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Cracking the code in tight shale

Oil moves through shale rocks, even though the physical structure of such rocks suggests it should not. That is only one of the confusing aspects of how best to exploit the shale oil plays, according to a rock mechanics expert.

Shale rocks, with their tiny structures, seem to suggest that oils and other fluids should not flow through them.

In conventional reservoirs — those composed of carbonates or sandstones, for example — oil and water flow through the porous rocks based on how much pressure is exerted on the fluid.

This flow formula, described by Henry Darcy, applies to many porous materials but would not be expected to extend to shale rocks.

Sidney Green, president and chief executive of Enhanced Production, and a research professor at University of Utah, says many will call any black rock a shale, but they are more appropriately called mud stones or mud rocks.

Green, a mechanical engineer, was a founder of the engineering company TerraTek, which was sold to Schlumberger in 2006.

Sid Photo: Enhanced Production

"Researchers will be pondering this for ten years or more, trying to understand the intricacies of how fluid mobility works in the shales."
Sidney Green, Enhanced Production

Shale rocks are made up of micron and nano-sized particles, along with clays, organic materials, hydrocarbons and water.

The fine structure of the shale rocks, with micron to nanometer pores, creates a lot of surface area for fluids such as oils to contact. The resulting surface tension, or capillary pressure, has a big effect on oil mobility, Green says.

“A couple of decades ago, everyone thought it was impossible to produce oil from these rocks. They thought the fluids couldn’t move because the conventional permeability is so low,” he says.

“The pore structures and pore throats are so small that we don’t understand how the liquids can move around in these very tight shales.”

Despite not fully understanding the basic physics of fluid flowing through shale rocks, the industry has nonetheless found a way to recover the oil, and drive up individual well production rates, with hydraulic fracturing.

However, operators are leaving as much as 90% of the oil trapped in the shale reservoirs. Solving the problem of fluid mobility in the shale rocks could lead to much better individual well production rates as well as increased overall recovery.

ShaleRock.jpg LOW PERMEABILITY: A photomicrograph of an unpolished shale surface shows the scomplex microstructure. Photo: Enhanced Production

At first, Green says, the industry thought perhaps the wells were not staying in the “sweet spots”.

“They got better at divining where the oil was in the rock in the formation, and to a large extent they solved that problem,” he says.

Next, the industry turned its attention to achieving more uniform hydraulic fractures across an ever-increasing number of lateral well stages.

Some of the fracture zones would produce a little, some might produce nothing, and some might be responsible for the bulk of the production stream.

“They used divergent agents to give a more uniform fracture,” he says.

The third problem – which the industry is now dealing with – is figuring out what in the rock is keeping hydraulic fracturing from consistently delivering the desired results.

“It must be something to do about the fracturing, and something about the rock that we don’t understand. They’re trying to understand what they can do about this,” he says.

“It’s all about creating a hydraulic fracture network in the right place that is connected and conductive.”

The industry is struggling to keep fractures from being pinched off, or not remaining propped open, and conductive for the life of the well.

“Knowing what is needed is not rocket science, but what is rocket science is how to do that. That’s what we’re struggling with,” he says.

“The drilling has had great advances but we don’t understand why we’ll fracture one well and production is high, and a well close by will only produce a quarter as much.”

m1353117.jpg DRILL, DRILL, DRILL: If companies drill enough wells, they do not have to understand fluid mobility in shale rock. They can calibrate it out, according to Enhanced Production chief executive Sidney Green. Photo: Bloomberg

The well-to-well production variation traces back to not understanding fluid mobility in the shales. Just a few years ago, the well-to-well variation might have been as much as six-fold or tenfold, he says. Now, it’s typically less than four-fold.

“But that’s still quite a lot of variation from well to well. People tend to blame the fluid mobility through the rock,” he says, but “we haven’t come together on what exactly the fracture looks like, and that’s because we don’t understand the rock.”

Researchers are working on the problem at oil industry laboratories, universities and national research centres.

“But you don’t know how the research is going to work out. I think it’ll take another decade. Researchers will be pondering this for 10 years or more, trying to understand the intricacies of how fluid mobility works in the shales,” he says.

Meanwhile, the industry keeps drilling. “If you drill enough wells, you don’t have to understand the fluid mobility. You can calibrate it out,” Green says.

“What does work is to drill, drill, drill. And, from an investor point of view, that is guaranteed to work, at least in some plays.”

In addition to drilling more shale oil wells, major advancements have driven up individual well production rates.

Several years ago, frac stages may have been placed every 400 feet, while now the stages may be every 80 feet.

The industry is also using finer sand grains for proppant – often 100 mesh sand – and far more proppant than before.

The changes are paying off. Early oil wells in shale plays produced hundreds of barrels per day of oil, he recalls. Now, a well produces thousands of barrels per day of oil. One well was recently noted as producing 6000 bpd at the outset, he says.

“We’ve seen a factor of about 10 in improvement in individual well production in the past three years. This is fantastic industry improvement, but they’ve done it the hard way,” Green says.

“Drill, drill, drill. If you drill enough wells, you don’t have to understand the rock. You don’t have to understand the fluid mobility.”

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