Despite the recent economic downturn and the associated unpredictability in oil prices after their historical high of ~$147/barrel, the long-term outlook for the oil industry is strong. The world's need for energy is increasing. In fact, the International Energy Agency (IEA) projects that total world consumption for marketed energy will increase by 50% through 2030. Despite recent work in developing renewable energy sources, oil will remain the world's main source of energy for many years to come, even under the most optimistic of assumptions about alternative technology. Although there is a common perception that the world is running short of oil, it is estimated that in actuality large reserves remain. The IEA estimates total conventional oil resources at 3.5 trillion barrels. Of that, only a third has been found. Resources whose geographical location has not yet been determined account for a third of the estimated remaining supply. It is clear that new oil exploration will continue to be a top priority for energy producing companies.
Technological advances in subsea oil extraction have allowed exploitation of resources at ever-increasing water depths (Transocean Ltd. recently announced the successful drilling of a well at a depth of six miles). Due to the technical challenges involved, however, deepwater wells represent a major capital investment. Therefore, strong economic factors drive a need for accurately locating and characterizing subsea resource fields.
Conventional methods of underwater oil exploration involve the use of seismic sensing, generally involving various combinations of hydrophones, geophones, and acoustic energy sources such as air guns. While these techniques can discriminate liquid from solid in the subsurface, it is still necessary to drill exploratory holes to discern whether the areas of interest identified through seismic modeling contain hydrocarbons of interest to oil companies. In shallow waters, this is a relatively simple and inexpensive procedure. However, operating in a deepwater environment poses considerable technical challenges not present on land or in shallow water. For example, the distance from the boat on the surface to the drill site below makes simple operation of the equipment challenging; the tremendous water pressure at depth requires specialized equipment optimized for this environment; and the hostility of the environment means that divers cannot be sent down to handle problems that may arise during drilling. These and other challenges make the drilling of numerous exploratory holes prohibitively expensive. Therefore, a technology was needed that was capable of providing more information about the subsurface content to limit the number of exploratory holes required.
MMT and CSEM
Marine magnetotelluric (MMT) and controlled-source electromagnetic (CSEM) surveying methods allow characterization of the resistivity properties of the subsurface content. Resistivity is a measure of how strongly a material opposes the flow of electric current. Hydrocarbons and salt water have different resistivity properties (water conducts electric current well and thus has low resistivity, whereas hydrocarbons such as oil conduct poorly and thus exhibit high resistivity). This information can assist in producing more accurate assessments of the subsurface content. By augmenting conventional survey data with CSEM and/or MTT data, experts can provide elaborate 3D maps of potential resource fields, enabling better targeting of drilling operations and more economical deepwater drilling. Incorporation of these data into the decision-making process can provide risk reduction to the drilling process, and thus result in cost savings to the oil industry.
MMT measurements exploit the natural low-frequency electromagnetic field of the Earth and thus do not require the use of a separate electromagnetic source to conduct a measurement. In this procedure, an electromagnetic receiver and data logging unit are deployed to the seafloor from a ship. The system incorporates an anchor to hold it to the seafloor, an acoustically triggered release mechanism to detach it from the anchor when signaled, and flotation to return it to the surface. For resource surveys, an array or a line of these receivers is arranged in a set pattern on the seafloor. The pattern is determined by a number of factors including the geological structure being imaged, the depth to the target region, and the desired resolution. Once the data collection period has passed, typically from 2 days up to as long as 2 weeks, vessels return to the area, trigger the release of the receivers, and pick them up so that the collected data can be analyzed. In interpreting the data, the electromagnetic (EM) field components measured on the seafloor are influenced by the composition of the subsurface (through the resistivity of the medium), and the resistivity of the subsurface as a function of signal frequency can be computed from the EM field measurements.
CSEM measurements are conducted in a similar fashion, but instead of using the ambient electromagnetic field, a low-frequency, high-powered dipole antenna is deep-towed behind a ship to provide an EM source and the resulting field is measured by the deployed receivers. In a large survey field, it is possible for receivers close to the source to be collecting CSEM data while those out of range collect MMT data (Figure 1). All of the resulting data are incorporated into a complex inversion resistivity map that is used as part of the process of assessing potential drilling sites.
Figure 1. Schematic showing standard marine CSEM/MMT data collection procedure (Click image for larger version)
The size and operational methodology of underwater EM measurement systems currently in use have been driven by: a) the need for a long baseline to mitigate the effect of electrical field noise, which demands large separation between electrodes; b) the fact that the current Ag/AgCL electrode technology requires storage in water to perform optimally; and c) the use of large, externally located magnetic field sensors. The result is a cumbersome system with sensing arms that span up to 10 m and sensors that must be installed into the system while it sits on the deck of a ship, just prior to deployment. Putting together so sensitive a system on the deck of a ship is a difficult task that reduces the measurement system's overall reliability when deployed.
A New Approach
Understanding this market and its challenges, Quasar Geophysical Technologies (QuasarGeo) was formed to bring new technologies to market to advance exploration operations and improve drill decision analysis. Building on its parent company's expertise in electromagnetic sensing systems, QuasarGeo has developed a new class of electric and magnetic field sensors and integrated them with the latest generation of ultra-low-noise, low-power electronics and data storage systems into the QMax EM3 ocean bottom receiver (Figure 2). This fully integrated sensor system powers a new compact receiver designed to optimize operational efficiencies for survey contractors and to produce the highest quality data for processing and analysis. As stated earlier, in conventional technology, the presence of electric field noise drives a requirement for a long sensing baseline, which is met by constructing devices with long, protruding arms with the electrodes located at the end (shown in Figure 1). Much of the operational difficultly of conducting an underwater EM survey can be traced to the awkwardness of handling these sensing arms. They are generally stored upright, with the electrode end submerged in a tank of water, and then placed onto the receiver right before deployment. The device is assembled on deck, then raised and swung overboard with a mechanical hoist/crane with personnel physically guiding its progress. The process is reversed when the receivers are brought back up onto the ship. With a rolling and slippery deck in the open ocean, the swinging device and swaying arms pose a hazard to personnel and the procedure can be very time consuming. In addition, there are a number of discrete modules that use pressure-resistant, waterproof electrical connectors that introduce additional potential failure points.
QuasarGeo's electrodes do not interact with the ion-rich underwater environment, resulting in lower electric field noise in their measurements. In addition, proprietary electronics, when combined with the new electrodes, produce a device with lower system noise. These two innovations have allowed QuasarGeo to produce a system in which the electric field sensors fit within the body of the unit as shown in Figure 2. In addition, the QMax's electrodes do not require storage in water as do conventional electrodes, and thus the system does not require assembly on deck, but rather can be purchased pre-assembled and ready to drop. Finally, the system's design innovations mean the QMax does not require as many electrical connectors, resulting in fewer opportunities for system failures at connection sites and thus a more reliable system.
Figure 2. Two QuasarGeo QMax EM3 units
Further, the QMax's reduced size and the elimination of deck assembly also enables the automation of deployment and recovery operations, reducing the number of (and perhaps completely eliminating altogether) personnel needed on deck. The system's compact nature also opens up the possibility of deployment and recovery by remote operated underwater vehicles (ROVs). ROV deployment could guarantee the position and orientation of the instrument on the seafloor and the ROV recovery procedure would allow for the elimination of the recovery system that is a component of current technology, thereby reducing the size and complexity of the instrument as a whole.
Because the system can be purchased pre-assembled, all that is required for deployment is to attach an anchor and install the battery memory unit (BMU) before deploying the unit. The receiver's smaller size means that more units can be transported in and deployed from a specific size of ship, resulting in a larger survey field, or alternatively a denser array of sensors on the seafloor than is possible with systems currently in use. The module-based system can be built to order, with integrated acoustic sensing modules included, offering customers a complete solution for marine surveys (Figure 3).
Figure 3. QMax EM3 Sensing System
The mission of Quasar Geophysical Technologies is to stay at the technological forefront of geophysical EM sensing and our vision for the future includes advanced systems for both underwater and ground-based sensing for resource exploration and geophysical science applications.
- World Energy Outlook, 2007, IEA
- World Energy Outlook, 2008, IEA
- Constable, S. and Srnka, L., "An introduction to marine controlled-source electromagnetic methods for hydrocarbon exploration," Geophysics vol. 72 issue 2; March–April 2007
ABOUT THE AUTHORS
Dr. Andrew D. Hibbs ([email protected]), Gayle Guy ([email protected]), and Dr. Thomas K. Nielsen ([email protected]) can be reached at Quasar Geophysical Technologies, San Diego, CA; 858-412-1839. For more information contact George Eiskamp, QuasarGeo's CEO, at 858-412-1821, [email protected] or Gayle Guy.