Hydraulic Fracturing of Oil & Gas Wells Drilled in Shale
What is Hydraulic Fracturing?
Hydraulic fracturing is a procedure that can increase the flow of oil or gas from a well. It is done by pumping liquids down a well into subsurface rock units under pressures that are high enough to fracture the rock. The goal is to create a network of interconnected fractures that will serve as pore spaces for the movement of oil and natural gas to the well bore.
Hydraulic fracturing combined with horizontal drilling has turned previously unproductive organic-rich shales into the largest natural gas fields in the world. The Marcellus Shale, Utica Shale, Barnett Shale, Eagle Ford Shale and Bakken Formation are examples of previously unproductive rock units that have been converted into fantastic gas or oil fields by hydraulic fracturing.
Successful Use of Hydraulic Fracturing in Shale
In the early 1990s Mitchel Energy began using hydraulic fracturing to stimulate the production of natural gas from wells drilled into the Barnett Shale of Texas. The Barnett Shalecontained enormous amounts of natural gas, however, the Barnett rarely produced natural gas in commercial quantities.
Mitchel Energy realized that gas in the Barnett Shale was trapped in tiny pore spaces that were not interconnected. The rock had pore space but lacked permeability. Wells drilled through the Barnett Shale would usually have a show of gas but not enough gas for commercial production. Mitchel Energy solved this problem by hydraulic fracturing the Barnett Shale to create a network of interconnected pore spaces that enabled a flow of natural gas to the well.
Unfortunately many of the fractures produced by the hydraulic fracturing process snapped closed when the pumps were turned off. The Barnett Shale was so deeply buried that confining pressure closed the new fractures. This problem was solved by adding sand to the fracturing fluid. When the rock fractured, the rush of water into the newly-opened pore space would carry sand grains deep into the rock unit. When the water pressure was reduced the sand grains "propped" the fracture open and allowed a flow of natural gas through the fractures and into the well bore. Today there are a variety of natural and synthetic products that are sold under the name of "frac sand".
Mitchel Energy further improved the yield of their wells by drilling them horizontally through the Barnett Shale. Vertical wells were started at the surface, steered to a horizontal orientation and driven through the Barnett Shale for thousands of feet. This multiplied the length of the pay zone in the well. If a rock unit was 100 feet thick, it would be have a pay zone of 100 feet in a vertical well. However, if the well was steered horizontal and stayed horizontal for 5000 feet through the target formation then the length of the pay zone was fifty times longer than the pay zone of a vertical well.
Mitchel Energy used hydraulic fracturing and horizontal drilling to multiply the productivity of Barnett Shale wells. In fact, many of their highly successful wells would have been failures if they were vertical wells without hydraulic fracturing.
Mitchel Energy realized that gas in the Barnett Shale was trapped in tiny pore spaces that were not interconnected. The rock had pore space but lacked permeability. Wells drilled through the Barnett Shale would usually have a show of gas but not enough gas for commercial production. Mitchel Energy solved this problem by hydraulic fracturing the Barnett Shale to create a network of interconnected pore spaces that enabled a flow of natural gas to the well.
Unfortunately many of the fractures produced by the hydraulic fracturing process snapped closed when the pumps were turned off. The Barnett Shale was so deeply buried that confining pressure closed the new fractures. This problem was solved by adding sand to the fracturing fluid. When the rock fractured, the rush of water into the newly-opened pore space would carry sand grains deep into the rock unit. When the water pressure was reduced the sand grains "propped" the fracture open and allowed a flow of natural gas through the fractures and into the well bore. Today there are a variety of natural and synthetic products that are sold under the name of "frac sand".
Mitchel Energy further improved the yield of their wells by drilling them horizontally through the Barnett Shale. Vertical wells were started at the surface, steered to a horizontal orientation and driven through the Barnett Shale for thousands of feet. This multiplied the length of the pay zone in the well. If a rock unit was 100 feet thick, it would be have a pay zone of 100 feet in a vertical well. However, if the well was steered horizontal and stayed horizontal for 5000 feet through the target formation then the length of the pay zone was fifty times longer than the pay zone of a vertical well.
Mitchel Energy used hydraulic fracturing and horizontal drilling to multiply the productivity of Barnett Shale wells. In fact, many of their highly successful wells would have been failures if they were vertical wells without hydraulic fracturing.
Hydraulic Fracturing in Other Shale Plays
As others learned of Mitchell Energy's success in the Barnett Shale of Texas the methods of horizontal drilling and hydraulic fracturing were tried in other organic-rich shales. These methods quickly succeeded in the Haynesville Shale and Fayetteville Shale of Louisiana, Texas and Arkansas - then in the Marcellus Shale in the Appalachian Basin. The methods worked in many other shales and are now being used to develop organic-rich shales in many parts of the world.
Hydraulic fracturing has also enabled production of natural gas liquids and oil from many wells. Rock units such as the Bakken Shale of North Dakota and the Niobrara Shale of Colorado, Kansas, Nebraska and Wyoming are now yielding significant amounts of oil from hydraulic fracturing.
Fracturing Fluids
Water is the driving fluid used in the hydraulic fracturing process. Depending upon the characteristics of the well and the rock being fractured a few million gallons of water can be required to complete a hydraulic fracturing job.
When the water is pumped into the well the entire length of the well is not pressurized. Instead, plugs are inserted to isolate the portion of well where the fractures are desired. Only this section of the well receives the full force of pumping. As pressure builds up in this portion of the well, water opens fractures, and the driving pressure extends the fractures deep into the rock unit. When pumping stops these fractures quickly snap closed and the water used to open them is pushed back into the borehole, back up the well and is collected at the surface. The water returned to the surface is a mixture of the water injected and pore water that has been trapped in the rock unit for millions of years. The pore water is usually a brine with significant amounts of dissolved solids.
Chemicals are often added to the water used in hydraulic fracturing. These additives serve a variety of purposes. Some thicken the water into a gel that is more effective at opening fractures and carrying proppants deep into the rock unit. Other chemicals are added to: reduce friction, keep rock debris suspended in the liquid, prevent corrosion of equipment, kill bacteria, control pH and other functions.
Most companies have been resistant to revealing the composition of their hydraulic fracturing fluids. They believe that this information should be kept private to protect their competitive research. However, regulators are starting to demand the information and some companies are starting to share the information voluntarily.
Environmental Concerns
There are a number of environmental concerns related to hydraulic fracturing. These include:
1) Fractures produced in the well might extend directly into shallow rock units that are used for drinking water supplies. Or, fractures produced in the well might communicate with natural fractures that extend into shallow rock units that are used for drinking water supplies.
2) The casing of a well might fail and allow fluids to escape into shallow rock units used for drinking water supplies.
3) Accidental spills of hydraulic fracturing fluids or fluids expelled during a fracturing job might seep into the ground or contaminate surface water.