Water Distribution in the Montney Tight Gas Play of the Western Canadian Sedimentary Basin: Significance for Resource Evaluation

Abstract

SummaryWater distribution in unconventional gas reservoirs is a key parameter that influences many aspects of resource exploitation including selecting geographic areas for multiwell development programs, identifying target zones for horizontal wells, calculating reserves, estimating permeability, and understanding variability in gas and water production. Insights on water distribution in the Montney tight gas play of northeast British Columbia and northwest Alberta were gained by integrating reliable water-saturation measurements from full-diameter core samples with other core, well-log, and geologic data. Water distribution in the studied Montney section was found to be related to stratigraphic architecture and rock fabric (defined by degree of bioturbation), and is interpreted to have been influenced by the displacement efficiency of mobile formation water updip through tight Montney siltstones during hydrocarbon charging. Low-gradient clinoform units with few shaly zones in the Lower Montney section enabled efficient water displacement and led to water contents at or close to irreducible water saturation. Higher-gradient clinoform units with greater facies variation and common shaly zones in the Upper Montney allowed less-efficient water displacement, and significant volumes of mobile water were retained in parts of the section. Water saturation varies widely and directly influences gas effective permeability. A simple method for determining gas effective permeability from well logs was developed on the basis of empirical relationships derived from the core and log data set. Gas-effective permeability logs generated by this method help to identify target zones with superior reservoir quality for exploitation by use of horizontal multistage-fracturing technology. A Pickett plot with gas-effective-permeability lines was found to be a useful tool for a log-based comparison of Montney rock quality by stratigraphic zone or geographic area. This work shows that understanding water distribution and its influence on gas effective permeability leads to the improved delineation of "sweet spots" for resource development in unconventional gas plays.