Summary
The Cadomin-Nikanassin (Cad-Nik) sandstone formations in the Lower
Cretaceous reservoirs along the reverse-thrust faulting belt of northeastern
British Columbia (NEBC), Canada, have emerged in recent years as a new tight
gas play. The low porosity (3?6%) of the rock matrix controls gas storativity,
while the presence of natural fractures in the form of clusters or swarms
allows significant and sustainable flow rates for commercial production. Newly
drilled wells are commonly hydraulically fractured to establish or enhance
wellbore connectivity to the natural-fracture network.
Seismic mappings of these structural unconventional-gas reservoirs provide
the early assessments of resource sizes and initial gas in place (IGIP), which
usually bear large uncertainties because of the difficulty in determining
reservoir structural closures and pay-porosity cutoffs. Regional analogue wells
are often used to guide development decisions. Meanwhile, estimating connected
reservoir volumes through conventional-gas material balances (p/z vs.
cumulative production) and production-data analysis [rate-transient analysis
(RTA)] has not been without challenges. Fairly long pressure buildups (PBUs),
on the order of hundreds of hours, are often performed without seeing the
pressure stabilization required to estimate accurately the reservoir pressure
needed for material-balance calculations. The applicability of pressure
extrapolation to these tests has not been systematically investigated;
therefore, no reliable methods for using shorter shut-ins to estimate reservoir
pressure currently exist. Thus, reliable average reservoir-pressure estimates
require significantly longer well shut-in times in order to perform meaningful
gas material balance. Because this is not practical, confidence in
material-balance results requires a second, independent method for establishing
connected well volumes to be used in comparisons and cross checking. One
possible choice is RTA, but, in these fields, numerous times wellhead-pressure
data are also unavailable or unreliable.
This paper presents two field-case studies that demonstrate the successful
application of the pressure/rate-deconvolution approach, combining a well?s
long, high-quality production-rate history with accurate downhole-pressure data
from relatively short buildup tests. This approach allows the reservoir
engineer to (1) reconcile the performance-based estimated-ultimate-recovery
estimates with the volumetric IGIPs; (2) establish, at the least, minimum
well-drainage size and connected volume; and (3) select possible
infill-drilling opportunities. A final benefit is that this often leads to a
better understanding of well/reservoir parameters.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
23 June 2011
- Meeting paper published:
15 April 2011
- Revised manuscript received:
20 December 2011
- Manuscript approved:
31 January 2012
- Published online:
24 April 2012
- Version of record:
1 May 2012
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