The skin factor is a number that tells you whether the area around a wellbore is damaged, undamaged or improved compared with the rest of the reservoir. It is one of the most important results of a well test, because it shows how much production is being lost, or gained, near the well. In this article you will learn what skin is, what causes it, how to calculate its effect with a worked example, and how engineers reduce it.
What Is Skin Factor?
In an ideal well, fluid flows from the reservoir into the wellbore through rock that has its original permeability all the way to the well. In real wells, drilling, cementing, perforating and production change the rock near the well. This zone is called the skin zone. It adds an extra pressure drop to the flow into the well. The skin factor, s, is a dimensionless number that measures this extra pressure drop.
| Skin value | Meaning | Well condition |
|---|---|---|
| s > 0 | Extra pressure drop near the well | Damaged |
| s = 0 | No extra pressure drop | Undamaged |
| s < 0 | Less pressure drop than expected | Stimulated (for example, fractured or acidized) |
Causes of Positive Skin (Damage)
The effect of damage on well productivity is introduced in well performance evaluation. Typical sources of damage include:
- Drilling and completion fluid invasion. Solids and filtrate enter the rock and block pores. This is the skin due to damage, sd.
- Perforation effects. Perforations that are too few, too short or crushed add skin, sp.
- Partial completion. If the well is not open over the whole producing interval, flow lines converge, spp.
- Perforation compaction. The rock around the perforation is crushed, sc.
- Heterogeneity and hole angle. The geometry of flow around the well changes.
- Production-related damage, such as scale, fines migration, asphaltene or wax deposits, and water or gas coning.
The total skin is the sum of all contributions: ST = sd + sp + spp + sc + …
Causes of Negative Skin
- Hydraulic fracturing, which creates a high-conductivity path; see hydraulic fracturing overview.
- Matrix acidizing that removes damage or enlarges pores.
- Natural fractures that connect to the well.
- Highly deviated or horizontal wells, which expose more reservoir to the well.
The Skin Equation
A simple model (Hawkins) treats the skin as a damaged zone of radius rs and permeability ks around a wellbore of radius rw. The reservoir permeability is k:
s = (k ÷ ks − 1) × ln(rs ÷ rw)
If the damaged zone is less permeable than the reservoir (ks < k), then s is positive.
Pressure Drop Due to Skin
In oilfield units, the extra pressure drop in the skin zone for a single-phase oil well is:
ΔPskin = 141.2 × q × B × μ × s ÷ (k × h)
- ΔPskin = extra pressure drop, psi
- q = flow rate, STB/d
- B = formation volume factor, rb/STB
- μ = viscosity, cp
- k = permeability, md
- h = net pay thickness, ft
- s = skin factor
Worked Example
An oil well flows at 500 STB/d. B = 1.2 rb/STB, μ = 1.0 cp, k = 50 md and h = 30 ft. A well test shows a skin of +5. The total drawdown is 800 psi.
- ΔPskin = 141.2 × 500 × 1.2 × 1.0 × 5 ÷ (50 × 30) = 423,600 ÷ 1,500 ≈ 282 psi
- Drawdown that is actually used for flow in the reservoir = 800 − 282 = 518 psi
So 282 psi of the 800 psi drawdown is wasted on the skin zone.
Flow Efficiency
Flow efficiency (FE) = (Pr − Pwf − ΔPskin) ÷ (Pr − Pwf)
In the example, FE = 518 ÷ 800 ≈ 0.65. The well produces only about 65% of what it could produce if there were no skin. If the damage were removed, the rate could increase by roughly 1 ÷ 0.65 ≈ 1.5 times for the same drawdown.
How Skin Is Measured
- Pressure transient (buildup and drawdown) tests analysed with semilog or type-curve methods. Test tools and procedures are described in introduction to flow tests.
- Bottom hole pressure surveys that provide the pressure data; see bottom hole pressure survey.
- Comparing the actual productivity index with the ideal one.
A test gives the total skin. Deciding which part comes from drilling damage, perforations or deviation needs further analysis.
How to Reduce Skin
| Cause | Possible remedy |
|---|---|
| Drilling or completion fluid damage | Use clean, compatible fluids; matrix acidizing or other chemical treatment |
| Poor perforations | Re-perforate with better charges, shot density and underbalanced perforating |
| Scale or fines | Chemical treatment, scale inhibitors, controlled production rates |
| Low natural conductivity near the well | Hydraulic fracturing or acid fracturing |
Frequently Asked Questions
Can skin factor be negative?
Yes. A negative skin means the well is more productive than an undamaged well, for example after fracturing or successful acidizing.
Is a skin of 0 always the best?
Zero skin means no damage, but a stimulated well (negative skin) can produce even more.
What is the difference between skin and formation damage?
Formation damage is the physical reduction of permeability. Skin is the number that measures the resulting extra pressure drop, including geometric effects that are not damage.
What is a typical skin value?
There is no single value. Many damaged wells show a skin between about +2 and +20, and stimulated wells show values near −3 to −5. It depends on the reservoir and the well.
Key Takeaways
- Skin factor measures the extra pressure drop near the wellbore: positive is damage, negative is stimulation.
- Total skin is the sum of drilling damage, perforation, partial completion and other effects.
- ΔPskin = 141.2 q B μ s ÷ (k h) in oilfield units.
- Well tests, with good pressure data, are the main way to measure skin.
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