Experimental Study On Increasing Oil Recovery Efficiency In Late Stage Of Waterflooding Development In Lunnan Oilfield

EOR by gas injection has been a mature technology to improve oil production since the20th century. Gas injection has been widely applied in the middle and later stage of sandstone oil reservoir development and water injection or natural depleting development before injection gas have been adopted by most of the reservoirs.After more than20years’water injection, oil recovery in Lunnan Oil Field has reached87.7%and entered the later stage of development. The horizontally unbalanced injection and production in the main reservoir LN2TI, big difference between interlayer physical properties, great interlayer interference, low oil recovery by water flooding, and high temperature and high salinity in Lunnan Oil Field impose strict restrictions on EOR by chemical flooding.This paper evaluated the feasibility of hydrocarbon gas injection in Lunnan Oil Field based on the present situation of water injection development. Direct at the relatively good but heterogeneous reservoir physical properties, great fluid property difference between various fluid districts, multiple layers which spread over large range of formation and plentiful natural gas resources, feasibility analysis research of EOR by hydrocarbon gas injection at the later stage of water flooding was based on the combination of laboratory experiment and numerical simulation.Core samples and fluid samples from three blocks, LN2TⅠ、LN2TⅡ、LN2TⅢ, were chosen for this research. Minimal miscible pressure (MMP) of hydrocarbon gas for these blocks, tested by slim tube experiment, were51.33MPa,54.31MPa and51.16MPa, which determined immiscible displacement in LN2TI and LN2TII and near miscible displacement in LN2TⅢ in comparison with present formation pressure in the three blocks. This research evaluated the oil recovery under different displacement methods by long core flooding experiments.9flooding experiments were carried out on these blocks, including gas injection, gas injection after water flooding and water alternative gas displacement after water flooding and results showed that oil recovery by water alternative gas displacement after water flooding was the highest for all three blocks.Block LN2TI was chosen for gas injection development numerical simulation. A five-spot injection-production unit conceptual model was established for this block, which considered the influence of vertical heterogeneity on different injection media (dry gas and water), recovery difference between different injection ways (general injection and layering injection) and macro mechanism. Simulation results of9development schemes showed that, for low permeable reservoir formation, recovery of gas injection was20.52%higher than that of water injection and, for high permeable reservoir formation,there was no distinct difference. What’s more, result of numerical simulation and that of laboratory experiment show the similar rule.