Research And Application Of Augmented Injection Technology To Injection Wells In Low-permeability Niu25 Block

Block Niu25 is located in the southwest of Niuzhuang oilfield.It is composed of 12 units,with the average permeability and the porosity being respectively 20.5×10^-3μm² and 19.2%,which indicates that this block should be classified as a medium porosity and low permeability lithologic reservoir.Poor reservoir properties and improper production methods result in significantly insufficient injections of water wells,which seriously restrict the efficient development of the block.Consequently,it is of great importance to carry out an efficient augmented injection technology to improve the water flooding recovery in Block Niu25.In this paper,geological characteristics,physical properties,pore throat characteristics and sensitivities of Niuzhuang oilfield have been detailedly investigated.The results show that the permeability of Block Niu25 reservoir is low,combined with the features of small throat radius,heterogeneous pore throat structure and large pore throat ratio.Considering these reservoir properties and that the reservoir is strongly lipophilic with a high residual oil saturation,the capillary resistance forces during water flooding should be quite large.Therefore,the relative permeability of water phase is low,resulting in a high injection pressure gradient around the water injection wells during the production of remaining oil.This should be the internal reason for the difficulty of water injection in Block Niu25.Based on the analysis of water qualities of injected water and formation water,the compatibility of injected water and formation water has been investigated.The results show that the calcium carbonate（or magnesium carbonate）precipitations are generated by mixing the two kinds of water at various proportions,and these sediments could obviously block the reservoir.The incompatibility of injected water and formation water is the main external cause of water injection difficulty in Block Niu25.The damage degree of suspended solids,sulfate reducing bacteria（SRB）,saprophytic bacteria（TGB）and iron bacteria（IB）in the injection water on cores from Block Niu25 have been studied by flow experiments.The results show that the damage of cores could be small while the content of suspended solids is less than 1.3 mg·L^-1,the median size of suspended solids is less than 1.5μm,and the bacterial content is less than 150 CFU·m L^-1.On this basis,the internal control standard of injected water is established.Based on the physical properties and the reason of insufficient water injection in Block Niu25,the technical ideas to increase water injection and decrease injection pressure are established,i.e.,reducing the capillary force and the residual oil saturation during water flooding and eliminating the carbonate scales.The adaptabilities of mud acid,polyhydric acid,fluoroboric acid and organic acid in Block Niu25 have been estimated based on the indexes including dissolution rate,reaction rate and the ability of preventing secondary injuries,and the results show that polyhydric acid has the effects of slowing down and preventing secondary injuries.In order to improve the displacement efficiency,nine kinds of surfactants have been estimated.Combineing the ability to reduce interfacial tension,adjust wettability and reduce residual oil saturation,a compound acid-surfactant depressurization and injection augmentation system suitable for applications in Block Niu25 has been developed.Field tests of this compound system have been carried out in NG42 and N101.The results show that the average injection oil pressure of two test wells were decreased by 24 MPa,and the average period of validity was more than 20 months.The oil production of the two well groups was increased by 1860 t,and the economic benefit was 2.18 million yuan.It has been proved that the compound acid-surfactant depressurization and injection augmentation system could meet the requirements of applications in Block Niu25.