Thursday, February 16, 2012
More than One Way to Frack a Well
Our ability to recover natural gas buried in deep geological deposits beneath the earth has increased dramatically due to advances in horizontal drilling which allows a vertically drilled well to turn and run thousands of feet laterally through the earth combined with advances in methods to "hydraulically fracture" or "hydraulically stimulate" the formation to generate cracks or "fractures" through which gases and liquids can flow more rapidly to the well. Hydraulic fracking as it is typically called is the pumping of millions of gallons of chemicals and water into shale at high pressure to increase the recovery of oil and natural gas from shale.
In hydraulic fracking on average 2-5 million gallons of chemicals and water is pumped into the shale formation at 9,000 pounds per square inch and literally cracks the shale or breaks open existing cracks and allows the trapped natural gas to flow. The use of water laced with chemicals to enhance oil and gas production has been very successful in the United States with many improvements in the technique since its inception in the 1950's. Water-based fracturing liquids (once called slickwater) are the most commonly used in the lower shale formations like the Marcellus. Generally, chemical additives are mixed with the water to improve its ability to transport the proppant, the sand or other substance used to prop open the fractures to allow the gas to flow. This is achieved through the addition of gels to increase the viscosity and also to reduce fluid loss from the fracture by temporarily blocking the natural permeability of the rock. Once the pumping is completed, the gel breaks down and the spent liquids can flow back to the surface after which the gas can flow up to the well bore. This works well in the higher pressure formations where the back pressure from the formation can push the liquids out of the formation rather than absorb the liquids into the formation.
While geologists and engineers believe that there is little risk that the fracking “water,” a mix of chemicals and water, will somehow infiltrate groundwater reserves though a fissure created by the fracking. It is believed that the intervening layers of rock would prevent a fissure from extending thousands of feet to the water table, but there are other risks in how we build wells and fracture the shale. There have been documented cases of seepage into drinking water wells through improperly sealed or abandoned drilling wells. There are also places where groundwater is only several hundred feet above the gas reserves as in Wyoming and groundwater is more easily directly impacted by fracking. In the past decade the advances in drilling and fracking technology have been adapted to exploit gas in the Barnett shale in the Fort Worth Basin in Texas and applied to a series of major shale gas deposits that could not have been viable without the advances in drilling and fracking techniques.
A mild winter combined with the newly available gas supplies has resulted in a crash in gas prices. At the current rate of natural gas consumption North America is reported to have a 100-year supply of proven, producible reserves and even with expanded use of natural gas to replace coal in fueling power plants, there is more than a generation of currently accessible reserves. The falling price of natural gas and disappointing life span of hydro fracked wells has renewed interest in other methods of fracturing a formation to increase the gas recovery over the life of the well and reduce the overall costs of fracking including the costs associated with waste water treatment to improve the economics of the project. Other methods of fracturing a formation have been used in formations where the gas reservoir is at a lower pressure and does not have sufficient energy to push the liquids back up the well. Without adequate pressure in the formation the liquids and chemicals used in the hydraulic stimulation process remain in the reservoir and impede the flow of oil and gas and shorten the lifetime of the well.
From the 1970’s until about 10 years ago it was standard to stimulate wells with nitrogen gas or nitrogen foam. Nitrogen gas and foam have a long history as the fracturing fluids of choice in the Antrim, New Albany, and Ohio (Lower Huron) shale where experience had shown a dry fracturing was superior. However, it was the Marcellus shale formations that tipped the balance to hydraulic fracturing. Water is non-compressible, drives net pressure better in shale stimulations than nitrogen foam fluids. Using water improved the chances of opening other planes of weakness or natural fractures with high pump rates and fluid volumes. It was believed that the Marcellus shale gas recovery was not significantly impacted by clays absorbing water and reducing overall gas production in a hydraulic frack; however, this belief may not prove to be true over the life of the wells.
Now, other methods of fracturing shale formations are being examined in response to the public outcry against the potential ground water contamination from hydraulic fracturing, excessive water use and earthquakes associated with some fracking water disposal wells combined with some hydraulic fracture wells experiencing a lower production yield than anticipated and the depressed price of natural gas. In the late 1990’s a series of test wells were drilled by industry and studied by the Department of Energy, DOE. These wells used liquid phase carbon dioxide, CO2, for fracturing and had significantly increased gas well yield over nitrogen fracked wells. In these wells, CO2 was pumped as a liquid then vaporized to a gas and flowed out from the reservoir leaving no liquid or chemical damage to the formation. This process can transport proppant in only limited volumes and requires a specialized blender to mix the liquid CO2 with proppant. These limitations were a problem for the lower shale formations where more proppant was needed.
Sometimes in a hydraulic frack the gels do not completely break down and even when they do, there is always some residue that remains in the well and can block and damage the gas reservoir. Hydraulic fracking gels create some damage, but the fractures were of sufficient length to offset the damage in the Texas wells which popularized the method. Some geological formations; however, do not respond as effectively to hydraulic stimulations. The fracturing liquids can become trapped in the formation because the reservoir is at a lower pressure and does not have sufficient energy to push the liquids back to the well bore. The gas well yields are diminished because the liquids and chemicals used in the fracking remain in the reservoir and impede the flow of oil and gas. These problems were slow to be addressed because of the high price for natural gas last decade, termination of the Gas Research Institute and a sharp decline in DOE gas research and technology program just as shale gas production was taking off.
Now with the low price for natural gas and contracts that require drilling, energy companies are looking for better methods of stimulating gas reserves. Chesapeake Energy Corp. has fractured a natural gas well in Ohio's Utica shale using a reported 471,534 gallons of water mixed with carbon dioxide, sand and chemical additives to create a foam that was pumped down the well under pressure to crack the underground rock. In nearby wells hydro fracked by Chesapeake the average water usage was 5.8 million gallons. Well production results from this CO2 foam and water fracked well will have to be evaluated over a period of at least 24 months and typically 36 months to know if this technique was successful, but it holds promise as a less resource damaging or wasting method to access shale gas.