SAN FRANCISCO, July 17 (Xinhua) -- A new software tool has helped researchers reveal that as some of the world's largest oilfields age, the energy required to keep them operating can rise dramatically even as the amount of petroleum they produce drops.
The findings, published in the journal Nature Climate Change, have implications for long-term emissions and climate modeling, as well as climate policy.
"Current climate and energy system models typically don't explore the impacts of oil reservoir depletion in any detail," said study co-author Adam Brandt, an assistant professor of energy resources engineering at Stanford University's School of Earth, Energy and Environmental Sciences.
"As oilfields run low, emissions per unit of oil increase. This should be accounted for in future modeling efforts."
Stanford postdoctoral researcher Mohammad Masnadi worked with Brandt to apply a new software tool developed at Stanford for calculating greenhouse gas emissions to oilfields around the world that have produced more than 1 billion barrels of oil over their lifetimes, sometimes called "super-giant" oilfields.
Developed in Brandt's lab, the software tool is called the Oil Production Greenhouse Gas Emissions Estimator (OPGEE).
For any given oilfield, OPGEE performs what's known as a lifecycle assessment, analyzing each phase of the oil production process, namely extraction, refinement and transportation. It then uses computer models to calculate how much energy is consumed during each step. From this, researchers can calculate precisely how much greenhouse gas each oilfield emits.
In comparison, conventional greenhouse gas estimates calculate emissions through a kind of economic reverse engineering, whereby an economic index is used to convert the monetary value of an oilfield's final products - whether it be processed oil, natural gas or petroleum-based products - into greenhouse gas emissions.
"This top-down approach for converting economic values into environmental and energetic costs misses a lot of underlying information," Masnadi said.
The new software tool does what he called "bottom-up type of analysis," which has not been done before because it is difficult. "For this study, we needed over 50 different pieces of data for each oilfield for each year. When you're trying to analyze an oilfield across decades, that's a lot of data."
However, most oil companies are reluctant to release this type of temporal data about their oilfields.
The two Stanford researchers developed two workarounds to this problem. First, they gathered data from places where transparency laws require oil production data be made publically available. These included Canada, Norway and Britain, and the state of California in the United States.
Secondly, the pair conducted a deep data mine of the scientific literature to seek out clues about oilfield production levels in published studies.
They ended up with data going back decades for 25 globally important super-giant oilfields.
Applying OPGEE to this group of super-giant oilfields, the researchers found that for many of the oilfields, oil production declined with time as the wells were depleted, but the energy expended to capture the remaining oil went up.
"The more oil that is extracted, the more difficult it becomes to extract the oil that remains, so companies have to resort to increasingly energy-intensive recovery methods, such as water, steam or gas flooding," Masnadi said.
Making matters worse, oil recovered through such methods has to undergo more intense surface processing to filter out the excess water and gas. In the latter case, this can result in an excess of carbon dioxide and methane gas that is typically eliminated through burning - a process called "flaring" - or venting into the atmosphere.
"We can show with these results that a typical large oilfield will have a doubling of emissions per barrel of oil over a 25-year operating period," Brandt was quoted as saying in a news release from Stanford.
How to stop this cycle? "Better regulation is certainly part of the answer, but a more progressive solution is to encourage energy companies to draw the energy they need to operate their aging oilfields from renewable sources such as solar, wind or geothermal," Masnadi said, citing the California-based company GlassPoint Solar, which uses solar-powered steam generators to reduce the gas consumption and carbon emissions of its oilfields by up to 80 percent.
The OPGEE tool has already been adopted by the California Air Resources Board to help reduce greenhouse gas emissions from transport fuels.