Michelle Ellis

Role of fine-scale layering and grain alignment in the electrical anisotropy of marine sediments

Electrical resistivities of seafloor sediments determined by controlled source electromagnetic (CSEM) surveys have been found to be significantly greater than those measured by electrical well logging, in some instances by a ratio of as much as 5:1. Because borehole logging techniques invariably measure electrical resistivity using currents circulating in horizontal planes and CSEM surveys are sensitive to the currents circulating in vertical planes, a possible cause of this discrepancy is strong electrical anisotropy of the sediments. We have examined electrical log data from vertical exploration and appraisal wells and deviated production wells in the North Sea. By correlating the same sedimentary units between wells, we are able to compare resistivities measured at different borehole inclination angles. As the inclination angle changes, the amount of vertical and horizontal resistivity contributing to the resistivity changes. Hence we can estimate the electrical anisotropy in the sediments. Results indicate that anisotropy ratios of 1:5 and greater are present within the shale-dominated units. We show that fine-scale horizontal layering can make only a relatively small contribution to this anisotropy, but that a model of horizontal alignment of highly oblate spheroidal grains can account for most or all of it.

An anisotropic model for the electrical resistivity of two-phase geologic materials

Electrical and electromagnetic surveys of the seafloor provide valuable information about the macro and microscopic properties of subseafloor sediments. Sediment resistivity is highly variable and governed by a wide range of properties including pore-fluid salinity, pore-fluid saturation, porosity, pore geometry, and temperature. A new anisotropic, twophase, effective medium model describes the electrical resistivity of porous rocks and sediments. The only input parameters required are the resistivities of the solid and fluid components, their volume fractions and grain shape. The approach makes use of the increase in path length taken by an electrical current through an idealized granular medium comprising of aligned ellipsoidal grains. The model permits both solid and fluid phases to have a finite conductivity useful for dealing with surface charge conduction effects associated with clay minerals and gives results independent of grain size hence, valid for a wide range of sediment types. Furthermore, the model can be used to investigate the effects of grain aspect ratio and alignment on electrical resistivity anisotropy. Good agreement was found between the model predictions and laboratory measurements of resistivity and porosity on artificial sediments with known physical properties.

The Potential of Controlled Source Electromagnetic Surveying in CO2 Storage Monitoring

Summary The capture and geological storage of CO2 is currently being investigated as a means of reducing CO2 emissions into the atmosphere. Before it can be accepted, the fate of the injected CO2 must be understood. Controlled Source Electro-Magnetic (CSEM) surveying could be used to monitor CO2 injection and migration, since the presence of CO2 causes resistivity within the reservoir to change. 1D EM modeling of potential storage sites can indicate whether CSEM monitoring would be viable. By altering the CO2 saturations in the models a range of CO2 storage scenarios can be investigated. The modeling shows that the changes in resistivity caused by the presence of CO2 are detectable by CSEM in some scenarios. However the vertical distribution of the CO2 plays a major role in determining the amount of change in the CSEM response.

Joint seismic/electrical effective medium modelling of hydrate-bearing marine sediments and an application to the Vancouver Island margin

Remote determination of the hydrate content of marine sediments remains a challenging problem. In the absence of boreholes, the most commonly used approach involves the measurement of Pwave velocities from seismic experiments. A range of seismic effective medium methods has been developed to interpret these velocities in terms of hydrate content, but uncertainties about the pore-scale distribution of hydrate can lead to large uncertainties in this interpretation. Where borehole geophysical measurements are available, electrical resistivity is widely used as a proxy for hydrate content, and the measurement of resistivity using controlled source electromagnetic methods shows considerable promise. However, resistivity is commonly related to hydrate content using Archie’s law, an empirical relationship with no physical basis that has been shown to fail for hydrate-bearing sediments. We have developed an electrical effective medium method appropriate to hydrate-bearing sediments based on the application of a geometric correction to the Hashin-Shrikman conductive bound, and tested this method by making resistivity measurements on artificial sediments of known porosity. We have adapted our method to deal with anisotropic grains such as clay particles, and combined it with a well-established seismic effective medium method to develop a strategy for estimating the hydrate content of marine sediments based on a combination of seismic and electrical methods. We have applied our approach to borehole geophysical data from Integrated Ocean Drilling Program Expedition 311 on the Vancouver Island margin. Hydrate saturations were determined from resistivity logs by adjusting the geometric factor in areas of the log where hydrate was not present. This value was then used over the entire resistivity log. Hydrate saturations determined using this method match well those determined from direct measurements of the methane content of pressurized cores.

Electrical Anisotropy Trends in the Snøhvit Region of the Barents Sea

In this study we use controlled source electromagnetics (CSEM), well log data and rock physics modelling to understand the controls and trends of electrical anisotropy in the Barents Sea. Electrical anisotropy in the Barents Sea is extremely variable, with values as high as 1:40. Disregarding resistivity anisotropy will lead to misleading CSEM survey feasibility studies, inaccurate CSEM data analysis, inaccurate estimations of hydrocarbon saturations and, consequently, erroneous interpretations.

Geological Parameters Effecting Controlled-Source Electromagnetic Feasibility: A North Sea Sand Reservoir Example

Seismic and electromagnetic data measure very different physical properties. To exploit these data to their fullest we must understand how each data type is affected by subsurface properties. To this end we have investigated the feasibility of using Controlled-Source Electromagnetic (CSEM) and/or seismic surveying at a sand reservoir in the North Sea. Different geological parameters such as fluid composition, reservoir depth, anisotropy and porosity etc. have been altered to determine which are important and must be considered when considering a survey, designing the survey and/or interpreting the results. Results show that for this site both CSEM and seismic methods could be used and that a combination of the two would be beneficial when interpreting the site. It also shows the value of investigating the effect of changing geophysical parameters. We also develop a joint CSEM and seismic feasibility workflow for the more general scenario.