A New Interpretation of Fracture Distribution in Waterton Sheet Ill: An Integrated Reservoir Characterization Study
Authors: Thomas M.B. , Nutakki R. , Baker R.O. , Gegunde G. , Geppert E.C.J. , Gould B. 
Published: December 2007

This paper outlines the results of a reservoir characterization study conducted on the low quality, fractured Mississippian limestones and dolomites in Waterton Sheet III, one of the largest gas fields in Canada, having an initial raw gas in place of approximately 100 billion cubic meters (BCM). Although average matrix porosity in the field is less than 4%, initial well potentials of greater than 1.2 million cubic meters/day were not unusual.

As part of this integrated study, data from well logs, cores, drilling and production records, pressure transient analyses, and reservoir engineering analyses were used to characterize the variations in fracture and matrix properties in the reservoir. The results of this characterization were utilized in a simulation study, described in a subsequent paper.

Conventional, tectonically-induced fracture intensity was found to vary with lithology (dolomite vs. limestone) and structural position. However, such 'conventional' descriptions were inadequate to account for large, but highly variable drilling mud losses in some zones, variable productivities that did not correlate strictly to structural position, and estimated volumes of initial-hydrocarbon-in-place.

In several key zones, solution enhanced fractures or karst development is proposed to account for these variations. This hypothesis is consistent with pressure transient analyses of most wells. The presence of such high porosity/permeability volume elements within the overall flow network of the reservoir has a profound influence on field development, in terms of recovering additional gas and liquid condensate. As a result, dual porosity models are required to diagnose dynamic reservoir behavior and investigate future development options.

Introduction

The Waterton Gas Field is situated in the southwestern corner of Alberta at the junction of the front ranges of the Rocky Mountains and the foothills disturbed belt (Figure 1). The field consists of westward dipping thrust sheets of Mississippian and Devonian carbonates with hydrocarbons trapped along the leading edges of what have been termed Sheets III, IV and IVC. The Waterton Field was discovered in 1957 when WAT-1 (4-21-4-1W5) encountered sour (15-20% H2S), wet gas in the Mississippian carbonates of Sheet IV.

The focus of this study is Sheet III (Figure 2), discovered in 1959 with the drilling of WAT-7 (7-34-3-1W5). The Waterton Field was put on production in 1962 upon completion of the initial phase of the gas plant. Twenty four wells have been drilled in Sheet III with the last (WAT-37) being completed in 1977. Drilling and seismic operations have been restricted to narrow stream valleys (Yarrow, Smith, and Drywood Mountain (Carpenter)) due to the rugged topography of the area and are prohibited in the Waterton Lakes National Park which extends from just south of the Yarrow Canyon to the Alberta/Montana border.

The gas initially in place (GIIP) in Sheet III was originally estimated to be 79 billion cubic meters (BCM) or 2.8 trillion cubic feet (TCF), although recent studies have shown that the GIIP may exceed 100 BCM (3.54 TCF). Production to date is in excess of 68 BCM of gas or 86% of the GIIP (68% if a GIIP of 100 BCM is used).

Sheet III, which contains a rich gas condensate with a maximum liquid dropout of 15%, has declined in pressure from 33 MPa to 5 MPa. It is estimated that 30 million cubic meters (150 million barrels) of condensate may still remain in the reservoir.