part 6 skin factor and its components

Skin Factor and Its Components - WTA 1

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• Mud filtrate invasion reduces effective permeability near p ywellbore.

• Mud filtrate may cause formation clays to swell, causing damage.

• Mud design is critical to avoid formation damage

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• In an oil reservoir, pressure near well may be below bubblepoint, , p y p ,allowing free gas which reduces effective permeability to oil near wellbore.

• In a retrograde gas condensate reservoir, pressure near well may be below dewpoint, allowing an immobile condensate ring to build up, which reduces effective permeability to gas near

llbwellbore.

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• Injected water may not be clean - fines may plug formation.j y y p g

• Injected water may not be compatible with formation water -may cause precipitates to form and plug formation.

• Injected water may not be compatible with clay minerals in jec ed a e ay o be co pa b e c ay e a sformation; fresh water can destabilize some clays, causing movement of fines and plugging of formation.

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• Consider an undisturbed formation of thickness h and permeability k.

• Assume that something (drilling the well, producing fluid from the well, injecting fluid into the well) changes the permeability near the wellbore. One simple model of this effect is to assume pthat this altered zone has uniform permeability ka and radius ra, and that the rest of the reservoir is undisturbed.

• For generality, we allow the permeability in the altered zone to be either smaller or larger than the permeability in the g p yundisturbed formation.

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Pressure profile with a damage zone presents an additional p g ppressure drop due to SKIN. This is represented by delta p due to skin.

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• We define the skin factor in terms of the additional pressure pdrop due to damage.

• As defined, the skin factor is dimensionless -- it has no units.Nomenclaturek = mdh fth = ftq = STB/DB = bbl/STBΔps = psiμ = cpμ cp

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The skin factor equation may be rearranged to give the additional q y g gpressure drop caused by a given skin factor.

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The skin factor may be calculated from the properties of the y p paltered zone.

If ka < k (damage), skin is positive.

If ka > k (stimulation), skin is negative.

If k k skin is 0If ka = k, skin is 0.

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• The equation on the previous slide can be rearranged to solve q p gfor the permeability in the altered zone.

• If we know the reservoir permeability and the skin factor and can estimate the depth of the altered zone, we can estimate the permeability of the altered zone using this equation.

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• If the permeability in the altered zone ka is much larger than the p y a gformation permeability k, then the wellbore will act like a well having an apparent wellbore radius rwa.

• The apparent wellbore radius may be calculated from the actual wellbore radius and the skin factor.

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The minimum skin factor possible (most negative skin factor) p ( g )would occur when the apparent wellbore radius rwa is equal to the drainage radius re of the well.

Drainage radius of the well: Related to well spacing……….Semi-steady state flow: all wells in the reservoir drain from areas proportional to the wells flow ratesp opo o a o e e s o a es

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For a circular drainage area of 40 acres (re = 745 feet) and a g ( e )wellbore radius of 0.5 feet, this gives a minimum skin factor (maximum stimulation) of -7.3.

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When a cased wellbore is perforated, the fluid must converge to p , gone of the perforations to enter the wellbore. If the shot spacing is too large, this converging flow results in a positive apparent skin factor. This effect increases as the vertical permeability decreases, and decreases as the shot density increases.To get ready of this geometric skin a redesign should be done and

h ti th ll ill d thi ff tre-shooting the well will decrease this effect.

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When a well is completed through only a portion of the net pay p g y p p yinterval, the fluid must converge to flow through a smaller completed interval. This converging flow also results in a positive apparent skin factor. This effect increases as the vertical permeability decreases and decreases as the perforated interval as a fraction of the total interval increases.

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Damage skin= true damage skin= sdg gCombined effect is worst than damage skin by itself

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These equations were developed by Papatzacos (SPE 13956)q p y p ( )It provides very good estimate of sp if rw/hp sqrt(kv/kh) ≤ 0.2

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When a well penetrates the formation at an angle other than 0 p gdegrees, there is more surface area in contact with the formation. This results in a negative apparent skin factor. This effect decreases as the vertical permeability decreases, and increases as the angle from the vertical increases.

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Angle units is DEGREES, and the equation given above for sθ is g , q g θvalid if θ ≤75 degrees, and presented by Cinco-Ley et al. JPT (Feb. 1975).

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Often to improve productivity in low-permeability formations, or to p p y p y ,penetrate near-wellbore damage or for sand control in higher permeability formations, a well may be hydraulically fractured. This creates a high-conductivity path between the wellbore and the reservoir. If the fracture conductivity is high enough relative to the formation permeability and the length of the fracture, there will be virtually no pressure drop down the fracture This distributesbe virtually no pressure drop down the fracture. This distributes the pressure drop due to influx into the wellbore over a much larger area, resulting in a negative skin factor.

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If the fracture conductivity is high enough relative to the formation y g gpermeability and the length of the fracture, there is virtually no pressure drop down the fracture, and the apparent wellbore radius and the fracture half length are related by the equations shown on this slide.

If the fracture conductivity is lower, there is still a negative skin factor, but a longer fracture half-length is required to give the same apparent wellbore radius as a high conductivity fracture. A high-conductivity fracture is one where the dimensionless fracture conductivity Cr is greater than 100.

100wkC f >

This is not a conversion of the area of the fracture open to flow to an equivalent circumference. It is a rigorous equation that applies whenever the fracture is high conductivity and the pressure transient has moved some distance beyond the tips of the

100kL

Cf

fr >

π=

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transient has moved some distance beyond the tips of the fracture.


For a well that has been perforated, there is an additional pressure drop across a zone surrounding the perforations. This pressure drop may be calculated using the radial flow form of Darcy’s law.NOTE: This expression does not include the effects of non-Darcy flow, which may be extremely important, especially in high-rate gas wells!Nomenclature:sp- geometric skin due to converging flow to perforationssp geometric skin due to converging flow to perforationssd - damage skin due to drilling fluid invasionsdp - perforation damage skinkd - permeability of damaged zone around wellbore, mdkdp - permeability of damaged zone around perforation tunnels, mdkR - reservoir permeability, mdkR reservoir permeability, mdLp - length of perforation tunnel, ftn - number of perforationsh - formation thickness, ftrd - radius of damaged zone around wellbore, ftrdp - radius of damaged zone around perforation tunnel, ft

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dp g p ,rp - radius of perforation tunnel, ftrw - wellbore radius, ft


When a well is gravel packed, there is a pressure drop through the gravel pack within the perforation. GRAVEL IS JUST COARSE SAND…!!!

NOTE: This expression does not include the effects of non-Darcy flow, which may be extremely important, especially in high-rate gas wells!GRAVEL PACK SKIN MAY BE VERY LARGE- greater than 10….GRAVEL PACKED WELLS MAY SHOW SKINS > 50PACKED WELLS MAY SHOW SKINS 50…Comparing this with FRACK PACKING usually is from zero to 5….It does not show a stimulation …. Paper written @ Holditch about Trinidad fields comparing SKIN for FRACPACKS and GRAVEL PACKS Nomenclaturesgp - skin factor due to Darcy flow through gravel packh - net pay thicknesskgp - permeability of gravel pack gravel, mdkR - reservoir permeability, mdLg - length of flow path through gravel pack, ft

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n - number of perforations openrp - radius of perforation tunnel, ft


• The productivity index is often used to predict how changes in p y p gaverage pressure or flowing bottomhole pressure pwf will affect the flow rate q.

• The productivity index is affected by

– Reservoir quality (permeability)

p

– Skin factor

DARCY’s Law applies….

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• We can express the degree of damage on stimulation with the p g gflow efficiency.

• For a well with neither damage nor stimulation, Ef = 1.

• For a damaged well, Ef < 1

For a stimulated well E > 1• For a stimulated well, Ef > 1

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We can use the flow efficiency to calculate the effects of changes y gin skin factor on the production rate corresponding to a given pressure drawdown.

qnew = Flow rate after change in skin factorq ld = Flow rate before change in skin factorqold = Flow rate before change in skin factorEfnew = Flow efficiency after change in skin factorEfold = Flow efficiency before change in skin factor

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Skin Factor and Its Components - PTT Interpretation and Analysis 32

Exercise 12Damage and Skin Factor CalculationsDamage and Skin Factor Calculations

1. Calculate the additional pressure drop due to skin for a well producing at 2,000 STB/D. Oil formation volume factor is 1.07 RB/STB, viscosity is 19 cp, permeability is 5400 md, net pay thi k i 175 ft ki f t i 11 d it i 1 2%thickness is 175 ft, skin factor is 11, and porosity is 1.2%.

skh

qB2.141p sμ

=Δ

2. Calculate the flow efficiency for the well in Problem 1, if the average reservoir pressure is 1,800 psi and the flowing bottomhole pressure is 1,600 psi.

kh

3. Calculate the apparent wellbore radius for the well in Problem 1, if the bit diameter is 8 in.

4. Calculate the new skin factor if we create a 100-ft fracture in the reservoir in Problem 1.

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skh

qB2.141p sμ

=Δ

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part 6 skin factor and its components

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