| The exploration and exploitation of shale gas has been rapidly developed in recent years,and the fracturing operation has been developed from the single well to well clusters,and the microseismic monitoring(MSM)which always plays the key role to estimate the effect of fracturing and became the hot spot in the oil and gas industry.The MSM is used to locate the microseismic event to help the engineer to analysize the fracture geometry and space distribution and calculate the SRV,furthermore,to real-time provide information of the target to direct the fracturing operation,and avoid the fracturing risk and direct the exploitation strategy for unconventional reservoir.However,it also meets the following difficulties,challenges and demands since of the complex geologic condition in China.(1)The specific factors such as the more complex structure of the near-surface geology compared to the North American,and the deeper target and sparse well clusters,therefore,the conventional MSM with the sensors deployed in deep wells nearby the treatment well is not available for all treatment wells.Therefore,the other MSM observation geometries are developed such as the surface monitoring,the multi-shallow wells monitoring and scattered monitoring and the recombination of them to improve location precision and reliability.However,the data for different geometries shows the completely different wave-field characteristics,which may decrease the location precision by the conventional location method.Therefore,the new location algorithm needs to be researched and developed.(2)Meanwhile,the location of microseismic epicenter always reveals the space relationship of the fractures,but can't detailed describe the source characteristic related to the crack type and activity.Therefore,how to describe the crack tendency and regularity of fracture activity during or after the fracturing operation is the profound requirement for MSM to describe the geometry and size of fracturing fracture description and estimate effect of fracturing.For example,the activity of fracture is open,close or shear? How to get the azimuth and dip of the fracture? All of the upper ideas need to the delicate description of fracture.In addition,the detective distance of the microseismic event plays the important role in geometry design of the MSM.We take the Sichuan and Changqing to be the examples.We always get the microseismic event with 1.5km away to the source in Sichuan shale,but we can't make sure to get the event with 300 m to 600 m away to the source in Changqing area.What results in such differences? Is the source radiation or the propagation of the wave in medium or the fracture type or activity? Therefore,how to design and optimize the geometry based on the source/fracture characteristic and the medium parameters is the emphasis and difficulty of the MSM observation geometry design.(3)The natural faults/fractures are developed in Sichuan shale,and we discovered that some events activated along the natural fault/fracture and the others activated in the area with network shaped.The natural fault/fracture always plays important role in the shale exploitation and engineer operation.One side,the natural fault/fracture increases the connectivity and permeability,but on the other side,it also may result in the leakage of fraturing liquid and the deformation of treatment well et al.,and trigger the small earthquake.Therefore,how to differentiate the natural fault and fracture to attain to goal of the risk prediction is the key task of MSM.The paper based on the theory of the earthquake and combined with the actual data of MSM,researched the epicenter rupture characteristic.To better direct the geometry design of the microseismic monitoring observation,the realtime processing and fracturing estimate,the paper provide the kinematics and dynamics rupture model of the induced microseismic based on the ray-tracing theory,the attenuation of the microseismic event and this model provides the theory foundation for geometry design and optimizaiton,processing and interpretation of the fracture net-work and those techniques have been widely used in the actual project of unconventional reservoir.The detailed ideas of the paper are shown as follows.(1)The energy of microseismic event is very weak because of the small rupture size and the un-elastic attenuation during the propagation of the wave in the medium.Noticeably,the higher frequency of the small rupture size means the strong un-elastic attenuation during the propagation.For different treatment area and different observation geometry,it is the emphasis and important problem to estimate the detective distance.Therefore,we provided the relative magnitude technique based on the characteristic of crack rupture and propagation of the microseismic event in medium to estimate the detective distance and to optimize the geometry and observation parameters.(2)Based on the wave-field characteristics of microseismic for four kinds of geometry(includes the surface,multi-shallow wells,deep well and scattered),we separately researched the location algorithm based on the kinematics and dynamics and also tested their location precision and suitability,and we got the ideas that specific location algorithm suits for the given observation geometry.Eventually,we separately used those location methods to project with four kinds of observation geometry and the result shows the surprising similarities.(3)We separately researched the mechanism inversion method based on the amplitude,polarity and waveform.Detailed speaking,we researched the moment tensor inversion method.Combined with the characteristic of microseismic wave and the regularity of crack rupture,we provided the source mechanism inversion technique characterized by the DC model.The simulation and actual data application show the better anti-noising and reliability.In addition,the method has the ability to reliably reveal the activity characteristics of fracturing fracture and its rapid inversion efficiency makes the real time inversion of source mechanism be true.(4)We provided a natural fault and micro-fracture differentiation technique based on the characteristics of microseismic event.This technique takes the mechanism of crack rupture to be the foundation,and incorporates the amplitude,frequency,relative magnitude,time frequency relationship and b value to differentiate the natural fault and micro-fracture.The application in unconventional reservoir showed the surprising suitability.All the ideas and techniques provided in this paper have been widely used in Weiyuan-Changning block of shale gas in Sichuan.Because of those methods,we profoundly understood the change of reservoir during and after the fracturing operation by epicenter location,focus mechanism inversion and description of the fracture and we also can provide more information to direct the exploitation,the fracturing operation design and the well design of the shale gas or tight sand. |
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