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Holten T.,Petromarker | Flekkoy E.G.,University of Oslo
SEG Technical Program Expanded Abstracts | Year: 2011

Recent technological advances have made time-lapse EM a real possibility. In particular, EM methods appear to be a promising tool to identify the process of salt water breakthrough between an injection and a production well. We will examine the CSEM method that uses vertical, stationary transmitters and receivers, as has been developed by Petromarker. A simple reservoir model is studied where small parts of a larger box-sized reservoir have reduced resistivity. The repeatability is affected by changes in weather conditions and ocean currents, which may cause slight inaccuracies in positioning and transmitter tilt. The seawater conductivity is measured routinely and may be compensated for. These uncertainties along with source waveform changes may accumulate to significant systematic errors, but they are controllable and their sizes easily estimated. An example of a repeated measurement where both the transmitter and receiver were moved is shown. The average deviation at late times of four such measurements is 16 nV. In our model, this corresponds to the detection of 10% reduced resistivity. Our conclusion is that time-lapse EM monitoring is indeed feasible for large reservoirs at the time scale of months, provided the saturation changes are sufficiently localized. © 2011 Society of Exploration Geophysicists.


Flekkoy E.G.,University of Oslo | Holten T.,Petromarker | Haland E.,Petromarker | Maloy K.J.,University of Oslo
72nd European Association of Geoscientists and Engineers Conference and Exhibition 2010: A New Spring for Geoscience. Incorporating SPE EUROPEC 2010 | Year: 2010

Underwater measurements of electric fields are a key component in most methods for marine hydrocarbon exploration based on electro-magnetism. Often these fields are very weak and their measurements highly challenging. We discuss the physics of a selection of the relevant noise sources in marine EM measurements and the technology designed to deal with them. These noise generating mechanisms include electronic noise in the receivers, electrode drift, noise induced by mechanical motion of the antennas, noise caused by the hydrodynamics of surface waves or the turbulent motion around antennas due to tidal currents, surface charges that move with the surface of the waves, atmospheric EM noise, and signal distortions due to ill-controlled antenna orientation. We address both the questions as how measurement design may be improved to deal with internal noise sources and how the effect of external noise sources may be reduced. © 2010, European Association of Geoscientists and Engineers.


Holten T.,Petromarker | Flekkoy E.G.,University of Oslo
73rd European Association of Geoscientists and Engineers Conference and Exhibition 2011: Unconventional Resources and the Role of Technology. Incorporating SPE EUROPEC 2011 | Year: 2011

Recent technological advances have made time-lapse EM a real possibility. In particular, EM methods appear to be a promising tool to identify the process of salt water breakthrough between an injection and a production well. We will examine the CSEM method that uses vertical, stationary transmitters and receivers, as has been developed by Petromarker. A simple reservoir model is studied where small parts of a larger box-sized reservoir have reduced resistivity. The repeatability is affected by changes in weather conditions and ocean currents, which may cause slight inaccuracies in positioning and transmitter tilt. The seawater conductivity is measured routinely and may be compensated for. These uncertainties along with source waveform changes may accumulate to significant systematic errors, but they are controllable and their sizes easily estimated. An example of a repeated measurement where both the transmitter and receiver were moved is shown. The average deviation at late times of four such measurements is 16 nV. In our model, this corresponds to the detection of 10% reduced resistivity. Our conclusion is that time-lapse EM monitoring is indeed feasible for large reservoirs at the time scale of months, provided the saturation changes are sufficiently localized.


Flekkoy E.G.,Petromarker | Flekkoy E.G.,University of Oslo | Haland E.,Petromarker | Maloy K.J.,University of Oslo
Geophysics | Year: 2012

Natural electric field variations are measured at the sea bottom over long periods of time by means of stationary, vertical, and horizontal galvanic antennas. We compare the power spectra of the vertical and horizontal field components and the extent to which they may be reduced by standard averaging techniques. Although the raw spectra of the vertical and horizontal components do not differ greatly, the difference in the spectra after averaging is significantly greater. Most significantly, in the frequency range between 0.0005 and 0.03 Hz, this averaging scheme suppresses the vertical electric field component more strongly than the horizontal component. © 2012 Society of Exploration Geophysicists.


Haland E.,Petromarker | Flekkoy E.G.,University of Oslo | Holten T.,Petromarker
73rd European Association of Geoscientists and Engineers Conference and Exhibition 2011: Unconventional Resources and the Role of Technology. Incorporating SPE EUROPEC 2011 | Year: 2011

Understanding the electromagnetic noise that reaches the bottom of the sea is of fundamental interest in a range of contexts. In magnetotellurics the natural electromagnetic field variations is used to detect resistive or conductive bodies in the subsurface. Effects of air-sea boundary give different noise spectra in the different E-field components, and it is of fundamental and technological interest to compare the two. Only recently has it been possible to measure the relatively weak vertical component with sufficient accuracy. We compare the vertical and horizontal components, in particular their relation to the theory of ocean wave generated fields. Electric field variations due to ocean waves have been studied widely. Here we generalize Weavers solution for an infinitely deep sea, to a 3-layer shallow sea model. This solution differs qualitatively from the Weaver solution and the solution quoted by Cox through the fact that there are no horizontal field components in our solution. This prediction is well confirmed by our measurements in the ocean wave range of frequencies. It is observed that the weaker vertical E-field component carries a clear effect of ocean waves, while the stronger horizontal components do not, and appears to be dominated by magnetotelluric noise.


Frafjord O.,PetroMarker AS | Holten T.,Petromarker | El Kaffas A.M.,Petromarker | Borven J.M.,Petromarker | Helwig S.L.,Petromarker
76th European Association of Geoscientists and Engineers Conference and Exhibition 2014: Experience the Energy - Incorporating SPE EUROPEC 2014 | Year: 2014

The uncertainty associated with measuring the vertical electric field in marine CSEM for different receiver types was investigated. Different designs for measuring the electrical field subsea have been published by a variety of authors. Our study focusses on the measurement of the vertical field component generated by a vertical transmitter. Three different receiver types, a pendulum design that aligns the measuring dipole towards gravity, a design with three orthogonal measurement dipoles and a design based on a tetrahedron are considered. The study utilizes published noise values derived from real measurements. Based on a ratio between vertical to horizontal noise of 1 to 20, we find that the vertical antenna solution delivers the best results. The design with three-perpendicular-antennas performs well on slightly sloping sea floor but its total error in vertical field measurement is up to 5 times higher for seabed tilts up to 10 degrees. The error is found to be relatively insensitive to realistic uncertainties in the tilt measurements. The tetrahedron design elegantly compensates the Ex component as long as the seafloor is flat. On a sloping seafloor the compensation may get lost and Ez has to be derived from components that contain relatively large parts of Ex.

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