Enhanced Gas Recovery for Edge and Bottom-Water Gas Reservoirs throughout the Full Life Cycle

Abstract

To investigate the water and gas flow patterns throughout the entire development life cycle of a water-drive gas reservoir, as well as to evaluate the enhanced gas recovery (EGR) effects of carbon dioxide (CO2) and surfactant injection, long-core displacement experiments under different permeabilities and depletion pressures before gas injection were conducted using a self-designed experimental setup for simulating inclined bottom-water gas reservoirs. Additionally, nuclear magnetic resonance (NMR) technology was employed to study the fluid occurrence states during the entire development life cycle of the gas reservoir. Studies show that for medium-to-high permeability bottom-water gas reservoirs, during the water-flooding depletion development stage, the favorable water–gas mobility ratio stabilizes the water–gas displacement front, making this the main stage of gas reservoir production. The recovery factor can reach 59.91–73.19%; however, an excessively high permeability can lead to severe water channeling, thereby reducing the recovery factor. The recovery factor of the relatively high-permeability core sample (19.1 mD) during the water influx stage is 13.26% lower than that of the relatively low-permeability core sample (4.0 mD). The water drainage gas production stage can further increase the recovery factor by 5.52–9.13%, which is conducive to subsequent gas/chemical injection-enhanced recovery. When the gas reservoir is depleted to a certain pressure (40 or 60% of the initial formation pressure), switching to the CO2 injection stage can further improve the recovery factor by 3.51–4.73% before CO2 breakthrough. Under the same conditions, a later depletion pressure before gas injection (40 versus 60% of the initial pressure) releases greater natural energy and discharges more water to increase the recovery factor by 7.59% during the dewatering gas production stage. Sufficient dewatering can also enhance the gas-phase flow capacity to increase the recovery factor by approximately 4.72% during the CO2 injection stage. The main role of CO2 injection is to replenish energy and release water trapping; once breakthrough occurs, the gas recovery factor increases significantly but the methane concentration drops rapidly. After CO2 injection, due to the small amount of remaining gas, the effect of switching to surfactant flooding is limited, only increasing the recovery factor by 0.27–1.22%. Results from on-site NMR transverse relaxation time (T2) spectroscopy of core samples during the full-life-cycle experiment indicate that during the water-flooding depletion stage, macropores, mesopores, small pores, and micropores are well-produced. In the CO2 injection and surfactant-enhanced recovery stages, mesopores are further produced. This reveals that methane production follows the ″all-to-mesopore″ pore-utilization sequence. The above results in this study were obtained from relatively homogeneous and small core plugs; therefore, the experimental conclusions cannot be fully extrapolated to actual gas reservoirs with strong reservoir heterogeneity and large scale. However, the relative performance trends among different schemes can provide directional guidance for field decisions. In the future, large-scale laboratory experiments or actual well-pattern field trials should be expanded.