Abstract
Cyclic solvent injection, known as solvent huff-n-puff, is one of the promising techniques for enhancing oil recovery from shale reservoirs. This study investigates the huff-n-puff performance in ultratight shale reservoirs by conducting large-scale numerical simulations for a wide range of reservoir fluid types (retrograde condensate, volatile oil, and black oil) and different injection gases (CO2, C2H6, and C3H8). A dual-porosity compositional model is utilized to comprehensively evaluate the impact of multicomponent diffusion, adsorption, and hysteresis on the production performance of each reservoir fluid and the retention capacity of the injection gases. The results show that the huff-n-puff process improves oil recovery by 4–6% when injected with 10% PV of gas. Huff-n-puff efficiency increases with decreasing gas-oil ratio (GOR). C2H6 provides the highest recovery for the black oil and volatile oil systems, and CO2 provides the highest recovery for retrograde condensate fluid type. Diffusion and adsorption are essential mechanisms to be considered when modeling gas injection in shale reservoirs. However, the relative permeability hysteresis effect is not significant. Diffusion impact increases with GOR, while adsorption impact decreases with increasing GOR. Oil density reduction caused by diffusion is observed more during the soaking period considering that the diffusion of the injected gas caused a low prediction error, while adsorption for the injected gas showed a noticeable error.
Highlights
Tight and ultratight hydrocarbon reservoirs have noticeably contributed to oil and condensate productions in the last decade, and it is projected to be doubled in the decade [1]
This study investigates diffusion, adsorption, and hysteresis effects when different gases are injected into ultratight reservoirs with different fluid types
The results show that neglecting the adsorption during the primary depletion underestimates oil recovery by 1.69%, 1.06%, and 1.16% for black oil, volatile oil, and condensate
Summary
Tight and ultratight (shale) hydrocarbon reservoirs have noticeably contributed to oil and condensate productions in the last decade, and it is projected to be doubled in the decade [1]. Cronin et al [11] investigated the impact of the reservoir fluids and injection gas and concluded that cyclic injection performed better for lower GOR than condensate They highlighted the important role of density reduction and composition dilution in explaining the low recovery factor from ultralight oil reservoirs compared to gas reservoirs. Adsorption is an essential mechanism when modeling hydrocarbon production or gas injection in organic-rich shales in order to accurately predict the reserves, recovery factors, and reservoir storage capacity of the injected gas. This study investigates diffusion, adsorption, and hysteresis effects when different gases are injected into ultratight reservoirs with different fluid types (black oil to condensate). We investigate hydrocarbon primary production in the presence and absence of diffusion and adsorption We examine their impact during huff-n-puff for different types of fluid and injection gas. We compare the performance of each injection gas for different fluid types and make recommendations on the injection gas based on the reservoir fluid type
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