Fracking Process: Well Casing & Cementing
Well casing and cementing is an important process primarily used to protect groundwater and aquifers during a drilling operation. It involves the placing and cementing of several steel pipes, or casings, within a well hole. The number of casing strings depends upon the needs of the well and state regulations, but can include conductor casing, surface casing, intermediate casing and production casing.
Casing and Cementing Basics
- Casing is used to line the inside of a drilled hole, or wellbore. It is typically a hollow steel pipe.
- The primary purpose of casing is to protect groundwater during a drilling operation.
- The American Petroleum Institute (API) sets the current industry standard for oil and gas casing, including thickness, length, composition and tensile strength of casing.
- A full length of casing is called a "casing string." A well usually has multiple casing strings—including a production string and a surface string—set in place starting from the largest diameter casing to the smallest.
- Casing strings are placed in the wellbore and cemented into place based upon specific state requirements. The cementing process creates a cement barrier around the casing strings.
- Cementing is one of the most important steps in the creation of fracking wells and combines several scientific and engineering disciplines.
- The type of cement used depends upon the type of well and state requirements, but the most common type of cement used in the oil and gas industry is Class A (Portland) cement.
Step 1: Set the Conductor Casing String
- This preliminary casing is placed to prevent the sides of the well hole from caving into the wellbore.
- Conductor casing is not always necessary.
Step 2: Set the Surface Casing
- Once the conductor casing is in place (if needed), drilling can continue within the conductor casing string.
- The well is drilled to below the lowest groundwater zone; depth depends on regulatory requirements for the given state.
- Surface casing is then set in place from just above the bottom of the hole to the surface of the well.
Step 3: Pump the Cement
- After the surface casing is in place, cementing can begin. A slurry of cement, cement additives and water are pumped down through the casing until it is forced up from the bottom, into the annulus (the space between the wellbore and the outside of the surface casing).
- The placement of the cement and the fluid (slurry) composition used depends on a careful analysis of the well parameters and the needs for the life of the well.
- After the annulus is filled with enough cement, fresh water is typically pumped into the casing until the cement begins to return to the surface of the annulus.
- This bottom-to-top method of cementing of casing is known as "circulation" and helps ensure that the entire annular space from top to bottom is filled with cement.
- This circulation of cement on surface casing is a requirement in nearly all states; however, in other states, cement is only required across the deepest ground water zone.
- Sealing the annular space with cement is important because it creates a hydraulic barrier to prevent fluid migration (both vertical and horizontal) and protect groundwater zones. It also creates zonal isolation; provides structural support for the casing, isolates the casing for subsequent drilling, and protects casing from corrosion.
- Zonal isolation keeps fluids from each zone from communicating with each other.
Step 4: Set the Intermediate Casing
- After the cement cures, drilling continues to the next zone of the wellbore.
- Intermediate casing may need to be placed at this time, but is only required in some states and for specific circumstances.
- Intermediate casing is generally only used when extra control of pressure/fluid flow is required or to protect additional underground resources (ex: gas storage zones or minable coals).
Step 5: Set the Production Casing
- Once the intermediate casing is set (if needed), the well is drilled to its target formation.
- Production casing can then be set at the top of or into the producing formation, typically using the same cement method used in surface and intermediate casing.
- How the production casing is set depends upon whether the well will be produced "open hole" (directly from the formation) or through openings in the production casing.
- If the well hole is not vertical, casing centralizers may be used on the outside of the casing to keep it centered in the hole and allow the cement to entirely surround the casing.
Other Important Info
- In some states and circumstances, tubing may be used instead of the casing and cementing process. Tubing is typically set using an internal seal (a "packer") at the bottom of the well and therefore no cementing is needed.
- In order to ensure proper well construction, state officials may be required to witness the casing and cementing process. In other states, a report is required that outlines the types/amounts of casing and cement used in well production.
- A few states—including Alaska, Michigan and Ohio—also require well drillers to use geophysical logs as an additional verification method. Examples include Variable Density Logs (VDL) and Cement Bond Logs (CBL).