CSG Formation Damage

Introduction

According to Bustin [2001], experience in CBM basins provide substantial evidence of significant damage to coal permeability by drilling fluids and completion practices. Some examples of operations/ fluids that lead to reduced permeabilty included:

• Brine (KCl/CaCl2) and water

• Hydroxypropyl Guar (HPG) fluids.

• Mud Filtrate (it has been documented that coals are heaviliy susceptible to cement filtrate [Cox, 2001])

• Over-balanced drilling (leading to potential coal stress reducing permeability)

• Coal fines

In previous years, operators often used quality CO₂ or nitrogen fracturing fluid allowing for proppant transport and placement, while reducing gel filtrate damage, and assisting in post-treatment clean-up (originally demonstrated in the Raton Basin and similar projects). Foam fracturing technology uses foam bubbles to transport and place proppant in fractures. Nitrogen Foam fracs have also been seen to create a wider fracture with enough height growth to intersect mutliple coal seams in a production horizon. An exception, according to Palmer (2010), is the San Juan Basin where XL gels are more successful.

For the Horseshoe Canyon coals (HSC, Canada), reports by operatiors have been inconsistent with Martin (n.d) indicating observed postive posf-frac skins using 100% nitrogen at high rates, while Leshchyshyn (2004) indicated that CO₂ based fluid (with proppant) provided a far more conductive fracture. Waffle et al (2009) suggested that 100% nitrogen at low rates produced reasonable success with the HSC (with the disclaimer that N₂ pumping rates are always evolving) based on thickness and depth of coal seams.

When exposed to water, coals may swell leading to permeability reduction. However, slickwater fracs have been common in Australia.

According to Palmer [2010], drilling slightly over-balanced or slightly under-balanced will reduce skin damage, and the chance of wellbore breakout problems (moderate to higher permeability). Presented below in the table is a comparison of between cavity wells and fractured wells from Palmer [2009]. It states the potential for stress and permeability damage is reduced with cavities completions while hydraulic fractures increase stress because of proppant. Under-reamed (UR) completions in the Powder River Basin have also shown great results for creating an elarged wellbore radius.

Formation damage can be treated as a skin in the various transient and/or PSS models. Refer to Wellbore Skin and Formation Damage

References

• Bustin, R. M.  Geology & Some Engineering Aspects of Coalbed Methane 2001, CBM Solutions.

• Dave Cox, PE 598D: Coalbed Methane, Colorado School of Mines, 2001

• Thakur, Pramod, Advanced Reservoir and Production Engineering for Coalbed Methane, 2017, Gulf Professional Publishing

• Palmer, Ian, Coalbed methane completions: A world view, International Journal of Coal Geology, 2010.

• Martin, Tony, Refracturing of Coal Seams, Presention, BJ Services Company.

• T. Leshchyshyn, and J. D. Tomson, Optimized Natural Gas From Coal Stimulations in the Western Canadian Sedimentary Basin, Part I: Database Collection, CIPC Paper 2004-284

• Chris Waffle, Darren Tisdale, and Cory MacNeil, The Horseshoe Canyon Coals of Central Alberta:
  A “Dry” CBM Play. Search and Discovery Article #80079 (2010), Adapted from oral presentation at Eastern Section AAPG Annual Meeting, Evansville, Indiana, September 20-22, 2009.

• Towler, Brian / Firouzi, Mahshid / Underschultz, James / Rifkin, Will / Garnett, Andrew / Schultz, Helen / Esterle, Joan / Tyson, Stephen / Witt, Katherine, An overview of the coal seam gas developments in Queensland,  2016, Journal of Natural Gas Science and Engineering