Skin Zone

In this article, we will discuss the graph above, whose Y axis displays pressure and X axis displays distance from well.  Rw represents the radius of the well, Re represents the distance of the reservoir limit from our well, while Rs is the distance of the skin zone from the well. The picture explains about the skin zone which is located near the well, created as the result from damages of our interventions to the formation. This skin zone will give vast amount of problems, as in the picture, the pressure will alter to be smaller near the well. But, the skin itself does not always make the pressure become lower. The skin also may be engineered to make the permeability near the wellbore become even higher than the natural permeability, thus increase the pressure higher than what supposed to be naturally. That might be the method to solve the problem which may be seen from the picture, as the higher pressure near the well will be advantageous for us. But first, let’s see why these conditions may be happened to our well.

The concept of skin zone was first proposed by Van Everdingen and Hurst in 1953, stating that the permeability may be reduced near the well due to damages during drilling and completion. This skin zone may be obtained from well test analysis such as pressure drawdown analysis or pressure build-up analysis. Skin factor play a key factor in the calculation. Skin factor is a number used to analytically model the difference from the pressure drop predicted by Darcy's law due to skin. The formula leads to the general results: if permeability near the wellbore altered to be lower than natural permeability, the skin factor will be positive, if permeability near the wellbore altered to be higher than natural permeability, the skin factor will be negative. If the permeability does not alter, the skin factor is 0.

The damage might come from a lot of things. In drilling, the mud particle & filtrate invasion and cement filtrate might be the reasons. Perforation problems, such as the perforating fluid, compacted zone, and perforation debris also have contributions.

Although the welltest analysis may find out the skin zone, it is not only determining the alteration because of drilling and completion damages such as stated by Van Everdingen and Hurst. The result such as shown in the picture is a composite parameter. Other parameters are borehole deviation, non-Darcy effect, partial penetration of well completion, reservoir shape, perforating, and perforation geometry. Each of these factors may be calculated partially using other data that we have beside the welltest result, thus we may find the biggest contributor of our skin problem.

Deviation angle may contribute in the skin factor calculation. The permeability of the rock is different horizontally and vertically, thus in a deviated well, the result of permeability will consist of vertical permeability and horizontal permeability. The result will be different in vertical well, where the horizontal permeability is the only one accounted.

Why we really concern about this skin zone? What will happen when the pressure near the wellbore become lower than what it should be?

The skin zone will bring quite a headache because it will decrease the pressure near the wellbore. As a common sense, the fluid will flow naturally from place with higher pressure to lower pressure, and the lowest pressure in this production system will be at the storage, as close as atmospheric pressure. Every places which are flown by the petroleum should have higher pressure than this atmospheric pressure, with the highest pressure is in the reservoir, so the fluid will flow from reservoir to storage. Unfortunately, the reservoir pressure cannot be maintained stable for years. The reservoir pressure will be depleted, and also the pressure near the wellbore.  As the pressure difference between the bottom hole and wellhead become smaller, the rate of production become smaller, until nothing can be produced and we need artificial lift such us pump to add pressure near bottom hole.  This condition might be explained with Flow Efficiency. Flow efficiency/ Productivity Ratio/ Completeness Ratio is the ratio between pressure drawdown with formation damage to without formation damage.

When the flow efficiency is high, then it might be implied that the skin is insignificant and the production might be accomplished as predicted.

When the skin zone has lower pressure, it might be expected that we will need use the pump sooner than scheduled.  We will need to have extra expenditure faster, and even worse, we need to abandon the well faster than it should be. We may see that this problem is very distinctive and need professional handling.

There are some efforts we may do to make things better. Hydraulic fracturing and acidizing might fix this skin zone to the level of natural permeability, or even higher, thus the skin factor might be less than 0. Hydraulic fracturing will be used to bypass the skin zone and create fracture which will help the fluid flow easier to the well. Acidizing might be used to try to remove the damaging substances, such us the filtrate from mud.

In a conventional homogeneous reservoir, there are some common skin factor values to describe the condition of the well. Skin factor ranged 0-2 means slightly damaged, 2-5 means medium damage, 5-10 means significant damage, and above 10 will be a serious damage. In a fractured reservoir, a non-damaged reservoir will have skin value of -3 since the permeability have been increased as the fracture created. 


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