Cementing operations in the oil and gas industry are a critical component of well construction and integrity. Proper casing and cementing hardware play a crucial role in ensuring the successful execution of these operations. In this article, we will explore the various equipment used in cementing operations, including guide shoes, float collars, centralizers, cement wiper plugs, and multi-stage collars.
Guide Shoes and Float Shoes:
Guide shoes are essential tools used to guide casing strings through the wellbore without encountering issues such as jamming in washed-out zones or deviations. They can be simple guides or more advanced float shoes, which incorporate valves to prevent cement from flowing back into the casing once it is displaced behind the casing. These shoes can be made with inner parts composed of either aluminum or cement, both of which are easily drillable. Cement offers greater impact resistance, making it a popular choice. However, it’s important to note that float shoes may add time to the casing running process due to the need to temporarily halt operations to fill the casing from the top. To expedite this process, orifice fill shoes (automatic fill-up shoes) can be used to fill the casing while it is being run in the hole.
Float collars serve as one-way valves placed one or two joints above the shoe. They serve the same purpose as float shoes, preventing fluid backflow into the casing during various stages of the cementing process, including mud backflow during casing insertion and cement slurry backflow after displacement. Float collars may employ ball-type or flapper-type valves, with flapper valves being preferred when minimal hydrostatic pressure difference is expected due to their superior sealing capabilities.
Casing centralizers are mechanical devices designed to maintain a uniform annular space around the casing, ensuring that cement can effectively seal the casing to the borehole wall. Two main types of centralizers are bow-spring and rigid blade designs. Bow-spring centralizers, while cost-effective, are better suited for vertical or slightly deviated wells. Rigid-blade centralizers, although slightly more expensive, are more rugged and can provide good centralization even in deviated wellbores. Proper centralization is crucial for cementing success, and the choice between these two types should consider wellbore conditions. In many cases, there will be mixed between bow-spring and rigid blade designs in order to maximize wellbore centralization in a cost effective way.
Cement Wiper Plugs
Cement wiper plugs are rubber plugs employed to separate the cement slurry from other fluids, maintaining the integrity and predictability of the slurry’s performance. Two types of cementing plugs are used during operations: the bottom plug and the top plug. The bottom plug precedes the cement slurry to minimize contamination, rupturing upon reaching the landing collar to allow cement flow. The top plug provides a clear indication of contact with the landing collar through increased pump pressure. These plugs play a vital role in ensuring the separation of mud and cement, preventing over-displacement of cement, confirming cement placement, and enabling casing pressure testing.
Multi-stage collars, also known as DV (Dual Valve) tools, are employed to facilitate cementing in two stages, mitigating excessive hydrostatic pressure on weak formations. These tools consist of a section of casing with similar strength properties as the rest of the string, featuring two internal sleeves and openings covered by the lower sleeve. The lower sleeve is opened by dropping a bomb, allowing cement to be pumped through the casing and placed around it. After reaching the desired cement volume, a closing plug is dropped, causing an upper sleeve to cover the holes in the stage collar. Multi-stage cementing is utilized to reduce pumping pressure or time, lower hydrostatic pressure on formations, selectively cement specific zones, and ensure complete casing cementation, with the positioning determined by the cement column length and formation strength.
Casing and cementing hardware are integral components of cementing operations in the oil and gas industry. Properly selected and deployed equipment, including guide shoes, float collars, centralizers, cement wiper plugs, and multi-stage collars, ensure the successful execution of these critical operations, contributing to well integrity and overall operational success.
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