Formation damage along well axis may be distributed unevenly. Especially in horizontal wells, the damage around the wellbore is highly non-uniform. The literature hosts many studies focusing on the effect of non-uniform skin distribution on the transient pressure behavior of horizontal wells. There are few studies concentrating on how the uneven distribution of the skin factor affects well productivity. Furui et al. and Frick and Economides presented simple models accounting for the non-uniform formation damage around horizontal wells.
In this study, we compare the simple heterogeneous skin distribution models against the results from a 3D multisegment model. We show that the non-uniform skin models proposed by Furui et al. and Frick and Economides for horizontal wells are incorrect although the methods are simple and easy to use. Additionally, we demonstrate that the horizontal well skin formulation suggested by Malekzadeh and Tiab is inaccurate.
Quantifying the effect of formation damage around horizontal wells is crucial for optimizing the production from such wells. Horizontal wells may be subject to non-uniform formation damage around them. It has been postulated that the shape and distribution of the formation damage around a horizontal well would be controlled by the duration of the exposure to drilling fluids. Therefore, damaged formation around a horizontal well may typically exhibit a conical shape. Non-uniform skin distribution may significantly impact the productivity and pressure transient characteristics of horizontal wells.
The performance of horizontal wells has been the subject of many studies.[1–26] Inflow performance models for horizontal openholes are well documented in the literature. [1–4] The impact of formation damage on the productivity of horizontal openholes has been investigated extensively. [4–7] In some field applications, horizontal wells may be selectively completed. Productivity models for selectively completed or perforated horizontal wells has also been presented in the literature.[8–14]
Pressure transient characteristics of horizontal wells influenced by formation damage have been studied by many researchers since the mid 1980s.[15–20] Goode and Thambynayagam were among the first to incorporate the effect of uniform skin factor into the transient pressure solutions for horizontal wells. Later, researchers[16–18] have developed pressure-transient models that can handle non-uniform skin distribution as well as selective completion along horizontal wells.
The details of the physical mechanisms responsible for the formation damage have been studied in the laboratory.[21,22] Damage potential of different types of drilling fluids has been also investigated.[22–24]
Most recently, a numerical model to characterize the formation damage due to use of water-based-mud and natural cleanup during production has been proposed. The numerical model presented by Ding et al.  is capable of predicting the horizontal well performance under the influence of unevenly distributed formation damage.
In this section, we review the methods for quantifying the skin factor due to formation damage. For convenience, we will classify the available methods as uniform skin models and non-uniform skin models.