Cementing

Cementing in well completion is the process of placing cement in the annulus between two casing string to support vertical and radial loads which will be applied to the casing.

Advantages Of Cementing
  • Cementing is used to permanently shut off water penetration from entering the well.
  • Cementing help protect the casing from corrosion obtain from fluids.
  • Its help isolate porous formations from the producting zone formation.
  • In part of completion process, 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. In other word, it can also be used to plug a well for abadonment.

Cementing is performed when the cement slurry is deployed into the well through 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. A specific time is also set for a cement to harden in the wellbore. This creates a seal so that foreign materials cannot enter the well flow, as well as permanently positions the casing in place.

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. Multi-finger 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.

To mix a cement together, 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 cement slurry. The cement 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.

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. A cement plug is run into the wellbore to sweep the inside of the casing and prevent mixing of the cement slurry with the drilling fluid.

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. The bottom plug is launched ahead of the cement slurry to reduced contamination by fluid inside the casing. The diaphragm you see in the body of the cement or wiper plug allow the cement slurry to pass through, after the plug has reaches the landing collar.

The essential function of the bottom plug is to wipes drilling mud off the walls of the casing and prevents it from contaminating the cement.

The top plug goes before the drilling mud so as to displace the cement from the casing and separate the mud from the slurry. 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 sets it takes for a cement to harden is called thickening time or pump-ability time.

Additives are substances which are added to a cement to help change its characteristics to satisfy specific conditions in the well. Additives may work as an accelerator, retarder, dispersant or other reactant. Let take a look at each of this categories of additives.

Generally, there are 8 categories of additives.
    • Accelerators: This is used to shorten the setting time and increase the rate of compressional strength.
    • Retarders: This is used to extend the setting time.
    • Dispersants: This is efficient for reducing viscosity.
    • Weighting agents: This help increase the density of the cement slurry.
    • Extenders: This is apply to reduce density of the cement slurry.
    • Fluid loss control agents.
    • Lost circulation control agents.
    • Specialty agent

This is a term often used to describe the process of injecting cement slurry into a zone using pump pressure especially for pressure isolation purposes. Its can also be defined in this way as the process of forcing cement slurry by pressure to specified points in a well to cause seals at the point of squeeze. This operation can be performed at any time during the life of the well such as Drilling to completion or production phases, in that case it is referred to as a secondary cementing method. This method is used to isolate a producing formation, seal off water, repair casing leaks e.t.c

Application of Squeeze Cement
    • Sealing of the thief or lost circulation zone.
    • Repair Casing leakage
    • Primary cement job repair
    • Abandon a non-productive or depleted zone or the entire well.
    • Unwanted water production
Methods of Squeeze Cement
      • Tools Method: Packers, Retainer, Coiled tubing and Bradenhead are Some tools are used in squeeze cementing operation.
      • Pumping Technique such as running and hesitation are the used. Running squeeze involves continuous pumping until final squeeze is attained. This process clean fluids in the hole and required large volumes of slurry without fluids lost control. It is applicable to Abandon perforation, lost circulation zone, Casing shoe, Liner top, Block squeeze, water flow and increase cement top. Hesitation squeeze requires Long job times, low pump rate, Small volume of slurry and intermittent pumping.
      • Placement technique: such as high pressure which set above formation frac pressure and Low pressure which are set below formation frac pressure.
High Pressure Squeeze

This type of placement requires large volume of cement slurry to be put in place. They are applicable block squeeze, liner top & Shoe. Acid is required to minimize pump rate required to initiate fracturing.

Low Pressure Squeeze

This is usually the best way to squeeze the pay zone. Its uses small volume of slurry and is applicable for multiple zone, Large interval, Low BHP wells and Naturally fractured formation.

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