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During 2009, Petroleum Geo- Services (PGS) successfully tested a novel method of conducting marine controlled-source electromagnetic (CSEM) surveys. The new approach to electromagnetic (EM) surveying has the objectives of aligning EM survey operations with those of seismic operations, improving EM data sampling, and broadening the application of EM technology for geophysical surveying. Building on the information gained from tests in 2009, a new suite of data was acquired during the summer of 2010 to further the development.
The test had two distinct objectives. First, Statoil funded a return to the Peon field in the Northern North Sea to gather sufficient data to perform 3-D analysis and to observe how the resistive anomaly associated with the reservoir appears from an areal perspective. It might be the case that the most productive zones of the reservoir can be highlighted by areas of higher resistivity, and analysis is ongoing to establish whether the EM data can provide valuable information for well placement. Second, to test longer offsets and the ability to detect a deeper resistive structure, a similar swathe of data was acquired over Troll field in the Norwegian North Sea. Troll also was the site chosen to test simultaneous EM and seismic data acquisition.
The EM streamer detects the electric field along its length. The basic construction (other than the receiver assembly) is
based on PGS seismic streamer technology, and for this test a streamer of 2.1-mile (3.5-km) active length prototype built at Teledyne UK was supplied. A key element in the design was the incorporation of configurable electrode pair lengths to determine optimal acquisition parameters for the two very different targets. The fact that the electrodes are integrated into the streamer and linked by custom acquisition telemetry to the recording system allows real-time data quality control (QC) and near real-time initial processing. The handling mechanisms and procedures are similar to that of a seismic streamer. The prototype is designed for tow depths of 330 ft (100 m), which is shallow for EM receivers, and features proprietary EM noise-reduction systems.
Importantly, the entire receiver assembly demonstrated robust build quality suitable for operations in marginal weather conditions.
The EM source is the EM equivalent of a seismic gun array. Instead of emitting sound energy, the EM source emits an
electric field by passing a very high current between two large steel electrodes in the seawater. The prototype source deployed on both test surveys provided up to 640 KiloAmpere-Meters and was towed 33 ft (10 m) below surface. Positioning was tracked using a relative global positioning system. The entire source assembly is transported and installed from a containerized system for flexible deployment.
Equipment performance during trial
The 2010 system was designed to deliver significantly higher field strength than had been used in the 2009 tests. The new streamer also was able to acquire data at much greater offset by adding dummy sections to the front end. There is a strong tie-in to the depth of target and the offset at which the greatest sensitivity is recorded. In looking for the Troll reservoir around 4,593 ft (1,400 m) deep, it was important to have much longer offsets than previously had been used to detect Peon (approximately 1,640 ft or 500 m deep). On the source side, the increased data volume and operating conditions served to confirm the robustness and reliability of the power supply and control equipment. Further, a specially designed package for online and offline data QC was tested. This provided the tools used to perform early data analysis through realtime data display and monitoring.
The Peon reservoir is in 1,237 ft (377 m) water depth, 558 ft (170 m) below the mud line. The 2010 survey comprised 13 lines of towed EM data (about 200 line miles or 330 line km) acquired in 3.5 days. The main aim was to survey a series of 2-D lines to allow 3-D evaluation of the gas distribution. In addition, data also were re-acquired over one of the lines surveyed in 2009 for comparison and calibration of the system performance. Due to the strong anomaly created by such a shallow gas reservoir, the data had a high signal-to-noise ratio. The early data QC, based on the electric field amplitude, confirmed detection of the reservoir and showed good consistency in the recorded amplitudes from one line to the next.
The initial processing step is frequency domain source signature deconvolution, which provides a set of frequency responses for each line.
The frequency response was modified and plotted as a convenient way to observe the location of the resistive anomaly.
The plot is called the “target response” and is a display of the frequency responses at each station along the line normalized by the first (westernmost) frequency response of each line. A new velocity-resistivity transform was tested on a Peon log, which enables the creation of background resistivity models to constrain the inversion, facilitating a quantitative estimate of the gas volume.
The survey area focused on the two main gas provinces of the Troll field. As was the case over Peon, the acquisition speed was four knots. The data were acquired in approximately 1,050 ft (320 m) water depth with the main reservoir at a depth of 4,659 ft (1,420 m). A range of reservoir thicknesses occurs between 131 and 330 ft (40 and 100 m). For this survey, 21,327 ft (6,500 m) of streamer was deployed with 11 active receiver offsets between 7,874 and 20,998 ft (2,400 and 5,400 m). An additional objective was to deploy a seismic streamer and source, in addition to the EM configuration, and test the simultaneous acquisition of EM and seismic with a view to assessing greater operational efficiencies in the future. Part of the strength and robustness of the data quality observed comes from the dense spatial data sampling achieved when compared to node-based EM methods. The source had 2,625-ft (800-m) electrode spacing and was tested using three different modes of operation. A very broad band signal using a pseudo-random binary sequence to control the source polarity switching was compared to a more conventional square wave signal, and both were compared to a newly developed “hybrid” approach currently referred to as an optimized repeated sequence. Twelve lines of high-quality data were acquired in weather conditions ranging from calm to sea state five. Maximum target response can be detected in the sensitive frequency and offset region of the electric field data for each shot point in all east-west survey lines. The noise and non-geophysical disturbances in the 2-D data image have been reduced by a low-rank matrix approximation according to the Eckart–Young theorem. The high resistive regions coincide well with the prescribed extent of the hydrocarbon areas. The red color indicates the most resistive parts.
Further work is ongoing to invert the EM data, and an evaluation of joint seismic and EM data acquisition and subsequent integration is under way. The main influence of the 2010 tests on the development plan is to focus on deeper subsurface penetration by improving the source capability and to develop deeper towing configurations. It is envisaged that the next series of tests to establish a case study for commercial capability will be scheduled around September 2011.
Acknowledgements We thank Petroleum Geo-Services and Statoil for the permission to present this work. We also thank Statoil for the permission to carry out this survey over the Peon discovery and Troll field and their assistance with the project.
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