Summary
Coalbed-methane (CBM) reservoirs commonly exhibit two-phase-flow (gas plus
water) characteristics; however, commercial CBM production is possible from
single-phase (gas) coal reservoirs, as demonstrated by the recent development
of the Horseshoe Canyon coals of western Canada. Commercial single-phase CBM
production also occurs in some areas of the low-productivity Fruitland Coal,
south-southwest of the high-productivity Fruitland Coal Fairway in the San Juan
basin, and in other CBM-producing basins of the continental United States.
Production data of single-phase coal reservoirs may be analyzed with techniques
commonly applied to conventional reservoirs. Complicating application, however,
is the unique nature of CBM reservoirs; coal gas-storage and -transport
mechanisms differ substantially from conventional reservoirs. Single-phase CBM
reservoirs may also display complex reservoir behavior such as multilayer
characteristics, dual-porosity effects, and permeability anisotropy.
The current work illustrates how single-well production-data-analysis (PDA)
techniques, such as type curve, flowing material balance (FMB), and
pressure-transient (PT) analysis, may be altered to analyze single-phase CBM
wells. Examples of how reservoir inputs to the PDA techniques and subsequent
calculations are modified to account for CBM-reservoir behavior are given. This
paper demonstrates, by simulated and field examples, that reasonable reservoir
and stimulation estimates can be obtained from PDA of CBM reservoirs only if
appropriate reservoir inputs (i.e., desorption compressibility, fracture
porosity) are used in the analysis. As the field examples demonstrate,
type-curve, FMB, and PT analysis methods for PDA are not used in isolation for
reservoir-property estimation, but rather as a starting point for single-well
and multiwell reservoir simulation, which is then used to history match and
forecast CBM-well production (e.g., for reserves assignment).
CBM reservoirs have the potential for permeability anisotropy because of
their naturally fractured nature, which may complicate PDA. To study the
effects of permeability anisotropy upon production, a 2D, single-phase,
numerical CBM-reservoir simulator was constructed to simulate single-well
production assuming various permeability-anisotropy ratios. Only large
permeability ratios (>16:1) appear to have a significant effect upon
single-well production characteristics.
Multilayer reservoir characteristics may also be observed with CBM
reservoirs because of vertical heterogeneity, or in cases where the coals are
commingled with conventional (sandstone) reservoirs. In these cases, the
type-curve, FMB, and PT analysis techniques are difficult to apply with
confidence. Methods and tools for analyzing multilayer CBM (plus sand)
reservoirs are presented. Using simulated and field examples, it is
demonstrated that unique reservoir properties may be assigned to individual
layers from commingled (multilayer) production in the simple two-layer
case.
Introduction
Commercial single-phase (gas) CBM production has been demonstrated recently
in the Horseshoe Canyon coals of western Canada (Bastian et al. 2005) and
previously in various basins in the US; there is currently a need for advanced
PDA techniques to assist with evaluation of these reservoirs. Over the past
several decades, significant advances have been made in PDA of conventional oil
and gas reservoirs [select references include Fetkovich (1980), Fetkovich et
al. (1987), Carter (1985), Fraim and Wattenbarger (1987), Blasingame et al.
(1989, 1991), Palacio and Blasingame (1993), Fetkovich et al. (1996), Agarwal
et al. (1999), and Mattar and Anderson (2003)]. These modern methods have
greatly enhanced the engineers’ ability to obtain quantitative information
about reservoir properties and stimulation/damage from data that are gathered
routinely during the producing life of a well, such as production data and, in
some instances, flowing pressure information. The information that may be
obtained from detailed PDA includes oil or gas in place (GIP),
permeability-thickness product (kh), and skin (s), and this can
be used to supplement information obtained from other sources such as PT
analysis, material balance, and reservoir simulation.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
28 February 2006
- Revised manuscript received:
5 February 2007
- Manuscript approved:
18 March 2007
- Version of record:
20 June 2007
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