Research On Low Frequency Electromagnetic Characterization Mechanism And Simulation Of Hydraulic Fractures

Hydraulic fracturing,as the core technology of reservoir stimulation and stimulation measures,is the main driving force for the development of unconventional oil and gas resources.A deep understanding of the geometry and orientation of hydraulic fracturing fractures is crucial for formulating oil and gas field development strategies and improving oil recovery.In this paper,aiming at monitoring the morphological characteristics of highly conductive propped fractures,the low-frequency electromagnetic characterization mechanism and characteristic simulation research of hydraulic fracturing fractures was carried out.The main completed work includes:By analyzing the fracture initiation conditions and propagation laws of fracturing fractures,a representative fracturing fracture morphology model of horizontal wells was established.The conduction mechanism of rock formation-fracture dual medium is studied,and the conductivity model of fractured reservoir is established from two perspectives of empirical model and equivalent medium theory.Using the selfconsistent approximate equivalent medium theory,the equivalent conductivity of the sand-carrying fluid and the fracturing area is solved under the condition of different component distribution ratios.By analyzing the influence of many factors such as the phase number of the mixture,particle shape and arrangement direction on the equivalent conductivity,the transformation law from the microstructure change to the macroscopic conductivity is revealed.By describing the conversion relationship between complex vector alternating electromagnetic field and current,the generation process of induced electromotive force related to fracture shape is analyzed,and the principle of fracture monitoring based on low-frequency electromagnetic induction is clarified.Aiming at the problem of fracture monitoring in open-hole wells,the characteristics and monitoring principles of three coil structures including multi-component three-axis orthogonal coil system,three-axis orthogonal transmitting-sector coil receiving,and three-axis orthogonal transmittinginclined coil receiving are explored.Aiming at monitoring fractures in the cased well with the near-field and far-field methods,an in-hole fracture monitoring device based on lateral electrodes + insulating rings and a well-ground fracture monitoring method based on a vertical line current source are respectively proposed.The optimization algorithm combining adaptive finite element and model equivalent simplification method is used to simplify the model with multi-scale structural characteristics,and solve the multi-scale solution problem of electromagnetic field forward modeling in the context of fracturing engineering.Using the transmission relationship of the AC electromagnetic field on both sides of the medium,the conductive thin crack and the casing are equivalent to transition boundary conditions.For the numerical solution of DC electric field,based on the concept of equivalent conductance,multiple nested structures such as wellbore,casing and cement sheath are equivalent to the parallel conductance of a single structure;keeping the product of fracture conductivity and thickness unchanged,the thin fractures are equivalent to thick fractures.And the reliability and efficiency of the algorithm are verified both in the open and cased well models.Using the measurement structure combining the three-axis orthogonal coil system and the circular rotation of the fan-shaped coil,in the low-frequency multi-source distance measurement mode,the numerical simulation of the open hole fracture morphology was carried out.The parameters of the instrument are optimized from the two perspectives of reducing the attenuation of the electromagnetic field and improving the detectability of the slit length.The corresponding relationship between different component signals and fracture position,scale,extension shape,dip angle,azimuth angle,secondary fracture and other morphological characteristics is clarified,and the reliability of fracture monitoring is improved through the linear combination of different component signals.The analysis of fracture monitoring characteristics in cased well and the sensitivity study of fracture parameters are carried out.By embedding insulating ring collars between the casings and applying differential voltage signals on both sides of the insulating rings,the problem of detecting weak signals in the traditional through-casing resistivity logging method is effectively solved.By analyzing the axial arrays in and between wells,as well as the signal response of the circumferential receiving array in the well,the influence of the position and source distance of the excitation source on the potential distribution inside the casing and the formation is clarified,and the fracture morphology in the cased hole is realized.Aiming at the well ground fracture monitoring,the influence of the casing on the transmission current and leakage current distribution is analyzed,and the influence of the depth of the fracturing area and the location of the excitation source on the well detection accuracy is explored.