Controlled-source electromagnetic modeling of the masking effect of marine gas hydrate on a deeper hydrocarbon reservoir
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The ability of marine controlled-source electromagnetic (MCSEM) methods to help image electrical conductivity contrasts below the Earth?s surface makes them useful for both initial reconnaissance surveying for hydrocarbons and for delineating prospective regions of high resistivity in development drilling. A 3-D finite-element MCSEM Fortran algorithm used for forward modeling was developed by Badea. Additional code was written and used for this thesis, with the goal of enforcing more realistic electromagnetic (EM) Dirichlet boundary value conditions. The results of the new boundary conditions on a MCSEM survey model, with a hydrocarbon-saturated region in the subsurface, show that the method does not work as hoped. Constant boundary values were applied to gauge the transmitter-receiver (TXRX) range at which results are not boundary influenced, using a hydrate/hydrocarbon model of the subsurface, at each of the three transmitter frequencies used in this study (1 Hz, 3 Hz, and 10 Hz). Results showed that electric field data were reliable to roughly 5000 m of TX-RX offset for the 1 Hz and 3 Hz cases, and to 6500 m offset for 10 Hz. The gas hydrate/hydrocarbon model was then run with zero-value boundary conditions. The goal was to determine what effect changing parameters of the gas hydrate, including hydrate radius, thickness, and depth, have on the EXEXS (xcomponent of secondary electric field inline with the transmitter dipole axis) curves at various offset, particularly in relation to a hydrocarbon-only model of the subsurface response, so as to evaluate the EM masking effect the hydrate has on the hydrocarbon. The results showed that the x-component of electric field in an inline survey is dominated by the hydrate response, in all cases studied, with a couple of exceptions. One exception is 1 Hz transmitter frequency at 2500 m to 3000 m offset when depth to top of the massive gas hydrate zone was greater or equal to 250 m. Receivers at these offsets would successfully detect the hydrocarbon target.