Summary
Worldwide Coal Bed Methane (CBM) resources are huge, estimated at 3,000 to
9,000 Tcf. The worldwide production from CBM is dominated by US production of
1.6 Bcf/year, where an estimated 20,000 wells are in production from CBM
reservoirs. Wyoming’s Powder River Basin (PRB) alone has 12,000 wells in
production, with an estimated 50,000 more wells to be drilled in the next 10-15
years. The production rate from CBM reservoirs is low, perhaps 50-100 mcf/day.
Various completion methods are being evaluated and new technologies are being
developed with the aim of increasing production rates.
Considering this interest and activity level, little attention has been paid
to the CBM completion fundamentals. Perforating is a critical part of this
process, especially considering the PRB development migration from single-coal,
open-hole completions into multi-zone, cased-hole completions Whereas the
industry has substantial knowledge of perforating characteristics in sandstones
and, to a lesser extent, carbonates, almost nothing is known about perforating
in coal. Predictions of perforation characteristics, such as depth of
penetration, are based on correlations developed for sandstones.
This paper will describe the first known laboratory-testing program to
investigate shaped charge penetration in coal targets. We will describe
mechanical properties of the coals tested, and penetration results for
different shaped charges (of different designs), shot at various stress
conditions. CT scan and cutaway imaging of the perforation tunnels will also be
discussed. Tests were conducted under dry and saturated conditions.
The preliminary experiments reported here indicate that shaped charge
penetration in coal is significantly less than expected, considering the
target’s density and strength. The authors will provide insight into what may
be the reasons for these unexpected results and recommend a path forward for
shaped charge testing, designs, predictive tools, and how to optimize CBM
completions.
Introduction: CBM Resources
Worldwide CBM resources are estimated to be in the range of 3,000 to 9,000
Tcf. The large land markets in Russia (400-6,000 Tcf), China (1,060-1,240 Tcf),
US (700 Tcf), Canada (263-2,700 Tcf) and India (525 Tcf) have the largest
reserves. (See Anderson et al. 2003 for extensive discussions of many aspects
of CBM production and reserves.)
The worldwide production from CBM is dominated by the US production of 1.6
Bcf/year. An estimated 20,000 wells are in production from CBM reservoirs.
Wyoming’s Powder River Basin has 12,000 wells in production and an estimated
50,000 more wells will be drilled in the next 5-10 years. Wells in this basin
are now trending towards a much higher percentage being cased and perforated
multizone completions rather than open-hole completions in a single
interval.
In Canada, the Horseshoe Canyon coal is expected to produce 3 Bcf/day by
2025. With an average rate of 80,000 cf/day per well, this corresponds to
37,000 wells in production. Overall, Alberta currently produces 21 Bcf/year,
but this is predicted to increase to 540 Bcf/year by 2014, with current booked
reserves of 263 Bcf. Alberta alone is predicted to have 500 Tcf of CBM to be
developed, equivalent to 83 billion barrels of oil. Approximately another 2,500
CBM wells/year are estimated in Canada for the next 10 years. By comparison,
conventional gas production in Canada is 5 Tcf/year, declining by 2.5% per
year.
Estimates of world coalbed methane reserves range from 4,416 to 8,556 Tcf
with 749 Tcf located within the US. Production of coalbed methane has steadily
increased in the last decade, particularly in the US and would seem to justify
additional perforating research.
The coal samples used in the present testing program came from a producing
formation in the Powder River Basin, a significant region for coalbed methane.
This region covers approximately 4.3 million acres and contains an estimated
800 billion tons of sub-bituminous coal. This CBM resource consists of many
shallow seams from 200 to 3,000 feet depth. Average well production is 163 MCF
per day with an average estimated life of 8 to 10 years. Gas produced from
Powder River Basin coal is almost 100 percent methane with an estimated 25 to
50 cubic feet of gas per ton of coal, although gas content varies appreciably
among the various coal formations.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
26 February 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
10 January 2008
- Manuscript approved:
10 January 2008
- Version of record:
20 June 2008
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