How Casing Protects Groundwater
Why are casing and cementing important in the hydraulic fracturing process?
Casing strings are an important element of well completion with respect to the protection of groundwater resources because they provide for the isolation of fresh water zones and groundwater from the inside of the well. Casing is also used to transmit flowback fluids from well treatment. In this regard, surface casing is the first line of defense and production casing provides a second layer of protection for groundwater. As important as casing is, it is the cementation of the casing that adds the most value to the process of groundwater protection. Proper sealing of annular spaces with cement creates a hydraulic barrier to both vertical and horizontal fluid migration. Consequently, the quality of the initial cement job is a critical factor in the prevention of fluid movement from deeper zones into groundwater resources. In some states it is common for state personnel to witness the running and cementing of casing strings, while in other states the submission of a completion report which details the amounts and types of casing and cement used in the completion of the well is considered sufficient evidence of proper well construction. In a few states such as Alaska, Michigan and Ohio, an additional verification method using geophysical logs such as Cement Bond Logs (CBL) and Variable Density Logs (VDL) may be required. By measuring the travel time of sound waves through the casing and cement to the formation, the CBL shows the quality of bonding between the casing and the cement. The VDL performs a similar function to measure the bond between the cement and the borehole. By measuring the quality of the cement to casing and cement to formation bond, the sealing quality of the cement in the space between the casing and the borehold (called the annulus)can be evaluated.
State Regulation of Well Construction
In a review of the regulations of twenty-seven state oil and gas agency regulations conducted in 2009 by the GWPC, the following percentage of states had the listed requirement for casing and cementing:
Although some states require complete circulation of cement from the bottom to the top of the production casing, most states require only an amount of cement calculated to raise the cement top behind the casing to a certain level above the producing formation. For example, in Arkansas, production casing must be cemented to two-hundred-fifty feet above all producing intervals.
There are a number of reasons why cement circulation from bottom to top on production casing is not always required including the fact that in very deep wells, the circulation of cement may be unnecessary due to the differences in depth between the production zone and fresh groundwater zones. Also, under certain circumstances, cementing must be handled in multiple stages which can result in a poor cement job or damage to the casing if not done properly. Finally, the circulation of cement on production casing prevents the monitoring of the space between the casing strings for changes in pressure which could indicate leakage through the casing or cement sheath.
For more information about the regulatory requirements of each oil and gas producing state, go to the Regulations By State page or the report State Oil and Natural Gas Regulations Designed to Protect Water Resources.