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AN OVERVIEW OF SKIN FACTOR

Skin factor (S), also known simply as “skin”, is a parameter in the Darcy Flow Equation, and it exists to account for the difference in theoretical flowrate versus the actual observed flowrate, for a given pressure drop. Another way to think of skin is this: to achieve a given flowrate in a well, the pressure drop across the reservoir can be predicted with Darcy’s Law. However, it is common that the actual test data shows a different pressure drop. This difference between the theoretical pressure and the observed pressure is the result of skin on the well.

To illustrate this mathematically, let’s look at Darcy’s Law in radial flow with skin factor:

Where: q = flowrate

                k = permeability

                h = formation thickness

                Pres = reservoir pressure

                Pwf = well flowing pressure (bottomhole pressure)

                µ = viscosity

                B = formation volume factor

                re = reservoir radius

                rw = wellbore radius

                S = skin factor

Because skin factor is in the denominator, it inversely impacts flowrate. As the skin factor becomes more positive, the flowrate will decrease for a given pressure drop. If the skin factor is zero, we are left with Darcy’s Law in its ideal form. If the skin factor is negative, the flowrate will be greater than what is ideally predicted by Darcy’s Law. Simply put, as it relates to production, a negative skin is a good thing, and a positive skin is a bad thing.

Perhaps a more practical way to think of skin is how it relates to the pressure drop when a well is flowing. We call this “pressure drop due to skin (ΔPs)”.

Pressure drop due to skin is calculated as the difference between the ideal pressure drop (at S = 0), and the actual pressure drop observed. Therefore, the pressure drop due to skin can be calculated only from actual well test data. Skin can also be calculated from ΔPs:

Skin factor is made up of many components. The equations above will calculate a total skin factor, which is sometimes denoted as S’. Total skin factor is the summation of the individual components of skin. These individual components include:

1.       Skin due to damage/stimulation

2.       Skin due to partial penetration

3.       Skin due to turbulence

4.       Skin due to perforations

5.       Skin due to inclination

The individual components of skin will be covered in more detail in upcoming Well Insights, but for now, here is a brief overview:

Skin Due to Damage/Stimulation

The most common form of skin comes from damage to the well face. This can occur when drilling fluid or some other chemical penetrates the sand face on a well, creating a layer of reduced permeability near the wellbore. This is very common in gas storage wells, as fluid and debris from the compressors gets pushed down the well during injection. Over time, a damaged layer builds up on the sand face which will reduce well deliverability over time.

The opposite of damage is stimulation. This typically occurs when a completion or workover treatment is performed on a well, such as a frac job or an acid treatment. Because these stimulation treatments improve the performance of a well, the skin factor associated with stimulation is negative.

Skin Due to Partial Penetration

Partial penetration occurs when the wellbore is not in contact with the entire pay zone. This can happen if the well is not drilled to the bottom of the pay zone, or if the perforations do not extend over the entire height of the pay zone. The result is either spherical or hemi-spherical flow, which is less efficient than true radial flow.

Skin Due to Turbulence

At extremely high gas flowrates, the velocity near the wellbore can become large enough that the flow becomes turbulent. Because turbulent flow is less efficient than laminar flow, the turbulence will result in a greater pressure drop, which will appear as skin

Skin Due to Perforations

In cased-hole completions, the perforations can act as a bottleneck. This typically occurs where there are not enough perforations in the pay zone for the amount of fluid flow. In this case the perforations can act like a choke, creating additional pressure drop.

Skin Due To Inclination

In some cases, the wellbore penetrates the pay zone at an angle, rather than perpendicularly. This can occur in either a deviated well or an inclined formation. If the angle of inclination is significant (greater than 10 degrees), a reduction in skin can be observed. This is due to an increase in contact area between the wellbore and the pay zone.

Summary

There are many different components of skin and each one has a different impact on the deliverability of a well. The first step to improving well performance is to identify and quantify skin with a well test. Once the skin is identified, a proper workover procedure can be developed.

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