Prospects of miscible gas injection to enhance oil recovery from low-permeable Jurassic reservoirs in West Siberia

Introduction. Standard waterflooding in the development of low-permeable reservoirs may be ineffective due to low water injection rates. In this regard, gas methods of enhanced oil recovery are particularly relevant due to the high mobility of gas which allows to reach the required injection rates. Also, if miscible injection is possible, this will maximize the oil displacement efficiency. However, due to the high mobility of gas and the presence of highly permeable channels in the reservoir, the gas efficiency can be significantly reduced due to its premature breakthrough to production wells. WAG injection can significantly reduce the probability of gas breakthrough and thereby increase the gas sweep efficiency while maintaining relatively high injection rates.

Objective. Evaluate the performance of miscible WAG (water alternating gas) injection in heterogeneous low-permeable reservoirs in West Siberia.

Materials and methods. The study evaluates gas injection technology on a composite flow simulation model based on laboratory studies of WAG miscible injection. In this paper, based on the results of a series of slim-tube experiments on gas-oil displacement, the possibility of achieving miscible displacement in reservoir conditions was assessed. Low-pressure gas of a treatment system (wet gas) demonstrated high displacement efficiency not only in a slim tube (ED — 0.94), but also on core samples (ED — 0.695). Experiments on additional displacement of oil from a core column at WAG conditions showed an insignificant increase of ED — 0.035. At the same time, the water and gas permeability in the WAG cycles decreased by 5 and 30 times, respectively (the value for gas is given relative to the basic gas injection cycle), which will have a positive effect on the process of redistribution of water and gas flows in a reservoir.

Results. Based on the results of numerical runs of sector reservoir models, it was found that despite the miscible nature of displacement and a high degree of permeability homogeneity within a reservoir, the wet gas, due to its high mobility, does not allow achieving high sweep efficiency and, therefore, high oil recovery factors. The option of wet gas injection in WAG conditions has a high potential for increasing oil recovery in both short and long term.

Conclusion. The WAG technology, despite an insignificant increase in ED in the core experiments, allows increasing the specific performance of gas injection by an average of 4.4 times compared to continuous wet gas injection due to an increase in sweep efficiency. At the same time, both options of wet gas injection allow achieving an increase in oil recovery compared to the basic flooding option: the average oil recovery factors for water injection, continuous wet gas injection, and wet gas injection options in the WAG conditions were 0.21, 0.3, and 0.48, respectively.

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