Investigation of Cyclic Gaseous Solvent Injection for Enhanced Oil Recovery in Unconventional Formations

Unconventional resources have been known to hold vast quantities of hydrocarbons. However, exploiting the oil and gas they hold was previously challenging due to their low permeabilities. Recent advances in horizontal well drilling and hydraulic fracturing have enabled oil and gas production from these reservoirs, leading to significant increases in energy supply to meet the growing energy demand. However, despite the relative technological success, considerable obstacles persist; oil production from unconventional wells often declines by more than 20% of their initial production rate within the first two years, leaving significant volumes of residual oil trapped in the formations. This overarching challenge warrants the need to develop techniques to improve recovery from these valuable resources. Enhanced oil recovery (EOR) from unconventional resources has been a primary subject of interest among the research community. Gaseous solvent injection has been touted as promising to enhance recovery from unconventional resources. Although several small-scale laboratory experiments and field-scale simulation studies have corroborated the efficacy of gaseous solvents in improving oil recovery, recent field pilot tests have failed to yield the expected improvement in oil production. Hence, the technique has not been widely adopted by industry players. A major contributing factor to this lack of adoption is the gaps in understanding the underlying multiscale (i.e., pore, core, and well scales) recovery mechanisms by gaseous solvents during the injection. Furthermore, there is an apparent knowledge gap in understanding the influence of critical operating parameters on oil recovery (and gas utilization), the variable interaction between these parameters, and their optimal settings at the well scale. This three-part project addresses some pertinent questions regarding gaseous solvent cyclic injection enhanced oil recovery in unconventional formations. The objective of the first part was vii to determine the efficacy of select gaseous solvents for EOR, determine the influence of critical operating parameters and the variable interactions with gas composition and miscibility conditions, and deduce the underlying dominant mechanisms during gaseous solvent cyclic EOR. To this end, core-scale experiments that simulate matrix fracture interactions were conducted using CO2 and ethane as gaseous solvents. The results showed that gaseous solvent huff-n-puff could achieve up to 87% recovery factor in small middle Bakken small core samples. The results also revealed the essential interactions of operating parameters with miscibility conditions and gas composition. Furthermore, the competition between diffusion and advection as a function of operating parameters was deduced from the results. The second part aimed to examine the pore scale displacement efficiencies and corroborate the underlying mechanism responsible for enhancing recovery during gaseous solvent EOR. Nuclear magnetic resonance relaxometry was used to measure the fluid-filled pore size distribution to assess the pore level displacement effectiveness of the gaseous solvent huff-n-puff. The results reveal that gaseous solvent huff-n-puff could displace fluids down to the micropores in tight core samples. They also revealed the influence of diffusion and advective force on pore-level displacement efficiency. The final phase consisted of well-scale studies to ascertain the large-scale effectiveness of gaseous solvents, the effects of the critical operating parameters and the interactions among them, and the optimization of these parameters for maximum recovery (and optimum gas utilization factor). Here, we leverage compositional reservoir simulation and response surface methodology (RSM) for this purpose. The result revealed the most important well-scale parameters for optimum recovery. Numerical optimization indicates that gaseous solvent huff-n-puff in the Bakken Formation can yield more than 7% incremental recovery with a gas utilization factor of viii more than 2 bbl/Mscf. We believe the findings in this work can better guide operators to design efficient gaseous solvent huff-n-puff strategies in the Bakken Formation and other unconventional reservoirs.