Controlled-source electromagnetic imaging on the Nuggets-1 reservoir

Abstract

The marine controlled-source electromagnetic (CSEM) sounding method is rapidly gaining acceptance as an exploration tool for detecting and delineating hydrocarbon reservoirs. Whereas seismic surveys can detect the structures that may contain hydrocarbons with great accuracy, distinguishing hydrocarbon fluids from water within these structures is more problematic. As a result, less than a third of exploration wells result in a commercial discovery.Originally developed in the late 1970s (Young and Cox, 1981), the CSEM method uses a high-powered horizontal electric dipole to transmit a low-frequency electromagnetic signal through the seafloor to an array of multicomponent electromagnetic receivers ( Figure 1). The direct signal through the water column is rapidly attenuated, with the result that at source-receiver separations of more than a few hundred meters, the signals received are dominated by fields that have interacted with the earth. By studying the received signal as the source is towed through the array, the bulk electrical resistivity of the seafloor can be determined at scales of a few tens of meters to depths of several kilometers. Transmission frequencies are typically between 0.01 and 10 Hz. At such low frequencies, the behavior of electromagnetic fields in the earth is governed by the diffusion equation. Resolution criteria based on wavelength arguments can therefore be misleading.The resistivity of silicate minerals is extremely high, and so the bulk resistivity measured in a CSEM survey is in general controlled by the volume, properties, and distribution of more conductive fluid phases (e.g., Schmeling, 1986). This makes the CSEM method ideal for studying fluid-dominated geologic systems (e.g., Greer, 2001; MacGregor et al., 2001). Marine sediments saturated with seawater typically have resistivities of 1-5 Ω-m. Replacing the seawater with resistive hydrocarbons can result in an increase in the bulk resistivity of the formation by 1 to 2 orders of magnitude. CSEM sounding exploits this dramatic change in physical properties to distinguish water-bearing formations from those containing hydrocarbons.CSEM surveys have been used successfully in a variety of settings including West Africa, Southeast Asia, the Gulf of Mexico and the North Atlantic (e.g., Ellingsrud et al., 2002; Srnka et al., 2005). Early surveys concentrated on Tertiary reservoir systems in deepwater areas (e.g., Ellingsrud et al., 2002). These demonstrated that in areas of relatively simple geologic structure, including deepwater turbidites and channel systems, positive results could be obtained from CSEM surveys. However, these settings represent only a small proportion of potential exploration targets. In particular to date the method has been limited to relatively deep water (300 m or more, e.g., Johansen et al., 2005). In shallow water, signals that have interacted with the (extremely resistive) air can have a severe impact on the recorded signals. This noise (known as the "airwave") dominates the CSEM response at source/rece...

Resistivity imaging with controlled-source electromagnetic data: depth and data weighting

“…While the non-linear conjugate gradient or the Newton method is often used in geophysics [2,14,25,39], we here decided to apply a quasi-Newton method. This approach is more suitable than the Newton method due to the size of the problem.…”

“…CSEM data are used in the oil and gas industry to detect hydrocarbons because they are sensitive to thin resistive horizontal layers [1,2,3]. The physical explanation is that the highly resistive layers strongly modify the current lines emitted by the horizontal electric dipole [4].…”

Imaging CSEM data in the presence of electrical anisotropy

“…One technique that has emerged in the last several years utilizes low frequency electromagnetic (EM) energy (less than 10 Hz) to map variations in the subsurface electrical resistivity of offshore oil and gas prospects (Constable, 2006;Eidesmo et al, 2002;Ellingsrud et al, 2002, MacGregor et al, 2006. In the marine controlled source electromagnetic (CSEM) measurement technique a deeptowed electric dipole transmitter is used to excite a low-frequency electromagnetic signal that is measured on the sea floor by electric and magnetic field detectors, with the largest transmitterdetector separations exceeding ~15 km.…”

Gulf Petro Initiative

“…Due to the low sensitivity of compressional wave velocity to changes in reservoir quality, however, seismic methods prove to be poor risk analysis tools. As few as one third of exploration wells assessed through traditional means lead to commercial quality discoveries (MacGregor, et al, 2006). It has long been known (and relied upon) that the presence of hydrocarbons-particularly gas-in the pore spaces of a formation significantly alters acoustic impedance.…”