Plug cementing

In addition to primary cementing of the casing and liner, there are other important well cementing operations. These are squeeze cementing and plug cementing Such operations are often called secondary or remedial cementing [161]. 

To prepare an oil-based cement, the cement is suspended in hydrocarbons instead of water. In this organic environment, no setting takes place and the cement particles remain fine. Therefore they may penetrate into small pores. The setting starts when the cement particles come in contact with water. This takes place in the formation. Oil-based cements are mainly used as plugging cements and squeeze cements. 

The addition of furfuramide to a plugging cement in portions of 1 % to 10% produces a plugging rock of increased corrosion resistance and reduced water permeability [1098]. 

Small amounts of modified methylcellulose and mono-substituted sodium phosphate (both 0.1%), may be added to plugging cement [1766]. The mono-substituted sodium phosphate weakens the effect of calcium ions on modified methylcellulose and prevents its coagulation. Stabilization of the plugging solution and increased strength of the cement rock result. 

Z. N. Kudryashova, B. V. Mikhajlov, A. P. Tamavskij, S. R. Khajrullin, V. M. Mustafaev, and P. F. Tsytsymushkin. Plugging solution for cementing oil and gas wells—contains plugging cement, ash, additional clay powder and crude light pyridine bases, and water. Patent SU 1765366-A, 1992. 

Plugging solution for low temperature wells—contains plugging cement, water and molten chloride melt obtained as waste from titanium-magnesium production as complex chloride additive. Patent RU 1091616-C, 1995. 

Once the cementation has been completed the rig will wait on cement (V OC), i.e. wait until the cement hardens prior to running in with a new assembly to drill out the plugs, float collar and shoe, all of which are made of easily drillable materials. 

Plug back cementations, i.e. cement placement inside the casing and across the perforations may be required prior to sidetracking a well or in the course-of abandonment. 

Cement is then placed across the open perforations and partially squeezed Into the formation to seal off all production zones. Depending on the well configuration it is normal to set a series of cement and wireline plugs in both the liner and production casing (see Figure 16.2), to a depth level with the top of cement behind the production casing. 

The cement is encountered when drilling a cement plug or out of new casing. 

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