From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, October 16, 2009
Marine Controlled Source Electromagnetic Modeling for Gas Hydrates
Dr. Hari Tirumalachetty, Visiting Scientist
Hydrocarbons (oil, gas, gas hydrates etc.) at subsurface depths characterize as high resistive layers. Research on gas hydrates has increased considerably in recent years as gas hydrate is considered as an alternate energy source to conventional fossil fuel. Although seismics help to locate the areas with BSR’s (Bottom Simulating Reflectors), for possible presence of gas hydrate stability zones, it is not easy in identification and estimation of the gas hydrate layers from seismics alone. Since the resistivity contrast with the surrounding medium is usually an order higher for gas hydrates, especially in a conductive marine environment and also in sedimentary basins, it is advantageous to apply electromagnetic methods for its detection, estimation of its parameters, and also for monitoring. The method acts as a complement to the seismics.
In the present talk, I discuss the modeling of marine Controlled Source Electromagnetics (CSEM) and test its efficacy for the above problems. The model considered here closely matches with the well known gas hydrate region in Gulf of Mexico - Walker Ridge Block-313. CSEM technique in marine environments is relatively new (Ellingsrud et al., 2002). Earlier modeling studies (Steve and Weiss, 2006; Orange et al., 2009; etc.), have considered horizontal transmitters to study the CSEM response—2 electrical and 1 magnetic component—for resistive subsurface layers. Here, I propose to study all 6 components—3 electrical and 3 magnetic. The proposed method reduces the ambiguity in interpretation considerably. Additionally, I show the results of CSEM response for a vertical transmitter—transmitter in a bore hole and receivers at the sea bottom. It is observed that CSEM response from a vertical transmitter helps to characterize the hydrocarbon layers more confidently.
Although the present study is aimed to study the gas hydrates in marine environments, the technique and methodology is applicable to other areas—for example in monitoring the CO2 diffusion at subsurface depths, which assists in CO2 sequestration research.