Abstract

Efficient production allocation plays a pivotal role in optimizing spacing and sequencing within plays that encompass stacked producing zones. This project focused on precisely allocating production from the primary producing zones within the Bakken/Three Forks development, i.e., determining the proportion of total production contribution derived from the Three Forks for wells landed in the Middle Bakken, and vice versa. Additionally, this initiative aimed to assess the contributions of the Upper and Lower Bakken Shales, recognized for their substantial STOOIP. Finally, the project sought to understand variations of zonal contribution associated with completion vintage and drainage changes through time for new vintage completions.

In the Bakken Petroleum System, production from the primary Middle Bakken and Three Forks reservoirs and the adjacent Upper and Lower Bakken source rocks is challenging for traditional geochemical allocation techniques; short-range geochemical changes laterally overwhelm the limited vertical geochemical heterogeneity. Multi-dimensional gas chromatography (or GC × GC) offers a solution by increasing the number of compounds available to characterize stacked producing units. In this project, GCxGC analyses were performed on a fresh core to build a geochemical baseline of the stratigraphy. Analyses were then applied to time-lapse oil samples collected over a one-year period from parent and child wells in adjacent DSUs to determine zonal allocations of different vintage completions.

The GC × GC analysis was successful at allocating production across the Bakken Petroleum System. Wells landed in both the Middle Bakken and Three Forks produce oils from all stratigraphic zones with the primary landing zone producing about 1.5X more oil than the other main landing zone. Older wells with smaller completions have more production from the landing zone; modern plug-and-perf completions have more equivalent production from both zones. Together, the Upper and Lower Bakken Shales yield about 10% of the total oil production, with a slightly higher fraction coming from newer wells with more intense completions. Careful interpretation of the data reveals the subtle impact of parent-child interactions, completion designs, and completion operations.

This project demonstrates that Middle Bakken-only development can effectively capture Three Forks resources while reducing the total water production from a DSU. It shows how early parent wells with small completions may have partially drained the non-landed zone, leaving infill opportunities. It also shows how new child wells will impact production in offset parent wells and how new child well drainage allocation changes with time.

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