The Damage Discussion: Its Effect On Well Flow
Authors: MacDonald R.G. , Chenery Dale 
Published: March 2010

Abstract
Damage induced by hydraulic fracturing fluids has been a topic of concern in the oil and gas industry for the past several years. Better ways are now available to quality the consequences of this damage and therefore plan the best stimulation treatment for the well completion.
Laboratory data, coupled with computer simulation, allows an examination of the more important damage mechanisms and their effect on a well's productivity. Using results from fluid compatibility and regained permeability core flow tests, it is possible to determine the effect of:permeability reduction by fluid invasion,invasion depth,relative permeability changes, andcapillary pressure changes
in both radial and linear flow cases.
This paper presents actual regained permeability data ranges and then uses this information in a reservoir simulator to assess the different damage mechanisms. Simulation results show that perceived "damage" problems are unrealistic, as even a very severe permeability reduction in the invaded zone does not significantly change well productivity.

Introduction
Hydraulic fracture treatments have become one of the most common and highly successful completion techniques used in western Canada. Much development work has been done on fracturing carrier fluids to avoid potential reservoir damage, though in reality, any fluid injected into a formation will do some damage. It is essential to understand basic reservoir mechanisms which control the flow from the reservoir into the wellbore. A step taken to understand and quantify this damage was made by Craft and Hawkinsw(1). They considered a concentric cylinder of altered permeability adjacent to the wellbore and the resulting well productivity in radially flowing wells. This altered permeability is now called "skin" and was defined by van Everdingen(2).
The problem was then expanded by van Pollen(3) to include a vertical hydraulic fracture with a zone of altered permeability adjacent to the fracture face. Prats(4) extended van Pollen's analysis by considering the effect of finite conductivity fractures with fracture face damage in a radial drainage area.
Well test theory provided a mathematical coupling of a vertical hydraulic fracture to a reservoir with wellbore storage and skin. Cinco Ley(5)) demonstrated the different flow regimes that were present in a fractured well. Linear flow was shown to exist in and adjacent to the fracture, while radial flow existed in the reservoir. Tannich(6) extended this work to include two phases in gas wells by the use of numerical simulation. Holditch(7) then included fracture face damage and capillary pressure effects for low-permeability gas wells.
With these works as a foundation, this paper will first compare the effects of damage on productivity for radial and linear single-phase flow. The analysis will then consider the complex combination of relative permeability, capillary pressure and induced permanent damage when two-phase flow occurs.

Laboratory Data
The degree of permeability reduction from fracture fluid invasion can be placed into a realistic framework from actual regained permeability tests. Regained permeability tests are used to evaluate the fracturing fluids' compatibility and depth of invasion with a representative sample of the formation core.