Study On Saturation Model And Water Cut Evaluation Method Of Tight Sandstone

In recent years,tight sandstone reservoir is a hot research topic,which is distributed in major basins in China.Due to poor physical properties,fracture development and strong heterogeneity of tight sandstone reservoir,logging interpretation is difficult.Fracture development not only improves the permeability of the reservoir,but also brings challenges to the logging evaluation of the reservoir.Water saturation is an important parameter for quantitative evaluation of reservoirs,and water cut is a key parameter for productivity prediction and reservoir evaluation.Both are of great significance for fluid identification and oil fields development.Firstly,the relationship between water saturation and gas column height and physical property index of reservoir types with different pore structures was established using capillary pressure curves.Then,considering the development of fractures in the study area,the fracture-porosity double-porosity medium model was optimized,and the theoretical derivation and practical application were completed.At the same time,we focus on the numerical simulation of fractured tight sandstone,established the digital core model with different fracture openings and different fracture angles,and established the fractured tight sandstone with different water saturation based on the mathematical morphology method and LBM.The relationship between rock resistivity and different fracture characteristics is explored by using the finite element method,and the mathematical model of cementation index,fracture opening and fracture dip angle is established.The results show that the resistivity of the rock decreases with the increase of the fracture opening and increases with the increase of the fracture dip angle.The resistivity increase rate and the water saturation are no longer a straight line under the double logarithmic coordinates,but an exponential relationship.Based on the simulation results,a new water saturation model is established based on the Archie principle.This model has a good application effect on the fractured tight sandstone of the Cretaceous Bashijiqike Formation in Kuqa Depression.The relative permeability of oil and water is a parameter that is crucial for the calculation of water cut.Taking Chang 81 Reservoir of Yanchang Formation in Ordos Basin as an example.Firstly,based on the results of core relative permeability experiments,the characteristics of relative permeability curves of reservoirs with different pore structures are analyzed,based on pore structure index and combined bound water saturation,the phase permeability curve is classified.Then,the calculation model of oil-water relative permeability is obtained by nonlinear fitting.Based on the relationship between model coefficients and pore structure index,the relative permeability models of different pore structures are established.Finally,the water content calculation models of different pore structures are obtained by using partial flow equation.Comparison of the calculated results of the model and the actual oil-testing results,the model in calculating water content is proved effective.It is worth mentioning that the radial basis function(RBF)neural network method provides a new idea for the study of relative permeability of tight sandstone.We investigated the principle of RBF neural network,choosing Gaussian function and nearest neighbor algorithm for the network training.With water saturation,nuclear magnetic irreducible water saturation,porosity and permeability as input and oil-water relative permeability as output,the best relative permeability prediction network model and parameters are determined according to error analysis.Then,we calculated the water cut using the partial flow equation.This method was applied in the Chang 8 reservoirs of the Yanchang Formation in Longdong area of Ordos Basin.The predicted oil-water relative permeability was consistent with the experimental results,and the calculated water cut is in good agreement with oil-testing results.