Research On Microseismic Fracture Interpretation Techniques Based On Focal Mechanisms

Microseismic monitoring is a technology of estimating rock stratum in hydraulic fracturing,which has been widely used in oil and gas reservoir dynamic monitoring and coal mining from 1960 s.This paper generally introduce the entire flow of data processing in microseismic monitoring,which includes data preprocessing,microseismic events location,focal mechanism,stress inversion and fractures interpretation.In consideration of that the rupture and growth of hydraulically created fractures is closely related to stress direction in rock,and that stress field impacts the well location optimizing,fractures development and improvement of fracturing scheme,we put the research emphasis on stress field and fractures interpretation.This paper quantificationally discuss the relationship between complexity of stress field and stress inversion accuracy,put forward effective stress inversion method that can be adapted to complex stress field,describe and explain the developmental and distribution characteristics of hydraulically created fractures.The concrete content include several aspects as follows:Firstly,we introduce basic process flow of microseismic monitoring,including the theoretical significance and method of detector correction,buildup of velocity model and its optimizing and necessity,noise suppression of monitoring data,pickup effective microseismic signal,and microseismic event location.Secondly,we systematically discuss the relationship between geostress and hydraulic fractures,including the effect of tectonic stress,stratum depth,fault type,lithology and pore pressure on geostess distribution,and measure horizontal principal stress based on hydrofracturing curve.We also analyze the effect of geological factors(geostress,natural fracture,fault)and engineering factors(fracturing technology,construction parameter)on hydraulic fractures.Then we study on the impact of complexity of stress field on stress inversion.We numerical model some focal mechanisms in homogeneous stress field and complex stress field,respectively.Stress inversion on these focal mechanisms is carried out with iterative joint inversion method base on true fault.According to error analysis of inversion result,we obtain the effect of the uncertainty of focal mechanisms and complexity of stress field on stress inversion result.Besides,two sets of real data that are similar to simulated data are chose to be implemented stress inversion and error analysis,which are used to be compared with the conclusion of numerical modeling.After that,we put forward clustering based stress inversion method using a thought for reference.The thought is that stress may appear to be inhomogeneous in space in local area.According to the study on characteristic of focal mechanism,we apply fuzzy clustering method to focal mechanisms of microseismic events based on principal press or tensile stress direction which can sufficiently represent stress property.Following on that,iterative joint stress inversion is implemented.The processing method not only improves the accuracy of inversion result,but also can present the inhomogeneity of stress state in space,which play an important role in cognizing the rupture and growing process of hydraulic fractures.At last,we do hydraulic fracture interpretation according to focal mechanism ellipsoid and stress tensor ellipsoid.The 3D ellipsoid is constructed by the direction and relative size of P,T,B axes of focal mechanism,or three principal stress axes of stress tensor.The long axis direction of ellipsoid is the extension direction of fractures.Also,the shape of ellipsoid and relative size of three axes can be used to confirm the anisotropic degree of local stress,so that the complexity of fractures can be learned.