Geophysical contractors have long beefed that oil companies are reluctant to spend money on seismic when the cost of the survey is a mere fraction of the cost of a dry hole. At least one oil company has gotten the message. At a recent Geophysical Society of Houston luncheon Dave Johnston, global geophysics coordinator for ExxonMobil Production, talked about his company’s increasing commitment to 4-D seismic as a way to increase reserves, optimize infill locations and field development, and reduce uncertainty. Johnston was the spring 2008 AAPG distinguished lecturer. (You would have been reading this blog Sept. 17 if Hurricane Ike hadn’t rearranged everyone’s plans.) Johnston discussed three deepwater fields offshore West Africa, including the Zafiro field. In this field the use of 4-D seismic made a considerable difference in the way it was managed. At Zafiro, a baseline survey was acquired in 1995, and the repeat survey was done in 2003. Johnston said the field development plan is designed to support an active drilling program, and since the 2003 survey more than 50 wells have been drilled. Zafiro is characterized by complex inter-connectedness between channels. Spatial relationships between the channels provide for both vertical and lateral connectivity. The goal of the 4-D survey was to locate flow pathways and possible baffles and to monitor gas and water movement. Where there was a decrease in impedance, it was interpreted as gas coming out of solution, indicating it was connected to another reservoir (there are 15 producing reservoirs at Zafiro). One part of the study saved the drilling of an additional well because it increased the understanding of what was swept and what was not. At another well location, 40 MMbw had been injected over a two-year period. The repeat seismic indicated that the water was not moving through the reservoir as anticipated because of a closure. Instead, it was displacing the oil and moving it downdip. “Overall, we used 4-D as a derisking tool on more than 20 wells,” Johnston said. “It’s been so successful that we completed a second repeat survey.” While the company has had success with streamer surveys, one of the questions regarded permanent monitoring such as BP has done at some of its fields. Johnston said that until recently the technology was not robust enough for deep water but that new systems that incorporate fiber optics and MEMS systems have potential in the future. When asked about how fields are high-graded for 4-D studies, Johnston replied that impedance contrast is key, but low impedance contrast doesn’t necessarily equate to ruling out that field. In one example he said that students were asked to high-grade a field with 20% impedance contrast, 10% impedance contrast, and 4% impedance contrast (3-4% being at the edge of detectability). Most of the students chose the field with the highest contrast. “What if I told you that the field with 4% impedance contrast was a 22 billion barrel field?” he asked. “You might be willing to take a risk if you know the potential is there.”