Abstract
Finding and developing continuous gas accumulations such as shales, coal
seams and tight sands requires the simultaneous development of accurate
geological models together with effective engineering technologies. This
linkage is well-established in conventional gas reservoirs where several
decades of experience have resulted in a clear and important role for geology
in exploration and production. Continuous gas accumulations, however, are
substantially different, in that they are regional in extent and lack obvious
seals or traps. Geologic controls on gas-in-place and production rates are no
less important in continuous reservoirs, but these controls operate on a
different scale. Moreover, geologic factors are often misunderstood and can be
difficult to quantify. This can lead to the erroneous belief that the geology
is either too complex to be understood, or that it is of little importance, or
that regional variability is effectively unpredictable, or that reservoir
behaviour can be sufficiently approximated with simplistic models. In all such
cases, the importance of geology is underestimated.
Many companies have relied largely upon engineering solutions in developing
continuous gas resources, while excluding basic geological data and analysis
from the project budget as a low-priority item. This makes little sense,
however, particularly for marginally economic projects that cannot sustain a
high proportion of sub-economic wells. In addition, this approach introduces a
high level of guesswork, and reduces the optimization of drilling and
completion strategies to trial-and-error. To alter this approach, and to
contribute more effectively to the exploration and development of continuous
gas accumulations, geologists must do a better job of defining their
responsibilities, demonstrating why geological knowledge is important, and
offering specific recommendations that will favourably impact the bottom
line.
Introduction
As compared with conventional gas reservoirs, continuous gas accumulations
typically extend over large stratigraphic thicknesses (tens to hundreds of
metres) and lateral distances (tens to hundreds of kilometres). Owing to their
size, continuous gas accumulations can encompass a variety of rock-types,
having widely varying reservoir properties. The three principal types of
continuous gas accumulations are: 1) coal seam gas (CSG), also known as coalbed
methane; 2) shale gas; and 3) tight sands.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
23 March 2005
- Meeting paper published:
7 June 2005
- Revised manuscript received:
23 March 2009
- Manuscript approved:
9 September 2009
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