The ultimate goal of hydraulic fracturing, or fracking, is to increase the ability of oil and gas to escape from a rock formation by creating systems of fractures in the rock along which liquids and gases can emerge and flow.
When combined with horizontal drilling, hydraulic fracturing is a powerful tool. Horizontal drilling gives drillers access to large volumes of unconventional reservoir rock, and hydraulic fracturing allows the hydrocarbons trapped in this reservoir to enter the wellbore.
Sounds good. But how are we going to create predictable fracture systems thousands of meters underground?
To learn about the details of the process, let’s turn to one of our experts.
Transcript
Fractures – Terry Engelder – Penn State
I would like to explain what geologists and engineers mean when they talk about fracturing rock during the stimulation or fracking phase of both oil and gas production.
Although the objective is to make it easier for oil and gas to flow out of a very impermeable rock and up a well, it is not commonly understood exactly what’s happening during the stimulation, and it isn’t as straightforward as most people imagine. The media often projects this image of pressurized water going down the pipe and into the borehole and just blasting away at the rock like dynamite. That’s not what happens.
Geologically speaking, deep in the earth rock may be full of naturally occurring fractures but they are very tightly squeezed together so that nothing can move through the space between the breaks. The squeezing is a consequence of rock pressure thousands of feet under the ground.
One way to think about this is to consider the seal between a canning jar and a lid. These are three separate pieces including the rubber gasket. The pieces are not glued or cemented together. They are even made of different substances. They are just squeezed together so tightly that not even an air molecule can fit through the space between them.
In geology this is also called a seal and it can happen even when rock has been split. The location of the split where one piece meets another is a discontinuity but discontinuities in the earth are not automatically places where gas can flow.
Other geological names for discontinuities are fractures, cracks, faults and joints. Actually, I prefer the name joint because it was used to describe these discontinuities as far back as the 19th century.
There are two points that I want to make concerning natural discontinuities or fractures. First is what the fractures look like and how they are affected during the industrial fracking process.
Fractures generally appear as planar features, sometimes growing to a height of 200 feet. But this 200 feet may be a discontinuity more than a mile below groundwater, for example. Although fractures in outcrop look like open discontinuities, cracks and joints at depth are tightly sealed.
By pumping high-pressure water along these discontinuities, a hydraulic fracture stimulation unseals or opens these natural discontinuities. Then, to keep the rock fracture from resealing, sand is pumped along the cracks and fractures.
The second thing in our discussion of fractures is how they form naturally.
Tectonic stress may constrain fractures to grow in a particular and predictable orientation but another mechanism actually causes the natural fractures to grow or propagate in the first place.
In the earth, heat and pressure working on organic matter cause chemical reactions that release high-pressure methane and other gases.
It is the high-pressure gas that splits rock, naturally, in a process that we call natural hydraulic fracturing.
Because tectonic stresses change orientation over time, a second joint set may be driven by the same high-pressure gases and these joints will all have similar characteristics as the original set but they may form in a different orientation.
The way these joints or fractures exist in the ground naturally is very important to the process of drilling.
The orientation of the horizontal leg, the industry calls it the lateral, of any shale well, ideally attempts to maximize the number of naturally occurring joints through which that lateral penetrates. Sometimes, industrial fracking will break rock to connect more than one of these natural fractures thereby increasing the permeability of the host rock, but the key point that I want to impart is that, for the most part, fractures exist under the ground naturally and the process of fracking is designed to open them up so that gas can flow to the wellbore.
In the interactive photos below, you can take a closer look at the pathways that high-pressure gases take as they force rock bodies apart in the subsurface. These are chalk rubbings of naturally fractured rocks that have been exposed by road construction. They look like this underground where no one can see them (minus the chalk, of course), but we have the luck of being able to examine the nature of fracture propagation on the surface thanks to an observant geologist who noticed these outcrops and took pictures of them. Click on the orange (rubbing) or blue (interpretation) text below the first photo to switch back and forth from an unaltered photo to an interpretation of the patterns that can be seen.
In the next photo, you can play the part of pressure, forcing fluids through the pore spaces of this rock until it splits apart. Notice how the gases leave pathways as they make their way through the rock, eventually pushing it apart.
Animations: Top Energy Training
Images: “Colorful Rock Slate” by TT via iStock