Evaluation of the multi-frequency electromagnetic method in exploration and imaging of hydrocarbon continental reservoirs
Keywords:
multi-frequency electromagnetic technique , reservoir imagingAbstract
Two geophysical experiments using the multifrequency electromagnetic method proposed by Dias(1968) were performed in the onshore portion of the Espirito Santo sedimentary basin to test its application in the petroleum industry. The first one was done in Fazenda Alegre oil field using a 5th generation prototype, operating at AC currents of 10A maximum amplitude and 54 values of frequency in the range from 1.0Hz to 10kHz. Six transverse sections 3.0km extent were surveyed to image electrically the subsurface down to 1.2km depth. The second one was in São Mateus oil field using a 6th generation prototype, which operates with AC current amplitudes up to 40A and 139 frequency values in the range 0.2Hz to 1.097kHz. Eight transverse sections 5.0km extent allowed to image the underground geology down to 2.8km depth. These experiments were designed to: (i) detect oil fields in distinct geological settings using the EMMF method; (ii) determine the geoelectrical and petrophysical characteristics of fault zones and other structures related to them; and (iii) evaluate the EM response of different lithologic reservoirs in their zones of hydrocarbon and water dominances. The results processed using a consistent procedure are presented as two-dimensional images of electrical apparent resistivity and a parameter of (induced) polarization, for the subsurface geology. These images, interpreted with the support of seismic sections and geophysical well logs, allow to distinguish the following geological features: (i) the interfaces between distinct lithologies, by the contrast of their electrical properties, such as sandstones, shales, limestones and salt-rocks; (ii) thin sub-vertical zones of high electric contrasts with their country-rocks, represented by faults and zones of dense fracturing (chimneys) filled with either conducting and IP source minerals or with hydrocarbon fluids and (iii) the indication of oil-water contact zones within a reservoir, through the lateral variation observed in both electrical resistivity and IP parameter. These results attest the adequacy of the complementary use of the EMMF method in exploration for the direct detection of hydrocarbon deposits, as well as to evaluate the efficiency of artificial injection of fluids during the enhanced oil recovery
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