Unconventional Rock Defies Old Rules, but New Rules are Far From Being Ready
Authors: Rassenfoss Stephen 
Published: August 2016

Conference review - 2015 Unconventional Resources Technology Conference

The label unconventional oil and gas stubbornly hangs on because these formations cannot be understood using the rules of conventional petroleum engineering.

To illustrate what makes it so different, Erdal Ozkan, a professor of petroleum engineering at Colorado School of Mines, pointed out that shale is often so tight that “you do not want to count the zeros” needed to measure the infinitesimally small permeability rate, and the pressure needed to force oil out sounds unattainably high.

The obvious conclusion is “you cannot produce any of this. How could you with this permeability? What are you doing, day dreaming? We are actually producing from these formations,” he said.

Ozkan used that observation to introduce a session at the 2015 Unconventional Resources Technology Conference in San Antonio, Texas, on the need to think “outside the conventional reservoir engineering perceptions.” That is a starting point to getting significantly more out of unconventional formations.

The four panel members focused on the need for a better understanding of the physics and chemistry of nanoscale pore spaces. Among their concerns is the major impact that miniscule pore throats can have on how much oil and gas is produced and what sort of hydrocarbons come out. These passages perform like a membrane, allowing natural gas liquids and gas dissolved in oil to get out, while trapping heavier oil molecules and perhaps free natural gas adhering on the pore walls, he said.

Beyond 10%

The message was that years of research will be needed to understand the rock and find ways to extract the oil and gas trapped inside. Research laboratories are still seeking ways to simulate reservoir conditions in ways that can be observed using delicate laboratory equipment. Low oil prices have apparently pushed thoughts of enhanced oil recovery (EOR) out of the minds of oil industry decision makers. Sustaining high levels of unconventional oil production in future decades will require producers to push ultimate recoveries past 10% of the oil in these formations.

“You better start working on this pretty soon because it will take a while to figure this out,” said Paul Leonard, manager of reservoir technology for Pioneer Natural Resources, before asking if production could be enhanced using chemicals.

The answer was that EOR using chemicals is being tested in research laboratories with some success. But what is known has to be adapted to formations that are poorly described by the current vocabulary for rock properties.

A few surfactants have proved promising in tests, said Mohammad Piri, a distinguished associate professor of petroleum engineering at the University of Wyoming.

Those tests, which will be detailed in an upcoming paper, are an outgrowth of fundamental work on whether shale is prone to attract water or oil, which would mean it is either water-wet or oil-wet. The answer has been both. When a tiny sample, which has been cleaned using air, is exposed to brine and then oil, the salty water adheres to certain sections of the rock and the oil to others.

For Piri, the challenge has been to find a chemical that allows the rock to release the oil in the oil-wet area, without changing the valuable water-wet properties. While he said there have been some promising surfactants identified, it is early work.