Looking for Oil From More Angles

Abstract

Seismic Data New generation of seismic promises big advances Selling new offshore seismic survey methods can be reduced to two goals: “We want more of everything at less cost.” That maxim was offered by Andrew Long, chief scientist of geoscience and engineering, imaging, and engineering for Petroleum Geo- Services, one of the players in this intensely competitive, economically depressed, technology-driven business. While the science behind using sound waves to image underground formations is baffling to outsiders, the two avenues for doing it better are not. There are improvements in data acquisition covering sound sources, receivers, and survey methods. And there is the mathematics and computer power needed to process and extract more information from the constantly growing amount of data gathered. Faster, Better Seismic Surveys by Making Many Sounds at Once The pace of a seismic survey can be measured by the number of shots taken per day. Huge onshore surveys have shown that allowing 10 or more sound sources working at the same time without any coordination can drastically reduce the time needed to gather data over a large area, and get better data as well. “One of the biggest surprises is we have gotten almost universally better data from our simultaneous source surveys,” said Craig Beasley, a Schlumberger Fellow and chief geophysicist at WesternGeco. More data are gathered from more angles for a more accurate picture of what is below, said Beasley, who explained that one flashlight can help you see in a dark basement, but it is better with two and more is better. The Hunger for the Lowest Frequencies Takes Data Gathering in New Directions The word “broadband” is used to sell a lot of what is new in offshore seismic. It can mean different things depending on who is speaking. But most often, it is applied to things used to gather scarce signals at the lowest frequencies. Gathering more low-frequency data is the “holy grail’ in broadband, said Craig Beasley, chief geophysicist at Western- Geco, which is part of Schlumberger. They are valuable for determining rock properties and imaging deep formations. Seemingly small gains can be big. “Moving down from “3 to 1.5 (Hz) does not sound like a lot. But in octaves, it is a whole other octave,” he said. Intractable Problem and Big Payoff Sell a Seismic Acquisition Innovation The Mad Dog field was a major discovery for BP in the late 1990s, but developing it looked like a high-risk proposition. The problem was the field was an early discovery beneath the thick layer of salt covering much of the deepwater US Gulf of Mexico. Shooting seismic through the salt resulted in murky images that increased the risk of development wells that produced little or nothing, which could turn a 4-billion-bbl field into a marginal investment. BP needed to solve the problem because it had leased a large number of deepwater blocks where the industry was drilling a growing number of dry holes due to poor subsurface imaging. “It was pretty common knowledge in the industry that you have this problem. But what are you going to do about it?” said John Etgen, a distinguished advisor in seismic imaging at BP. The problem is salt distorts sound waves as they travel down and are reflected back up. The salt is like broken eyeglasses. It acts as “big, very complex lens distorting the sound waves.”