Accelerators are chemicals added to cement slurry to reduce the setting time of cement and increase the rate of compressive strength development. Accelerators are beneficial in low density slurries required in low temperature applications. Setting time is impacted by the proportion of accelerator added to the slurry. Depending on application and cementing operation time, slurry is designed and tested in the lab with different proportions of accelerator to maintain a balance between achieving the intended objectives of the job and having enough time to complete the cement pumping operations.

Balanced plug is a technique of placing a cement plug in such a way that the levels of different fluids inside and outside drill pipe are same. It is the most common method of plug placement. Having the same level of fluids in drill pipe and annulus provides stability to the fluid system in the slurry. It minimizes the contamination risk and enhances the plug effectiveness.

Cement slurry pumped for casing cementation or setting a cement plug need to fulfill several property requirements like weight, fluid loss, thickening time etc. Mix water is prepared by adding various different chemicals for achieving required properties. These chemicals are called cement additives. Every cementing company has its own proprietary chemicals that they use as cement additives.

Cement Bond Logs (CBL) and Variable Density Logs (VDL) are conducted after the primary cementing operation for evaluating the quality of the cement bond behind a cemented casing or liner. It is a sonic tool that is run on wireline and uses a combination of transmitter and receivers to measure sonic wave arrival for assessment of cement bond quality. The principle used is that when no cement is bonded to the casing, the pipe is free to vibrate, whereas when the casing is bonded to hard cement, casing vibrations are attenuated proportionally to bonded surface.

A bit and scrapper run is recommended before the CBL-VDL log to remove any cement or scale from the casing wall. The tools should be properly centralized and the wellbore should be fluid-filled for accuracy of results since gas or air bubbles induce inaccuracies.

Radial tools were developed to overcome some limitations of conventional CBL-VDL tools. The Radial CBL-VDL tool permits a more accurate evaluation of cement distribution. It uses more sensitive transducers and evaluated the cement quality around the circumference providing precise location of partial bond and channeling.

Cement plugs are placed in the well for either repairing a leak, sealing a severe loss circulation zone, for side tracking or abandoning the well. It is a process of placing the cement slurry at desired depth and allowing it to set. Various different placing techniques are used depending on requirement and scenarios.

Cement Squeeze is a method of carrying out a remedial cementing job. Cement slurry is forced into the holes, fractures or splits in casing / formation. As the slurry is forced under pressure, the solid particles bridge the opening and the aqueous part escapes the slurry under squeeze pressure. The slurry is squeezed until the pressure reaches a predetermined level.

Dispersant is a chemical that is used for reducing the viscosity of the cement slurry. Since cement slurry is prepared by mixing dry blended cement with mix-water which has many chemicals, achieving desired rheological properties in the final mixture could be a challenge. It is very important for the slurry to have good rheological properties to be able to pump it through long narrow annuli. Dispersant helps in reducing the viscosity and making slurry pumpable without too high surface pressure.

Dump Bailer is a mechanical device used for spotting a cement plug above a permanent bridge plug. It is a vessel that is filled with measured quantity of cement slurry and is run in the well on wireline. As the Dump Bailer touches the permanent bridge plug, it is opened through a mechanical or electrical opening mechanism. As the vessel opens, the cement slurry is dumped on the plug by raising the bailer. Dump Bailer provides a quick and relatively inexpensive means of spotting cement plug. It is used quiet effectively at shallow depths. At deeper intervals, special cement slurry design is required to maintain desired slurry properties considering well temperature and static time for running the bailer in the well to spotting depth.

The External Casing Packer (ECP®) has an Inflatable Packing Element that is run as an integral part of the casing string to provide a mechanism to seal between casing OD and the well bore wall. ECP features a fully continuous mandrel as part of the casing or liner, and an Inflatable Packer Element as the main component. ECPs can be used to support primary cement, isolate lower zones during multi-stage cementing and isolate lost-circulation zones during cementing. Most packers are bladders that are inflated by fluid pressure, and have different pressure ratings.

Expandable Tubular

Fluid loss control agent is an important cement additive that reduces the fluid loss from slurry. When cement slurry is pumped into the well, it comes across several open formations. If a formation is porous and permeable, the aqueous phase of the slurry escapes into the formation under pumping pressure. This changes the rheological properties of the slurry due to improper solid and liquid fraction. If the loss of fluid is not controlled, the slurry could become too thick and eventually unpumpable causing failure of cement job.

Microannulus is a very small gap that can form between the casing or liner and the surrounding cement in primary cementing operations. Microannulus can jeopardize the hydraulic efficiency of a primary cementing operation and act as a channel for reservoir fluid migration, compromising the well's integrity. Microannulus could be created due to temperature & pressure variations during or after the cement job. These variations can cause cement shrinkage, cracking, or reverse ballooning of casing to create these micro gaps. Improper removal of mud or film of oil on the pipe before displacing with cement can also result in microannulus.

Micro-annuli - Causes, Prevention & Cure

Primary cementation is the process of securing the casing in the well with cement. Cement is pumped through the casing and is placed in the annulus between casing and formation. Objective of primary cementation is to create zonal isolation between the casing annulus and hydrocarbon or water bearing formation. Good cementation is important for safety of the well and also for successful well testing or completion.

Objectives of Well Cementing

Remedial cementing is carried out to repair the cement if the primary cement job fails to achieve desired results. As part of remedial cement job, cement is either squeezed into the casing annulus or placed at specified depth as a plug.

Retarders are the chemicals added to cement slurry for delaying the setting time. Class G ad H are most commonly used cements for well cementing. Neat slurry prepared with these cements do not have long enough setting time or thickening time at temperatures above 100 deg F. Depending on the well depth, downhole temperature and total cementing operation time, the slurry prepared with neat cement could start thickening before the completion of cementing process and make the slurry un-pumpable. Hence cement retarders need to be added to the slurry in required proportion depending on specific application.

Variable Density Logs (VDL) and Cement Bond Logs (CBL) are conducted after the primary cementing operation for evaluating the quality of the cement bond behind a cemented casing or liner. It is a sonic tool that is run on wireline and uses a combination of transmitter and receivers to measure sonic wave arrival for assessment of cement bond quality. The principle used is that when no cement is bonded to the casing, the pipe is free to vibrate, whereas when the casing is bonded to hard cement, casing vibrations are attenuated proportionally to bonded surface.

A bit and scrapper run is recommended before the CBL-VDL log to remove any cement or scale from the casing wall. The tools should be properly centralized and the wellbore should be fluid-filled for accuracy of results since gas or air bubbles induce inaccuracies.

Radial tools were developed to overcome some limitations of conventional CBL-VDL tools. The Radial CBL-VDL tool permits a more accurate evaluation of cement distribution. It uses more sensitive transducers and evaluated the cement quality around the circumference providing precise location of partial bond and channeling.

Ultra-Sonic Imaging (USI) Tool is an advancement on conventional sonic tools used for cement evaluation. It uses a single rotating transducer and receiver to emit and receive ultra-sonic waves and provide complete circumferential casing coverage. Another improvement of the USI Tool is the presentation of data in a color map, which allows the interpreter of the log to quickly identify intervals of good cement and the segments with problems in the cement sheath. Since its introduction, USI tools have proved to be a superior cement evaluation tool that overcomes most of the problems found with the limitations and interpretation constraints of other cement bond logging tools.