Study On Mechanism And Development Law Of Low Salinity Water Flooding For Carbonate Reservoirs

In recent years,Chinese oil companies have encountered a large number of porous carbonate reservoirs in the process of oil and gas field acquisition overseas,especially in the Middle East.There are obvious differences between this kind of carbonate reservoir and domestic carbonate reservoir.The main pore types of domestic carbonate reservoirs are fracture and karst cave(such as Tahe Oilfield).The carbonate reservoirs in the Middle East are mainly pore-type,and there are few or no large karst caves.Fractures are only locally developed with a small amount.Because of the special wettability(neutral-partial oil wettability)and strong heterogeneity of the reservoirs,abnormal waterlogging is very easy to occur for conventional water flooding development in such carbonate reservoirs,resulting in poor development effect.According to the development practice of similar reservoirs abroad,in order to improve the recovery of such reservoirs as much as possible,many methods for water control and oil recovery enhancement have been put forward by researchers.Among them,as a simple,efficient,low cost,low risk and environmentally friendly technology to enhance oil recovery,low salinity water flooding has attracted great attention.However,a unified understanding of the law of low salinity water flooding has not been formed yet.Different carbonate reservoir rocks have different effects during low salinity water flooding.Even some experimental results are contradictory.There are also some unsuccessful cases in laboratory experiments and field applications.For this reason,in this paper,the mechanism and development law of low salinity water flooding in carbonate reservoirs are studied profoundly by combining physical experiment,theoretical analysis and numerical simulation.1.Experimental study on the mechanism of wettability alteration for carbonate rocks during low salinity water flooding.The research shows that for carbonate rocks with specific mineral components,with the decrease of salinity of the injected water,the maximum variation of rock wettability increases first and then decreases,that is,there exists an optimum salinity which makes the maximum variation of rock wettability.The optimum salinity value is between 1192.3 mg/L and 4769.2 mg/L,and with the increase of calcite content in the rock,the optimum salinity value increases gradually.Under the action of the brine with a certain salinity,the maximum change of wettability of carbonate rocks is linearly and positively related to the content of calcite in the rock.Under different salinity brines conditions,the sensitivity of rock wettability to calcite content is different.The sensitivity is the lowest when salinity is 238460.5mg/L,and the sensitivity is the highest when salinity is 4769.2mg/L.Based on the experimental results,the constitutive equation of rock wettability varying with salinity of the brine and calcite content in rock is established.2.Experimental study on the mechanism of oil-water relative permeability variation during low salinity water flooding for carbonate rocks.The results show that compared with formation water flooding,when flooded with low salinity water flooding which comes from diluted formation water,with the decrease of water salinity,oil relative permeability increases first and then decreases,and water relative permeability decreases first and then increases.That is to say,there exists the optimum salinity value for water flooding,and the optimum salinity value is about 2384.6 mg/L.The normalized oil-water phase permeability curve shows good logarithmic function characteristics.When salinity of the injected water is the same,the coefficient of oil-water phase permeability curve is linearly and positively correlated with the content of calcite in the rock.With the increase of calcite content in the rock,the oil relative permeability increases and the water phase permeability decreases.Based on the experimental results,the constitutive equation of oil-water relative permeability during low salinity water flooding varying with salinity of the brine and calcite content in rock is established.3.Study on numerical simulation method of low salinity water flooding in carbonate rocks considering changes of injected water salinity and rock mineral composition.A new formula for calculating interpolation coefficient of oil-water relative permeability and capillary pressure is derived.The existing constitutive equation of numerical simulation of low salinity water flooding is improved.The defect that the original constitutive model can not consider the influence of rock composition and the oil-water relative permeability and capillary force interpolation coefficient at arbitrary salinity can not been calculated is corrected.The variation law of oil-water relative permeability and capillary pressure interpolation coefficients with water salinity and calcite content in the rock are obtained with this method.The interpolation coefficients of oil-water relative permeability and capillary force are concave with salinity.Under the same salinity of injected water,the interpolation coefficients of oil-water relative permeability and capillary pressure have a good linear relationship with calcite content in the rock,and with the increase of calcite content in the rock,the interpolation coefficients of oil-water phase permeability and capillary pressure increase gradually.4.Study on the development law of low salinity water flooding in carbonate reservoirs.Using the established numerical simulation method of low salinity water flooding,the development law of low salinity water flooding in carbonate reservoirs is studied.With the increase of reservoir thickness,the advantages of low salinity water flooding to high salinity water flooding weaken gradually.The higher calcite content in the rock,the higher oil recovery.The recovery of low salinity water flooding in reservoirs with positive lithological rhythm distribution(calcite content increaes from top to bottom)is higher than that of lithologic reverse rhythm distribution reservoirs,and the advantages of low salinity water flooding in the former reservoir are more obvious than in the latter one.In field implementation,the best choice of reservoir development is carried out with low salinity water flooding in the first half of the development and followed with formation water flooding in the second half of the development.In summary,this paper develops the mechanism of wettability alteration,oil-water relative permeability change and numerical simulation method of low-salinity water flooding in carbonate reservoirs by combining physical experiment,theoretical analysis and numerical simulation,and the development law of low-salinity water flooding in carbonate reservoirs is given.The research results have significant theoretical significance and applied reference value for efficient development of carbonate reservoirs using the technology of low salinity water flooding.