Part of the process of preparing a well for further drilling, production or abandonment, cementing a well is the procedure of developing and pumping cement into place in a wellbore.
Used for a number of different reasons, cementing protects and seals the wellbore. Most commonly, cementing is used to permanently shut off water penetration into the well. Part of the completion process of a prospective production well, cementing can be used to seal the annulus after a casing string has been run in a wellbore. Additionally, cementing is used to seal a lost circulation zone, or an area where there is a reduction or absence of flow within the well. In directional drilling, cementing is used to plug an existing well, in order to run a directional well from that point. Also, cementing is used to plug a well to abandon it.
Cementing is performed when the cement slurry is deployed into the well via pumps, displacing the drilling fluids still located within the well, and replacing them with cement. The cement slurry flows to the bottom of the wellbore through the casing, which will eventually be the pipe through which the hydrocarbons flow to the surface. From there it fills in the space between the casing and the actual wellbore, and hardens. This creates a seal so that outside materials cannot enter the well flow, as well as permanently positions the casing in place.
Preparing the Cement
In preparing a well for cementing, it is important to establish the amount of cement required for the job. This is done by measuring the diameter of the borehole along its depth, using a caliper log. Utilizing both mechanical and sonic means, multifinger caliper logs measure the diameter of the well at numerous locations simultaneously in order to accommodate for irregularities in the wellbore diameter and determine the volume of the openhole.
Additionally, the required physical properties of the cement are essential before commencing cementing operations. The proper set cement is also determined, including the density and viscosity of the material, before actually pumping the cement into the hole.
Special mixers, including hydraulic jet mixers, re-circulating mixers or batch mixers, are used to combine dry cement with water to create the wet cement, also known as slurry. The cement used in the well cementing process is Portland cement, and it is calibrated with additives to form one of eight different API classes of cement. Each is employed for various situations.
Additives can include accelerators, which shorten the setting time required for the cement, as well as retarders, which do the opposite and make the cement setting time longer. In order to decrease or increase the density of the cement, lightweight and heavyweight additives are added. Additives can be added to transform the compressive strength of the cement, as well as flow properties and dehydration rates. Extenders can be used to expand the cement in an effort to reduce the cost of cementing, and antifoam additives can be added to prevent foaming within the well. In order to plug lost circulation zones, bridging materials are added, as well.
Cementing the Well
After casing, or steel pipe, is run into the well, an L-shaped cementing head is fixed to the top of the wellhead to receive the slurry from the pumps. Two wiper plugs, or cementing plugs, that sweep the inside of the casing and prevent mixing: the bottom plug and the top plug.
Keeping the drilling fluids from mixing with the cement slurry, the bottom plug is introduced into the well, and cement slurry is pumped into the well behind it. The bottom plug is then caught just above the bottom of the wellbore by the float collar, which functions as a one-way valve allowing the cement slurry to enter the well.
Then the pressure on the cement being pumped into the well is increased until a diaphragm is broken within the bottom plug, permitting the slurry to flow through it and up the outside of the casing string.
After the proper volume of cement is pumped into the well, a top plug is pumped into the casing pushing the remaining slurry through the bottom plug. Once the top plug reaches the bottom plug, the pumps are turned off, and the cement is allowed to set.
The amount of time it takes cement to harden is called thickening time or pumpability time. For setting wells at deep depths, under high temperature or pressure, as well as in corrosive environments, special cements can be employed.