The performance of a well depends not only on the properties of the reservoir but also on the conditions in the near-wellbore region. Fluid flow toward a well typically occurs in a radial pattern, and the pressure distribution in the reservoir decreases logarithmically with distance from the well. As a result, a large portion of the pressure drop occurs very close to the wellbore.
An important consequence of this behavior is that the near-well region strongly influences well productivity. For example, for a constant production rate, approximately the same pressure drop may occur within the first meter from the wellbore as in much larger portions of the reservoir farther away. Therefore, even relatively small changes in permeability in the near-well zone can significantly affect the production or injection rate.
During drilling, completion, or workover operations, the permeability near the wellbore may be altered. In many cases, permeability in this region is reduced by drilling fluid invasion or other operational effects. This phenomenon is commonly referred to as formation damage. The impact of such near-well alterations on well performance is represented by the skin factor, s.
When the skin factor is included, the steady-state inflow equation becomes
where s represents the additional pressure drop associated with the near-well region.
The skin factor quantifies how the flow conditions near the well differ from those of the undisturbed reservoir.
- Positive skin (s > 0): indicates additional resistance to flow near the wellbore. This usually results from formation damage and reduces production rates.
- Zero skin (s = 0): represents an ideal well with no additional flow resistance beyond that predicted by radial flow theory.
- Negative skin (s < 0): indicates improved flow conditions near the wellbore. This occurs when stimulation treatments increase the conductivity of the region surrounding the well.
Stimulation treatments are designed to reduce the resistance to flow in the near-well zone. Matrix stimulation methods, such as acidizing, remove formation damage and restore permeability near the well. In some cases, the permeability in the stimulated region may even exceed the original reservoir permeability.
When matrix stimulation is insufficient to achieve the desired production rate, hydraulic fracturing may be used. Hydraulic fracturing creates highly conductive fractures that provide additional pathways for fluid flow and significantly reduce the effective resistance near the well, leading to much higher production rates.
