The dimensionless parameter “skin” is commonly used to describe actual well performance with reference to an ideal case, and a number of analytical correlations have been derived over the years to account for the effect on well productivity caused by well geometry, completion design, formation damage and production rate. In a previous publication (Byrne and McPhee, 2012) we outlined the case for reducing the industry's reliance on analytical skin factors for well performance evaluation, particularly since the various “components” of skin, or “pseudo skins,” are often lumped together, and their interaction and interrelationship are often confused.
The concept of well design without recourse to a “total” or lumped skin parameter merits further description. For example, whilst hydraulic fractures may present an opportunity for significant enhancement in well performance, the fact remains that the fractured well itself, while an improvement on the unfractured case, may still not be producing at an optimum rate, especially if there is damage in the formation, at the fracture face or in the proppant pack. An overall total “negative skin,” which arises naturally when characterising the fractured well productivity with reference to an ideal vertical, unfractured well, can actually mask the true well potential. There is a tendency to believe that performance has been optimised but this is simply not the case.
If the geometry of the wellbore, including the completion and induced fractures, are relatively well understood and can be described then the well productivity can be modelled based on the system architecture without the need to compare it to some hypothetical simpler geometry. The quantity and location of formation damage can then be imposed on the model to determine the impact of damage in the fracture, at the fracture face and deeper within the formation.
This paper will illustrate how an indiscriminate use of skin factors can lead to sub-optimal well design and resulting performance prediction, and how this can be improved by representing the real system geometry as rigorously as possible. Decoupling formation damage from its association with other pseudo skins enables a proper description of the impact of damage and its contribution to well performance.