COMMENT—EOR can extend the promise of unconventional oil and gas

With the viability of huge unconventional gas resources in shale plays confirmed, the US Geological Survey's estimate of US proved gas reserves jumped by more than 50% in 2010. Further growth is expected as natural gas prices recover. The latest estimates suggest that US natural gas reserves could meet current levels of demand for more than 100 years. Moreover, unconventional gas sources (shales, tight sands, and coalbed methane) are expected to account for more than three fourths of (much-increased) US gas production by 2035, paving the way for the nation to become a net exporter of natural gas (Fig. 1b).

Was it just a few years ago that the major concern for US energy planners had been the speed with which a fleet of LNG import facilities could be developed in North America? And that controversy had raged over whether US oil and gas production had peaked or was about to peak before collapsing into irretrievable decline?

Developing this new oil and gas potential in the US also avoids the risks that come with imports. In fact, a surge in onshore Lower 48 liquids production directly attributable to unconventional oil development was a major factor in a dramatic fall since 2005 in US dependency on imported crude oil (from about 60% to about 49%), according to EIA. North Dakota alone quintupled its oil production to more than 500,000 b/d in the past 5 years, owing solely to Bakken shale development.

The resource potential goes well beyond US borders. The International Energy Agency, acknowledging these new resources, doubled its estimate of global gas reserves life to 250 years at current consumption rates.

EOR for unconventionals

Even with that already brightening outlook for oil and gas, the marriage of EOR to these vast unconventional resources creates a new frontier for research and development that, if successful, could economically sustain the world's hydrocarbon supplies for decades.

It is easy to argue, in fact, that the marriage has already occurred, insofar as the industry loosely defines improved oil recovery (IOR), an umbrella term that includes EOR. In essence, IOR is any practice that improves oil recovery. That definition certainly encompasses horizontal drilling, and it does not seem much of a stretch to apply it to highly tailored multistage hydraulic fracturing. Advances in technology and best practices in these two areas proved the key to unlocking these giant resources.

Where the real technology frontier lies is in the adaption of EOR processes to unconventional resources. One definition of EOR held that the term should refer to reservoir processes that recover oil not produced by secondary processes that entail injectants—typically water or gas—used to repressurize a reservoir and to displace oil to producing wells. EOR processes target what is left, focusing on the rock-oil-injectant system and on the interplay of capillary and viscous forces.¹

To a certain degree, this marriage also has already occurred, with the application of thermal EOR to high-viscosity crudes that may also be thought of as unconventional resources. But targets in these emerging unconventional resource plays typically are low-porosity and low-permeability reservoirs containing crudes within an otherwise acceptable API gravity range. In many instances the target formations in these efforts are source rocks. Accordingly, what is revolutionary about the unconventional resources boom is that the industry effectively has found a way to commercially exploit source rock—a concept that might have had its proponents regarded as deranged a generation ago. As these unconventional plays begin to mature, it isn't too soon to consider which EOR technologies might be best adapted to this so-old-it's-new-again resource.

But along with the evolution of EOR science, the oil and gas industry also has made great strides in being able to characterize the subsurface—both in terms of imaging, such as with 3D seismic methods, and in terms of obtaining real-time data, such as with intelligent wells. The industry has reached new thresholds in being able to tailor EOR solutions to many different types of reservoir rock. Additionally, there is a vast trove of data and knowledge about EOR process performance parameters under a wide range of conditions that could apply to the shale and tight sands dominating US oil and gas development today.

It is not unreasonable to assume that implementing advanced EOR technologies could represent yet another step-change for this already game-changing resource.

It's equally reasonable to consider that the evolving "hydrocarbon factory" business model that has underpinned unconventional resource development in the US might also apply to the aging conventional reservoirs that have produced for decades and still—in the aggregate—constitute a significant portion of US oil and gas production via stripper wells.

In short, it doesn't have to be a shale or tight sand to qualify for the unconventional drilling business model. A good example is the Mississippi Lime play in northern Oklahoma and southern Kansas. A conventional—if somewhat complex—carbonate reservoir, this shallow formation has produced from vertical wells for more than 50 years. The introduction of the unconventional development business model is turning the Mississippi Lime into the hottest new play in North America. Shallow pay zones, massive amounts of well history data, significant infrastructure—it all sounds like a play amenable to consideration for EOR.

Research efforts

The frenetic leasing and drilling in these plays has garnered much attention (and controversy), so it's not surprising that less attention has been paid to EOR research targeting them.

But such research is, in fact, under way.

Researchers from North Dakota University and the New Mexico Institute of Mining & Technology are investigating chemical imbibitions using surfactant or brine formulations to stimulate oil recovery from the Bakken shale of North Dakota. The focus here is on surfactant formula optimization, specifically, attempting to identify a surfactant formulation that promotes imbibitions while minimizing clay swelling and formation damage.

New Mexico Tech researchers also are working on water-soluble polymers to recover oil from unconventional reservoirs on Alaska's North Slope with chemical EOR, with funding from the US Department of Energy's National Energy Technology Laboratory (NETL). The goal is to improve injectivity during polymer injection for fractures and horizontal wells without compromising sweep efficiency.

Perhaps the next logical place for a marriage of EOR and unconventional resources is the Permian basin. This stalwart of US oil and gas production—the most prolific oil basin in the nation to date—is home to the vast majority of the world's CO₂ floods. It also houses some of the hottest unconventional development activity in plays such as the Wolfberry and Avalon Shale. It is especially serendipitous that some of the major CO₂ EOR project developers in the Permian basin also have become key players in the Permian unconventional plays.

Among the CO₂ EOR research projects in the Permian Basin funded by NETL are:

• A project to improve mobility control in CO₂ EOR by using silica-polymer initiator gels.

• Research into developing engineered nanoparticle-stabilized CO₂ foams to improve volumetric sweep of CO₂ EOR processes.

• Efforts to devise novel CO₂ foam concepts and injection schemes for improving CO₂ sweep efficiency in sandstone and carbonate hydrocarbon formations (see following article).

Improving sweep and mobility control will be a major hurdle for operators seeking to introduce EOR processes into tight formations that have been extensively fractured.

Nascent effort

The effort to wed EOR to today's booming unconventional plays remains nascent. But the industry needs to take the long view.

According to DOE's NETL, the US holds more than 400 billion bbl of discovered oil that has not been tapped or pushed into the proven reserves column. More than half that total lies at depths of 5,000 ft or shallower (Fig. 2).

This enormous potential of oil alone is an irresistible target for both unconventional resource drilling and completion methods and EOR technology. Combining the approaches would change the game yet again.

Reference

1. Stosur, G., Hite, J.R., Carnahan, N.F., Miller, K., "The Alphabet Soup of IOR, EOR, and AOR: Effective Communication Requires a Definition of Terms," SPE International Improved Oil Recovery Conference in Asia-Pacific, Kuala Lumpur, Oct. 20-21, 2003.

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