Simultaneous estimation of hypocenter and velocity in microseismic monitoring: A case study at Shengli Oilfield, China
Abstract:The oil and gas are often trapped in low-porosity/low-permeability formations such as shale or tight sands. Hydraulic fracturing is a commonly used technique to create fractures in the rock and allow oil and gas to flow into the production well. During this process, microseismics are usually accompanying fractures. These signals can be recorded by 3-component geophone arrays to provide effective monitoring to the fracture process. By locating microseismic events associated with fractures, it is possible to determine the geometry of the fracture zone and the effective production volume; observe any interactions with existing faults or events located out of fault zone.
A set of quasi-linear equations relating observed P and S wave arrival times with source locations and velocity parameters were formulated. With this equation, both source and medium parameters can then be simultaneously determined by a least squares method. As an example, we document here the successful application of this method to hydraulic fracturing data observed at Shengli Oilfield. The proposed joint inversion can reduce the misfit between the picked and modeled traveltimes while provide correct locations for microsesmic sources and velocity model parameters. The final estimation of hypocenter for all 12 phase after 7 iterations is shown in Figure 1, where their colors denote fractures in different hydraulic stages and sizes indicate the event magnitudes. The microseismic activities are mostly around the horizontal fracturing well or close to the monitoring well, with rest events scattering throughout the entire model space.