Evaluation of Directly Simulated WAG Hysteresis at Pore Scale and its Effect on Injectivity Index

Abstract

Water Alternating Gas (WAG) injection is a widely practiced EOR method for many reservoirs. One drawback of WAG is the decreased injectivity when gas, often CO2, is injected into a previously water-flooded reservoir, and a further decline of injectivity is observed as water and gas injection are alternated. We present a workflow which allows the estimation of injectivity decline using pore scale displacement simulations and reservoir simulations. In this approach, we use a multiphase Lattice Boltzmann method to directly simulate the alternating water-gas injection at pore scale resulting in a relative permeability curve for each injection phase. The simulation input accounts for injection rate, fluid properties and spatially varying wettability for each cycle during WAG. The final distribution of fluid phases in pore space of each displacement test is used as the starting point for the next displacement cycle. This enables the simulation of imbibition-drainage cycles. Any hysteresis effects present are typically captured in the resulting relative permeability curves. These are then used in a reservoir model to obtain an injectivity index for each injection phase. We observe a strong decline of water relative permeability after the first gas injection cycle in an oil-wet rock. Detailed analysis of the fluid phases, in particular the water phase, shows that water is well connected after the initial water flood before gas injection. As gas is injected large water blobs are partially displaced and their size significantly reduced. For this wettability scenario, water and gas are competing for the large pore system. We find that capturing the hysteresis effect in a WAG requires the direct simulation of the displacement process, in particular known pore scale phenomena such as trapping and retraction. The novelty of this approach is to directly capture the hysteresis effect of a WAG workflow in a direct simulation of displacement at pore scale. Emphasis is put on a detailed analysis of the multiphase displacement, including visualizations and an explanation for why the injectivity during WAG is reduced, namely, water and gas are competing for the same pore space. The presented workflow enables an a priori estimate for injectivity losses in a WAG EOR approach.

Oil production and reservoir damage during miscible CO2 injection

“…It has already been noted that residual oil distribution is affected by the pore-throat microstructure (Chatzis et al, 1988). In H2 and H3, the sweep volume of CO2 is larger, which implies that oil is displaced from more pores on whose pore surfaces more asphaltene precipitation adsorption occurs, resulting in the rock becoming more oil wet, and hence able to imbibe more oil once again; a propensity recently noted by Fager et al (2019) in lattice Boltzmann modelling of pore scale rock models. More oil production means more asphaltene precipitation, but leading to a smaller permeability decline, which again proves that the type of pore-throat microstructure of H2 and H3, especially H3, is more resistant to damage to rock properties by asphaltene precipitation.…”