Shales: How About Just the Rock?

Geophysicist offers perspectives on shale properties and behavior

Speaking before delegates at ShaleTech 2010 in Vienna, Austria, Rune Holt, Professor at the Norwegian University of Science and Technology admitted that he knew nothing about shale gas.

“I got a call to come and speak and said ‘I don’t know anything about shale gas, but I do know something about shales,’” he recalled, noting his institute’s 25 years of lab studies of shale properties. “So I hope you pick up something from this as well.”

Professor Holt’s speech was entitled Shale Properties & Shale Behavior in which he pledged to speak about borehole stability and the response of overburden seismics; he said there was some overlap between the two.

“Borehole stability drilling, hole collapse in overburden, interbedded reservoir shale is a main reason for tight hole/stuck pipe incidents,” he said. “In 4D seismics, time shifts caused by stress changed in the overburden can be seen as fingerprints of depletion within the reservoir.”

Holt presented a graph to the audience, which showed time shifts for identifying depleting pockets in the reservoir.

To answer the question “what is a shale?” he turned to Wikipedia, which called it a fine-grained sedimentary rock.

“I translated this into rock mechanics  - it should also have a constitutive load bearing framework. On the other hand, shales have low permeabilities (nanometer pore sizes), surface size is large, and water is absorbed on surfaces, bound inside clay platelets.”

He aimed to outline the challenges associated with shale.

“There is a problem that cores are usually not available,” he explained. “Usually cores are taken by accident and are not very well preserved. They’re usually sealed off.”

Holt continued, “Is the core representative of the rock in situ? Factors can lead to core damage by stress, pore pressure and temperature changes as well as exposure to non native fluids.”

“The degree of saturation is never known. We assume to have full water saturation, which is often required for testing.”

Professor Holt said that low permeability lead to long test durations and spoke of how core plugs must be drilled carefully.

“The mechanical behavior of shale is extremely sensitive to saturation,” he explained. “We did experiments where we tried to re-saturate a shale core and the result was negative, and led to damage. We apply stress to the core, and in that way we induce micro-cracks and believe that it is fully saturated. When we do the tests we have to make sure of that and look at strains in addition to measuring pore pressure evolution. This can be tricky if the shale is creepy.”

He showed diagrams of brittleness  which he said was controlled by stresses and stress history, porosity, fluid exposure.

“There are many definitions of brittleness,” said Holt. “There was a paper in the 1970s that looked at the different definitions and none of them gave us an end result. What is it we really mean?” he queried.

Shale characterization was another topic touched upon.

“It’s important to have a complete characterization: core testing, petrographical mineralogical analysis, total organic content, specific surface area and nuclear magnetic resonance.”

He added, “If you’ve done all of that you can start to make correlations. The lower the clay content, the more brittle the rock.”

“The strategy has been to do detailed lab measurements and then make correlations. Usually we don’t have a lot of core material, but maybe we can do something if this is fractured. We have a setup for doing things on centimeter-scale samples, or we can measure even smaller samples.”

Holt continued, “The short track is to get something that mimics the shale like a core scratch. The average value of that force gives a very nice correlation to the strength of the rock. We can get a log on over 12 centimeters of core. We can see that the shale is very heterogeneous. Another technique we’re currently experimenting with is the shale puncher: the force correlates with the cohesion of the shale.”

He also spoke of potential future developments regarding shale behavior.

“There’s technology to go much further with the imaging,” he said. “The pores are now a few nanometers large and what happens at that scale is important to the core. This gives us the option to get a 3D image of the core at the grain scale.”

Holt said that discrete element modeling used spheres as building blocks. “Spheres are clustered to create grains and we can see how bonds break in a very realistic way.”

He proceeded to speak about what a gas shale was.

“Now that I’m talking to a shale gas audience, it’s a dense, low perm rock, preferably black, that contains gas.”

Holt spoke about the geomechanical and rock physics challenges of shale gas plays.

“The reservoir rocks are essentially tight rocks, not necessarily shales, and are often heterogeneous,” he said. “For successful development one needs high gas saturation, large matrix permeability and high pore pressure in strata which are continuous, homogeneous and extensive.”

“Fracturing is necessary, fracture containment is needed. It should be easy to fracture and rock-fluid sensitivity should be low.”

Holt said his last slides were an attempt to model the fracturing process in the field. “Traditionally hyrdofracturing modeling predicts planar tensile fractures and we are not able to account for interaction with natural fractures. This technique captures branching.”

“Shale characterization pays off,” concluded Holt, explaining, “Billions have been saved on borehole stability. For shale gas we have to address anisotropy, heterogeneity, initial stresses and fracture system which are all important.”

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