Abstract
We conducted a comprehensive analysis of approximately 7000 horizontal wells drilled in the Middle Bakken formation between 2007 and 2016 to assess the impact of well orientation on cumulative production. While it is common practice to drill horizontal wells "on-azimuth", that is, in the direction of the minimum horizontal stress (Shmin), there is a diversity of well orientations in the Bakken. Shmin is consistently oriented N42°W throughout the production area. Our analysis clearly demonstrates that wells drilled in the direction of Shmin ("on-azimuth") produce more barrels per foot than wells in other directions, both in the core area and across the entire Bakken play. However, the amount of uplift gained from drilling on-azimuth wells decreases as the field matures, which we hypothesize is due to depletion. We found that the relationship between production and well orientation is consistently observed, regardless of the amount of proppant used. An economic analysis indicated that for wells of equal length, it is clearly beneficial to drill wells in the direction of Shmin. However, wells in the direction of Shmin are consistently shorter in length than off-azimuth wells, and it is generally more efficient to drill longer laterals on a given leasehold. Nevertheless, using the average oil price at the time the wells we studied were drilled, we find that the shorter wells in the on-azimuth direction have a significant economic uplift of several million dollars per well relative to the longer wells drilled in the off-azimuth direction.
Hydraulic fractures propagate in a plane perpendicular to the least principal stress (Hubbert and Willis, 1957) which normally means that hydraulic fractures propagate in vertical planes, normal to Shmin in areas characterized by strike-slip or normal faulting. When exploiting unconventional oil and gas reservoirs, it is common to drill horizontal wells with multiple hydraulic fracturing stages in the direction of Shmin. If the spacing between adjacent wells reflects the drainage area associated with the propped half-lengths of the hydraulic fractures of the wells, it would seem to result in optimal recovery. This said, in some areas wells are drilled in north-south or east-west directions regardless of the stress orientation to 1) optimally exploit available acreage with the highest number of wells and 2) take advantage of the fact that drilling longer wells decreases drilling and completion costs on a per foot basis.
In this paper we study the relationship between well orientation (relative to the stress field) in the Bakken play to assess its effect on production. The Bakken shale play is in the Williston Basin straddling a region of 200,000 square miles across western North Dakota, eastern Montana, Saskatchewan, and Manitoba. Unconventional oil production started in 2006 and peak oil production reached its maximum to date in 2019 with approximately 1.5 million barrels per day (EIA, 2020a). We have restricted this study to wells drilled in the Middle Bakken formation to avoid intermingling data from the less productive Three Forks.
