Fracture skin is a flow impediment that can exist in two distinct forms (Cinco-Ley and Samaniego-V.) The form considered in this paper is fluid loss damage (SFS). It can occur when capillary pressure exceeds drawdown pressure (i.e., initial or average reservoir pressure minus pressure in the fracture) and there is an excess of water in the system. Imbibing water can then cause a stable increase in water saturation, often dramatically reducing the relative permeability to gas in the area of the reservoir immediately adjacent to the hydraulic fracture faces. SFS is a function of fracture half-length (xF), damage-zone width (bS) and the ratio of k (undamaged permeability) to kS (permeability in the damage zone.) It results in a measurable pressure drop in the immediate vicinity of the fracture but not within the fracture itself. Yet the extra pressure drop near the fracture serves to make the fracture appear shorter or less conductive than it is physically and can dramatically reduce well productivity.
This paper is an in-depth study of the existence and consequences of the fluid loss damage form of fracture skin. The focus is a hydraulically-fractured well completed in a low-permeability (i.e. k=0.03 md) gas sand. Drawdown analysis of 12 months of daily rate and pressure data was correlated with a pressure buildup test done at the end of this flow period. A downhole shut-in configuration significantly reduced the duration of wellbore storage during the buildup test, allowing detailed characterization of the fracture and surrounding reservoir rock. The derived SFS from pressure buildup analysis was found to be +3, much higher than evaluated in examples shown by Cinco-Ley and Samaniego-V. The analytic results were verified by numerical evaluation with a multi-cell compositional reservoir simulator. It was found that a moderate conductivity fracture (FCD=7) with a xF of 160 ft was connected to the wellbore, but over time a 70-fold reduction in permeability to gas occurred in a 4-5 ft zone surrounding the fracture (non-unique solution.) The extra pressure drop caused by this damage zone reduced the effectiveness of the fracture to a zero-skin equivalent xF of approximately 5 ft. It is shown that the fracture skin probably existed from the onset of production but intensified with late-time water influx from the wellbore to the fracture. Remedial measures are described and evaluated for effectiveness in reducing the skin and improving well productivity.