Drilling Induced Formation Damage of Horizontal Wells in Tight Gas Reservoirs
Authors: Coskuner Gokhan 
Published: March 2010

Abstract
As the oil and gas industry matures and known reserves continue to be depleted, the focus moves towards more challenging environments. To help moderate the decline trend in reserves or to reverse it, the industry is expanding its exploration and development efforts to include horizons with permeabilities in the same range as common cement, that is, microdarcies. While horizontal wells are being used with increasing frequency in exploiting tight gas reservoirs, many questions have arisen as to the optimum practices to drill such wells.
The experience of drilling horizontal wells in one of the deep basin tight gas reservoirs in Alberta is presented in this article. The initial results were below expectations. Consequently, a systematic study was undertaken to investigate the controllable factors that are in play during the course of drilling these wells. It was found that formation damage played a signifi cant role in reducing the initial productivity. This was particularly true because all horizontal wells were completed in an open hole fashion where bypassing damage by perforation is usually not an option.
The investigation into the formation damage in horizontal wells and its results are presented. The study led to using a new drill-in fl uid which resulted in signifi cantly reduced formation damage. Well tests conducted in the wells drilled with the new fluid appear to support the laboratory results. Finally, general guidelines are provided for selecting the most suitable drill-in fluid and implementing it in the field to minimize the horizontal well drilling formation damage in tight gas formations.

Introduction
As the oil and gas industry matures and known reserves continue to be depleted, the focus moves towards more challenging environments. One such environment is the Deep Basin area of West Central Alberta where vast reserves of natural gas and associated liquids are present in a number of low permeability zones(1). Similar areas also exist in the U.S., such as Powder River Basin and the Permian Basin where in situ permeabilities are in the 100 microdarcy (0.1 mD) range or less. It should be noted that while routine permeability measurement of the cores in the laboratory may indicate permeabilities up to 1 mD for such reservoirs, the effective gas permeability in the reservoir will be significantly less(2).
Horizontal wells are being used with increasing frequency in exploiting such reservoirs in an attempt to increase production rates by maximizing reservoir exposure, targeting multiple zones, reducing drawdowns and increasing the potential to intersect zones of enhanced permeability, such as natural fractures(3–9). However, the production results from many horizontal wells have been below expectations(9–11). While it appears easier to attain technical success with horizontal wells, economic success has been more difficult to achieve(10). This has been partly attributed to limitations in current completion technology, reservoir heterogeneity, well geometry, and formation damage(9, 12). A further complicating factor, which affects the production rates adversely, arises when the produced gas is rich in liquids(13).