Analysis of factors influencing the magnitude of microseismic events in coal reservoirs

In recent years,with the development of microseismic technology,the application depth of microseismic is also expanding,for example,microseismic monitoring from the initial simple source locating to source mechanism inversion,formation main stress state and reservoir permeability and other highlevel interpretation parameters.Among many interpretation parameters,the magnitude of microseismic events is a very important source parameter,which is representing the intensity of induced source in production.The magnitude distribution characteristics of induced events in different regions are different.At present,the influencing factors of magnitude are not very clear.Taking the surface microseismic monitoring of hydraulic fracturing of coalbed methane wells in the south of Qinshui Basin as an example,the influencing factors of the magnitude of fracturing events in this well were analyzed based on the existing data.First of all,the average pressure and peak pressure of oil pressure during the hydraulic fracturing were calculated quantitatively through the fracturing hydraulic fracturing curve,and then the intersection analysis was carried out with the magnitude of microseismic events respectively.Secondly,GutenbergRichter law was applied to study the distribution characteristics of events magnitude in space and time frequency.Finally,based on the attributes of threedimensional seismic coherence body,the natural fracture of reservoir was interpreted,and the influence of natural fracture on fracture pressure was analyzed by using Mohr Coulomb fracture criterion.The results show that ① the average magnitude or peak magnitude of the events in hydraulic fracturing has a significant positive correlation with the hydraulic fracturing pressure,and the pressure is the direct factor affecting the magnitude of the microseismic events.② The area with strong magnitude concentration is characterized by obvious low b value,which indicates that the lithology near the area is relatively more brittle,and the stress accumulation capacity is greater than the surrounding area.③ The natural fractures will reduce the fracture open pressure of the formation,so the induced events magnitude in the fracture development area of the coal seam is relatively low.The induced events in hydraulic fracturing is a complex process,limited by the data,there are still some factors affecting the magnitude that have not been analyzed,such as insitu stress,reservoir brittleness and so on.The next step is to collect as much data as possible to make a more comprehensive analysis of the magnitude impact.Through the analysis of the factors influencing the magnitude of the hydraulic fracturing events,it is helpful to understand the occurrence law of the magnitude of the events,so as to ensure the safety and smooth progress of the hydraulic fracturing