Well Construction & Groundwater Protection
What are casing and cementing?
The existing industry standard for oil and gas casing was established by the American Petroleum Institute (API) in Specification 5CT. It specifies the length, thickness, tensile strength and composition of casing for a given situation and is the most commonly used standard for the selection of oil and gas casing.
Each full length of casing is often referred to as a casing string. Wells are typically constructed of multiple casing strings including a surface string and production string. These strings are set in the well and cemented in place under specific state requirements. The API has also established standards for cement types. These standards are covered by Specification 10A, which lists a variety of oil and gas cements.
Cementing is the process of placing a cement sheath around casing strings. Although Class A (Portland) cement is the most common cement used in the oil and gas industry, the actual type of cement can be tailored to the individual well, depending on what the state’s rules allow. For example, some wells penetrate formations that are difficult to cement because of their porous nature or due to a substantial water flow within the formation. In such cases, additives like cellophane flake and calcium chloride are sometimes added to the cement to seal off such zones, quicken the cement hardening process, and prevent washout of the cement.
The casing and cemening process
In general, the casing of oil and gas wells, whether vertical or horizontal, is accomplished in multiple phases from the largest diameter casing to the smallest.
The first phase often involves the setting of conductor casing. The purpose of this casing is to prevent the sides of the hole from caving into the wellbore. It is not always necessary.
After the conductor casing string is set in place, drilling continues inside the conductor casing string to below the lowest ground water zone depending upon regulatory requirements. Surface casing is then run from the surface to just above the bottom of the hole. Cement is pumped down the inside of the casing, forcing it up from the bottom of the surface casing into the space between the outside of the surface casing and the wellbore. This space is called the annulus.
Once a sufficient volume of cement to fill the annulus is pumped into the casing, it is usually followed by pumping a volume of fresh water into the casing until the cement begins to return to the surface in the annular space. The cementing of casing from bottom to top using this method is called circulation. The circulation of cement behind surface casing insures that the entire annular space fills with cement from below the deepest ground water zone to the surface.
While nearly all states require the circulation of cement on surface casing, it is not a universal requirement. In some states, cement is required only across the deepest ground water zone. Regardless, such variations from the circulation of cement on surface casing are still designed to ensure that ground water zones are isolated and protected from oil and gas production zones.
Once the surface casing is set and the cement has had time to cure, the wellbore is drilled down to the next zone where casing will be set. In some states this results in the placement of intermediate casing.
Intermediate casing is usually only required for specific reasons, such as when additional control of fluid flow and pressure is needed, or to protect other underground resources such as minable coals or gas storage zones.
After the surface (and if needed, intermediate casing) strings are set, the well is drilled to the target formation. Upon reaching this zone, production casing is typically set at either the top of, or into, the producing formation. The placement of production casing depends on whether the well will be produced directly from the formation (“open-hole”) or through perforations in the production casing.
The production casing is typically set into place with cement using the same method as the one used for surface and intermediate casing. In drill holes that deviate from vertical, casing centralizers like the one shown above are placed on the outside of the casing to center it in the hole. This ensures that cement will completely surround the casing.
Tubing is also used under certain circumstances in some states. Typically, tubing is set into an internal seal called a packer at the bottom of the well rather than cemented in like casing.
Why casing and cementing are an important part of groundwater protection.
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 borehole (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.