Measuring perforation ‘skin’ in absolute terms can be extremely challenging due to the complex nature of the damage. In simple terms, perforation skin is often referred to as being the thin layer of crushed rock created between the perforation and the reservoir during the perforation event. However, as the illustration below outlines, there are many more factors that play a hand in the overall perforation damage.
Factors at play include;
- Failure to penetrate beyond drilling damage zone
- Reduced permeability due to presence of a crushed zone around the perforation
- Blocking of the perforation due to charge debris, well-bore and other solids.
- Ineffective perforation through poor gun centralisation
- Increased flow convergence (tortuosity) due to limited number of open perforations
A primary objective for any perforation system is to establish an effective hydraulic communication between the wellbore and the formation (reservoir) beyond. To do so means bypassing any drilling induced damage (Deep Penetration, Good Centralisation), while delivering an effective hydraulic conduit for reservoir fluids to flow into the well bore (Minimised Crushed Zone, Clean Tunnel, Improved Geometry).
Crushed Zone Damage – kc/k
Conventional shaped charge perforating techniques create single perforated flow channels by punching discrete high pressure shock waves into the formation at high velocity. This unavoidably compresses the formation resulting in crushed / stress altered rock immediately surrounding the tunnel. As highlighted below, this often results in a region of significantly reduced permeability that severely impedes the effectiveness of flow.
In the case of the Crushed Zone (Cr_z) two key features are present. The first is the thickness of the crushed zone, and the second, and perhaps more important, is the permeability of the Cr_z.
Again a simple example helps illustrate the effect of the Crushed Zone Permeability (kcr) on well productivity.
This example assumes that the perforation is subject to a crushed region of 0.2” in thickness. When only the permeability of the crushed region is altered, it is clear that there is a definite transition in the PI reduction influence where permeability values have fallen below 15md.
The same example looked at in a different way, considers the effect of the changing thickness of the crushed zone for a range of kcr values. Assumed within the model is a crushed damaged region extending no more than 0.5”. Although this is not necessarily the case in real life, it helps place each kcr scenario in a similar context.
The above example again helps to illustrate / reaffirm the understanding that the overall steepness of the slope is largely down to the permeability (kcr) within the crushed region, and the thickness, although important largely has a second order effect.
Testing methods such as API Section IV & Section II, have helped greatly reduce some of the uncertainties regarding the amount of anticipated damage for known rock types under controlled conditions. This can help greatly in the optimisation of the design, however, there often remains a significant amount of residual uncertainty over the nature of the damage that can / has occurred following the perforation event.
The focus of this blog post has been an introduction to Perforation Skin, and more specifically how the Crushed Zone can influence the well performance. However, in our next blog post we will go onto look at perforation the other elements related to perforation skin (Sp), and describe how each can also have an influence on production performance.
More from ‘The Perforator’s Guide to Formation Damage’
The Perforator’s Guide to Formation Damage. Part 1: Measuring Formation Damage as ‘Total Skin’
The Perforator’s Guide to Formation Damage. Part 2: How Skin Can Greatly Affect Flow Performance