Feature

The statuesque derricks that dot Postle Oil Field work relentlessly in the fierce winds that sweep across the farmlands of Oklahoma's panhandle. For half a century, the pump jacks have been plunging more than a mile deep into the ground to squeeze up the black gold that powers large parts of the mid-continent. But what they have brought up to the surface in recent years is only a shadow of what it used to be. When the wells first started operating they were producing nearly pure oil; today much of it contains water. "For every 100 barrels, you pump up 95 or more of water and only about five of oil," says Tom Davis, professor of geophysics at Mines, who earned his doctorate from the department in 1974.

The field isn't running out of oil; about two-thirds of the reservoir's original 320 million barrels are still buried underneath the nearly 100-square-mile patch, but producing them at economic rates is a tough challenge. A few years ago, oil companies started to flood parts of Postle with liquefied carbon dioxide to reduce the oil's viscosity and make it easier to squeeze out of the sandstone, a technique called enhanced oil recovery, or EOR. But making such operations successful requires detailed knowledge of the subsurface, and that's where Davis and his team from Mines come in.

Geophysics Professor Tom Davis stands by a hydraulic vibrator truck that drops 60,000 pound weights onto the ground, creating seismic waves that "light up" the subsurface.

To image the migration of carbon dioxide through an oil reservoir 6,000 feet below the surface, the gas is injected through well heads at 2,100 psi.

Once injected into a reservoir, carbon dioxide acts like a scrubbing agent. It mixes with the oil, turning it into a slippery fluid that more readily flows through rock. At the wells, operators capture the blend and recycle the gas. However, only part of the carbon dioxide dissolves in oil and returns to the surface. As much as half of it remains underground, where it dissolves in water and over time becomes part of the reservoir rock. "These two processes of enhancing recovery and storing go hand in hand," Davis says. But they need to be optimized to be successful and seismic remote sensing technologies are the key for that.

To develop accurate three-dimensional reservoir models, Davis and his team "light up" the subsurface with different seismic waves. In the same way that our brains form visual images from light reflected by surrounding objects onto our optic nerve, Davis and his team image the underground by producing seismic waves at the surface and recording the resulting series of echoes with an array of sensors. While conventional seismic techniques often measure only sound or compression waves, Davis and his team also use shear waves, which are more difficult to detect and analyze but much more useful for finding cracks and other formations that could channel fluids, he says.

At Postle the team has wired up a six-square-mile patch that is scheduled for EOR in the coming months. A mile-long string of digital sensors reaches through a defunct well bore all the way down to the reservoir and nearly 2,000 others blanket the dirt surface like a gigantic spider web. These sensors are hooked up to million-dollar instruments that monitor the reservoir like a patient in intensive care. Trucks carrying hydraulic vibrators roam the field and stop every few feet to drop 60,000-pound weights that shake the ground like a miniature earthquake, sending seismic waves deep into the surrounding strata. Listening on the surface, Davis and his team record the seismic waves that echo back up after each 60,000-pound hammer blow.

Once the data is collected, it is interpreted back on campus by staff and students of the Reservoir Characterization Project.

At Mines, Davis and his students constantly develop new and better algorithms that can translate the enormous mountains of raw seismic data into three-dimensional models. Many conventional seismic techniques leave it at that, but Davis adds a fourth dimension to the process: time. He plans to return to Postle to image the ground again several times in coming years to check up on the carbon dioxide flow after the EOR process has started.

The four-dimensional maps he can create through this process are invaluable for oil companies, such as Postle operator Whiting Petroleum Corporation, who would work completely in the dark without them. "Usually, operators are at the mercy of what the reservoir dictates in terms of what comes out of the producing well," says Scott Wehner, Operations and Engineering Manager at Whiting Petroleum and member of RCP's Advisory Board. "But the seismic sensing technology basically gives us an X-ray of the reservoir and we can see into it at different points in time." If the carbon dioxide is not moving the way they want, operators have the ability to be proactive and make changes to influence pressure and flow direction to help push the plume back on track. The maps also reveal blockages that can trap carbon dioxide, Davis says. "You need to see where such barriers are so that you can inject carbon dioxide in the most effective location." The ground underneath Postle consists of a channel system made of ancient riverbeds. Once operators have a model of this labyrinth, they can align wells to ensure that they actually inject into the channels. And there is always the exploration aspect, Davis says. "We may find another, deeper oil reservoir or outlying pockets of this reservoir that haven't been discovered yet."

The remote sensing technology that Davis employs has enormous advantages over old-fashioned drilling and sampling operations used in the past. Not only do Davis and his team leave a minimal footprint on the field, consisting of no more than a few truck tracks, but their technique also minimizes safety concerns. And in fields where liquefied carbon dioxide is forced through rock at 2100 psi, things can and do go wrong. Some oil fields still contain defunct open well bores that no one knows about because operators cut the wellheads off years ago and covered them up, Davis says. It's important to find these zones by mapping the underground because if there is a leakage, tons of the asphyxiating gas can rapidly escape into the atmosphere.

Davis' team has used the technology in numerous oil fields, including the Weyburn field in Canada, where it played an integral part in the International Energy Agency (IEA) Greenhouse Gas Programme's Carbon Dioxide Monitoring & Storage Project. One of the project's objectives was to test various carbon dioxide tracking methods, including Davis' four-dimensional seismic remote sensing technology. "Four-D seismic is very good at measuring small changes in concentration, which means that it's very good at following the front of the carbon dioxide plume," says Brendan Beck of the IEA. And because seismic sensing can be done remotely, it will likely become the predominant monitoring technology once a formation has been filled and closed.

Since Davis started the RCP some 23 years ago, he and hundreds of students worked in numerous locations in North America and Canada, including the Permian Basin—America's energy hotbed—that spreads from Texas into New Mexico. But when Whiting Petroleum requested assistance at Postle, RCP took on their greatest challenge to date—the reservoir is much thinner and lies at greater depth than anything the RCP has encountered so far. "We are trying to make this technology work for thinner and thinner bed forms to give it a much more widespread application," Davis says. "The Weyburn Field, with a thickness of about 100 feet, was a breakover point because we could see this thin reservoir and see the carbon dioxide move even at a depth of close to a mile." Postle is no more than 60 feet thick and lies even deeper, at more than 6,000 feet. And the carbon dioxide flooding projects that the RCP is investigating at Postle are located at the field's outer limits. That's a particularly important area for sequestration, Wehner says. "The question is ‘Can we see the outer channels nice and crisp, and can we see whether the carbon dioxide will leak into the surrounding area?'"

Questions like these need to be answered before large-scale storage can become reality. But if validated, sequestration in geologic formations could be an effective way to store carbon dioxide captured in power generation operations. Even though sequestering carbon dioxide in depleted oil and gas fields is a finite solution—they
would eventually all fill up—these formations could provide an excellent starting point. Wehner points out that the common understanding is we will eventually have to store it in saline aquifers as well. But sequestration in oil reservoirs in conjunction with EOR can help develop the network of pipelines that is needed to move large volumes of carbon dioxide around, because using the gas for EOR generates revenue. "That means that there is a way to pay for the required infrastructure," Wehner says. "Once you move to sequestration into a saline aquifer, there is no offset, it's just full cost unless a viable carbon credit market develops."

Ironically, as scientists and engineers around the world deliberate on how to get rid of carbon dioxide, the 74 EOR projects in the United States are using 33 megatons each year, most of it mined from natural underground reserves. And the price of carbon dioxide is going up all the time because natural sources can't satisfy demand from EOR operations. "There are existing [EOR] projects that cannot expand because the supply of natural carbon dioxide is limited or constrained," says Michelle Michot Foss MS '85, chief energy economist and head of the center for energy economics at the University of Texas at Austin. Part of the problem is that pipelines built in the early 1980s are not large enough to handle the demand there is today. That's why many operators are looking at the economics of capturing carbon dioxide from industrial sources. But "this is extremely complex—a very large number of conditions have to be satisfied for captured carbon dioxide to work and for the captured carbon dioxide-EOR link to be commercially feasible," Foss says. Collecting the stream of gases that power plants spew into the atmosphere and separating carbon dioxide from it can drive up the price well above economic thresholds, Davis adds.

The carbon dioxide that is currently flowing through Postle comes from the Bravo Dome, a large natural reserve some 127 miles down the pipeline in northern New Mexico. However, a smattering of ethanol plants that are currently being built around the Postle area could, in principle, provide an industrial source in the future. But the process of capturing and liquefying the carbon dioxide exhaust from such ethanol plants is still cost-prohibitive, Wehner says. Technologies such as coal gasification utilized in a handful of power plants fare slightly better. These vent carbon dioxide at higher pressures, making it more attractive for EOR customers who need carbon dioxide in its liquid form. One of the first EOR users to tap into such manmade sources of the gas was Weyburn Field operator EnCana. "In this particular case, natural carbon dioxide sources were remote—as far away as Wyoming," Davis says. That made it economically attractive for EnCana to purchase from the Great Plains Synfuel Plant, a coal gasification facility near Beulah, ND.

The largest hindrance to large-scale adoption of such schemes is the lack of incentives, such as tax breaks to capture and store carbon dioxide, Beck says. "Reducing the costs, getting the incentives and working out a regulatory regime for storing carbon dioxide which governments are currently working on, are the biggest issues right now."

The potential could be enormous. Over the past five decades, Postle has undergone a sequence of recovery operations typical for North American oil fields. Operators first produced the wells naturally for a few years and once recovery declined they began to inject water, a process called secondary recovery, that pushed the oil to the wells. But because water doesn't mix with oil, it tends to bypass much of it. "Primary and secondary recovery operations typically only get a total of 25 to 30 percent of the oil out." The remainder, sometimes called attic oil, won't come out voluntarily and diluting it with carbon dioxide is sometimes the only way to mobilize it. As oil fields around the world are aging, even some of the world's largest producers, such as Saudi Arabia, have begun to talk about carbon dioxide injection. "We are definitely going to see more and more EOR projects in the future," Davis says, adding that the process can increase the recovery of unconventional oil and gas resources as well. Seismic remote sensing will play an important role in all of these projects. "I think we have a very bright future ahead of us by doing this kind of work and we are excited to be involved at the forefront."