| The development of low permeability and ultra-low permeability reservoirs has become an important potential for the stable development of China's petroleum industry and the guarantee of National Petroleum security.However,the characteristics of low permeability,low abundance and fracture development is the main question in low permeability reservoirs which lead to low single well production,fast production decline,serious water channeling,small sweep coefficient,low oil displacement efficiency.The average water flooding recovery is less than 20%.And surfactant flooding technology has been proved to be effective in improving oil recovery of low and extra-low permeability reservoirs by field tests.Aiming at the characteristics of fluid properties and complex oil-water percolation relationship in low permeability and ultra-low permeability reservoirs,a screening evaluation method of surfactant flooding reservoirs is established combined with surfactant flooding mechanism.The BP neural network model with Fuzzy Analytic Hierarchy Process(FAHP)is introduced to reduce the subjective factors existing in current screening methods.A surfactant flooding system with ultra-low oil-water interfacial tension,strong emulsifying ability and low adsorption capacity has been developed through laboratory experiments.Considering the percolation characteristics of low permeability and ultra-low permeability reservoirs,a mathematical evaluation model for enhanced oil recovery by surfactant flooding has been established,which is suitable for low permeability and ultra-low permeability reservoirs.The theory and evaluation technology of surfactant flooding in ultra-low permeability reservoirs have played a certain role in promoting the popularization and application of surfactant flooding technology in low permeability and ultra-low permeability reservoirs.This paper carries out research from the following four aspects:(1)In order to determine the development priority of surfactant flooding,a set of screening and evaluation models for surfactant flooding reservoirs with low permeability and ultra-low permeability were established.In this paper,the fuzzy comprehensive evaluation method is used as the basic model for screening and evaluation of surfactant flooding reservoirs in low permeability and ultra-low permeability oil,and the weights of evaluation indexes are determined by using the neural network model.In view of the complexity and diversity of evaluation indexes for surfactant flooding reservoir,10 evaluation indexes for reservoir screening are proposed based on the mechanism of surfactant flooding and percolation characteristics of ultra-low permeability reservoirs,which include geological characteristics,percolation characteristics and surfactant flooding characteristics.The evaluation index value is determined by BP neural network algorithm,which reduces the subjectivity of determining the index weight.Finally,the evaluation method of surfactant flooding reservoir screening is established.Taking six reservoirs to be developed by surfactant flooding in Yanchang Oilfield as an example,the weights of each index are calculated by using the model,and the evaluation values are calculated.The evaluation values determines the priority of the six blocks to be developed by surfactant flooding.The sequencing results and the prediction of the final recovery of the six blocks are obtained.The ranking results are in good agreement,which verifies the reliability of the screening evaluation model for surfactant flooding reservoirs.(2)Considering the percolation characteristics and surfactant flooding mechanism of low and extra-low permeability reservoirs,the main controlling factors affecting oil recovery by surfactant flooding in normal pressure and abnormal low pressure reservoirs are analyzed by reservoir numerical simulation method.Because the main reservoir in Yanchang Oilfield is a typical low pressure and low permeability reservoir,the research and development of surfactants should pay attention to the performance indicators of oil-water interfacial tension,injection concentration and adsorption capacity.Based on the above research knowledge and the principle of molecular structure design,the macromolecular surfactant PAS of anionic amphiphilic random copolymer was prepared by using hydrophilic monomers,alkyl long chain acrylic monomers and functional monomers through laboratory experiments.Polymer/small molecule surfactant POA was prepared.The surfactant has good interfacial activity and emulsification,and is suitable for low permeability and ultra-low permeability reservoirs.(3)Based on the theory of Berkeley-Levitte flooding calculation,considering the factors of heterogeneity,starting pressure gradient,stress sensitivity,adsorption and interfacial tension effect,the paper establishes an evaluation model for the potential of surfactant flooding to enhance oil recovery in ultra-low permeability reservoirs.At the same time,in order to improve the convergence of numerical calculation,the two-dimensional plane displacement calculation is transformed into one-dimensional displacement calculation by taking into account the treatment method of diversion model,and the evaluation model of surfactant flooding for enhanced oil recovery in low permeability and ultra-low permeability reservoirs is revised.Based on the established evaluation model,a software for evaluating the potential of surfactant flooding for enhancing oil recovery in low and extra-low permeability reservoirs is developed by using C# language.The software can realize the analysis of injection and production parameters(development mode,injection rate,injection volume,well pattern,etc.)and the design and optimization of development scheme.The potential of surfactant flooding to enhance oil recovery in Chang 2 reservoir of Zhaike Reservoir in Yanchang Oilfield is evaluated by using evaluation software.The final oil recovery calculated by the new model is only 3% different from that calculated by Eclipse software.The main reason for the difference is that the effect of start-up pressure gradient is considered in the new model.When there is start-up pressure gradient,seepage resistance increases and recovery factor decreases.Therefore,the effect of starting pressure gradient should not be neglected when evaluating the development effect of water flooding and surfactant flooding to improve oil recovery.(4)210 blocks in Yanchang Oilfield were screened and evaluated by the established screening and evaluation method of surfactant flooding.The evaluation results show that 153 of 210 blocks are suitable for surfactant flooding.By using the developed software for evaluating the potential of surfactant flooding to enhance oil recovery,153 blocks were evaluated by surfactant flooding.The evaluation results show that surfactant flooding can improve oil recovery by 7.7 percentage points compared with water flooding.The field test of surfactant flooding was carried out in Yujiaping 8355 well area,a typical block of Yanchang Oilfield.Based on the polymer/small molecule composite surfactant system suitable for the main reservoirs in Yanchang Oilfield,the injection parameters were optimized by surfactant flooding enhanced oil recovery evaluation software: injection concentration 0.5%,injection rate 55 m3/d,total injection volume 1 PV,predicted oil recovery increased by 10%;cumulative injection table in test area Two months after the injection of 25 tons of surfactant,the block as a whole showed the characteristics of "two liters and one drop" of liquid,oil and water cut.By the end of May 2018,the block had accumulated 598 tons of net oil increase,with a stage input-output ratio of 1:2.75,and the test effect was good. |
PDF Full Text Request