Lucy MacGregor

Using CSEM to monitor production from a complex 3D gas reservoir—A synthetic case study

The marine controlled-source electromagnetic (CSEM) method is most commonly used in targeted exploration or appraisal studies as a tool for detecting and delineating hydrocarbon reservoirs (see, for example, Constable and Srnka 2007). Whereas seismic surveys can detect the structures that may contain hydrocarbons with great accuracy, distinguishing hydrocarbon fluids from water within these structures is more problematic. Originally developed in the late 1970s (Young and Cox, 1981), the CSEM method uses a high-powered horizontal electric dipole (HED) to transmit a low-frequency electromagnetic signal through the sea floor to an array of multicomponent electromagnetic receivers. Bulk electrical resistivity of the sea floor can be determined at scales of a few tens of meters to depths of several kilometers. In many cases this resistivity is driven by the fluid content and saturation of subsurface formations.

2.5D inversion of CSEM data in a vertically anisotropic earth

The marine Controlled-Source Electromagnetic (CSEM) method is a low frequency (diffusive) electromagnetic subsurface imaging technique aimed at mapping the electric resistivity of the earth by measuring the response to a source dipole emitting an electromagnetic field in a marine environment. Although assuming isotropy for the inversion is the most straightforward approach, in many situations horizontal layering of the earth strata and grain alignment within earth materials creates electric anisotropy. Ignoring this during interpretation may create artifacts in the inversion results. Accounting for this effect therefore requires adequate forward modelling and inversion procedures. We present here an inversion algorithm for vertically anisotropic media based on finite element modelling, the use of Frechet derivatives, and different types of regularisation. Comparisons between isotropic and anisotropic inversion results are given for the characterisation of an anisotropic earth from data measured in line with the source dipole for both synthetic and real data examples.