Introduction to Well Cementing

Cementing is the process of pumping cement slurry to a predetermined point in a well.

The main reasons for primary cementing are to:

• Isolate subsurface formations from each other and thereby restrict fluid movement.
• Anchor the casing by bonding it to the wellbore wall.

Additional benefits include:

• Protecting the casing from corrosion or erosion due to exposure to subsurface fluids
• Protecting the casing from external fluid pressure or formation stresses that could cause it to collapse
• Preventing blowouts during drilling
• Preventing shock loads to the casing during drilling

While primary cementing is the most widely used application, remedial cementing is also done to:

• Temporarily or permanently plug a well.
• Remedy an improperly performed previous primary cement job by squeezing cement into an unwanted void space behind the casing.

Remedial cementing can also be used to seal off lost circulation or “thief” zones.

It is very important that every step in a cement job go smoothly and without delay. When the slurry is mixed and pumping begins, the setting time has started and unexpected shutdowns can have disastrous impacts.

History of Cementing

The casing in most early wells drilled in the late 1800s and early 1900s was not cemented. While attempts to cement casing may have taken place as early as 1883, the first effort to use cement to seal casing within a well using a modern method was in California in 1903 (Piot 2009; Analog, 2010). In 1919, Erle P. Halliburton introduced the cementing process to Texas and Oklahoma (Figure 1).

Over the next 20 years Halliburton and the company he founded were responsible for developing and patenting many of the basic well cementing tools and techniques used during the first half of the 20^th century (Pike 2007). Soon after the process was popularized, it became normal operating procedure for the casing in every oil and gas well to be cemented.

Today, the global pressure pumping service sector, which includes cementing as well as acidizing and hydraulic fracturing, is done primarily by major service companies. Some national oil companies carry out their own cementing operations and many smaller firms operate on a regional basis.

Types of Cement Jobs

There are two basic categories of cementing operations:

• Primary cementing
• Remedial cementing, which includes
  • Squeeze cementing
  • Plug cementing

Primary Cementing

Primary cementing involves placing cement in the annulus between the casing and the formations exposed to the wellbore. The major objective of cementing is to provide zonal isolation –to preventing fluids from one zone from travelling to another zone.

The most basic process for accomplishing a primary cementing job employs the two-wiper plug method for pumping the cement slurry and the displacing fluid ( Primary cementing Animation ). The plugs, which fit snugly within the casing, are pumped ahead of the cement slurry and then behind it:

• The bottom plug is the first plug to drop and prevents the cement slurry from mixing with the drilling mud that the cement is displacing.
• The top plug drops after the cement, preventing the displacing fluid from mixing with the cement slurry.

Sufficient cement is pumped into the casing and displaced around the bottom of the casing into the annular space to completely fill the annulus between the casing and the wellbore from the bottom to at least the top of the producing zones, and sometimes to the surface.

Remedial Cementing

Remedial cementing includes two broad categories: squeeze cementing and plug cementing.

• Squeeze cementing involves pumping cement slurry into a formation under pressure to seal off a void caused by fractures within the formation, or spaces behind the casing that were not properly filled during primary cementing, or perforations or splits in the casing that occur after a primary cement job. Cement squeezes that involve placing cement behind the casing are usually preceded by perforating the casing to ensure smooth passage of the cement slurry.