Resolution of reservoir scale electrical anisotropy from marine CSEM data

Abstract

A combination of 1D and 3D forward and inverse solutions is used to quantify the sensitivity and resolution of conventional controlled source electromagnetic (CSEM) data collected using a horizontal electric dipole source to transverse electrical anisotropy located in a deep-water exploration reservoir target. Since strongly anisotropic shale layers have a vertical resistivity that can be comparable to many reservoirs, we examine how CSEM can discriminate confounding shale layers through their characteristically lower horizontal resistivity. Forward modeling demonstrates that the sensitivity to reservoir level anisotropy is very low compared to the sensitivity to isotropic reservoirs, especially when the reservoir is deeper than about 2 km below the seabed. However, for 1D models where the number of inversion parameters can be fixed to be only a few layers, both vertical and horizontal resistivity of the reservoir can be well resolved using a stochastic inversion. We find that the resolution of horizontal resistivity increases as the horizontal resistivity decreases. We show that this effect is explained by the presence of strong horizontal current density in anisotropic layers with low horizontal resistivity. Conversely, when the reservoir has a vertical to horizontal resistivity ratio of about 10 or less, the current density is vertically polarized and hence has little sensitivity to the horizontal resistivity. Resistivity anisotropy estimates from 3D inversion for 3D targets suggest that resolution of reservoir level anisotropy for 3D targets will require good a priori knowledge of the background sediment conductivity and structural boundaries.3

The use of rotational invariants for the interpretation of marine CSEM data with a case study from the North Alex mud volcano, West Nile Delta

“…This error has two contributions. The first contribution is a relative error expressed as a percentage of the amplitude, which could account for uncertainties in the measurement geometry or instrumental errors and is assigned to be 1 per cent for this sensitivity study (similar in Brown et al 2012or Edwards 1997. The second contribution, is inversely proportional to the square root of time.…”

“…A minimum error floor of 3 per cent of the measured value was assumed to account for errors in the geometry (i.e. distances, tilts, headings; similar in Brown et al 2012) and to avoid a dominant influence of measurements taken at short offsets, which often have very small relative errors.…”

Accuracy and effectiveness of three-dimensional controlled source electromagnetic data inversions

“…However, Brown et al (2012) investigate 3D anisotropic inversion of a simple slab model. They show that the resulting resistivities are less accurate than those produced by 1D inversion of a 1D version of the slab model, but they do not provide a quantitative estimate of accuracy.…”

Electromagnetic field analysis in the marine CSEM detection of homogeneous and inhomogeneous hydrocarbon 3D reservoirs

“…The latest research papers have considered only the responses from the solid (homogeneous) reservoir models with isotropic (Abubakar et al, 2009;Constable and Weiss, 2006;Da Silva et al, 2012;Kang et al, 2012;Key and Ovall, 2011;Løseth et al, 2008;Myer et al, 2012;Ray and Key, 2012;Sasaki, 2011;Schwarzbach et al, 2011;Tehrani and Slob, 2010;Weiss and Constable, 2006) or anisotropic (Brown et al, 2012;Li and Dai, 2011;Ramananjaona et al, 2011;Wiik et al, 2011) resistivity.…”