Research On Key Technologies Of Borehole Microseismic Signal Acquisition For Borehole-Surface Monitoring

Hydraulic fracturing technology is an effective technique for the exploitation of low-permeability and unconventional oil and gas fields.Among the existing hydraulic fracturing monitoring methods,the most effective and feasible one is microseismic monitoring.Microseismic events which is caused by hydraulic fracturing can be recorded using the seismic signal acquisition system arranged on the fracturing site.By inverting and deriving the collected data,the result can be used to infer the information of the trend,density,and dimensions of the fracture in the subsurface.There are two types of microseismic monitoring: borehole monitoring and surface monitoring.For borehole monitoring,it is required high precision of the equipment,so it is complicated to operate and many monitoring wells are needed which has a great impact on the construction.For surface monitoring,there are few effective monitoring event,and the signal to noise ratio is low,thus leading to poor reliability of the positioning results.Therefore,reducing the difficulty of on site construction and improving the accuracy of microseismic event location are the main problems that need to be solved in current microseismic monitoring.Comparing the analysis results of the different monitoring method,we found that the borehole monitoring has higher vertical resolution than surface monitoring,while surface monitoring has strong horizontal resolution because it has a wide range of observation arrays.Therefore,to obtain better monitoring angles in both horizontal and vertical directions and improve the positioning accuracy of microseismic events effectively,we combine the two methods together which is called borehole-surface monitoring.In this method,the presence of surface monitoring arrays increases the lateral viewing angle,so placing borehole instruments in one monitoring well is sufficient enough to meet the demand.And compared to multiple-well monitoring,borehole-surface monitoring can reduce the cost and the complexity of constructionThere are many factors that can influence the positioning accuracy of microseismic monitoring,including the velocity model,noise processing,positioning methods,the accuracy of initial picking,the azimuth angle of the detector,and the sampling interval.Therefore,to find a better way to solve the problem of mismatched data collection between different types of systems,improve the positioning accuracy and make it applicable to the borehole-surface monitoring,for borehole instruments,there are still some key techniques need to be studied.In order to improve the accuracy of first picking,the acquisition time of borehole and surface equipment must be strictly synchronized.For ground system,we can use the GPS to synchronize.And when GPS is used to synchronize the acquisition node in the well,the signal will be delayed by the logging cable transmission,which will cause errors between the acquisition time of the instrument in the borehole and the surface instrument.A cable delay measurement and correction method is designed to solve this problem to compensate for the deviation of the timing signal due to long-distance transmission.At the same time,a kind of pulse compensation method of AD clock source is designed,to solve the high error problem caused by high-temperature crystal oscillator,which can maintain the stability and accuracy of data acquisition when AD working for a long time.The position of the geophone in the well is random and uncontrollable.Therefore,the attitude of the borehole geophone needs to be corrected before the data processing.In this article,we measured the attitude change of the geophone by installing attitude sensors in the acquisition section.And the Kalman Filter is used for multi-sensor data fusion to obtain the detector’s attitude information.Aiming at solving the problem caused by the influence of local geomagnetic anomaly on the measurement output of magnetometer due to geological structure or existence of minerals,an algorithm based on Unscented Kalman Filter was designed to correct the error.At the same time,a quaternion-based least squares method was deduced to obtain the relative rotation between the geophones using the data collected by the geophone.Attitude sensors can be placed in only one or a few acquisition nodes according to this relative rotation,and all geophone attitude can be derived by the algorithm.Well logging cables act as a medium for data transmission has a rather poor electromagnetic characteristic.In long-distance signal propagation,the logging cable will exhibit the characteristics of an approximate low-pass filter,which will affect the transmission of high-frequency signals.Based on the existing cable communication solutions,this paper applies the FBMC filter bank technology to the logging cable data transmission.The FBMC system and the OFDM system both use the multi-band communication method.The FBMC system uses a prototype filter with a longer impulse response,so it has good time-frequency focusing characteristics.Compared with OFDM technology sub-band does not need to insert a cyclic prefix,thus improving the spectrum utilization.The research on FBMC is mostly focused on wireless communication systems.Therefore,to apply it to wired transmission based on logging cables,the system has been optimized and improved in order to meet the long-distance data transmission needs of logging cables.The existing monitoring systems were developed specifically for the acquisition of seismic signals in borehole or on surface.There is no specific instrument system designed for borehole-surface monitoring.Due to the lack of effective real-time communication links between the collection units of distributed work,we cannot analyze it in real time,all the research remains at the post-assessment level.In real applications,the real-time presentation of the results of fracturing crack monitoring can truly reflect the core of the relevant research value.In order to solve this problem,a borehole-surface monitoring system was established on the basis of the existing wireless network,the application of database management technology and server cluster technology in the existing borehole and surface data acquisition systems.The system has been able to meet the requirement of the microseismic real time monitoring.In order to verify the borehole-surface monitoring method and the designed system,a hydraulic fracturing monitoring experiment was conducted at Daqing Oilfield in Heilongjiang Province.In order to verify the positioning accuracy of borehole-surface monitoring,we used the borehole monitoring data,surface monitoring data,and borehole-surface monitoring data separately to invert the position of perforation points,and compared the positioning results of different monitoring methods.Neglecting the influence of speed model and other factors on positioning,compared with the surface monitoring method,the positioning error declined from 32 m to 14 m by using the borehole-surface monitoring method.In order to verify the timeliness of borehole-surface monitoring system,the monitoring system was used to monitor the hydraulic fracturing construction in real time,and using the microseismic events to evaluate the fracturing fractures.The analysis results verified that the location of the fracturing point is consistent with the mechanism of hydraulic fracturing cracks and the orientation of the fracture is consistent with the local stress distribution.