Per Atle Olsen

Casing effects in the sea-to-borehole electromagnetic method

The sea-to-borehole controlled source electromagnetic CSEM method has the potential of becoming a useful new exploration and monitoring technique. Modeling shows that the electromagnetic fields at reservoir depths can be 1-2 orders of magnitude larger than those on the seabed, thus opening up the possibility to see deeper or detect and monitor smaller targets. In the case of measurements inside metal casing, the use of reciprocity to interchange the source and receiver allows the casing conductortobeconsideredasanantenna.Thisantennaisexcited by a source inside the casing, which induces a current flowing along the casing conductor. Based on the current distribution, a casing dipole moment and an equivalent length were derived for thetransversemagneticTMmode.Inthefrequencyrangeofinterest 0.1‐10 Hz, the increase in field strength resulting from sea-to-borehole measurements is not lost to the casing attenuation for either the TM or the transverse electric TE mode. The ratio of the dipole moment of the casing antenna to that of the sourcecurrentcanbeconsideredasthecasingattenuationeffect. Thisratioisnotfarfromunityforfrequenciessmallerthan1 Hz, but it decreases rapidly with frequency.The equivalent length of the casing antenna can be of the order of a few hundred meters. As a consequence, the field distribution changes significantly in the presence of casing, and the vertical resolution of TM measurementsisdegraded.Thehorizontalresolutionisnotdegraded significantly.MeasurementoftheTMmodehastheadvantageof being sensitive to thin resistive targets and being less affected by sea-air interactions in shallow water. The TE mode measurement, having a localized casing effect, contributes valuable informationaboutthebackgroundresistivity.

3D reservoir characterization of a North Sea oil field using quantitative seismic & CSEM interpretation

The marine controlled source electromagnetic (CSEM) method for hydrocarbon exploration was introduced ten years ago (Constable, 2010) and is now an established exploration tool. The majority of the CSEM surveys acquired so far have focused on prospect ranking and derisking (Hesthammer et al., 2010). Traditionally, the survey geometry for these purposes has been a single or a few 2D lines due to limitations in acquisition and processing efficiency. Such datasets may qualitatively differentiate between a low resistivity brine saturated reservoir, and a high resistivity hydrocarbon saturated reservoir. However, interpretations based on 2D are only reliable in cases where 3D effects can be ignored, and when the reservoir outline is known with high confidence so that the towline can be positioned optimally.

Detecting Skrugard by CSEM — Prewell prediction and postwell evaluation

AbstractThe discovery of Skrugard in 2011 was a significant milestone for hydrocarbon exploration in the Barents Sea. The result was a positive confirmation of the play model, prospect evaluation, and the seismic hydrocarbon indicators in the area. In addition, the well result was encouraging for the CSEM interpretation and analysis that had been performed. Prior to drilling the 7220/8-1 well, EM resistivity images of the subsurface across the prospect had been obtained along with estimates of hydrocarbon saturation at the well position. The resistivity distribution was derived from extensive analysis of the multiclient CSEM data from 2008. The analysis was based on joint interpretation of seismic structures and optimal resistivity models from the CSEM data. The seismic structure was furthermore used to constrain the resistivity anomaly to the Skrugard reservoir. Scenario testing was then done to assess potential alternative models that could explain the CSEM data in addition to extract the most likely re...

Coarse-scale resistivity for saturation estimation in heterogeneous reservoirs based on Archie's formula

Relations between porosity, saturation, and resistivity are essential when estimating in-place hydrocarbon volumes. The most common relation is the Archie formula. During the last decade, several new coarse-scale electromagnetic techniques for resistivity measurements have been developed. Applications of coarse-scale resistivity must consider the underlying geologic variability of porosity and saturation to be used in such relations. I have investigated the effect of small-scale (logging-scale) variability in porosity and water saturation on coarse-scale resistivity for Archie relations. An analytical expression for coarse-scale resistivity is derived based on the assumption of lognormal distributed porosity and saturation. Similarly, coarse-scale water saturation is derived assuming lognormal distributed porosity and resistivity. The results suggest that the coarse-scale equivalent Archie resistivity is normally less than the geometric average and larger than the harmonic average of the heterogeneous res...

3D Reservoir Model and Resource Estimation for a North Sea Oil Field from Quantitative Seismic and CSEM Interpretation

Understanding reservoir properties and fluid distribution is the aim of petroleum geophysics, in particular in the field appraisal and development phases. The ultimate goal is not only to identify and delineate hydrocarbon charged reservoirs, but to quantitatively determine the volume and distribution of oil and gas contained. Since no single measurement has the required response properties to achieve this, it is now recognized that integration of different types of data with complimentary sensitivity will be essential. In this study, we describe a quantitative, joint interpretation of 3D seismic and 3D controlled source electromagnetic (CSEM) data from the Troll western oil province (TWOP), an oil and gas field in the Norwegian North Sea.