Research And Application Of CO₂ Huff And Puff In Tight Oil

At present,the development of tight reservoirs urgently needs effective energy supplement mode to replace the depletion mode.The traditional flooding mode cannot be used in the tight reservoir due to high injection pressure or channeling problems.Therefore,huff and puff has been focused in the development of tight reservoirs.For the selection of injection fluid,CO₂ has attracted the most attention because of its good oil displacement mechanism in traditional reservoirs and better injection performance than water in tight reservoirs.But the tight reservoir is different from the conventional reservoir.Its pore throat is narrow,the micro-scale effect is remarkable,thus the speciation of crude oil is special.After CO₂ is injected into the formation,whether its displacement for crude oil is effective in this micro-scale needs to be investigated and studied by more effective methods.At the same time,after artificial fracturing,tight matrix and fracture coexist in the tight reservoir.The basic seepage unit in tight reservoir is fracture-tight matrix system.CO₂ huff and puff were significantly affected by the fracture in the process of crude oil displacement in tight matrix.Therefore,in view of the above scientific problems,this paper is divided into several parts to study CO₂ huff and puff in tight reservoirs.Firstly,molecular dynamics?MD?method is used to simulate the molecular dynamics process of the system of CO₂ and crude oil?different alkane molecules?on the rock surface??-SiO2 is used in this research?at the micro-scale.The detachment and erosion process of CO₂ on crude oil in the micro-scale are analyzed.Compared with N₂ and water,the advantages of CO₂ are clarified.Secondly,core-scale numerical models of fracture-tight matrix seepage unit,which is the most basic seepage element in tight reservoirs,are established by numerical simulation method.Based on this model,the displacement process of crude oil in core by CO₂ huff and puff is studied,and the difference between CO₂,N₂ and water huff and puff is compared,and the possible mechanism of the difference is analyzed.The influence of fracture properties?including permeability,density and fracture dispersion characteristics?on CO₂ huff and puff EOR?enhanced oil recovery?in core scale is analyzed.Then,a three-dimensional large-scale model is also designed based on similarity criterion,which can reflect the more real complex fracture characteristics in tight reservoirs.The saturation distribution characteristics in the 3D similarity model are further analyzed,and CO₂ huff and puff mechanism are also analyzed and compared with the core scales model.Because the EOR effect evaluation based on the 3D similarity model is more in line with reservoir system.Orthogonal experiments with 8 factors?reservoir properties,huff and puff parameters,stress sensitivity and CO₂ diffusion,etc.?and 8 levels were designed,to evaluate the reasonable oil recovery range of tight reservoirs using CO₂ huff and puff.The sensitivity of recovery to each factor is also analyzedFinally,based on this research,the CO₂ huff and puff parameters are optimized?such as huff and puff timing,gas injection intensity,soaking time,etc.?for one typical fractured horizontal well in Lucaogou Formation of Jimsar.The oil production in optimal scheme is predicted.The economic oil exchange rate index of CO₂ is synthesized to optimize the cycle rounds to guide efficient development in the fieldIn conclusion,this study has a deep and extensive study on the development technology of CO₂ huff-n-puff in tight oil,with some theoretical and practical significance for application of CO₂ huff-n-puff in tight reservoir.