EM for Exploration and Production
Electromagnetic techniques are important for hydrocarbon E&P because they can distinguish between oil and water saturated rock, responding directly to water-filled porosity. Their sensitivity decreases slowly with depth, so they can often be applied when seismic fails due to abrupt physical property change. However, their real strength lies in their complementary use, along with seismic, either to improve the seismic velocity interpretation or to provide additional information such as inferred porosities.
In this course we review the historic foundation of electromagnetic methods and introduce three techniques: Magnetotellurics (MT), frequency domain CSEM and time domain CSEM (tCSEM™). MT uses the Earth’s natural electromagnetic field as its source, and makes tensorial (multi-channel) measurements to obtain information about the resistivity structure. CSEM uses its own transmitter with magnetic and electric field component measurements, also to obtain information about the resistivity structure. Each technique has an optimum depth range and preferred application targets, which depends on the resistivity of the subsurface and the noise environment. When optimizing the recovery factor of hydrocarbon reservoirs, integration between borehole measurements and surface measurements is crucial to understand the scale limitation of the surface data and also the limitations of borehole data, as it is being use for calibration. A better understanding of the influence of the rock properties leads to select the appropriate Earth model.
Understanding borehole electromagnetics tools allows to design better Earth models and to get a more insight into the reservoir. Numerous case histories are shown where MT, CSEM, and tCSEM™ measurements confirm independently other measurements and help getting better resolution in the interpretation. The applications range from hydrocarbon exploration, geothermal exploration to sub-salt exploration and deep crustal studies. MT is widely used in many parts of the world for a variety of applications. Its major use is for oil exploration in seismically difficult areas, including karsts in East and SE Asia, and for sub-salt and sub carbonate exploration in the Mediterranean and Gulf of Mexico. At 'audio' frequencies it is used for mineral exploration at depths beyond the reach of Transient EM while, at still higher frequencies, it is being applied to a range of environmental problems.