Study On Numerical Simulation Of Nuclear Magnetic Resonance Logging Response Characteristics Of Carbonate Rocks Containing Fractures And Caves

The fracture-cavity carbonate reservoir has large reserves and high production capacity and is rich in oil and gas resources.This kind of reservoir is affected by diagenetic transformation.The kind of reservoirs are dominated by secondary pores.A large number of fractures and caves are developed,and the pore size distribution is wide.The NMR relaxation characteristics of different fractures and caves are different.The actual pore structure of the core is very different.It is difficult to analyze the NMR T2 spectral response characteristics of fractures and caves through physical experiments.Therefore,it is necessary to systematically study the T2 spectral characteristics of different types of fractures and caves by numerical simulation,and provide a theoretical basis for the evaluation of carbonate rock NMR logging.In this paper,the characteristics of matrix pores,fractures and caves are summarized.Corresponding pore network models are established respectively,and spatial superposition is carried out at the same resolution to construct a fracture-cavity carbonate rock pore network model.The attenuation curve of NMR echo data is obtained by random walk method,and the corresponding T2 spectrum is obtained by Tikhonov regularization method.Through the control variable method,the pore space structure parameters of the fracture model and the cave model are changed respectively,and the variation law of the nuclear magnetic resonance T2 spectrum is analyzed.The results show that when the pore size of the fracture and the matrix pore is more than three times,the fracture signal is separated from the matrix pore signal,the T2 spectrum is multimodal,the short relaxation peak is the matrix pore signal,and the long relaxation peak is the fracture signal.The relaxation time of the short relaxation peak is not changed compared with the original matrix pore,but the relaxation time corresponding to the right end of the peak decreases with the increase of the fracture width,and the relaxation time corresponding to the long relaxation peak is higher than that of the fracture.The relaxation time is longer;the relaxation time of the fracture within a certain width decreases with the increase of the specific surface area;the relaxation time of the cave and the matrix pore is different,the T2 spectrum is multimodal,and the short relaxation peak is the matrix signal,the long relaxation peak is the cave signal,and the cave improves the connectivity of the matrix pore.The short relaxation peak does not change from the original matrix pore relaxation time,but the relaxation time corresponding to the right endpoint of the peak will follow As the radius of the cave increases,the relaxation time of the long relaxation peak is longer than that of the cave.