Chapter 2 - Formation Damage

2. FORMATION DAMAGE

1TUNIO, May' 2011,,, Courtesy AP Aung

LESSON OUTCOMES

• Types of formation damage

• Pre-treatment well tests analysys

• Potential formation damage caused by matrix stimulation• Potential formation damage caused by matrix stimulationfluids

• Explain when and how formation damage contributes topoor well performance

• Identify the major source of formation damage


FORMATION DAMAGE CONCEPTS

• Formation damage concerns the formation of a volume ofrock with a reduced permeability in the near well-borezone.zone.

• Formation damage means reduced current production.



• Stimulation treatments are designed to increase the wellproductivity, either by:

– Reducing or completely removing the formation damage by– Reducing or completely removing the formation damage bychemical matrix treatment (e.g. acidizing)

OR

– Bypassing the formation damage by creation of a highpermeability channel by hydraulic fracturing treatment.



• Basic causes of formation damage

– Contact with a foreign fluid is the basic cause.

– This foreign fluid may be

• Drilling mud

• Clean completion fluid or work-over fluid

• A stimulation fluid

• Well testing fluid

• Sometimes even the reservoir fluid itself if the originalcharacteristics are altered



• Most oil field fluids consist of two phases -liquid and solids.

• Either liquid or solid can cause significant damage throughany one of several possible mechanisms:

– Plugging by solids occurs on the formation face, in the– Plugging by solids occurs on the formation face, in theperforation, or in the formation.

– Solids may be weighting materials, clays, viscositybuilders, fluid loss control materials, lost circulationmaterials, drilled solids, cement particles, gravel pack orfrac sand fines, precipitated scales, paraffin orasphaltenes.

TUNIO, May' 2011,,, Courtesy AP Aung


• Liquid may be water containing various types andconcentrations of solids particles and surfactants.

• When liquid is circulated or forced into porous zones by• When liquid is circulated or forced into porous zones bydifferential pressure, displacing or commingling with a portionof a virgin reservoir fluid, it may create blockage due to one ofseveral mechanisms.

TUNIO, May' 2011,,, Courtesy AP Aung


• Classification of Damage mechanism

– Reduced absolute permeability of formation-- results fromplugging of pore channels by particles

– Increased viscosity of reservoir fluid– results from emulsions orhigh-viscosity treating fluids


Pre-Treatment Well Tests

• Diagnosis of Formation Damage

– It is possible to determine whether formation damage or ‘skineffect’ exists in a particular well.

– This can be done through well tests such as infectivity or– This can be done through well tests such as infectivity orproductivity test.

– From pressure build-up or pressure draw-down tests, themagnitude of damage or ‘skin effect’ can be determined.

– Production logging surveys may show zones which are notcontributing to the total flow stream.


DIAGONOSIS OF FORMATION DAMAGE

• Formation Damage during special well operations

1. Damage during drilling of oil and gas zones in wildcat ordevelopment wells

a) mud solids may block pores and natural fractures.

b) mud filtrate invasion into pay zones

c) pores or fractures near well-bore may be sealed by the drillstring.



2. Damage during Casing and Cementing

a. Cement or mud solids may plug large pores andnatural fractures.

b. Chemical flushes may cause changes in clays in theb. Chemical flushes may cause changes in clays in theproducing formation.

c. Filtrate from high fluid loss cement slurries may bringabout changes in producing formation



3. Damage during Completions

a) During perforating

b) While running tubing and packer

c) During production initiationc) During production initiation



4. Damage caused by cleaning of paraffin

a. When cleaning paraffin from a well with hot oil or hot water,formation and perforations will be plugged unless melted paraffinare swabbed.are swabbed.

b. While cutting paraffin from the tubing (using scrapers)andcirculated down the tubing and up the annulus, some particlesmay be pumped into perforations and into pores.



5. Damage during well servicing and work-over

a. When killing or circulating a well, perforations or pores or

fractures may be plugged.

b. Filtrate invasion by incompatible water, oil, or chemicals mayb. Filtrate invasion by incompatible water, oil, or chemicals may

cause water blocks, emulsion blocks, or changes in formation

clays.



6. Damage during production phase

a. Corrosion inhibitors or paraffin inhibitors, if contacted to theformation zones, may reduce permeability.

b. Precipitated scales may plug the well bore, perforations, andb. Precipitated scales may plug the well bore, perforations, andformation.

c. Well bore opposite the producing interval in both carbonateand sandstone wells may become plugged with silt, shale, mud orfracturing sand.

d. Screens or gravel packs may become plugged with silts, mud,or other debris.



7. Damage during Water Injection

a. While injecting water, emulsion may occur in the formationadjacent to the well

b. The tubing, casing, perforations, screen, gravel packs, andb. The tubing, casing, perforations, screen, gravel packs, andformation fractures may be plugged with mud, silt, clay, paraffin,emulsions, and corrosion inhibitors.



8. Damage during Gas Injection

a. Well bore, perforations, formation fractures and pores may beplugged with solids scoured by injection gas.

b. The injection of corrosion inhibitors will reduce well injectivity.


• The value of the Total Well skin (Stotal) measured during aproduction test has many sources other than formationdamage.

Type of Formation Damage - Skin

damage.

• It is very important to be able to identify the formationdamage component (Sd), since this can be reduced bybetter operational practices or possibly, be removed orbypassed by stimulation treatment.


• The total well skin is a composite parameters:



• Formation damage skin

– Most forms of formation damage reduce the rock permeabilityto a certain depth away from the well


– Figure 1 illustrates the resulting producing pressure profile andcompares it with the equivalent pressure profile for anundamaged well.


– Well geometry

• The well geometry skin reflects geometrical considerations


• The well geometry skin reflects geometrical considerationswhich alter the skin value form due to the well design (limitedentry, well not at the centre, well orientation (slanted))


– Completion skin

• Insufficient perforation (density, penetration depth, incorrectphasing)


• Fractures – either naturally occurring or (artificially) createdpropped hydraulic fractures – will lead to increase inflow andnegative skins by placing a high permeability pathway fromdeep in the formation to the wellbore


– Production skin

• A rate dependent skin is often observed in high rate gas well(and very high rate oil wells).


(and very high rate oil wells).

• Its presence can be a useful indication that the well is apotential stimulation candidate


– The resulting extra pressure drop has to be compensated foreither by a reduced pressure drop across the choke or by asmaller production rate


Figure 1


• The total skin effect for a well, s, consists of a number ofcomponents. Generally these can be added together, andtherefore

Formation Damage Concepts – SkinComponents

Where,

– Sd = damage skin

– Sc+θ = partial completion and slant skin

– Sp = perforation skin

– ΣSpseudo = pseudo-skins (all phase and rate dependent effect)


• It is alerted that once a hydraulic fracture is generated,most pretreatment skin effects (Sd, Sc+θ, Sp) are bypassedand have no impact on the post-treatment wellperformance

Formation Damage Concepts – SkinComponents

• Similar case in deep penetrating perforation, may bypassthe near wellbore area

• In general, it is not correct to add pretreatment skineffects to any post-fracture skin effects


• These two skin effects can be determined using themethod of Cinco-Ley et al. (1975) using the dimensionlessparameters;

hD = h/rw : zw/h ; hw/h and {degrees}

Skin From Partial Completion and Slant

hD = h/rw : zw/h ; hw/h and {degrees}

where;

– hw = height of the perforation interval,

– h = height of the reservoir,

– zw = elevation of the midpoint of the perforations relative to theformation base, and

– = slant angle relative to the vertical axis




• Table 5-1 and 5-2 give the results for reservoirdimensionless thickness, hD= (h/rw) of 100 and 1000.

• The composite skin effect, Sc+θ, and the individual parts, sc

and sθ, are listed


and sθ, are listed




A well with a radius, rw= 0.328 ft is completed in a 33 ft reservoir. Inorder to avoid severe water coning problems, only 8 ft arecompleted and the midpoint of the perforations is 29 ft above thebase of the reservoir.

(Also find the composite skin effect for a vertical section)

Exercise – Partial Penetration and Slant SkinEffect

1. What would be the composite skin effect if θ=45o?

2. Calculate composite skin effect if h=330 ft, hw=80 ft andzw=290 ft.

1. Determine the composite skin effect when h= 230 ft, hw=58 ft and zw=120 ft when θ=30o ,60o ,75o


Base


h = 33ft hw = 8ft

zw = 29ft


Answer

1. hD = 100

Sc+θ = 8.6 (vertical)

Sc+θ = 6 (45o slant)


2. hD=1000

Sc+θ = 15.7 (vertical)

Sc+θ = 10.4 (45o slant)

3. ????


• Perforation creates a flow path for fluids from thereservoir through the cement and casing to the wellbore.

Skin From Well Perforation


• The perforation skin (Sp) isa function of perforationlength (l), perforationdiameter (d) and phasing


diameter (d) and phasingangle ().

• hperf = 1/SPF


• Karakas and Thariq (1988) performed perforation skineffect calculation consisting of following components;

– Plane flow effect, sH

– Vertical converging effects, sv

– Wellbore effect, s


– Wellbore effect, swb

Sp = sH + sv + swb


Calculation of plane flow effect, sH


• where r’w(θ) is the effective wellbore radius and is afunction of the phasing angle θ.


• The constant aθ depends on the perforation phasing andcan be obtained from table by Karakas and Tariq (1988).

• However, the total contribution is usually small.


• However, the total contribution is usually small.




Calculation of vertical converging effect, sV

• To obtain sV, two dimensionless variables (hD and rD) mustbe calculated.


be calculated.

V

H

perf

perf

Dk

k

l

hh

• Where kH and kV are the horizontal and verticalpermeabilities, respectively


• And,

• The vertical pseudo-skin is then


The vertical pseudo-skin is then

with

and



The constant a1, a2, b1 and b2 are also functions of the perforationphasing and can be obtained from the table.


Calculation of wellbore effect, swb

• For calculation of swb, a dimensionless quantity iscalculated first


• And

• The constant c1 and c2 also can be obtained from thetable




Assume that a well with rw = 0.328ft is perforated with 2SPF, rperf = 0.25 in (0.028ft), lperf = 8 in (0.667ft), and θ =180o. Calculate the perforation skin if the kH/kV = 10.

Exercise – Perforation Skin Effect

Repeat the calculation for θ = 0o , 45o, 60o , 90o, 120o


• Solution

r' 667.0328.05.0


4.0

5.0

328.0ln

H

H

s

s

wr' 5.0

667.0328.05.0


• Knowing that hperf = 1/SPF,

Hperf

Dk

k

l

hh 10

667.0

5.0Dh


Vperf kl

H

V

perf

perfD

k

k

h

rr 1

2

37.2

667.0

Dh

037.0

1.015.02

028.0

D

D

r

r


• From table and equations,



Sv = 3.852

33.0

328.0667.0

328.0

wD

wD

r

r


1.0

106.2 33.0532.42

wb

wb

s

es

552.3

1.0852.34.0

p

p

s

s


Thank You….Thank You….


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Chapter 2 - Formation Damage

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