71st EAGE Conference and Exhibition - Workshops and Fieldtrips

Subsurface resistivity mapping based on Controlled Source Electromagnetic (CSEM) measurements are attractive for Shell because they offer the possibility to distinguish between hydrocarbon and brine bearing reservoirs where conventional seismic methods are sometimes inconclusive. Indeed, the resistivity of a reservoir rock is directly related to the amount and type of fluid filling the pores while its acoustic properties are rather insensitive to it. CSEM can therefore be a valuable tool to compliment seismic data for prospect evaluation. In Shell, we have applied the CSEM method on a worldwide scale since 2003 to both de-risking and portfolio polarization in a marine settings.

Looking back from the very first test of using CSEM to detect hydrocarbon reservoirs in 2001, we have seen a tremendous improvement in technology and processing. We have moved from 2D lines to true 3D acquisition and from attribute analysis to 3D inversion. We have seen significant improvements in data quality and acquisition efficiency, and we have seen an increased effort in integrating EM data with seismic data. All these improvements have contributed to better resolution, deeper penetration, less ambiguity, and, as a consequence, better interpretation and understanding of the subsurface.

The Barents Sea represents tremendous geologic challenges in oil and gas exploration. This is illustrated by drilling of numerous disappointing wells, emphasizing the need for new exploration techniques. Previous CSEM experiences in the Barents Sea showed that EM can detect commercial hydrocarbon accumulations, while clearly non‐economic accumulations do not give significant EM responses. Therefore resistivity measurements could be a valuable indicator of commercial volumes of oil and gas in the region, allowing a possible reduction in the number of dry and non‐productive wells.

Improved reservoir management and production optimisation demands require accurate characterisation of reservoir properties and their changes through time. Advances in geophysical data acquisition and interpretation have led to significant improvements in the remote imaging of earth structure and properties. However, when only a single data type is considered, ambiguities in the interpretation can remain. Integration of disparate geophysical data types allows the strengths of each to be exploited. Here we will concentrate on three contrasting methods: surface seismic, marine controlled source electromagnetic (CSEM) and well-log data.

Increasing production efficiency and monitoring water/CO2 floods are key issues to be addressed with borehole and surface technologies. At the same time linking the information to 3D surface seismic data and borehole data is required to extrapolate in the inter-well space and find the sweat spots between wells and in the 3D reservoir space. Electromagnetic has the strongest coupling to the fluid content of the reservoir while seismic can delineate impedance contrasts or lithological boundaries.

The workshop will be concerned with current practice and new developments in conditioning reservoir models to dynamic data. Aspects covered include: case studies; importance in making robust decisions; getting the static model right; matching 4D seismic data; and assisted history matching. The objectives of this workshop are to share knowledge, understand current practice and discuss new developments on conditioning reservoir models to dynamic data. The deliverable from the workshop will be a summary document from discussions posed by keynote speakers.

Dispersion curves estimated from field data can be inverted to supply near surface velocity models. Since the problem is hill posed, strongly non linear and mix-determined, the inverse problem suffers from strong solution non uniqueness particularly for complex velocity models. Global search methods explore the solution space and supply a “picture” of the solution non uniqueness enabling a proper model parameterization to be used for linearised inversion. A possibility for reducing the broadness of the possible equivalent solutions is the introduction of constraints to the solution. Constraints are traditionally obtained from other geophysical tests or boreholes, but significant improvement in the inversion results can be obtained also introducing the higher modes of propagation or inverting simultaneously the dispersion curves relative to different locations in the case of complex 2D/3D systems.

This paper focuses on the Spectral Analysis of Surface Waves (SASW) method for the determination of stiffness and damping parameters of shallow soil layers. The paper consists of three parts, addressing (1) the in situ SASW test, (2) the determination of the dispersion and attenuation curves from the measurement data, and (3) the inverse problem where the soil profile is identified. The existing practice is critically reviewed, and a number of improvements to the SASW method are presented. These include a technique to improve the efficiency of the in situ test, and a new method to determine the experimental attenuation curve. The efficiency of the test is improved by monitoring the signal-to-noise ratio during the experiment. The experimental attenuation curve is determined by means of a frequencywavenumber analysis, using the half-power bandwidth method.

Surface-wave methods offer relative straightforward experimental, signal processing and inversion procedures to infer materials one-dimensional shear-wave structures. Dispersion inversion can moreover be considered particularly efficient when strong {a priori} information about the probed medium is available. The laboratory experiment presented here shows its ability to estimate the shear-velocity gradient power-law exponent in an unconsolidated granular medium.

ed by seismic phases velocity analysis. In this aim, we performed an experimental study in the experimental platform of Tournemire (Aveyron, France), which is a site that is operated by IRSN (Institute for Radiological Protection and Nuclear Safety). A comparison between the hammer source and a vibrator source showed the efficiency of the vibrator that improved the accuracy of evaluation of the phases velocity. A numerical study showed that the phases velocity of seismic surface waves is sensitive to the EDZ characteristics under the layer of concrete. In this way we have been able to invert the phases velocity picked between 200 Hz and 480 Hz. The inversed velocity profile includes an EDZ layer 60 cm thick with a velocity gradient from 300 m/s to 600 m/s. However, introducing the second mode in the inversion process does not give satisfactory results. This disagreement is probably due to the artifact introduced by the way we calculate the dispersion diagram that can lead up to define “effective mode”.