Formation Damage and Horizontal Wells - A Productivity Killer?
Authors: Bennion D. Brant , Thomas F. Brent , Bietz Ronald F. 
Published: December 2007

Abstract
The use of horizontal drilling is gaining widespread frequency throughout the world. Production results from many horizontal wells have been disappointing, and it is believed that when this has occurred in situations where viable reservoir quality has been present, near wellbore formation damage effects have been a major contributor to the marginal flow performance. Due to the fact that most horizontal wells are completed in an open hole fashion, even relatively shallow invasive near-wellbore damage (that would be penetrated by conventional perforation practices in cased and cemented vertical completions) may substantially impede flow. Drilling induced damage may include fines mobilization, invasion of mud solids, mechanical glazing, phase trapping or chemical reactivity between invading fluids and the formation matrix or in-situ fluids. Calculations illustrate how the permeability of horizontal wells can be reduced dramatically by high near wellbore skins and how this damage effect is attenuated as horizontal to vertical permeability ratio is increased (such as in highly laminated sands). They also illustrate how damage effects are reduced in situations of high vertical permeability, such as formations containing natural vertical fractures which are penetrated by the horizontal well. Underbalanced drilling is discussed as a solution to some horizontal well formation damage problems, and the importance of maintaining a continuous underbalance pressure condition during the entire drilling operation to obtain optimum results is emphasised. A list of reservoir parameters which should be evaluated to design an effective drilling program is presented, and a brief discussion of special core analytical techniques used to optimize drilling fluid and drilling process design is presented.

Introduction
Horizontal wells are being utilized throughout the world in an ever increasing fashion to attempt to increase production rates by maximizing reservoir exposure, targeting multiple zones, reducing drawdowns to minimize premature water or gas coning problems, exploit thin pay zones and, more recently in such processes as steam-assisted gravity drainage and as injectors and producers in secondary and tertiary enhanced oil recovery processes. Underbalanced drilling using horizontal well technology has also increased as a means of attempting to increase productivity from horizontal wells by reducing formation damage, improve ROP and reduce drilling and stuck pipe problems in severe lost circulation zones.
The use of conventional technology to drill and complete horizontal wells has resulted in disappointing results in many applications, due to what is believed to be formation damage effects. This paper reviews near wellbore skin damage from a mechanistic view in horizontal vs vertical completions, highlights reasons why formation damage effects may be more significant in horizontal vs vertical well applications and reviews current technology levels which are being utilized to attempt to reduce formation damage effects in horizontal well applications.

Mechanism of Formation Damage During Drilling of Horizontal Wells
Mechanisms of formation damage which may be operative in reducing the productivity of horizontal wells have been discussed in the literature by various authors.
These damage mechanisms can be grouped into several major categories, these being:
Fines Migration. Fines migration is the motion of naturally pre-existing particulate matter in the pore system. P. 827