Viability of EOR Processes in the Bakken Under Geological and Economic Uncertainty

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For tight oil reservoirs, primary depletion using multistage hydraulically fractured horizontal wells typically only recovers between 5 to 15% of original oil in place, and can exhibit an exponential decline in production rates within a few months. Various enhanced recovery techniques exist, which offer great potential to increase the recovery in conventional reservoirs, however their feasibility within tight formations is not proven. This paper investigates the applicability of several different recovery techniques using a simulation-integrated workflow to both optimize the hydraulic fracture and well configuration design, and determine under which conditions these EOR methods are feasible.

In addition to natural depletion, the EOR techniques investigated included immiscible and miscible gas flood and a cyclic Huff n Puff process. Using a simulation model representative of the Elm Coulee reservoir in the Bakken formation, a net present value optimization was performed with comprehensive economic analysis. Optimal operating constraints, fracture design and well configurations were found for each of the techniques investigated. Upon completion of the optimization, each optimal case was subjected to a sensitivity analysis on geological and economic parameters which cover the expected range of the Middle Bakken formation within the Elm Coulee region. Based on this uncertainty, a greater confidence can be achieved in each of the techniques as well as their versatility.

From the simulation study, it was discovered that the optimal configuration was different for each of the recovery techniques evaluated. For natural depletion and immiscible flooding, the optimal well spacing was 400m combined with the largest fracture spacing of 125m. Conversely, the miscible flooding method achieved optimal NPV values at the closest well spacing. The cyclic process, which used CO2 for the injected fluid, produced its optimal NPV at a closer fracture spacing when compared to the other 3 methods. Additionally, the Huff n Puff process required further optimization on cycle time to achieve the ideal conditions. The uncertainty analyses performed showed that for each of these recovery techniques the results varied significantly depending on the combination of reservoir, hydraulic fracture design and operating input parameters. Economically, the oil price was the most influential parameter, while geologically the connate water saturation and formation thickness carried the greatest impact. A change in conditions could greatly influence the application of the recovery techniques and possibly yield different optimum configurations.

This paper provides an extensive workflow for optimizing EOR techniques and insight into their applicability in tight oil formations. It also demonstrates the importance of understanding the geological parameters prior to determining the ideal recovery technique and corresponding well configuration.