Electromagnetic monitoring with local mesh for time-lapse monitoring of hydraulic fracturing

The controlled-source electromagnetic method (CSEM) is a useful tool for monitoring underground fluid movement due to hydraulic fracturing. To understand the characteristics of time-lapse monitoring, an efficient three dimensional(3D) forward modeling is a premise. However, 3D finite element method (FEM) employs the normal mesh that applies fine grids to model the whole sensitive area, resulting in a large degree of freedom (DOF) of the equation that is associated with expensive computation cost. Moreover, time-lapse monitoring requires continual expensive calculations, which lead even an unacceptable computational burden. Inspired by the fact that during the hydraulic fracturing, the resistivity changes occur in a limited domain, we propose a method called as the FEM with local mesh, which is based on secondary field equation to model the EM field perturbation due to hydraulic fracture, and uses fine grids on the fracturing area with resistivity change and as large grids as possible to represent other areas without resistivity change. Therefore, the DOF can be reduced significantly. The numerical tests demonstrate that our method is accurate and has only 10% of the DOF of conventional method.