页岩气压裂微地震监测中的裂缝成像方法

Fracture imaging of the surface based microseismic monitoring in shale gas fracking:Methods and application

微地震监测是页岩气储层水力压裂改造过程中的关键配套技术,但目前国内微地震监测技术还不能直观描绘水力压裂改造储层的裂缝网络。为此,将地面微地震层析成像和细化算法相结合,提出了一种人工压裂裂缝反演方法——细化地震发射层析成像方法 (TSET),即将图像处理中的骨架提取算法引入到地面微地震监测结果中,提取能量最强位置的骨架结构来反演压裂裂缝的位置及形态。将该方法应用于河南中牟页岩气区块MY1井的水力压裂微地震监测:(1)对原始微地震数据进行滤波,应用基于Semblance算法的地震发射层析成像(SET)技术对微地震数据进行处理,得到压裂区域储层的能量叠加结果 ;(2)采用克里格网格化将能量图插值为连续的图像,并对网格化结果进行奇异值分解降噪(SVD);(3)应用细化算法提取叠加能量图的骨架结构,得到能直观显示的水力压裂裂缝或裂缝带成像结果。结论认为:(1)水力压裂活动可以改变天然裂缝的应力状态,即激活天然裂缝并在远离作业井段部位诱发破裂,在压裂液无法到达的部位形成新的"诱发破裂裂缝";(2)当压裂裂缝与天然裂缝方向平行或近于平行时,天然裂缝对人工压裂裂缝会产生延伸诱导作用,反之则产生屏障阻隔作用。

Micro-seismic monitoring is a critical supporting technology in hydraulic fracturing of shale gas reservoirs. In China, however, the current technique is incapable of directly imaging the fracture network generated in shale gas reservoirs stimulated hydraulically. In this paper, an inversion method for artificially induced fractures, i.e., thinning seismic emission tomography （TSET）, was proposed, which integrates surface micro-seismic tomography with thinning algorithm. This method adopts the skeleton extraction algorithm and can realize the inversion of position and geometry of hydraulic fractures through extracting the skeleton structure at the position with the strongest energy. It was used in micro-seismic monitoring during the hydraulic fracturing of Well MY1 in the Zhongmou shale gas block in Henan. Firstly, energy stacking was obtained after the original micro-seismic data filtering and the seismic emission tomography （SET） processing with the Semblance algorithm. Secondly, energy diagram was interpolated through Kriging gridding into continuous images, and singular value decomposition （SVD） was conducted on the gridding results to reduce noise. Thirdly, the skeleton structure of the stacked energy diagram was extracted using the thinning algorithm to image hydraulic fractures or fracture zones that can be displayed visually. It is concluded that the stress state of natural fractures may be changed by hydraulic fracturing. Specifically, natural fractures are activated and fractures are induced at positions far away from the well sections in operation in order to generate new induced fractures at positions beyond the reach of fracturing fluids. Moreover, if hydraulic fractures run parallel or nearly parallel with natural fractures, the latter may guide the former to extend further; otherwise, the latter may function as a barrier to the former.