Coalbed methane can be described as a permeability “challenged” reservoir.
Permeabilities in Alberta’s Horseshoe Canyon coals range from one to 100 millidarcies, often resulting in formation damage during drilling and completion operations.
Accordingly, some wells are drilled with air, to reduce formation damage while drilling.
The key to commercial success -- from the exploration phase to field development -- hinges on understanding the fracture or cleat systems that form part of the coal’s fabric. Successful companies have identified geological fairways or sweet spots where large densities of open fractures are developed, ensuring good reservoir permeability or conductivity.
Classified by rank or degree of metamorphism, coal grades from peat (at the lowest rank) to anthracite and graphite at the upper end of the continuum. The networks of cleats or fractures formed during coalification result from the stress that the rocks are under and from the dewatering process.
Face cleats form the dominant fracture set in coal -- they are parallel to the maximum stress and extend laterally for up to a meter, cutting through bedding. Butt cleats, the secondary fracture set, are oriented perpendicularly to the face cleat and parallel to minimum stress.
Butt cleats frequently terminate in face cleats.
Coals in Alberta have a severe tectonic overprint -- a relict compressional stress -- due to the influences of the Laramide Orogeny. Even in Alberta’s adjacent prairies, this relict compressional stress has resulted in face cleats that are generally oriented perpendicular to the Rocky Mountain Front (southeast-northwest), and are vertical and usually open -- unless filled with mineralization.
Cleat spacing is on the order of millimeters to centimeters.
According to Brian McKinstry, manager of geology for Western Gas Resources Canada Co. and a co-author of the Horseshoe Canyon paper presented in Perth, the insitu stress fields of Alberta’s coals also have been influenced by their shallow depths -- or lack of overburden -- and by isostatic rebound due to glacial retreat.
(At writing, Western Gas Resources Co., including its Canadian subsidiary, had just been acquired by Anadarko Petroleum.)
“There is a reorientation of the stress field through production of the well,” said McKinstry, who described coal as a very dynamic reservoir. “You get alteration of the stress field, often requiring re-fracs.”
Tools That Help
The Canadian industry in a very short period of time developed state-of-the-art laboratories to characterize Alberta’s coals, McKinstry said -- their physical properties including permeability and gas content.
Working with the Alberta Research Council, E&P operators built state-of-the-art mobile laboratories to measure desorption of gas from cores. Towed behind trucks, these portable labs are deployed to the lease during drilling and coring operations.
McKinstry also used an innovative split tube wireline core retrieval system, resulting in excellent core recovery rates.
“I needed core back fast,” he said, “to minimize the amount of lost gas.”
Many Canadian operators are completing previously untested coal zones in standing wells, in efforts to characterize the Horseshoe Canyon.
“It’s a very good and cost effective reconnaissance tool,” said Paul Gagnon, lead author of the Perth paper.
In fact, some Canadian operators are “saving” the Horseshoe Canyon coals for uphole completions, after deeper, productive zones have been depleted.
Hyrdraulic fracturing completions are used to prop open the production pathways in CBM reservoirs, connecting the cleat systems.
To date, nitrogen hydraulic fracturing has been the most successful stimulation process used in the Horseshoe Canyon -- nitrogen is pumped at high rates, without proppant, through coiled tubing and using a selective cup-type packer to isolate each coal seam during the completion.
Leveraging off its experience in measuring hydraulic fracs in CBM reservoirs, American-based Pinnacle Technologies opened its Calgary office in late 2002. Pinnacle uses its tiltmeters in the field to measure the characteristics of downhole fracs -- their height, width, orientation and distance propagated from the borehole.
Tiltmeters are deployed on the surface of the ground, down the treatment well on a wireline where the frac is propagating and often down an adjacent, observation well.
Data collected from tiltmeters are crucial to understanding how to position CBM wells in a field development scenario.