Research On Microseismic Monitoring Methods For The Fracture Expansion Process Of Tight Reservoir

The key technology of shale gas revolution is reservoir fracturing and stimulation.The study of fracture propagation and distribution is very important for fracturing design,operation and productivity evaluation.The micro seismic monitoring technology of hydraulic fracturing(HF)on the reservoir is an important method to improve reservoir permeability,and how to describe the crack formation and the development of the network using the temporal and spatial distribution of micro seismic events and the focal mechanism is the key part of many researcher studies,and many scholars have performed a lot of research on this field.However,the depth of the buried reservoir is generally about 3000 meters below surface,and it is very difficult to verify the process of crack spreading and crack expansion by other methods.Because there is a close link between hydraulic fracturing production and laboratory rock acoustic emission,we can study the fracture propagation of the rock failure process in laboratory.The previous study on general location method focus on the use of time or energy information,there is a big error of location when inhomogeneous or anisotropic velocity model becomes large;in addition,they put the micro earthquake as a point source,which losts a lot of useful information,so the results cannot accurately describe the micro cracks of seismic information.This study will launch the experiment based on the acoustic emission(true three axis hydraulic fracturing),recording signal waveforms during the fracturing process,using double difference method to locate relative position of AE(Acoustic Emission)events accurately,meanwhile the method of back projection can be used to image of the fracture migration,finally getting the fracture shape and orientation and the angle information;CT technology can be helpful to verify our location results.By comparing the results obtained by the two methods,we deeply analyze the accuracy of microseismic technology in the process of retrieving fracture propagation,and establish the relationship between microseismic and micro fracture,then applying this relationship into real hydraulic fracturing operation.Through this study,the following conclusions are obtained:(1)Bilateral monitoring is better than unilateral monitoring considering their location error and the error of unilateral monitoring will make the crack tend to smaller inclination;large estimation on speed will lead the location results diverge,and vice versa;source location error increases with the increase of distance from the center point of the detector,the shallow events have larger error than deeper ones for the same epicentral distance;the velocity anisotropy will cause large location error,it's better to consider the anisotropy when doing location method.The energy back projection method can make use of waveform information to get the morphology of microseismic rupture surface during acoustic emission or hydraulic fracturing;phase weighted superposition can greatly improve the imaging accuracy of the energy back projection method;the imaging resolution is controlled by the Rayleigh criterion:(S2-S1)/V>=T/2-L/Vo.(2)The rate of acoustic emission characteristics vary consistently with pump pressure,that is when the pump pressure increases,the acoustic emission rate increased;morphological information of of the cracks can be obtained by double difference location method,which can be verified by compareing with CT scan slices,the results show that the event locations will deviate away in the direction perpendicular to the crack and little alignment between the small magnitude AE events and CT scan can be found;energy projection can outline the rupture process of micro cracks in rock dynamic migration,and can estimate the rupture speed;the fracture morphology of the reverse projection method deviates from the CT image.(3)Double difference location method using surface stations can well determine the horizontal position of micro seismic events and the relative position between close events,overcoming the resolution in fine structure than traditional method;sub event identified by the peak wave packet method can be used to determined the rupture direction considering their origin time difference;treatment results of nineteenth fracturing data show that the flow of the obtained results indicate the direction of the fracturing fluid;the seismic interferometry has great potential to image natural fractures and to monitor the temporal evolution of subsurface fractures during HF.