Numerical Modelling of the Hydrodynamically Trapped Milk River Gas Field, Western Canada
Authors: Lies H. , Letourneau J. 
Published: March 2010

Abstract
The Milk River Gas Field, located in southeastern Alberta, is the largest single gas accumulation in the Western. Canadian Sedimentary Basin, with recoverable reserves estimated at 140 000 X 106m3. This hydrodynamically trapped gas field, positioned over the-Sweetgrass Arch, covers an area greater than 200 townships and has no identifiable up-dip structural closure. A steady-state, finite difference, single-phase, numerical fluid flow model was constructed for a 24 X 24 township area (Townships 1 – 24, Ranges 25W3-19\V4M). The water migration from the Belly River to the Bow Island was modelled in three dimensions. The primary input data included aquifer structure thickness and permeability. The permeability of the aquitards was varied until a good match was obtained with observed water potentials.
The structural top and the modelled potentiometric surface for the Milk River aquifer were combined using the method described by Hubbert in 1953 (UVZ). The resulting gas isopotential map illustrates 'the migration paths and accumulation sites of gas within a hydrodynamic system. Since the gas pool interferes with the water migration, the model was sensitized to account for the effective aquifer and aquitard permeabilities with the presence of the gas. This altered the position and gross pay distribution of the gas pool.
The model was used to estimate the significance of cross formational flow. When the aquitards were made impermeable, the resulting water potentials showed no resemblance to field data. A minimum of three layers are required to represent flow within the aquifer. The middle layer represents the aquifer and the adjacent layers account for vertical, cross-formational flow into and out of the aquifer- The model demonstrates that aquitard permeability is a controlling factor, and that hydrocarbon pools will alter water migration in their vicinity and therefore need to be taken into account.

Introduction
This paper illustrates the utility of hydrogeological modelling in explaining the trapping of the Milk River Gas Field. An understanding of the nature of this hydrodynamic trap will greatly assist future exploration efforts in similar environments. The project area illustrated in Figure 1 extends from the U.S. border up to Township 24, and from Range 24W3M in Saskatchewan to Range 19W4M in Albena. The first well drilled to produce gas from the Milk River formation was spudded in 1904(1). The Medicine Hen #2, located at JO-31-012-05W-t-M the centre of the city of Medicine Hat supplied the city with natural gas. In 1911, three additional Milk River gas wells were put on production within the dry of Medicine Hat and ten more in 1913. It wasn't until 1914 that the first well was drilled outside of the city limits. Some of the wells drilled within the city limits were owned by local businesses (e.g., Alberta Clay and Maple Leaf).
Development of the field around the Suffield Military Reserve was controlled by gas prices and drilling areas offsetting existing production. Today, the Milk River Gas Field has grown to cover over 200 townships, and has 30,000 gas wells and total proven recoverable reserves exceeding 140.000 × 106m3.