Hydrodynamic Model for Biogenic Gas Trapped in Low Permeability Sands - Western Plains of North America

By

 Kaush Rakhit¹, Dave Hume¹, Daniel Barson¹

(1) Rakhit Petroelum Consulting Ltd, Calgary, AB

 Gas trapped in tight reservoirs in the Western Plains and Rocky Mountain Basins of North America forms a substantial unconventional gas resource. Three play types with distinct hydrodynamic signatures are recognized:

1. Shallow biogenic gas

2. Deep Basin gas

3. Basin-Centred gas

All are hosted in Cretaceous-Tertiary clastic reservoirs. Deep Basin and Basin-Centred gas have been discussed in the literature in recent years. This paper focuses on current developments in understanding of the shallow biogenic gas (SBG) play. SBG generally occurring at depths of less than 1,000 m (3,300 ft) represents a poorly understood by-passed resource. A potential for greater than 70 TCF of gas-in-place has been determined in the Western Plains region extending from central Alberta in Canada into the U.S. mid-west. The play potentially continues south to the Gulf Coast.

A broad areal extent, subnormal formation pressures ranging from 20 to 70% of hydrostatic and occurrence in low permeability sand-shale sequences characterizes the resource. Subnormally pressured gas-charged sands often show a transition updip to normally pressured water-wet sands. Downdip flow, which is usually observed in the water-wet section, may enhance the trap in some cases. SBG is often by-passed due to deep invasion, relatively high water saturation (45-75%) and fresh formation water (<10,000 ppm), which together invalidate conventional petrophysical analysis and testing techniques.

Recognition of the unique hydrodynamic signature and an understanding of the basin evolution required for its occurrence are key to identifying and exploiting the shallow gas resource and extending the play into other basins.