More 1IWRP highlights

As I reported on Wednesday, I've been at 1IWRP, a workshop on rock physics in the petroleum industry. Topics ranged from lab core studies to 3D digital scanners, and from seismic attenuation and dispersion to shales and anisotropy. Rock physics truly crosses a lot of subject areas.

Here are a few of the many great talks that really stood out for me:

Mark Chapman from the University of Edinburgh, submitted a new formulation for frequency dependant AVO analysis. He suggested that if a proper rock physics model of the rock is described, frequency can be decomposed from seismic gathers for improved reservoir characterization. Some folks in the crowd warned that the utility of this work might be limited to select cases with a full band impedance change, but his method appears to be a step beyond the traditional AVO workflow.

Arthur Cheng from Halliburton talked about modeling techniques to estimate anisotropic parameters from borehole measurements. He descibed the state of the art in acoustic logging tools, and used a ray-tracing VSP forward model to show a significant smear of reflection points through an anisotropic earth layer. He touched on the importance of close interaction between service companies and end users, especially those working in complex environments. In particular: service companies have a good understanding of data precision and accuracy, but it's usually not adequately transfered to the interpreter.

Colin Sayers from Schlumberger presented several talks, but I really enjoyed what he had to say about sonic and seismic anisotropy and how it is relevant to characterizing shale gas reservoirs. Fracture propagation depends on the 3D stress state in the rock: hard to capture with a 1D earth model. He showed an example of how hydraulic fracture behaviour could be more accurately predicted by incorporating anisotropic stress dependant elastic properties. I hope this insight permeates throughout the engineering community. 

Rob Lander from Geocosm showed some fresh-out-of-the-oven simulations of coupled diagenesis and rock physics models for predicting reservoir properties away from wells. His company's workflow has a basis in petrography, integrating cathodluminescence microscopy and diagenetic modeling. Really inspiring and integrated stuff. I submit to you that this presentation would be equally enjoyed at a meeting of AAPG, SPE, SPWLA, SEG, or SCA — that's not something that you can say about every talk. 

← Every break heralded a new discussion. The delegates were very actively engaged. 

Today, I am going on a field trip to the Niobrara Shale Quarry. After four days indoors, I'm looking forward to getting outside and hammering some rocks! 

Shattering shale

In shale gas exploration, one of the most slippery attributes we are interested in is fracability. The problem is that the rocks we study have different compositions and burial histories, so it's hard to pin down the relative roles of intrinsic rock properties and extrinsic stress states. Glass could be considered an end member for brittleness, and it has fairly uniform elastic parameters and bulk composition (it's amorphous silica). Perhaps we can learn something about the role of stresses by looking more closely at how glass fractures. 

The mechanics of glass can be characterized by two aspects: how it's made, and how it breaks.

Annealed glass is made by pouring molten glass onto a thin sheet of tin. Upon contact, the tin melts allowing for two perfectly smooth and parallel surfaces. The glass is cooled slowly so that stress irregularities dissipate evenly throughout, reducing local weak points. This is ordinary glass, as you might find in a mirror.

Tempered glass is made by heating annealed glass to near its softening point, about 720˚C, and then quickly cooling it by quenching with air jets. The exterior surface shrinks, freezing it into compression, while the soft interior of the glass gets pulled out by tensional forces as it freezes (diagram). 

How glass is made is directly linked to how it breaks. Annealed glass is weaker, and breaks into sparse splinters. The surface of tempered glass is stronger, and when it breaks, it breaks catastrophically; the interior tensional energy releases cracks from the inside out.

A piece of tempered glass is 4-6 times stronger than a piece of annealed glass with the same elastic properties, composition, density and dimensions. This means it looks almost identical but requires much more stress to break. Visually and empirically, it is not easy to tell the difference between annealed and tempered glass. But when you break it, the difference is obvious. So here, for two very brittle materials, with all else being equal, the stress state plays the dominant role in determining the mode of failure.

Because natural permeability is so low in fine grained rocks, production companies induce artificial fractures to connect flow pathways to the wellbore. The more surface area exposed, the more methane will be liberated.

If we are trying to fracture-stimulate shale to get at the molecules trapped inside, we would clearly prefer shale that shatters like tempered glass. The big question is: how do we explore for shale like this?

One approach is to isolate parameters such as natural fractures, anisotropy, pore pressure, composition, and organic content and study their independent effects. In upcoming posts, we'll explore the tools and techniques for measuring these parameters across scale space for characterizing fracability. 

Update on Tuesday, April 19, 2011 at 12:56 PM by Matt Hall

You might also like some of our other recent posts about shale gas:

• What is shale?, a look at how to define shale
• Shale vs tight, a comparison with tight gas plays
• What is unconventional?, a look at how to define unconventional resources