Optimization of Balanced-Plug Cementing
- Kristian Harestad (BJ Services) | Thor P. Herigstad (BJ Services) | Arne Torsvoll (Statoil) | Nils E. Nodland (Statoil) | Arild Saasen (Statoil)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 1997
- Document Type
- Journal Paper
- 168 - 173
- 1997. Society of Petroleum Engineers
- 1.6 Drilling Operations, 2 Well Completion, 1.14.3 Cement Formulation (Chemistry, Properties), 1.10 Drilling Equipment, 1.11 Drilling Fluids and Materials, 3 Production and Well Operations, 1.6.9 Coring, Fishing, 1.14 Casing and Cementing, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6.6 Directional Drilling, 4.3.4 Scale
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Placing properly balanced cement plugs in wells has been a serious problem to the drilling industry for decades. The cement plug is rarely found at the desired depth. Because of the fact that the cement slurry usually is more dense than the well fluid, the cement plugs are found deeper than anticipated, and the quality of the cement plug further down may be doubtful. This often results in setting a new cement plug to proceed with operations such as kickoff in the wellbore, abandonment, or pulling the blowout preventer. In the North Sea area cement plugs fail in more than 25% of the cases. Several techniques have been developed to keep the cement in place, but they are either very time consuming or they have a low degree of success.
A tool has been developed that is deployed below the cement plug, preventing the cement slurry from escaping down the well. This tool is very easy to install, does not take extra rig time and covers all sizes of hole diameters, from 152.4 to 584.2 mm(6 to 23 in.), with one tool. Before field application, the system was tested in full scale, where cement plugs were set in casings both with and without the new tool. The result from this testing showed that the new tool had a very high degree of efficiency compared to the reference tests that were performed without the tool. This paper presents the results from the tests and case histories where the tool was used.
Balanced cement plugs in oil and gas wells are used for several reasons, including the following.
. To sidetrack above a fish or to initiate directional drilling.
. To plug back a zone or a well.
. To solve a lost-circulation problem during drilling operation. . To provide an anchor for an openhole test.
. For other remedial work.
Such work often demands a cement plug at a specific place in the well, and rarely at the bottom of the wellbore. As the well fluid commonly has a lower density than the cement slurry, it has been experienced that the cement slurry tends to fall or flow down through the well-fluid column, leaving the top of cement (TOC) deeper in the well than anticipated. Smith et al. studied plug cementing in detail and made excellent recommendations for setting successful cement plugs. These recommendations included the placement of a gelled fluid underneath the cement plug and use of a diverter tool. Bour et al. recommend using a reactive fluid system placed in the well before pumping cement. This fluid develops a rapid gel structure upon contact with the cement slurry. The objective of this gel is to keep the cement slurry in place. These practices have been further improved by Heathman et al. by optimal management planning.
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