Wednesday, June 08, 2005
Underpressure and Gas-saturation in the Upper Cretaceous and Tertiary of Central Alberta
This talk was the result of some consulting work that I did for Rick Kunimoto (then at Stellarton Energy, which was bought by Tom Brown which in turn was bought by Encana) and my UofA research on gas seeps. This work predicted the "dry" nature of the Horseshoe Canyon coals.
Underpressuring in the Cretaceous of Western Canada is predominantly due to Darcy flow of natural gas to outcrop (it leaks to atmosphere). Decreasing pressures allow natural gas to desorb from coals and shales which helps to maintain gas saturations (see conclustion iv from Carsten Büker's modelling work).
The Upper Cretaceous and Tertiary sandstones of Central Alberta (Paskapoo, Scollard, Edmonton, Horseshoe Canyon and Bearpaw) form a major shallow, low-pressure gas resource. Potential sources of this gas are from associated coal seams and bacterial activity at the water/gas contact. These zones were initially bypassed due to formation damage and low gas prices.
A hydrostatic water gradient is up to 350% overbalanced in some of these formations. A detailed examination of over 800 pressure tests confirmed that the Scollard member and Edmonton Group are pervasively gas-saturated. Formation pressures decrease with proximity to formation outcrop along the Red Deer River. Conventional hydraulic head calculations using these data result in an east-west hydraulic gradient.
The extreme underpressures in these zones have been attributed to pore volume expansion associated with erosional unloading. This paper proposes an alternate hypothesis. Gas pressure gradients show a predominantly west to east gradient. The lowest measured formation pressures (<200 kPa) were usually found within 20 km of the formation outcrop. It appears that the bulk of the gas is migrating up dip in response to the regional gas pressure gradient. Post-Tertiary methane desorption has decreased coal gas content.
Lateral migration of gas towards outcrop along the Red Deer River Valley is proposed as a mechanism to create underpressures in the Upper Cretaceous and Tertiary sandstones in Central Alberta.
Underpressuring in the Cretaceous of Western Canada is predominantly due to Darcy flow of natural gas to outcrop (it leaks to atmosphere). Decreasing pressures allow natural gas to desorb from coals and shales which helps to maintain gas saturations (see conclustion iv from Carsten Büker's modelling work).
The Upper Cretaceous and Tertiary sandstones of Central Alberta (Paskapoo, Scollard, Edmonton, Horseshoe Canyon and Bearpaw) form a major shallow, low-pressure gas resource. Potential sources of this gas are from associated coal seams and bacterial activity at the water/gas contact. These zones were initially bypassed due to formation damage and low gas prices.
A hydrostatic water gradient is up to 350% overbalanced in some of these formations. A detailed examination of over 800 pressure tests confirmed that the Scollard member and Edmonton Group are pervasively gas-saturated. Formation pressures decrease with proximity to formation outcrop along the Red Deer River. Conventional hydraulic head calculations using these data result in an east-west hydraulic gradient.
The extreme underpressures in these zones have been attributed to pore volume expansion associated with erosional unloading. This paper proposes an alternate hypothesis. Gas pressure gradients show a predominantly west to east gradient. The lowest measured formation pressures (<200 kPa) were usually found within 20 km of the formation outcrop. It appears that the bulk of the gas is migrating up dip in response to the regional gas pressure gradient. Post-Tertiary methane desorption has decreased coal gas content.
Lateral migration of gas towards outcrop along the Red Deer River Valley is proposed as a mechanism to create underpressures in the Upper Cretaceous and Tertiary sandstones in Central Alberta.
# posted by Jim : 1:12 PM
