Mention porosity, and it undoubtedly conjures up thoughts about holes, or pores, contained within rocks.
In turn, this tends to trigger visions of empty spaces, for the uninitiated.
Au contraire.
Pores, in fact, are never voids. They are always occupied.
This content varies and often includes desirable substances awaiting recovery, such as petroleum liquids, gas and water. They also can be filled with material for storage, such as porous rock accumulations that provide a repository for carbon dioxide.
It’s fortunate, particularly for the petroleum industry, that this whole arena captivates the attention of skilled researchers like AAPG member Kitty Milliken, senior research scientist at the Bureau of Economic Geology, University of Texas at Austin.
Milliken, who received the AAPG Robert R. Berg Outstanding Research Award in 2015 has studied fine-grained rocks and the pores therein since the late 1970s.
“My focus is not on pores so much as it is fine-grained rocks, and pores are a part of the rock,” Milliken said. “If I’m going to understand this class of rocks, then understanding pores is part of it.”
Right Tool for the Right Pores
The variety of material that can fill the pores calls for measurement techniques that are sensitive to the particular fill.
These techniques entail putting material into pores, such as epoxy, mercury, nitrogen gas and argon gas. The ensuing measurements reflect the pores into which a certain material is able to go. There may be other pores in the rock that are detectable by a different technique.
“It’s important to remember that the total pore system is something we only partially sense with any one technique,” Milliken said. “My colleagues and I try various techniques on the same sample, working in a collaborative manner.”
She noted that when measuring and viewing the pores in a subsurface sample, one is seeing only the final state, which is a consequence of a long process.
“If we’re going to predict the outcome of that process, we need to understand the historical process by which we arrived at that final state,” she emphasized.
Today, there are new methods that are transforming geoscientists’ ability to make predictions.
One of these is a sample preparation technique called argon-ion milling, or Ar-ion cross section polishing, which has become the industry standard for imaging pores in fine grained rocks, according to Milliken. It allows the scientists to look at really tiny pores without some of the ambiguities created by mechanical polishing.
Another technique is X-ray mapping.
“This is really allowing us to look at the components of shale at sufficient resolution that we can begin to assess how they’re impacting the porosity,” Milliken said.
“Although fine-grained rocks have components that are really small compared to those in sandstone, you still see the same range of component parts to the rock that are subject to many of the same processes that cause pore evolution in sandstones,” she noted.
“We can demonstrate that similar processes impact the pore evolution in these fine-grained rocks. In other words, things like compaction, cementation, development of secondary pores all happen in both sandstones and fine-grained sedimentary rocks,” Milliken pointed out.
She regards X-ray mapping as a key to turning all of this into something predictive.
“It’s wonderful to be able to see pores, describe them and classify them and understand what they look like,” she said. “But ultimately, what we really want to do is predict them, and to do that we have to integrate our understanding of the pores with the material around the pores.
“We have to understand the origin of the pore walls, and to do that we have to look at imaging that allows us to see the composition of the rock – and x-ray mapping is key to that,” Milliken declared.
It’s All About Prediction
When a team of petroleum geologists is assigned to a basin, they must decide where to drill. To do so, they need some sort of conceptual basis on which to determine where it’s preferable to site the borehole.
Milliken weighed in on that.
“Trying to link our understanding of pores to understanding of the composition of the rock is geared toward giving people this kind of exploration model they can take and apply to make better drilling decisions, which are predictions,” she noted. “That’s the most important point.
“We’re not there yet, but we’ve come a long way,” she noted. “To get there it’s this combo of being able to see not just the pores but the minerals around the pores and to understand their history.
“That’s what’s going to yield a predictive understanding of where you do and you don’t get pores in rocks,” Milliken emphasized.
It’s a given that the operators recognize that unconventionals have “sweet spots” where the best production can be anticipated.
But there’s no such thing as a divining rod to zero-in on them.
“The question is how one goes about predicting where those are going to be,” Milliken noted. “I think the big element of gaining that predictive power depends on understanding how grain composition affects pore evolution.”