Active Electromagnetic Monitoring Technology For Hydraulic Fracture Diagnostics

The recovery rate of unconventional oil and gas can be improved by increasing the length of horizontal wells and continuous multi-stage fracturing.To develop the dry hot rock geothermal resources,we can apply hydraulic fracturing to form interconnected fracture network systems.Hydraulic fracturing is a key technology for oil and gas production and energy conversion,and real-time monitoring is an important means to ensure that the fracturing operation achieves ideal results.Many fracturing monitoring methods and technologies have been developed,and the indirect fracture diagnostic techniques based on model analysis and the direct near-wellbore fracture diagnostic techniques can not meet the demand of real-time monitoring.and direct far-field fracture diagnostic techniques.Although the commonly used microseismic method can monitor the fracturing process in real time,it can also record the microseismic events caused by the change of stress field in the nonhydraulically connected area.In addition,the microseismic energy caused by hydraulic fracturing is weak,and the velocity model is not easy to determine.In the process of hydraulic fracturing,a large amount of highly conductive fluid is injected,which has a significant electrical difference from the reservoir.So,the electromagnetic method has a unique advantage in the identification of fracturing fluid diffusion range.Therefore,it is very important to develop an economical and effective real-time electromagnetic hydraulic fracturing monitoring technology with strong anti-interference ability to know the fracture propagation,analyze and evaluate the quality of fracturing construction and guide the adjustment of fracturing operation.This thesis takes shale gas and dry hot rock reservoirs as the research object,focuses on the physical properties and fracture characteristics of hydraulic fracturing reservoir,reveals the electromagnetic response characteristics of low resistance abnormal body or charging conductor fracture of fracturing,develops electromagnetic real-time monitoring system,and realizes efficient and high-precision electromagnetic data collection and transmission.The hydraulic fracturing monitoring technology has been formed to solve the problems of large environmental noise interference,small observation anomaly and low timeliness of traditional electromagnetic detection in the process of hydraulic fracturing,and to realize the electromagnetic dynamic monitoring of fracturing horizon.Firstly,the fracturing monitoring technology based on controlled source electromagnetic method(CSEM)is studied.The fracturing model of Well JY2 is constructed,and numerical simulation of controllable source electromagnetic method is carried out based on integral equation method.We analyze the influences of field source intensity,offset,reservoir reconstruction scale,thickness,depth and conductivity on observed electromagnetic anomaly.The characteristic frequency of electromagnetic response for hydraulic fracturing monitoring is clarified,and the applicable conditions of the WFEM monitoring technology are clarified.Secondly,a method based on charging conductor for hydraulic fracture diagnostics is studied.In this thesis,based on the basic theory of electromagnetic method,the charging conductor fracture model is established,the calculation formula of electromagnetic field in the whole space is derived,and the electromagnetic anomalies caused by fractures are obtained.Numerical simulation and experimental analysis on fracture models of horizontal wells with different orientation,length and depth show that the anomaly distribution curve can effectively indicate the fracture orientation,and the anomaly has a positive linear correlation with the fracture length.The anomaly decreases with the increase of the depth,and the anomaly is greater than 140 μV for the depth of 8000 m.At the same time,the feasibility and reliability of the hydraulic fracturing monitoring technology based on charging conductor are verified by physical simulation experiments of fracture models for vertical well in the water tank and horizontal well in the sand tank.In view of the continuity of fracturing operation,timeliness of monitoring and complexity of on-site construction environment,the electromagnetic system based on pseudo-random signals and the real-time monitoring software platform based on "4G + Cloud platform" for hydraulic fracture diagnostics are developed.Through the system and measured signal tests,the database structure and the parsing program of data packet are optimized,and the average parsing time is less than 36 s,which greatly improves the system operation efficiency.The electromagnetic real-time monitoring software can be logged in through identity authentication and carry out remote monitoring on the cloud platform.The background system analyzes and transmits data in real time,while the Web displays monitoring data and abnormal curves.It can meet the requirement of field continuous monitoring data storage and query,and ensures the simultaneous fracturing operation of multiple well platforms.Finally,a real-time monitoring technology for hydraulic fracturing based on charging conductor is formed,which is directly stimulated at the fracturing site through the wellbore power supply,with a power supply voltage of less than 36 V,and a signal strength of more than 10 m V.According to the field tests of Hydraulic fracturing,the natural field is less than 0.01 m V and the noise proportion is less than 0.2 %.During the whole fracturing process,the observed anomalies increase with the expansion of fracture network,and the potential changes caused by hydraulic fracturing can be observed on the ground.Fracture parameters can be obtained based on model calculation,which can effectively identify the fluid diffusion range.Based on the electromagnetic monitoring results of Well GR1,the effects of different fracturing processes are evaluated.By comparing electromagnetic and microseismic results of Well JY11,it is further verified that electromagnetic method has certain advantages over microseismic in the effect of hydraulic fracture diagnostics.This thesis,from the electromagnetic theory to the numerical simulation analysis,from the physical model experiment to field fracturing monitoring tests,is systematically studied.The active electromagnetic monitoring technology for hydraulic fracture diagnostics is formed,which provides a new means for evaluating reservoir reconstruction effect,and has important practical significance for enhancing recovery efficiency in the stage of oil and gas development.