It is just about impossible to talk about the identification of pore networks in mudstones and shales, not to mention the possible energy within, without talking about hydraulic fracturing.
And you can’t talk about that without talking about politics.
Which is exactly why one of this year’s AAPG Robert R. Berg Outstanding Research Award winners won’t talk about it.
To be fair, he doesn’t have to.
“Understanding pore systems in mud rocks and shale is a scientific question, not a political question.”
What comes next, he seems to be saying, is what comes next.
“The [fracturing] is necessary to connect and drain these nanopore networks,” he said.
So, with that in mind, he wants to talk about what comes first.
Robert G. Loucks, a senior research scientist at the Bureau of Economic Geology, University of Texas at Austin, said that there were two aspects of these small pores that first caught scientists’ attention:
♦ Their size – Most average only 100 nanometers (a nanometer is one billionth of a meter).
♦ Their location – Loucks said organic matter pores are formed during thermal maturation and are not seen in immature organic material.
“These organic matter pore types were a complete surprise to the geologic community when we started this research in 2006,” Loucks said, and here he credits fellow BEG researchers Robert Reed and Steve Ruppel, and Dan Jarvie with EOG Resources in Humble, Texas. All are AAPG members.
“Producing shale gas was still in its early stages and pore systems in these tight rocks were unknown,” Loucks said. “Many people thought all production and storage was from microfractures.”
He said his team’s initial research discovered that in the Barnett Shale, in the example found there, the major types were pores associated with organic matter.
“We showed that the visible pores using a scanning electron microscope (SEM) were in the nanometer range.”
And that was exciting, especially considering the same could probably be said about other sites in the state.
“We later showed that there were many more pore types in shale systems such as the Eagle Ford in south Texas,” he said.
It was there, he and his team found the reservoirs contained common nanometer to micrometer mineral pores, which are the pores between mineral grains and within mineral grains. They then published a mudrock pore classification in the AAPG 2012 BULLETIN, summarizing their research.
Questions … And More Questions
Loucks, who also has previously won the Wallace E. Pratt and A.I. Levorsen Memorial awards as author of award-winning works, said he began wondering about these networks of nanopores after a simple request and subsequent finding.
“I had a service company do some conventional core analysis on Barnett Shale samples,” he said. “They ran a few NMR samples, which showed a distribution of pore sizes.”
And that’s when he started wondering.
“When I got the results, there were no measurements of pore sizes provided.”
He then asked what those pore sizes were, and was told that nobody knew because of the extreme small sizes of the pores.
“So I asked if anyone had looked for them using the SEM,” Loucks continued. “They said they didn’t think so.”
Didn’t think so?
And then he got really curious.
♦ Step One:
“Therefore, wanting to know the size and origin of the pores in mud rocks, I got together with Rob Reed and we prepared samples for the SEM,” Loucks said. “The initial samples were rock chips, but we found that we could not clearly differentiate pores because of the amount of plucking of small grains.”
A setback.
“The pores we at first thought were real were actually artifacts of sample preparation.”
♦ Step Two:
“We then tried polished thin sections, but they also didn’t work because of too much relief on the surface related to differential hardness,” he said.
Ok, another setback.
♦ Step Three:
“At that point we heard from (AAPG member) Kitty Milliken (BEG senior research scientist and former AAPG Distinguished Lecturer) about Ar-ion milling, a technique used in the computer industry to prepare very flat surfaces,” Loucks said. “We decided to try this technique, but the machines to do this type of sample preparation cost approximately $80,000.”
Not just another setback, but an expensive one – one that could have ended the search.
But then some luck.
“We were lucky that the companies who made these millers were willing to prepare test samples for us,” Loucks said, “and it is with these samples that we first saw the organic matter pores in the Barnett Shale.”
Without hesitation, Loucks will tell you this was the ballgame – a new ballgame.
“It was the Ar-ion milling sample preparation technique,” he said, "that allowed for our major breakthrough."
The Key: Pore Networks
Looking back, Loucks said it was the mystery of these pores that drove his team.
“We wanted to know what the pore networks were in shale-gas and shale-oil systems,” he said. “These pore networks are the basis for creating permeability pathways for gas and oil out of the mineral matrix into the induced fractures.”
And here’s the money shot.
“These pores are the storage for much of the oil and gas in mudrocks and shales.”
And where the work begins.
“I have worked on carbonate and sandstone reservoirs for years,” he said, “and a basic understanding of pore networks is an essential element for knowing how to approach these reservoirs relative to exploration and production.”
These pores in shales and mudstones, all agree, is the where future economic reservoirs will come.
“I think we have established the basic pore networks in mudrocks and shales,” he said, “and many researchers are now perusing this work in more detail.”
Loucks also said there needs to be more work on quantifying how different pore types affect permeability, how different types of organic matter pores evolve, and how migrated bitumen also produces pores and an extended permeability system.
Mostly, though, he said the key will be to “ … predict ahead of the drill bit what facies will have the best porosity and permeability.”
Looking back, Loucks was right in suggesting there was potential in something that, literally, could not be seen – even after numerous attempts.
“I just didn’t think answering this question would be as hard as it was.”