Study On Enhanced Oil Recovery By Surfactant Flooding For Low-permeability Reservoir

Low permeability reservoirs are the main areas of increasing production for oil field.Surfactant flooding enhancing oil recovery of low permeability reservoirs has obtained someachievements, but its theory research is no perfect. Defining the mechanisms of surfactantflooding enhancing oil recovery of low permeability reservoirs is of great theoreticalsignificance and academic value for developing low permeability reservoirs. The mechanismsof surfactant flooding enhancing oil recovery of low permeability reservoirs were studied bymacroscopic displacement experiments and microscopic flow experiments. The mainresearch results are as follows.(1) Whether it was the formation water flooding or surfactantflooding, the low-permeability cores had starting pressure gradient in the course of fluid flow.Through the surfactant flooding, minimum starting pressure and threshold pressure decreasedsignificantly. The higher the surfactant concentration in the water-phase was, the more therelationship curve between seepage velocity and pressure gradient deviated from horizontalaxis, the shorter the length of curved section was. Starting pressure gradient of oil and watertwo-phase in low permeability core gradually decreased, and water saturation graduallyincreased, as the ratio of displacing velocity of oil and water two-phase decreased. At thesame ratio of displacing velocity, starting pressure gradient of oil and water two-phasegradually decreased, and water saturation gradually increased, as the interfacial tensiondecreased. And the change of water saturation was larger with the increase of the proportionof surfactant.(2) The influence of oil-water interfacial tension on oil recovery of lowpermeability core was studied. Average diameter of emulsified oils decreased with thedecrease of interfacial tension of oil and water. The relative permeability curves of waterflooding had the following characteristics: relatively narrower two-phase region, lower waterrelative permeability at residual oil saturation, higher residual oil saturation, and lowerultimate oil recovery. While oil/water interfacial tension decreased, the two-phase region became wider, residual oil saturation decreased, water relative permeability at residual oilsaturation increased, and ultimate oil recovery increased. Through core displacementexperiments, water flooding recovery of low permeability core was low. After injectingsurfactant slug with0.5PV, the recovery rose in some extent. And, the lower the oil-waterinterfacial tension, the better the effect of enhancing oil recovery. The experiment of reducinginjection pressure showed that injection pressure rose sharply during the first water flooding,and the peak is achieved when the volume of injecting water reaches2~2.5PV, then thepressure becomes gradually stable after showing a small drop. The injection pressure droppedafter injecting surfactant slug with0.5PV. And, the lower the oil-water interfacial tension, thebetter the effect of reducing injection pressure.(3) The wettability of low permeability coreswas measured which have the similar permeability. The effect of wettability on oil recoveryof low permeability core was studied. And after injecting the same surfactant, the changes ofoil recovery and injection pressure of low permeability cores were researched. Results showthat, irreducible water saturation decreased with the enhancement of hydrophobicity of coreswhile the permeability of the low permeability cores of different wettability was similar. Theeffect of wettability on water flooding recovery was very great. The recovery of the core forslightly water wet was the highest, followed by the core for neutrally water wet, and therecovery of the oil-wet core was the lowest. And, there is similar tendency of the totalrecovery of the first water flooding, surfactant flooding and the second water flooding. The effectof reducing injection pressure of the core for neutrally water wet after surfactant flooding wasthe strongest, followed by the core for slightly water wet, and the effect of the oil-wet corewas the least. And, there is similar tendency of the effect of enhancing oil recovery aftersurfactant flooding.(4) The existence form of residual oil after water flooding, themechanism of surfactant flooding reducing residual oil were studied by using glass etchingmodel. The main microscopic morphology of residual oil after water flooding can becategorized into oil film, columnar-type, broken and stuck oil droplet, Y-shape, blind-type,and cluster-type residual oil. The injection of surfactant solution can obviously decrease thesaturation of residual oil. The mechanism of surfactant active mainly includes: reducinginterfacial tension of oil-water, reducing capillary force of pore, altering the wettability ofrock surface, reducing adhesive energy between residual oil and pore surface, emulsifying crude oil, increasing the surface charges, conglomerating oil drop, forming oil zone,solubilization capacity, changing the rheology of crude oil and so on. For the same surfactant,the lower the oil-water interfacial tension, the higher recovery it can get. But the surfactantconcentration has little effect on the recovery if the difference value of oil-water interfacialtension is very small.