significantly higher resistivity than its surroundings. Terefore, hydrocarbon-saturated reservoir rock, unless its resistivity is significantly reduced by, for example, the presence of clay matrix or a salty water component, can be directly detected by transmitting CSEM fields into the subsurface and recording the returning signal that has been ‘guided’ in the thin resistive hydrocarbon layer. (Guiding means that the energy’s direction of propagation is effectively parallel to the layer boundaries.) Similar to the way seismic
velocity depends on direction, electric resistivity depends on the direction of current flow in the rock. Te earliest observations of electrical anisotropy were noted in the 1930s when surface measurements using electrodes laid in different directions were seen to give different results depending on whether layers were dipping or flat. In most situations the resistivity is quite uniform in the horizontal directions, but larger in the vertical direction. Today, surprisingly large anisotropies have been observed
Figure 1.55: Approximate range of electrical resistivity of rocks, taken from several sources. From petrophysical measurements in oil wells, it is well known that the resistivity increases when hydrocarbons are present in sandstones. The challenge for the CSEM interpreter is to discriminate between resistive layers caused by the presence of hydrocarbons and other layers that also might have high resistivity.
decreasing frequency. In travelling one wavelength the amplitude drops by the factor
, or 55 dB. 1.4.2 Controlled Source Electromagnetics (CSEM)
from log measurements – 10:1 or greater – and not just in shales and laminated sand-shale sequences. Some of the largest anisotropies are found in what in the past had been assumed to be clean, homogeneous sands. In these formations, the anisotropy is believed to be caused by variations in grain size and irreducible water saturation. Electrical anisotropic resistivity must therefore be taken into
account in EM exploration where the goal is to investigate the nature of sedimentary formations and is especially relevant to understanding water saturation in hydrocarbon reservoirs.
1.4.1.2 Velocity and Skin Depth
In a vacuum, EM waves travel at the speed of light, 299,792,458 m/s, often rounded to 300,000,000 m/s. In air, they travel slightly slower at 299,702,534 m/s. In rocks, however, the velocity is much slower, and depends strongly on frequency and resistivity. Te phase velocity – the velocity at which the phase of any one frequency component of the wave travels – is . At a frequency of 1/4 Hz, the phase velocity in
sea water, where resistivity is 0.3 Ωm, is 922 m/s. In a layer with resistivity 2.5 Ωm, the phase velocity is 2,500 m/s, whereas in a layer with resistivity 50 Ωm, it becomes as high as 11,180 m/s. Based on electromagnetic theory, the EM amplitude is, as we
have noted, exponentially damped with distance. Skin depth is a measure of how far into a material an EM wave can go. It has quite practical applications in geophysics, where low-frequency EM waves are sent through water and rocks. Te skin depth is given as
, showing that, for fixed resistivity, skin depth, and thus EM depth, penetration increases with
Marine controlled-source electromagnetics (CSEM) uses electrical resistivity contrasts to investigate the subsurface. CSEM techniques play an important role in oil and gas exploration since the physical property used, resistivity, is higher in hydrocarbon-bearing sediments than in water- bearing ones. As such, CSEM is a complementary tool to seismic – it provides another key parameter which characterises the subsurface. CSEM is sensitive to resistivity and can thus be indicative of hydrocarbon saturation. Te first commercial application of CSEM for hydrocarbon
exploration was performed by Statoil in 2002 (Ellingsrud et al., 2002; Røsten et al., 2003), and spurred a period of extensive data acquisition and development of the technology. It became clear that successful use of CSEM in exploration requires a thorough understanding of the CSEM method. Different resistivity models can explain the same measured CSEM data, so one has to select a model that fits the seismic interpretation and geological understanding from among the resistivity models that explain the data. It helps to have well calibration data, which enables the selection of resistivity models that have the best match to well-log resistivities in the surveyed area. In Section 1.3.1.2 we presented the spectacular seismic
example from the Barents Sea, the Johan Castberg (previously Skrugard) field, found in 2011. Te history of mapping Skrugard by CSEM is reviewed in Løseth et al., 2014. CSEM multiclient data were acquired over the area in 2008 for the 20th Norwegian Offshore licensing round. In 2010, before the first exploration well was drilled at the Skrugard prospect, these data were revisited, together with a variety of other data analysis and preparation work. New data analysis software
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