Compositional and Relative Permeability Hysteresis Effects on Near-Miscible WAG

Abstract

Evaluation of compositional effects and fluid flow description on near-miscible (water-alternating-gas) WAG modeling have been studied for a North Sea oil field starting production in 1998. A sector model with four wells was applied to simulate a heterogeneous sandstone reservoir, and a compositional model was used to compare different production strategies e.g. waterflooding and a near-miscible (WAG) injection. In the WAG scheme both dry and wet (rich) hydrocarbon gases have been considered for injection. The phase behaviour was quantified by comparing the performance of the different injection gases. Result obtained shows the WAG injection gives improved recovery compared to water injection, due to better sweep and lower residual oil saturation. Simulations with and without relative permeability hysteresis (two-phase model) were compared. The effect of trapped gas on oil recovery does not seem significant with the compositional model. The WAG process has been optimized with respect to slug size and the water-gas ratio. A black-oil-model was generated tuned to fit the results from the compositional simulations. A WAG three-phase relative permeability hysteresis model using cycle dependent relative permeabilities for both wetting and non-wetting phases, have been compared to the standard two-phase Killough and Carlson hysteresis models. The results show significant lower gas ratio and a higher oil recovery for the WAG injection when using cycle dependent relative permeabilities. The simulations show sensitivity toward the three-phase model whereas Carlsen/Killough type hysteresis has little/no influence on the oil recovery. Simulations indicates that the recovery by WAG-injection may be underestimated in the compositional model due to lack of possibility for cycle dependent relative permeability hysteresis, whereas the black oil model underestimates the phase behaviour effect. The results indicate that the WAG- injection might have an upside potential with respect to the influence of combined phase behaviour and relative permeability effects. P. 233

Assessment of Oil Recovery Methods for Reservoirs in the Flemish Pass Basin

“…From the simulations, WAG is determined to result in a higher recovery factor than a standard water flood recovery, increasing the sector model recovery factor by approximately 3% [15]. Using continuous gas flood alone results in a recovery factor increase of 1% over water flood [15].…”

“…In reality, it may prove difficult or impossible to maintain the reservoir pressure above the minimum miscibility pressure for the entire duration of a field's production. A 1998 paper by Christensen et al [15] approaches a real-world simulation study by using both compositional and black oil fluid models, and a heterogeneous sandstone reservoir -representing a producing oil field in the North Sea -to compare water flooding to near-miscible WAG flooding. This is a very reasonable perspective as injection conditions often fluctuate between miscible and non-miscible reservoir conditions in real field conditions.…”

“…This is a very reasonable perspective as injection conditions often fluctuate between miscible and non-miscible reservoir conditions in real field conditions. Additionally, this study evaluates relative permeability models, with and without hysteresis, in order to evaluate the dynamic fluid effects on recovery [15].…”

An Overview of Oil Production Stages: Enhanced Oil Recovery Techniques and Nitrogen Injection

“…The initial proposed ratio of water and gas was 0.5:4 in frequencies of 0.1 to 2% Pore Volume slugs of each fluid that was being adopted according to the reservoir conditions [20]. Miscible WAG injection has been implemented successfully in a number of fields around the world [43]. In principle, it combines the benefits of miscible gas injection and water flooding by injecting the two fluids either simultaneously or alternatively [33].…”

Experimental investigation of simultaneous water and CO2 (SWACO2) injection for oil recovery in immiscible and near‐miscible conditions: A comparative study

“…Injection strategies like water alternating gas (WAG) and simultaneous water and gas (SWAG) have been devised to reduce the gas mobility and hence increase the sweep efficiency. In WAG injection, water and gas are injected as separate slugs, whereas in SWAG injection, water and gas are injected simultaneously in the porous media . WAG injection process has been extensively studied in the literature.…”