NHDES Technical Bulletin WD-WS-1-3
Hydrofracturing, commonly referred to as hydrofracking, is a well development process that involves injecting water under high pressure into a bedrock formation via the well. This is intended to increase the size and extent of existing bedrock fractures, thereby enlarging the network of water bearing fractures and size of the area supplying water to the well.
The procedure is often used to increase well yields of new deep drilled wells with marginal or inadequate production rates. It may also be applied to older existing wells that have progressively diminished recovery rates over time usually caused by mineralization and incrustation of rock fractures.
History
Hydrofracturing was originally developed in the oil fields to increase production volumes and is a relative newcomer to the drinking water well industry. It is now the method preferred by most contractors for developing bedrock wells because it is a controlled process. Older methods included blasting with dynamite and more recently, the application of dry ice to a capped well was used. These methods were somewhat uncontrolled and sometimes resulted in well failure.
The use of dynamite frequently caused wells to collapse from the force of the blast rendering the well useless. The use of dry ice was common only a few years ago and was a much less violent approach to applying high pressure to the bedrock formation. The phase change from solid carbon dioxide (CO2) to gaseous CO2 results in a large increase in volume. The well is capped immediately after the introduction of dry ice into the well. The CO2 gas is trapped in the well as it expands and the result is an extreme increase in pressure. The pressure generated theoretically has the same affect on bedrock fractures as the hydrofracturing process does today. Unfortunately, in some wells the increased pressure on the well casing forced the casing in the upward direction causing the drive shoe seal to lift consequently breaking the seal and allowing surface water into the well. At least in one instance the well casing actually shot out of the ground into the air.
The hydrofracturing process eliminates this problem by controlling the pressure with packers below the well casing.
The Hydrofracturing Process
The procedure involves the installation of an inflatable or mechanical packer which is placed in the well bore at least 20 feet below the well casing and drive shoe seal and at least 60 feet below the ground surface to insure that the process does not "break" the seal or allow surface water contaminants to enter the well. The packer is inflated or locked into position and water is pumped through the packer under pressure. Most applications require between 500 and 2000 pounds per square inch (psi) pressure and in some cases 3000 psi pressure may be needed in tight rock formations.
If successful, pressure will steadily rise to a maximum level as the rock formation resists flow then will suddenly drop off and stabilize at a lower pressure. The drop in pressure indicates that the formation is accepting water and the resistance to flow is diminished. Water is pumped into the formation for 5 to 30 minutes. Injection pump delivery rates of 50 to 75 gallons per minute (GPM) have proven successful. Generally, 1500 to 2000 gallons of water or more are pumped into the bedrock formation.
It is extremely important that only clean, disinfected water is used for injection water because of the extreme pressures involved and potential for forcing contaminants deep into the bedrock aquifer.
One or two packers may be used for hydrofracturing. When utilizing one packer, it is set near the top of the well but at a minimum safe distance below the drive shoe. After the initial frac sequence, the packer is deflated or unlocked and lowered further into the hole and the process repeated as many times as necessary. Commonly two hydrofrack sequences are performed.
Zone fracturing utilizes a two packer system where the packers are placed in series and water is pumped into the zone between the packers. This system can be more effective because it concentrates pressures within a small area, typically 30 to 60 foot intervals, and individual fractures can be isolated and hydraulically fractured. With this method, the procedure starts within a specified section of the well targeted by the Water Well Contractor. Each successive frac sequence stresses one interval higher than the last. In this way, all potential water bearing fractures or fracture zones are worked independently within the section being hydrofracked. This differs from, the single packer, one or two frac sequence method which probably only affects the weakest, least resistant point(s) in the well but still may produce adequate results for private wells serving individual households.
Zone fracturing is a much more expensive and time consuming procedure and is generally used only on very difficult wells or public supply wells or if a larger flow of water is desired.
Yield Testing
Well yield increases, if successful, are generally modest but may constitute a significant increase if the original well yield was very low. Occasionally, large increases in well recovery rates are realized.
A typical well yield rate after hydrofracturing is 1/2 to 5 gallons per minute. Water well contractors report a high success rate, however, in some instances due to geologic conditions hydrofracturing will not increase well yield. If initial pressure continues to build and reaches the operator's maximum equipment capacity, the formation will not accept water and the procedure may not be successful. Many water well contractors do not include yield testing as part of their hydrofracking contract because injection water volumes are large enough that it may require an extended period of time to allow the bedrock aquifer to reach equilibrium. If the yield test is conducted immediately after the well is hydrofracked, the amount of water injected into the bedrock formation must be a consideration to avoid an over estimation of the increased well yield.
Contractors Must Be Licensed
Only licensed New Hampshire Water Well Contractors are authorized to hydrofrack wells. If you are thinking about having your well hydrofracked, it is generally advisable to first consult with your water well contractor of choice or perhaps several water well contractors. These professionals will help you evaluate your water supply needs and the probability of success in your geographic area to help you determine if hydrofracturing is a viable choice for your well. The equipment capabilities of different contractors and the type of contract offered are other considerations.
For further information call the N. H. Water Well Board at 603-271-3406.