Time-lapse seismic is one of the great success stories in the geophysical industry. From dubious roots in the 1980s to widespread adoption today, the technology gives oil companies repeat seismic pictures of the subsurface to help them make decisions about the development of their fields.
The issue with time-lapse seismic is that by its very definition it needs to jump the not insubstantial hurdle between geophysicists and reservoir engineers. Geophysicists mostly understand what the data are telling them but aren’t used to working within the time scales that engineers need to make timely decisions. But more frequent surveys are an expensive proposition.
To overcome some of these issues, Heriot Watt University in Edinburgh has, for the past 15 years, run a consortium devoted to time-lapse seismic, not within its geophysics department but within its petroleum engineering department. The consortium is gearing up for its sixth three-year phase, which will kick off in June. According to Colin MacBeth, professor of reservoir geophysics, this phase will involve about 12 Ph.D. candidates as well as a few research assistants. Their goal is “to balance our existing strengths established over many years of experience with the desire to inject new ones into the mix to ensure that studies remain fresh and vibrant,” according to the project summary.
What’s been established
MacBeth said that time-lapse seismic had a rocky start in the 1980s. The concept, also known as 4-D, “failed on land before it was successful in marine,” he said. “In the late ’80s they started working on it and concluded that it wasn’t going to work. The technology hadn’t developed enough.”
The procedure was more successful in the marine environment, but due to the cost of marine surveys it was only feasible to do repeat surveys every few years. The marine environment also suffered from the repeatability issue. For a time-lapse survey to be successful, both sources and receivers need to be as close as possible to the same location as the baseline survey. With towed streamers this is quite difficult.
“Technically there are huge challenges,” he said. “It’s like trying to fly a spaceship to Pluto.”
Over time these challenges have been addressed. On the acquisition side, the use of permanent sensors or recoverable nodes has gained tremendous traction, and several fields in the North Sea have permanently entrenched systems. This greatly increases the repeatability and also reduces the cost of repeat surveys, meaning they can be done more frequently.
Permanent sensors also are bringing the technology back to land, MacBeth said. The greater frequency of the surveys means that time-lapse has moved from “snapshots taken once every several years” to almost real-time monitoring. This, in turn, is providing exciting information about the reservoir.
“In waterflooding you can see where the reservoir has not flooded and see the blank patches so that you can go back and target them,” he said. “There are some great examples, and the industry has found a lot of bypassed oil. Sometimes they’ve changed the trajectory of the well to avoid excess water. We’ve made some nice strides to improve recovery efficiency.”
What’s to come
But even relatively mature technologies can use some tweaking, and time-lapse has plenty of hurdles yet to overcome. MacBeth’s project relies on data donated from its sponsoring companies, so it has no control over the acquisition method that was employed. “Our aim is to look at the interpretation side,” he said. “We’re involved with reservoir simulation and how to understand the seismic at a quantitative level. We’re doing more with seismic than was ever thought possible.”
The project’s model is broken down into seismic analysis, which includes seismic-to-simulation and simulation-to-seismic studies, reservoir shales, and petro-elastic models; dynamic properties including pressure and saturation estimations, well-to-seismic techniques and geomechanics; and model updating, which includes accurate seismic history-matching, well performance and model paradigms. All of these facets are interrelated and lead to the 4-D seismic signature.
By using real data the team hopes to overcome some of the current limitations in time-lapse studies. One of these is pressure. “People don’t understand how pressure manifests itself in the seismic data,” MacBeth said. “It’s unlike saturation, which you can see quite well. If we could track the pressure evolution, it would tell us a lot about the reservoir.”
History-matching is also a challenge, he said. “We’d like to put the output of the simulation into the seismic response,” he said. “Petro-elastic models are calibrated but not well understood. There is a lot of debate over their effectiveness. We’re trying to work our way around it.”
There are some things that time-lapse can’t do well enough yet, he said, like monitoring gas reservoirs with complex overburdens. “In a gas reservoir that doesn’t have strong aquifer response, when you’re looking for a pressure signal, you could be struggling to interpret those data,” he said. “This is especially true in older rocks.”
Stacked reservoirs are another issue due to the constraints of seismic’s vertical resolution. MacBeth said tracer data can show trends, and advances in linking injector/producer pairs with 4-D seismic is “appealing to the engineering community.”
Combining communities
Despite the fact that the project is housed in the petroleum engineering department, about 75% of the people who attend the meetings are geophysicists. MacBeth would like to see more engineers participate.
“We do still get people who are more dominantly seismic, although the engineers are starting to attend. However, all now recognize that how you deal with seismic data depends on engineering. Gas, pressure or water in the reservoir influences how you process and subsequently interpret your data. The post-stack 4-D response may or may not reflect that accurately.
“We cannot afford to ignore this.”
While some degree of polarization of disciplines still remains, he said that most companies today are looking for people with more of an integrated mentality. “My Ph.D. students get snapped up because of that,” he said. “Our industry needs people who are able to understand and talk across the different disciplines.”
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