The most common critical factors when setting a balanced cement plug is avoiding contamination, slurry design which includes density contrasts and yield point progression, slurry volume, stability of any liquid base below and temperature estimation. This section will give insight into these critical factors and suggest measures to avoid failure by correct addressing the different challenges of setting a competent balanced plug.
Contamination of the cement slurry during placement can cause failure of setting a competent plug. Contamination is the
69 process cannot be reversed [50]. The most severe consequence is longer hydration time but it may cause the cement not to set or harden at all. Only ten percent contamination of the cement plug will lead to three to five times longer cement setting time [46]. Contamination will also decrease the compressive strength of the cement when set. Sensitivity for contamination is especially high when the cement volume is relatively low, which it is during plugging operations. The degree of fluid mixing cannot be assessed until the plug has been weight and pressure tested and will increase rig time significantly if remedial operations are needed. There are four different phases during placement where contamination may occur [51].
1. During flow through workstring 2. During flow up the annulus
between workstring and casing 3. During pulling of stinger
4. During hydrating or during placement by fluid swapping at the base of the plug
Contamination during flow through workstring is a problem that is usually avoided by using a spacer or fresh water pill as a separator between cement and displacement fluid. In coiled tubing applications, fluid separators may not always prevent undesired degree of mixing. Then mechanical separators, such as darts or spunge balls can be used in addition.
Well properties such as pressure, temperature, inclination, wellbore geometry and the rheology of slurry and of fluids below and above will contribute to the mixing process. If the fluids are mixed before leaving the workstring there is a high probability of cement settling in pockets and not providing any hydraulic isolation. Especially in deeper wells it can be optimal to use a mechanical separator due to the longer traveling distance and higher temperatures.
To avoid contamination during flow up the annulus between workstring and casing, it is crucial to do a correct calculation of the displacement volume. An incorrect calculation will lead to under-displacement or over-displacement. This is illustrated in Fig. 7.5 and Fig. 7.6.
Overestimating or underestimating the displacement volume will consequently lead to imbalance between fluid levels operational factors will make it difficult to calculate the volume displaced by cement [16].
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Figure 7.5 - Underdisplacement of cement during plug setting.
Figure 7.6 – Overdisplacement of cement during plug setting.
Primarily the stinger is smaller than the drillpipe and the flow area is not the same, making the cement flow faster through the stinger compared to the drillpipe. Secondly cement may set on the inner side of the tube during the operation or the abrasive cement slurry may cause erosion on the inner wall, creating small changes in inner tubing diameter, and consequently affecting flow capacity. Thirdly drilling fluids are compressible when exposed to
high pressures in the well and the actual volume pumped is impossible to monitor without knowing the exact pump efficiency. In other words, calculating the exact displacement volume is unrealistic.
Overdisplacement will place spacer or mud in the cement plug before it sets and thereby contaminate the plug. Hence it is common practice to underdisplace. As the stinger is pulled through the upper interface, the cement will be pushed out of the stinger at a higher rate to fill the volume occupied by the workstring as illustrated in Fig. 7.7.
Underdisplacement will create a mixing zone at the top of the plug. This zone containing excess cement and spacer is usually circulated out before the cement thickens. If the cement slurry has already developed a high yield point before the stinger is pulled, the top of competent plug may end up at a lower depth than anticipated.
Figure 7.7 – Underdisplacement at the time the stinger leaves the top of the plug.
71 Yield point progression is one of the
reasons for using a cement stinger instead of larger pipe; a smaller pipe will disrupt the interface less than a bigger pipe. To lower degree of mixing it will also help to pull the stinger out slowly. However, pulling out the stinger too slow will induce risk for getting the stinger stuck in the interface a source of contamination, but the interface is also fundamental to prevent the cement slurry moving downwards in the bottom interface stable and to stop gravity from channeling the slurry downwards before hardening. This fluid will need to be placed below the planned cement plug bottom prior to cement placement. It has to have sufficient gel strength to support the gravitational forces which are acting when heavy cement is placed on top. A fluid
Wellbore inclination, inner casing diameter and slurry design is crucial when placing a plug with no mechanical barrier underneath [53]. It is more difficult to
place cement plug in a deviated wellbore compared to a vertical or horizontal one.
The particles in the cement slurry will have a tendency to settle at the lower side of the pipe and slide down, and the more buoyant cement upper layer of the cement slurry will move upward. This effect, called the Boycott effect, will accelerate itself, and create instability within the slurry [46]. It is also harder to keep a stable interface in a larger diameter casing than in a smaller diameter, because it requires higher gel strength to withstand the gravity forces from the heavy liquids above. Cement may penetrate the layer of gel below and move downwards. point, have a reasonable waiting-on-cement (WOC) time, be stable in well conditions and still be able to be pumped through the string and placed in the well. If placed in open holes, fluid loss must be addressed as well.
Correct amounts of water are added to tailor the optimum cement density during mixing. Stabilizers will be needed to keep the particles from settling by sustaining the viscosity. The concentration of retarders should be optimized according to temperature in the specific well. Too high retarder concentration will cause long WOC time and make it difficult to estimate when the cement has settled, or the cement may not set at all. The contrary will cause premature settling and consequently operational problems. The optimal concentration of retarders is dependent on the environment in which the plug shall be
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set. Estimating the temperature of this environment is critical for success.
7.3 BALANCED PLUG