, Tea leaves are fines or solids of drilling fluids.

Solids of drilling fluids are of larger size than reservoir pore throat, so it accumulates on reservoir face in well bore (outside of formation ‘’Mud Cake’’) and only filtrates passes though formation.

Image referred from internet

Both Mud cake and filtrates contribute to formation damaged and creates problem to production. this mechanism is known as Invasion of foreign materials.

Mud cake restricts entry of reservoir fluids to well, and hence reduce production. Type of problem filtrate creates inside reservoir formation depends on type or base of drilling fluid.

If drilling fluid is of water based,

 Water reacts with specific clay types. When water and clay come into contact, clay swells (volume of clay increase, hence it reduces pore size of formation, hence reduce permeability and hence reduce production). This is known as Formation damage due to clay swelling.

If drilling fluid is oil based,

Access amount of surfactant used in fluid causes mainly 2 problems.

i)               Emulsion Blocking: emulsion created due to formation water and Oil based mud results into clogging of pores, hence reduces permeability near well bore. (detailed description will be shared in part 2)

ii)             Wettability reversal: Generally, water wet reservoir converts into oil wet reservoirs when they come into contact with surfactant. Oil wet reservoir has less oil permeability than water wet reservoir. (detailed description will be shared in part 2)

Image Courtesy Internet

Other than this, Mechanical damage of the area near well bore may take place due to many physical or mechanical actions during drilling, eg. Rock compression during drilling may occur due to successive increase in the pressure differential (caused by surge pressure). The drill bits and the perforation can also adversely affect the unconsolidated and mechanically weak reservoirs. This all physical and mechanical activities leads to closing of pores and hence damage the formation.

 

(Perforation causes one kind of formation damage. When jet fluids enter into reservoir while perforation, it compresses reservoir to make space for jet fluid to go. This creates Small carrot zone around each perforation, compressed enough to lower the permeability of near perforation or near well bore.)

Image Courtesy  internet

Any cement displaced or cement filtrates if get into contact with reservoir formation, it will create blockage to reservoir fluids to flow into well bore.

for, Formation Damage part 2.

  

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