The Research On Microseismic Location And Velocity Optimazation For Microseismic Monitoring On Surface

The research on microseismic monitoring of low permeability reservoir is a kind of technical to locate the microseismic event by utilizing the rock bed dislocation underground. In recent years, with the development of the theory of microseismic monitoring, the evaluation of fracture based on microseismic monitoring technology, has became the most intuitive and reliable technology in the process of exploitation of low permeability reservoir. There are two main types of micro seismic monitoring: surface based monitoring and borehole based monitoring. Compared with surface based monitoring, the borehole based monitoring is with the advantage of high signal to noise ratio(SNR), and can avoid the interference of the noise on the ground, which is widely accepted by both domestic and foreign. But the borehole based monitoring requires rather high precision instrument, and the operation of construction is rather complex, we cannot carry out the monitoring work in the absence of monitoring wells. However, because the surface based monitoring of hydraulic fracturing is not restricted by borehole geometry or the difficulties in maintaining subsurface equipment, it is becoming an increasingly common part of microseismic monitoring. This paper aimed at the surface based monitoring. In this paper, based on the microseismic monitoring method, theory, numerical simulation and field experiment, we mainly studied on the key technology of microseismic monitoring, such as forward modeling, velocity optimization, microseismic location, and 3D visualization software of seismic monitoring.The forward modeling which takes the seismic ray tracing as the main cause directly affects the quality of the source location and computational efficiency. Therefore, we need to find a flexible, efficient and accurate seismic ray tracing algorithm. The shortest path method is the method which is most widely used in the seismic exploration. But this method has the problem that the computer memory was seriously occupied and costs a lot of computing time and other issues. For the three-dimensional layered structure, this paper enhances the efficience of ray tracing forward calculation by using the dichotomy to improve the traditional iterative method. For the three-dimensional cube grid structure of low density, we designed a new three-dimensional seismic ray-tracing algorithm, which uses a step-by-step iterative ray-tracing method, and mid-perpendicular point search method. Compared with the shortest-path ray-tracing algorithm, this algorithm has the unique flexibility and efficiency, therefore has a high application potential in the field of microseismic monitoring.Microseismic event location is the key problem in microseismic monitoring; traditional microseismic event location is to pick the first arrival of the microseismic signal. And optimize the solution of the overdetermined equation to determine the location of the source. But its main defection is that the method can only use the uniform velocity model. In this paper, the microseismic location technology based on grid search is used to locate the underground microseismic event. This method can introduced a more precise velocity model to improve the reliability of microseismic location. However, this method can not take into account the calculation efficiency and location accuracy. Therefore, the conventional microseismic location method was improved by using the method of grid successive subdivision in this paper. On the basis of conventional algorithm, a further subdivision search was carried out near the grid point with the maximum energy focus after the first division to find global maximum energy focus value. This will avoid a great deal of unnecessary grid calculations, and higher location accuracy was obtained with smaller computational cost. The influence of the velocity model on this algorithm was also analyzed and discussed. Finally, the advantage of calculation efficiency and location accuracy of the improved method was verified by synthetic test and field data processing.Velocity model is the main factor which affects the accuracy of microseismic location. Firstly, this paper studies the construction of the ground microseismic velocity model based on the first arrival time difference. The perforation shot of the fracturing is important prior information for the optimation of velocity model. However, the origin time of the perforation cannot be acquired accurately in the actual monitoring process. In order to solve this problem, we propose to use the root mean square error of time double-difference which is based on the time difference of first arrivals to describe the discrepancies between the initial velocity model and that of the actual medium. The root mean square error of time double-difference is reduced thanks to very fast simulated annealing, and a set of root mean square error of time double-difference samples are reserved during the process of simulated annealing. A threshold value is set near the minimum value of the samples, and an appropriate amount of them are selected again within the range of the threshold and the corresponding velocity models are used to relocate the perforation-shot. The velocity model which has the optimal source relocation result as the basis for microseismic location. However, for microseismic monitoring on surface, the seismograms of perforation shot with a low signal-to-noise ratio(S/N) do not allow for obtaining accurate information on the time-picking of arrivals. We propose a non-linear method based on amplitude stacking to optimize the velocity model. The energy focus effect of the perforation-shot is improved thanks to very fast simulated annealing(VFSA), and then the accuracy of the relocation of perforation shot is taken as the standard to evaluate whether the velocity model can be used for microseismic location. Because the unreasonable priori velocity information and the interference of Gaussian noise are the two major factors that cause the location inaccuracies of the perforation shot, we take the velocity model optimization as a kind of error compensation scheme aiming at improving the accuracy of perforation relocation. Synthetic tests show that when the S/Ns are as low as 0.1 at individual(pre-stacked) receivers, the relocation inaccuracy of perforation shot is within 2 m by utilizing the proposed technique.Due to the lack of 3D visualization software for micro seismic monitoring in China, Qt Creator is implanted in the Visual Studio 2010 in this paper, and a 3D visualization software for micro seismic monitoring was developed by utilizing the drawing technology of Open GL and C++ language. The processed microseismic data are imported into the calculation module by coordinate transforming and then, the microseismic events are intuitively displayed on output devices. The 3D visualization software provides more convenient man-machine interactive tools.In this paper, a three-dimensional microseismic software using surface arrays are developed, which includes source identification, ray tracing, microseismic event location, velocity optimization, and 3D visualization software, and this system was applied to Ning Wu basin field fracturing experiments. Practical application shows that this system is feasible in a simple geological condition. This system will be extended to more complicated geological conditions by improving the velocity model optimization methods.The innovation in this paper is that the commonly used ray tracing method is improved, the computational efficiency is higher than other method, and is more suitable for the microseismic monitoring field; on the velocity optimization for surface microseismic monitoring, for the situation of high signal to noise ratio of the perforation shot, a velocity model optimization method is proposed based on the first arrival time difference, which can avoid the problem of the unknown origin time of perforation shot, and the unreasonable priori velocity information, the problem of the nonuniqueness of the velocity model inversion can be also solved by perforation relocation, for the situation of low signal to noise ratio of the perforation shot, a new velocity model optimization based on amplitude stacking method is also proposed to avoid the problem of inaccurate time-picking of arrivals; In terms of microseismic location, the grid successive subdivision method is proposed to improve the conventional grid search method, which greatly improving the calculation efficiency and accuracy. At last, a 3d visualization module is developed which provides a convenient visualization tools.