To help understand how Skin can affect flow performance, the following illustration shows, for a vertical well with a diameter of 8½”, how these two factors, damage radius & damage ratio, can control the magnitude of the Skin factor, and hence the productivity of the well.
In order to better understand why this is matters, it is best to place it in the context of an example.
Consider a well where the permeability of the damage zone has been reduced by a factor of 10 (kd/k = 0.10), extending a distance of 3ft (36”) away from the wellbore. The dashed line, shown on the above figure, illustrates that with such conditions imposed on the well yields a total Skin (S’) = 20.
An alternative is to express the impact of formation damage on well productivity as a “Flow Efficiency” (FE), rather than simply as a skin value. The Flow Efficiency is simply the ratio of the damaged (Qd) to undamaged production (Q) rates. So, for pseudo- steady state flow FE can be expressed:
Again, using the same example as previously discussed, where the permeability of the damaged zone has been reduced to by a factor of 10 for a radial distance of 3 feet, the resulting Flow Efficiency would be 0.25 or 25% of the theoretical undamaged case.
The Skin factor S, in the above equations is actually composed of two terms, the Darcy skin term, S, and the turbulent or non-Darcy skin term D, as shown in the following equation:
S’ = S + Dq
Darcy Skin (S)
The Darcy skin (S) term is considered a flow restriction that is essentially independent of the flow rate, each component is dependent on the physical attributes of the well and/or completion, such as:
- Near Wellbore Damage (Sd)
- Completion or partial penetration (SC)
- Well Deviation / Geometric (SƟ)
- Perforation damage (Sp)
Other Darcy Skin can manifest on the resulting reduction in flow efficiency, these can be from Skin from Sand control measures and Stimulation Related Skin. It is important to note that the skin terms can be negative as well as positive, where negative Darcy Skins exhibit an enhancement on Flow Efficiency as would be expected through Stimulation treatments if executed effectively.
Non-Darcy Skin (Dq)
In contrast, the non-Darcy or turbulent skin is dependent on flow rate. In low rate wells the non-Darcy skin can, and is, frequently neglected, but in high rate wells (and in particular high rate gas wells) the non-Darcy skin can become the dominant factor controlling well performance.
Total Skin (S’)
Total skin is therefore a combined effect stemming from a number of different skin elements. As highlighted above there are several skin mechanisms affecting the Darcy Skin component. Therefore Total Skin (S’) can be expressed in complete form as follows:
S’ = SƟ + Sd + SC + Sp + Dq
It should be noted that a total skin S’ = 0 is simply the sum of the individual skin components and therefore, does not necessarily imply a total absence of any formation damage. Other factors affecting flow, e.g. completion or perforation skin, can result in both negative and positive skin values, giving rise to an overall zero, negative or positive total skin.
This concept is extremely important, as to maximise well productivity, all the factors that could potentially contribute to the S’ – skin factor, need to be managed effectively to a minimum and wherever possible minimised to the fullest possible degree.
The focus of this blog post has been on Skin as a generally expressed term. However, in our next blog post we will go on to look at the influence of perforation related skin (Sp) in more detail, and its influence on production performance.
More from ‘The Perforator’s Guide to Formation Damage’
The Perforator’s Guide to Formation Damage. Part 3: Perforation Skin and its influence on production
The Perforator’s Guide to Formation Damage. Part 1: Measuring Formation Damage as ‘Total Skin’