Niobrara shale field trip

Mike Batzle explaining rock physics in the fieldOn my last day in Colorado, I went on a field trip to learn about the geology of the area. The main event was a trip to the Lyons Cemex quarry north of Boulder, where they mine the Niobrara formation to make cement. Interestingly, the same formation is being penetrated for oil and gas beneath the surface only a few thousand metres away. Apparently, the composition of the Niobrara is not desireable for construction or building materials, but it makes the ideal cement for drilling and completion operations. I find it almost poetic that the western-uplifted part of the formation is mined so that the eastern-deeper parts can be drilled; a geologic skin-graft, of sorts...
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News of the week

New site for seismic related work

There is a new job site for the seismic and geophysical industry called Seismic Works. Search for jobs, post jobs, advertise, or find crews, vendors, or equipment. Still very much in its early days, the site is nicely designed and very full-featured. You can also connect with SeismicWorks on Facebook. We look forward to seeing more sites and social connectors like this in our community.

SonicScope, a new real-time geophysical tool

Schlumberger has introduced a new logging-while-drilling tool called SonicScope that can be run directly behind the bit while drilling. The tool has potential to improve rock mechanics measurements and fracture identification by getting to the borehole wall immediately after penetration, minimizing the effects of washout and invasion. Real-time sonic measurements could enable pore pressure monitoring, time-to-depth information for seismic-well ties, and borehole damage assessment. We hope to see technology like this increase the relevance of rock physics. 

Geophysicist on last shuttle

Fellow geoscientist Andrew Feustel will be flying on space shuttle Endeavour's last trip into orbit. Andrew has an MSc in Geophysics from Purdue University in Indiana, USA, and a PhD in seismology from Queen's University in Ontario, Canada. He's even a fully paid-up member of the American Geophysical Union and the Society of Exploration Geophysicists. In addition to his scientific prowess, Andrew brings considerable mechanical tinkering skills, and will serve as the crew's repairman during the voyage. You can read more about him on his NASA profile page. Good luck, Andrew!

Nexen joins Marathon to explore shale gas in Poland

Canadian based Nexen Inc. will acquire a 40% working interest in ten of Marathon Oil's land agreements in Poland's Palaeozoic shale. Nexen brings experience in unconverntional drilling and completion technologies and shale gas experience from their assets in the Horn River shale in British Columbia, Canada. Europe's development of shale gas resources are complicated by population density, but activty in Poland's shale gas resources is evolving rapidly in the wake of successful shale plays in North America. Read more about the deal.

See all of our News of the week posts.

This regular news feature is for information only. We aren't connected with any of these organizations, and don't necessarily endorse their products or services. Image of Feustel is courtesy of NASA.

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. 

News of the week

Paradigm showcases new geophysical software

Paradigm will preview its latest exploration and development software technologies and workflows at the AAPG convention 9–13 April 2011. Their agenda covers workflows for multi-disciplinary subsurface teams, next generation geologic software, and a Windows 7 interpretation platform. Paradigm is one of the sponsors of the 2011 AAPG Imperial Barrel Award student propsect competition. Follow them on Twitter: @ParadigmLtd.

Ikon Science releases RockDoc 5.5

Ikon Science has teamed up with the experts at Statoil and immersed rock physics modeling templates into the software interface, allowing users do rock physics all in one place. And with a new extension, External Interface, users can add their own C and MATLAB code to RockDoc. As a MATLAB users, we find this is a very appealing step. Click here to read more.

Third beta release for OpendTect 4.2.0

dGB Earth Sciences, creators of OpendTect, the purveyors of the Open Seismic Repository, have announced their third Beta release of OpendTect 4.2.0. The roll-out of the official version 4.2.0 is due in mid-April. If you aren't using OpendTect, why not download it, and start using this software today. And while you're at it, grab some data from the Open Seismic Repository

PetroChina drills first horizontal shale gas well

China sprang into the embryonic stages of shale gas exploration and development this week when PetroChina completed the drilling of its first horizontal shale gas well in Sichuan Province. It will be exciting to watch the results China strives to access its massive shale gas resources, which up until now have been beyond its technological reach. Click here to read more.

University of Aberdeen opens seisLAB

Thanks to industry sponsors BP, Chevron, BG Group, Halliburton, and Schlumberger, the University of Aberdeen will soon be decked-out with state-of-the-art geoscience software and infrastructure. seisLAB will accelerate training, research, and teaching in one of Europe's energy capitals, pushing innovation and collaboration in the field. Click here to read more.

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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