Posts Tagged ‘hydraulic fracturing’

Horizontal Drilling to Penetrate More Fractures

The fractures (also known as “joints”) in the Marcellus Shale are vertical. So, a vertical borehole would be expected to intersect very few of them. However, a horizontal well, drilled perpendicular to the most common fracture orientation should intersect a maximum number of fractures.

High yield wells in the Marcellus Shale have been built using the horizontal drilling technique. Some horizontal wells in the Marcellus Shale have initial flows that suggest that they are capable of yielding millions of cubic feet of gas per day, making them some of the most productive gas wells in the eastern United States. Although some experts are very optimistic on the long-term production rates of these wells, it is too early to determine their productive life or long-term yield.

A second method is used to increase the productivity of a well. That is to increase the number of fractures in a well using a technique known as “hydraulic fracturing” or “hydrofracing”. This method uses high-pressure water or a gel to induce fractures in the rock surrounding the well bore.

Hydrofracing is done by sealing off a portion of the well and injecting water or gel under very high pressure into the isolated portion of the hole. The high pressure fractures the rock and pushes the fractures open.

To prevent the fractures from closing when the pressure is reduced several tons of sand or other “propant” is pumped down the well and into the pressurized portion of the hole. When the fracturing occurs millions of sand grains are forced into the fractures. If enough sand grains are trapped in the fracture it will be propped partially open when the pressure is reduced. This provides an improved permeability for the flow of gas to the well.

Copyright: Geology.com

Both vertical and horizontal wells are used in shale gas drilling and completion; however, horizontal wells are the increasing trend due to both environmental concerns and economic efficiency (DOE, 2009). Horizontal drilling allows more exposure within the formation to optimize capture of natural gas as well as reducing the environmental footprint of drilling activity (DOE, 2009). The United States Department of Energy’s recently released document on shale gas development in the United States explains that “a vertical well may be exposed to as little as 50 ft of formation while a horizontal well may have a lateral wellbore extending in length from 2,000 to 6,000 ft within the 50-300 ft thick formation” (DOE, 2009, p.47). As such, surface disturbance and impacts to wildlife and communities are reduced while providing optimal gas recovery; considering 16 vertical wells per 640-acre section of land would disturb 77 acres, the equivalent in horizontal wells (4- horizontal wells) would disturb approximately 7.4 acres (DOE, 2009). In addition to reductions in surface disturbance, horizontal wells allow for development in areas previously considered unavailable, primarily urban and environmentally sensitive or protected areas. Well pads can be located away, or ‘setback’, from residences, roadways, wildlife habitats and other protected areas without hampering access to available gas reserves.

In order to recover the shale gas after drilling a well, current industry practice is to hydraulic fracture the formation to stimulate the near wellbore area and facilitate the release of natural gas trapped within the shale. Hydraulic fracturing is a process whereby a fracturing fluid, primarily water, is pumped into the formation under pressure at a calculated rate to form fractures and cracks within the formation, providing a pathway for the gas to migrate to the wellhead for recovery. Sand or other granular materials are added to the fracturing fluid to help ‘prop’ open the newly created fractures after the fluid has been removed from the formation (ALL, 2008a). Additional chemicals may be added to the fracturing fluid for specific engineering purposes; these additions may include friction-reducing agents, biocides and various stabilizers to prevent corrosion of metal piping in the well (DOE, 2009; ALL, 2008a). Depending on the formation and well characteristics, multiple fracturing procedures may be performed in order to fully develop the well for gas recovery (DOE, 2009). While each well and geologic formation is unique, continuing advances in horizontal drilling and well completion practices provide additional reductions in environmental impacts from oil and gas activities while providing the nation’s critical energy supply.
Copyright: GoMarcellusshale.com

Throughout most of its extent, the Marcellus is nearly a mile or more below the surface. These great depths make the Marcellus Formation a very expensive target. Successful wells must yield large volumes of gas to pay for the drilling costs that can easily exceed a million dollars for a traditional vertical well and much more for a horizontal well with hydraulic fracturing.

And rock units are not homogeneous. The gas in the Marcellus Shale is a result of its contained organic content. Logic therefore suggests that the more organic material there is contained in the rock the greater its ability to yield gas. John Harper of the Pennsylvania Geological Survey suggests that the areas with the greatest production potential might be where the net thickness of organic-rich shale within the Marcellus Formation is greatest.

Copyright: Geology.com