Systematic Study of Bakken Well Performance Over Three Well-Completion-Design Eras

Abstract

Oil production from the Bakken formation has been active for 60 years and has applied three well-completion-design strategies in three different eras: hydraulically fractured vertical wells before 1987; horizontal wells before 1991; and multiple-transverse-fracture horizontal wells since 2006. Reported production data enable comparisons of well performance during these eras. This study employs the log-log graph of rate (q) vs. material balance time (MBT) (Q/q) to diagnose transient (Slope 2;1=4 or 2;1=2) and boundary-dominated flow (BDF) (Slope 2;1) behaviour. Wells from the three eras show mainly three types of flow-regime sequences observed as straight trends on the log-log graph of rate vs. MBT: 2;1=4 to 2;1, 2;1=2 to 2;1, and 2;1=4 to 2;1=2 to 2;1. Flow geometries corresponding to various flow-regime sequences are related to specific well and formation characteristics. Then, we extrapolate the BDF behaviour to estimate the expected ultimate recovery (EUR) for each well when possible. We observed that both the average well EUR and the average well rate at start of BDF behaviour are highest for the multiple transverse- fracture-horizontal-well completion design. This project also investigates the behaviour of the gas/oil ratio (GOR) vs. MBT. Three types of GOR behaviour were observed: constant; constant followed by sharp increase; or scattered. In all three eras, the EUR was highest in wells with constant-GOR behaviour followed by a sharp increase. The sharp increase likely signals flow below the bubblepoint pressure. The lower EUR in wells that did not produce below the bubblepoint pressure shows that solutiongas drive behaviour enhances the EUR. Lower EUR in wells with scattered GOR behaviour may be attributed to unstable well production. This study shows how to use long-term production behaviour to gain important insights describing well designs and why some wells have higher EUR and rate behaviour.