Successful Applications of Pressure-Rate Deconvolution in the Cad-Nik Tight Gas Formations of the British Columbia Foothills

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.