Abstract
Unconventional reservoirs produce large volumes of oil; however, recovery factors are low. While enhanced oil recovery (EOR) with cyclic gas injection can increase recovery factors in unconventional reservoirs, the mechanisms responsible for additional recovery are not well understood. We examined cyclic gas injection recovery mechanisms in unconventional reservoirs including oil swelling, viscosity reduction, vaporization, and pressure support using a numerical flow model as functions of reservoir fluid gas–oil ratio (GOR), and we conducted a sensitivity analysis of the mechanisms to reservoir properties and injection conditions. All mechanisms studied contributed to the additional recovery, but their significance varied with GOR. Pressure support provides a small response for all fluid types. Vaporization plays a role for all fluids but is most important for gas condensate reservoirs. Oil swelling impacts low-GOR oils but diminishes for higher-GOR oil. Viscosity reduction plays a minor role for low-GOR cases. As matrix permeability and fracture surface area increase, the importance of gas injection decreases because of the increased primary oil production. Changes to gas injection conditions that increase injection maturity (longer injection times, higher injection rates, and smaller fracture areas) result in more free gas and, for these cases, vaporization becomes important. Recovery mechanisms for cyclic gas injection are now better understood and can be adapted to varying conditions within unconventional plays, resulting in better EOR designs and improved recovery.
Highlights
Unconventional reservoirs such as the Eagle Ford, Bakken, and others have had tremendous success over the last decade
This paper focuses on the recovery mechanisms for huff-n-puff natural gas injection projects to determine why additional oil is recovered
While huff-n-puff gas injection into unconventional reservoirs is different to thermal recovery, a similar approach can be used to evaluate the mechanisms, including determining the recovery as a function of fluid properties
Summary
Unconventional reservoirs such as the Eagle Ford, Bakken, and others have had tremendous success over the last decade. While huff-n-puff gas injection into unconventional reservoirs is different to thermal recovery, a similar approach can be used to evaluate the mechanisms, including determining the recovery as a function of fluid properties. Vaporization are more significant for lighter oils While this is a qualitative view of recovery as a function of the oil density, a more quantitative evaluation was conducted for a medium-weight oil (22 API) as a function of pore volumes of steam injected [23]. For this oil, all mechanisms contribute significantly; this work showed that the relative importance of the mechanisms changes slightly. GOR of scf/STB and evaluate wide range reservoir parameters to understand the recoveryhow mechanisms are mechanisms impacted. are impacted
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