Hydrofracturing

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Geology 101

Large near-vertical fractures along Trail Ridge Road in Rocky Mountain National Park.

Most mountain wells are drilled into geologic materials that are known as crystalline rocks. These include granite, schist (shist), gneiss (nice), and volcanic rocks. Ground water moves through these materials along inter-connected cracks, fractures, and joint systems that are present in the rock. These features are often visible along road cuts as "lines" or flat surfaces.

Much of the technical and descriptive material presented on this website is taken from a paper, "Hydrofracturing as a Way to Enhance Water Well Production", written by Ray Boyle and is protected by appropriate copyright laws. Additional references are shown at the end of this section.

Numerous photos are displayed throughout this section to illustrate the wide variety and configuration of fractures which can exist Although the photos show fractures along road cuts and in surface exposures, these same fracture patterns can exist hundreds of feet below the ground surface. It is at these greater depths that we attempt to access these fractures by drilling or hydrofracturing.  As the photos show, the presence and interconnection of the fractures can vary greatly with only minor changes in well location and depth.  One reasonable generalization is that in the subsurface, the size, number, density, and aperture, of the fractures will decrease with depth.  Having said that, many outstanding wells have been obtained at 1000 feet or greater with the encounter of some prominent “regional” fracture.

Very distinct fractures in otherwise impermeable granite material along Hwy 67 west of Sedalia, CO.

If the borehole does not pass through the right combination of fractures and only penetrates the crystalline rock matrix material, the result may be a dry water well or low producing well as the matrix material without fractures offers no permeability.

Local intense fracturing due to minor folding along Hwy 285 near Tinytown, CO. The fracture zone, if penetrated during drilling could produce a lot of water.

Regional factors such as fracture frequency, orientation, length, interconnection, continuity, and the presence of individual fractures vs. fracture zones control ground water flow.  Locally, fracture aperture at the borehole intersection, fracture smoothness and irregularity, mineralization, and the degree of subsurface weathering and clay formation are factors of consideration (Domenico, 1990). The fracture characteristics at both scales can show wide variation in character over short distances laterally and vertically.  Overall, these factors will exist in complex combination controlling the flow into or out of the well (Johnson, 2007) .

Near vertical 'bedding' of materials present special challenges during drilling as the vertical borehole will not pass through many different formations. Hwy 34, Big Thompson Canyon, west of Loveland, CO.

The fractures that are present in the rock can result from several mechanisms (Long, 1996).  Those mechanisms include: 1) the cooling of the material as formed deep below the earths surface; 2) lithostatic unloading (erosion); and 3) tectonic activities associated with mountain building.  The stress distribution of any of these processes results in the development of a fairly consistant fracture pattern made up of two near-vertical fracture sets perpendicular to each other and one near-horizontal fracture set.  Although variations on this theme are common, the overall picture seems to hold. Unfortunately, the best water bearing and water transmitting fractures happen to be the vertical sets which are under the least amount of stress and tend to be wider and more continuous laterally.  Thus, the driller is challenged to intercept these near-vertical fracture planes with a vertical borehole, to avoid a dry water well or a low producing water well.

Road cut along Hwy 287 near Colorado/Wyoming state line showing 3 fracture sets prominently displayed in a granite material.

Without a source of recharge, the largest, most open, most continuous fracture will be dry.  It is generalized, that for practical purposes, most of the recharge to a mountain domestic well is derived from within about a 1/2–mile radius. Precipitation (primarily snowfall) is the main source of ground water recharge meaning that slope, vegetation density, and soil development overlying the fractures will influence recharge.  Surface water features can play a part in recharging fractures in the vicinity of a well with consideration of  distance from, elevation above, and orientation and continuity of fractures between the well and the surface-water feature (Johnson, 2007). 

For example, a poor hydrofracturing candidate might be a well 100 feet deep with a 90-foot static water level, on top of a hill, 300 feet above, and ½ mile from the nearest stream which only flows three months of the year, and is separated by a prominent fault containing abundant gouge and clay minerals providing an effective ground water barrier.  Some common sense items for sure, but this illustrates the need to consider the bigger picture.

Road cut along Hwy 285 near Tinytown, CO showing dramatically different materials over a short distance. The contact between the light and dark materials can be a good water bearing zone. These same material variations can exist hundreds of feet below the ground surface	Prominent fracture (minor fault) breaks the connection between materials on either side. However, if this fracture is penetrated during drilling or is within reach of fractures opened during hydrofracturing, it could be a very good production zone. Off Hwy 285 near Bailey, CO.

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