Investigating Enhanced Oil Recovery in Unconventional Reservoirs Based on Field Case Review, Laboratory, and Simulation Studies

Abstract

Aimed at advancing gas injection enhanced oil recovery (EOR) technologies in unconventional reservoirs, this study comprised a series of activities to bridge the gap between the theoretical study and actual field applications. Twenty-four EOR pilot tests were collected from the major unconventional plays in North America to evaluate the performance of different EOR technologies. Fit-for-purpose experiments and simulations were performed to investigate the effects of injection rate and pressure on EOR performance, as well as to reveal the effectiveness of huff "n" puff (HnP) cycles in actual field operations. The selection of injection rate and pressure as key parameters for investigation was based on field observations and communications with oil and gas operators because these two parameters play critical roles in both facility design and overall cost for an EOR project. Results showed that miscible EOR with a high gas injection rate and pressure is required for field operations because the injected gas needs to penetrate and extract oil from the tight matrix. Experimental results indicated that there is a correlation between oil recovery and the logarithm of core volume for miscible EOR. Immiscible gas EOR could not yield a satisfactory EOR response in actual reservoirs because the injected gas tends to flow through fractures instead of penetrating the matrix to interact with oil. Results also showed that reaching minimum miscibility pressure (MMP) does not guarantee an optimum EOR operation in unconventional reservoirs. Pressure higher than MMP is preferred in field operations. When designed properly, up to a tenfold oil production rate boost is achievable in field applications within a short period. However, such a high-performance operation is only effective in the first several HnP cycles due to the limited gas penetration depth into the rock matrix.

Minireview of Microscopic CO2 Interactions with Fluids and Minerals in Shale: Advances and Outlook

“…During CO 2 -EOR, injected CO 2 not only interacts with reservoir fluids to change oil properties but can also react with minerals to change shale properties. Because the permeation depth of CO 2 is small, CO 2 -induced mineralization mainly changes rock properties near the wellbore, such as mechanical properties, pore sizes, permeability, and wettability. The mechanisms and effects of CO 2 -induced mineralization are both complicated as a result of the multicomponent nature of shale.…”

Fractal Pore-Scale Model for Effective Thermal Conductivity of Multiscale Unsaturated Porous Media

“…The relative roughness depends on the porosity, fractal dimension, and size range of rough elements. As illustrated in Figure 1c, the equivalent capillary diameter should be revised as λ ' = λ(1 + ε) and the equivalent porosity can be rewritten as ϕ ' = ϕ(1 + ε) 2 55,56 Figure 1b shows that, under certain capillary pressure, the gas (nonwetting) and liquid (wetting) phases occupy pores larger and smaller than a critical value λ c , respectively. The capillary pressure can be estimated by the critical capillary diameter, p = σ cos θ/λ c .…”

“…Water-based enhanced oil recovery (EOR) techniques have been proven profitable in conventional reservoirs for decades; however, the extremely low permeability and mixed wettability of unconventional formations present foremost obstacles to success. , Meanwhile, thermally enhanced oil recovery (TEOR) techniques use thermal energy from hot fluid injection, in situ combustion, and electrical and electromagnetic heating to reduce oil viscosity, specific gravity, and interfacial tension, which facilitate oil flow in the production well . TEOR is a highly promising EOR method in the development of unconventional hydrocarbon resources including heavy oil, shale oil and gas, coal bed methane, etc. − Heat conduction is pivotal in designing of TEOR systems, where the estimation of thermal conductivity of unconventional formations becomes significant .…”

Molecular Insights into Effects of Methanol Additive on CO2 Enhanced Oil Recovery in the Dead-End Nanopore

“…4,5 However, unlike conventional reservoirs with considerable oil recovery factors, unconventional reservoirs often leave a large percentage of trapped oil, posing a challenge that requires innovative approaches to extract hydrocarbons throughout the reservoirs. 6,7 The low oil recovery factors of unconventional reservoirs, typically below 10% after primary oil recovery, can be attributed to their unique reservoir environment, including nanoscale pore sizes, low rock permeabilities, limited pore connectivity, and high oil viscosity. 8 Gas injection-enhanced oil recovery (EOR) is a promising approach in tertiary oil recovery.…”

“…Global demand for crude oil in the energy sector remains high, despite the rapid development of renewable energy sources. − The discovery of new conventional oil reservoirs, however, generally declined in the past decades. To meet the growing demand, huge efforts have been devoted to unconventional reservoirs due to the vast potential for oil reserve growth. , However, unlike conventional reservoirs with considerable oil recovery factors, unconventional reservoirs often leave a large percentage of trapped oil, posing a challenge that requires innovative approaches to extract hydrocarbons throughout the reservoirs. , The low oil recovery factors of unconventional reservoirs, typically below 10% after primary oil recovery, can be attributed to their unique reservoir environment, including nanoscale pore sizes, low rock permeabilities, limited pore connectivity, and high oil viscosity . Gas injection-enhanced oil recovery (EOR) is a promising approach in tertiary oil recovery .…”