So, Where Did All This Gas Come From Suddenly?
November 13, 2011 § Leave a comment
Few will dispute that shale gas has changed the very make up of the petroleum industry. At every twist and turn new resource estimates appear, each vastly greater than the previous. The estimate in 2008 exceeded the one from 2006 by 38%. As with all resource estimates, be they for rare earth metals or gas, disputes abound. But through all the murk is the inescapable fact: there certainly is a lot of the stuff. How could this suddenly be so? The last such momentous fossil fuel find in North America was the discovery of Alaskan oil. But a discovery out in the nether regions is understandable. In this case we were asked to believe that all this was happening literally in our backyard.
To appreciate what happened we first need to understand how oil and gas is formed and recovered. Millions of years ago marine organisms perished in layers of sediment comprising largely silt and clay. Over time additional layers were deposited and the organic matter comprising the animals and vegetation was subjected to heat and pressure. This converted the matter into immature oil known as kerogen. Further burial continued the transformation to oil and the most mature final form would be methane. By and large the only real difference between oil and gas is the size of the molecule. Methane is the smallest with just one carbon atom. One of the lightest oil components, gasoline, averages about eight carbon atoms. Diesel averages about twelve. So, although we refer to them as oil and gas, chemically they are part of a continuum. So, it is easy to understand that they could come from a single source.
The key word is source. The rock in which the oil or gas originally formed is known as source rock. The figure shows a schematic representation of the location of one such source rock. This is almost always shale, which we told you was some mixture of silt and clay and sometimes some carbonates. Conventionally, the fluid in this rock will migrate to a more porous body.
This is depicted as the sandstone shown, which is predominantly silica, an oxide of silicon. It may also be a carbonate, predominantly calcium carbonate. These two minerals are host to just about every conventional reservoir fluid in the world. The fluid (and by the way gas is a fluid, although not a liquid) migrates “updip” as shown to the upper right. This is because the hydrocarbon is less dense than the water saturated rock and essentially floats up, not unlike oily sheens on your cup of coffee.* This migration continues until stopped by a layer of rock through which fluid does not easily permeate. This is known as a seal, and more colloquially, a cap rock. Ironically this is most usually a shale, not unlike where the fluid originated. The trapped fluid is then tapped for production.
The trap is often a dome as shown in the upper left. It can also be a fault. This is when earth movements cause a portion of the formation to break away and either rise or fall relative to the mating part it just separated from. In some instances a porous fluid filled rock will now butt up against an impermeable one, and a seal is formed laterally.
In the schematic shown the yellow zone would be the sandstone, and the updip fluid shown in red now finds itself abutting an impermeable zone shown in green.
In the early days of prospecting they looked for surface topography indicative of a dome type trap below. These days sound waves reflected back produce excellent images of the subsurface.
Unconventional Gas: We have described how conventional gas, and oil for that matter, are found and produced. The current flurry of activity in shale gas is concerned with going directly to the source. This was previously considered impractical, primarily because the rock has very poor permeability, which is the ease with which fluid will flow in the rock. The permeability of shale is about a million times worse than conventional gas reservoir rock. In fact, as we observed earlier, shale acts as a seal for conventional reservoirs. The breakthrough was the use of hydraulic fracturing. Water is pumped at high pressures, causing a system of fractures. These are then propped open with some ceramic material to hold the cracks open. Without this the sheer weight of the thousands of feet of rock above would close the cracks. The propped open fractures now comprise a network of artificially induced permeability, allowing the gas to be produced. This is akin to pillars and beams used in underground mines.
The sheer ability to extract gas from source rock is now well understood as feasible. But some still doubt the magnitude of the estimated resource. Here is the explanation of why one would expect this resource to be plentiful. Consider that for a conventional reservoir to be formed one needed a confluence of two events. First there needed to be a proximal porous and permeable rock and second, a trap mechanism had to exist. So it would be easy to believe that more source rock did not have these conditions than did. In other words the probability of source rock without a release mechanism was greater than with. This is why it is reasonable to conjecture that the total resource trapped in source rock is greater than the resource that escaped into permeable trapped rock. Further adding to the potential is that this is fresh territory, relatively unexploited. Decades of exploitation have denuded conventional reserves, while the source rock remains relatively untapped.
A word on the nomenclature of resource estimation. A resource estimate indicates the quantity of estimated hydrocarbon accumulation, whether economically recoverable or not. A subset of that is a reserves estimate. Reserves are the portion of the resource that one could recover economically and bring to market. Typically in a new play one would expect reserves to keep getting revised upwards. This is because every new well put on production increases the certainty of the extent and quality of the reservoir, and the reserves can confidently be increased. In reading the popular literature it would be well to keep the distinctions in mind; they are often confused.
*Darker roasts produce more oil. One way to minimize oily sheen is to brew with cold water; also results in a “sweeter” coffee. This is analogous to “sun tea”.