Shales and technology

Day three of the SEG IQ Earth Forum had more organizational diversity than the previous two days. The morning session was on seismic for unconventional plays. This afternoon was for showcasing novel implementations of seismic attributes.

Resource shale plays aren’t as wildly economic as people think. This is not only because geology is complex and heterogeneous, but also because drilling and completions processes aren't constant either. Robin Pearson from Anadarko presented a wonderful shale gas science experiment: three systematic field tests designed to target key uncertainties:

  • List all of your uncertainties and come up with a single test for evaluating each, holding all other variables constant.
  • Make sure you collect enough data so that results are statistically valid.
  • Make your experiment scalable — 10 measurements must be extrapolatable to influence hundreds.

To better understand production heterogeniety, they drilled and fracked three wells in exactly the same way. Logging and microseismic surface monitoring showed a tight limestone zone that was liberating gas from a strike slip fault, previously unseen. 

The best talk for interpreters so far was from Randy Pepper, who has done basin-scale modeling to define the erosional and non-depositional periods of geologic history not captured in the rock record. He used Wheeler diagrams to transform between two different representations of the same data, so that interpreters could work interactively between the structural and stratigraphic domains. It reminded me of dGB's Horizon Cube technology, allowing interpreters to explore between the mappable horizons in their data. Next step: allowing interpreters to perturb structural restorations on the fly. 

If you showed a seismic amplitude map to a radiologist, they might form completely rational arguments for arteries and other anatomical structures. Interpreters sometimes see what they want to see, which can be a problem. My favorite talk so far was from Jonathan Henderson from ffa. He is dedicated to keeping art and expertise in the interpretation process. His company has developed software for building data-guided geobodies with an organic and intuitive design. Automatic data classification can only go so far in elucidating what the human brain can perceive. Read his article.

I repeat his principles here:

  • Understanding the imaged geology: the art of interpretation,
  • Measurements and Uncertainty: a need for science
  • Adaptive Geobodies: combining art and science.

Kudos to John for ending the talk with a video demo of the software in action. Gasps from the crowd were a plenty. I'm hoping for more of this tomorrow!

What is shale?

Until four or five years ago, it was enough just to know that shale is that dark grey stuff in between the sands. Being overly fascinated with shale was regarded as a little, well, unconventional. To be sure, seals and source rocks were interesting and sometimes critical, but always took a back seat to reservoir characterization.

Well, now the shale is the reservoir. So how do we characterize shale? We might start by asking: what is shale, really? Is it enough to say, "I don't know, but I know it when I see it"? No: sometimes you need to know what to call something, because it affects how it is perceived, explored for, developed, and even regulated.

Alberta government

Section 1.020(2)(27.1) of the Oil and Gas Conservation Regulations defines shale:

a lithostratigraphic unit having less than 50% by weight organic matter, with less than 10% of the sedimentary clasts having a grain size greater than 62.5 micrometres and more than 10% of the sedimentary clasts having a grain size less than 4 micrometres.
ERCB Bulletin 2009-23

This definition seems quite strict, but it open to interpretation. 'Ten percent of the sedimentary clasts' might be a very small volumetric component of the rock, much less than 10%, if those 'clasts' are small enough. I am sure they meant to write '...10% of the bulk rock volume comprising clasts having a grain size...'.

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Shale vs tight

A couple of weeks ago, we looked at definitions of unconventional resources. Two of the most important play types are shale gas and tight gas. They are volumetrically important, technologically important, and therefore economically important. Just last week, for example, Chevron bought an unconventional gas company for over $4B.

The best-known examples of shale gas plays might be the Barnett in Texas, the Marcellus in eastern US, and the Duvernay in Alberta. Tight gas plays arguably had their hyper-popular exploration boom five or so years ago, but are still experiencing huge investment in areas where they are well-understood (and have nice reservoir properties!). Prolific examples include the Bakken of northern US and the Montney of Alberta.

So if we were to generalize, perhaps over-generalize: what's the difference between shale gas plays and tight gas plays?

Shale gas Tight gas
Grain-size Mostly mud Substantially silt or fine sand
Porosity up to 6% up to 8%
TOC up to 10% up to 7%
Permeability up to 0.001 mD up to 1 mD
Source Mostly self-sourced Mostly extra-formation
Trap None Facies and hydrodynamic
Gas Substantially adsorbed Almost all in pore space
Silica Biogenic, crypto-crystalline Detrital quartz
Brittleness From silica From carbonate cement

Over-generalization is a problem. Have I gone too far? I have tried to indicate where the average is, but there is a space in the middle which is distinctly grey: a muddy siltstone with high TOC might have many of the characteristics in both columns; the most distal facies in the Montney are like this.

Why does this matter? Broadly speaking, the plays are developed in the same way: horizontal wells and fracture stimulation. The difference is really in how you explore for them.

What is unconventional?

Subsurface science in the oil industry has gradually shifted in emphasis over the last five, maybe ten, years. In 2000, much of the work being done in our field was focused on conventional oil and gas plays. Today, it seems like most of what we do has something to do with unconventional resources. And this is set to continue. According to the American Petroleum Institute, unconventional gas production accounts for almost 50% of today's US Lower 48 production total of about 65 billion cubic feet per day, and is expected to reach 64% by 2020. In Canada, where unconventional gas is also very important, unconventional oil is at least as significant to geoscientists, especially bitumen. According to the Alberta govermnent, production from the Athabasca oil sands in 2011 will be about 2 million barrels per day.

But what does 'unconventional' mean? The short answer is "not conventional", which is more helpful than it sounds, and the long answer is "it depends who you ask". This is because where you draw the line between conventional and unconventional depends on what you care most about. To illustrate the point, here are some points of view...

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