Plug Cementing | What is Plug Cementing?

(adsbygoogle = window.adsbygoogle || []).push({}); Learning Objectives After completing this topic "Plug Cementing", you will be able to: Define plug cementing and associated terms. Summarize the reasons for setting plug cementing (Cement plugs). Describe the types of plug cementing (Cement plugs) and the context within which they are used. List the major considerations involved in designing a cement plug job. Describe methods for evaluating the success or failure of a cement plug job. Perform simple volume/displacement calculations, in particular using balanced plug method. List circumstances under which a cement plug can fail. What is a Plug Cementing? https://youtu.be/3w2Jx-SSTvw (adsbygoogle = window.adsbygoogle || []).push({}); Plug cementing is the placement of cement within a wellbore or casing to prevent flow. There are several common reasons for setting a cement plug in a well (Figure 1): Complete well suspension or abandonment Depleted zone abandonment Sealing off lower intervals in a well so that the well can be sidetracked or recompleted in a shallower zone Protecting weak formations from high pressure operations that may take place in shallower intervals Cement plugs can also be used during drilling operations. For example, in cases of lost circulation during drilling, a cement plug can be spotted to seal off the loss zone. In rare situations cement may be used as a means of well control during the drilling process if the well is at a point where there is no difference between pore pressure and fracture pressure. Figure 1: Cement plug applications (adsbygoogle = window.adsbygoogle || []).push({}); General Cement Plug Job Design Considerations (adsbygoogle = window.adsbygoogle || []).push({}); Cement plugging essentially consists of placing a cement column in an open or cased hole. It is a relatively simple process, although problems may be encountered such as fluid migration and cement contamination. Cement plug failures can be minimized by: Using a caliper log to accurately determine the hole gauge Using sufficient cement slurry to cover the plug volume Using a spacer or preflush ahead of and behind the cement slurry volume to minimize contamination Maintaining drill pipe or tubing movement during cement placement Moving the pipe slowly when lifting it out of the cement to minimize contamination Using diverter tools and viscous pills to minimize contamination at the base of the plug Other important factors to consider include slurry composition and volume, statutory requirements, placement technique, and well conditioning. Cement volume needed for a specific plugging operation depends on plug length and hole/casing diameter. Government regulations and safety considerations may also dictate plug length for well abandonment. (adsbygoogle = window.adsbygoogle || []).push({}); For sidetracking, sufficient volume is required to minimize drilling-fluid contamination that occurs at the plug top. Additionally, allowances are usually made for dressing off the drilling-fluid-contaminated top of the cement plug before attempting to sidetrack. Fluid spacers should be used both ahead of and behind the cement slurry to minimize mixing of cement and displacement fluid. Also, spotting a viscous pill spacer at the intended plug bottom can improve cement-plug stability. Use of a diverter tool, which forces fluid to flow directly at the wellbore face, provides a more uniform placement of both spacers and cement slurry. There are three common methods for setting cement plugs in a well: Balanced plug method Dump bailer method Two-plug method Each of these is described in the following sections. (adsbygoogle = window.adsbygoogle || []).push({}); Balanced Plug Method (adsbygoogle = window.adsbygoogle || []).push({}); The balanced plug method is the most common placement method. A tubing or drill pipe work string is run into the hole to the desired depth of the base of the plug, preceded by a spacer and followed by a spacer. The volumes and densities are calculated and displaced in such a manner that they align at the same levels in both the annulus and the casing. Animation 1 illustrates the sequence of the balanced plug method: Work string is positioned above viscous fluid plug base Spacer is circulated ahead of cement slurry Slurry is followed by tail spacer Spacers and cement in hydrostatic balance inside and outside work string Pipe is pulled up out of cement to circulate out excess https://youtu.be/ApFLU-sSGkg (adsbygoogle = window.adsbygoogle || []).push({}); It is common practice to pump the displacement fluid 2 or 3 barrels shy of full displacement, so that the plug can reach hydrostatic balance without any mud flowing back on the rig floor. After the plug is balanced, the pipe is slowly pulled out of the cement to a depth above the plug, and excess cement is reversed out. A risk of cement contamination during the displacement process is the primary drawback to the balanced plug method. If a high gel strength fluid is placed in the hole below the plug depth as a base, it can help to prevent migration of the cement slurry downward. For example, Animation 2 illustrates how 18 [latex]\tfrac{lbm}{gal}[/latex] cement slurry can fall through a 9 [latex]\tfrac{lbm}{gal}[/latex] drilling fluid but remain intact if a viscous bentonite pill spacer is used as a base when pumping a balanced plug. A mechanical cement retainer is another option. https://youtu.be/ZOvdGngtbQI (adsbygoogle = window.adsbygoogle || []).push({}); Dump Bailer Method (adsbygoogle = window.adsbygoogle || []).push({}); A dump bailer is a tool generally run on wireline that contains a quantity of cement slurry and is designed to open when it encounters a bridge plug previously set inside the casing below the desired plug location. The cement is “dumped” on the bridge plug, either by mechanical action or electronic signal sent through the electric wireline (Animation 1). A casing collar locator can also be used to precisely locate the bailer. https://youtu.be/l1uXyL06mlM This method is generally used for setting plugs at shallow depths, but can be used at greater depths if the cement is properly retarded. It is a relatively inexpensive approach since both bridge plug and bailer can be run on wireline and a rig is not needed. Multiple runs are necessary if a plug volume larger than the volume of the bailer is needed. Generally, dump bailers hold between 1 and 10 gallons of cement, depending on the length and diameter of the tool. (adsbygoogle = window.adsbygoogle || []).push({}); Two-plug Method (adsbygoogle = window.adsbygoogle || []).push({}); The two-plug method (Figure 1) refers to dual wiper darts or foam balls that displace cement down a work string and place it in the casing in a manner similar to the balanced plug method but with less likelihood of cement contamination. A spacer is pumped down the work string followed by the first of two wiper darts/balls to isolate the cement from the spacer. The cement slurry is pumped after the dart. When the dart reaches a locator sub near the bottom of the string, pressure is increased until pins shear, rupturing the dart and allowing the cement slurry to move through it. Figure 1: Two-plug method for setting a cement plug in casing (adsbygoogle = window.adsbygoogle || []).push({}); The cement is followed by a top dart to isolate it from the displacement fluid. When this plug arrives at the seat an increase in pressure provides a positive indication that the entire cement slurry volume has been displaced without contamination ahead of or behind it. The work string is then pulled up until the lower end of the tailpipe reaches the calculated depth of the top of the cement plug. Excess cement is circulated out. The tailpipe may be constructed of aluminum, making it easy for the pipe to be broken off and the work string retrieved if the tailpipe becomes stuck in the cement plug. Some advantages of the two-plug method are: It provides isolation ahead of and behind the slurry. The pipe is cleaned to the bottom of the tailpipe. The breakable tailpipe can be readily abandoned if stuck. Reverse-circulation is accomplished through the bottom of the tailpipe. It allows accurate placement of the cement plug. It provides positive surface pressure indications of proper displacement. (adsbygoogle = window.adsbygoogle || []).push({}); Plug Cement Slurry Design (adsbygoogle = window.adsbygoogle || []).push({}); Cement slurry design is determined by the objectives of the plug and depends on well depth, bottom hole circulating temperature (BHCT), and drilling-fluid properties. Key design considerations are: Rheology Density Compressive strength Thickening time Waiting on cement (WOC) time For lost-circulation plugs, slurry density should be controlled to reduce bulk slurry loss. Bentonite and silicate extenders are useful slurry-density-reduction additives, but they lower the ultimate cement compressive strength. Gilsonite, a natural asphalt, is also a lightweight cement additive used in lost-circulation plugs. Low compressive strength is desirable in a lost-circulation cement plug because it will need to be drilled through, and a high-compressive-strength plug is more likely to cause the bit to deviate. Slurries used to correct lost circulation are also designed to have thixotropic properties to help prevent total cement loss into the lost-circulation zone. Success of a sidetracking or whipstock plug depends to a large extent on high compressive strength. A densified cement (API Class A, G, or H) is recommended. Densified cements: (adsbygoogle = window.adsbygoogle || []).push({}); Provide high strength Tolerate drilling-fluid contamination Expand upon setting Decrease fluid loss Have negligible permeability when set Provide strong bonding Table 1 shows typical compressive strengths of densified Class G and H cements used for setting sidetracking cement plugs. Additives such as calcium chloride, dispersants, or retarders can be used in densified slurries. However, because slurries with dispersants have lower consistency and viscosity, the amount of dispersant should be kept to a minimum for better plug stability. Laboratory testing to ensure maximum viscosity, ample thickening time, and appropriate compressive strength should be performed before each job. Table 1: Compressive strengths of densified Class G and H cements Slurry Density(lbmgal) \left (\tfrac{lbm}{gal} \right ) 100° F and1,600 psi 140° F and3,000 psi 170° F and3,000 psi 200° F and3,000 psi After 12 hrs. 16.5 2,075 4,000 7,800 9,035 17.0 2,850 6,535 8,375 10,025 17.5 3,975 6,585 8,550 10,675 After 24 hrs. 16.5 5,475 8,985 9,750 10,460 17.0 6,035 9,060 11,075 12,660 17.5 7,025 10,125 11,860 12,875 Some operators using whipstock plugs claim that higher compressive strengths and better success ratios are achieved when 10 to 20% sand and/or silica flour are included in the cement composition. A possible explanation of this phenomenon is that sand may improve drilling-fluid removal by its scouring action, and thus may reduce drilling-fluid contamination, affecting compressive strength in a way unrelated to any silica/cement chemical reaction. To further reduce the risk of cement slurry contamination with drilling mud, it is advisable to pump a spacer fluid ahead of and behind the cement. The density of the spacer fluid should be equal to or greater than that of the drilling fluid. Because of the density difference between mud and cement, the cement will tend to migrate downward in the well. This is another reason for including a preflush in the plugging treatment. Because of the small cement volumes associated with plug cementing, the slurry should be batch mixed or at least mixed at a rate that will ensure uniform slurry density. (adsbygoogle = window.adsbygoogle || []).push({}); Testing Cement Plugs (adsbygoogle = window.adsbygoogle || []).push({}); After placing the plug and waiting on cement, the first step in testing the plug is to “tag” the top of the cement with the drill pipe or tubing. This step is very important, because the actual top of cement may be different from the pre-job calculation. The second test is usually a performance test. For example, if a cement plug has been drilled for lost circulation and full returns are observed during further drilling, the operation is considered to be successful. It is very important to allow adequate setting time before tagging a cement plug. Waiting on cement (WOC) time is determined using tables. With primary cementing, drilling operations may resume after the cement has attained a compressive strength of approximately 500 psi. But when sidetracking off of a cement plug, the plug compressive strength must be substantially higher before drilling is initiated. In general, for a successful sidetrack, the cement plug must be harder than the formation. Past experience in a given location should indicate the compressive strength requirement for a whipstock plug. (adsbygoogle = window.adsbygoogle || []).push({}); Reasons for Failure of Cement Plugs Cement plug failure can occur for any of the following reasons: Lack of adequate compressive strength (sidetracking plugs) Poor isolation (plug back, well abandonment plugs) Improper setting depth (all types of plugs) Plug either sinks in the wellbore fluid (all types of plugs) or it is lost to a thief zone (lost circulation plug) Reasons for such failure can generally be traced to the following input or operating errors: Equipment failure Poor slurry design Inadequate WOC time Drilling-fluid contamination (adsbygoogle = window.adsbygoogle || []).push({}); Assignment Plug Cementing (adsbygoogle = window.adsbygoogle || []).push({}); Given the conditions of the Assignment well shown here, determine: A. The depth to the top of the planned cement plug B. The volume of post-flush needed behind the cement slurry in order to maintain a balanced pressure condition between the tubing and the tubing/casing annulus C. The volume of mud needed following the post-flush to displace the slurry to the calculated top of the cement plug The formula for determining the height of a balanced cement plug is: [latex]H = \dfrac{V_{Cmt}}{C_{Annulus}\ +\ C_{Pipe}}[/latex] Where: H = height of cement plug, ft CAnnulus = Capacity of tubing/hole, tubing/casing, drill pipe/hole or drill pipe/casing annulus, ft3 per linear ft CPipe = Capacity of tubing or drill pipe, ft3 per linear ft Volume of cement used for plug: 100 ft3 Casing: [latex]7\ inch,\ 23\ \frac{lb}{ft} (Capacity\ =\ 0.2210\ \frac{ft^3}{ft}[/latex], or [latex]0.0393\ \frac{bbl}{ft})[/latex] Tubing: [latex]2.875\ inch,\ 6.4\ \frac{lb}{ft}\ (Capacity\ =\ 0.0325 \frac{ft^3}{ft}[/latex], or [latex]0.00579\ \frac{bbl}{ft} )[/latex] Annular capacity: [latex]0.1760\ \frac{ft^3}{ft}[/latex], or [latex]0.0313\ \frac{bbl}{ft}[/latex] Bottom of plug: 2500 ft Solution (adsbygoogle = window.adsbygoogle || []).push({}); A. Depth to top of cement plug: [latex]H = \dfrac{V_{Cmt}}{C_{Annulus}\ +\ C_{Pipe}} =\ \dfrac{100}{0.1760\ +\ 0.0325}\ =\ 480\ ft[/latex] Depth to top of cement plug [latex]= 2500\, ft - 480\, ft\ =\ \textbf{2020 ft}[/latex] B. Volume of post-flush needed in tubing in order to balance plug: 10 bbl of pre-flush in annulus [latex]\Rightarrow \dfrac{10}{0.0313}\ =\ 319\ ft\ \Rightarrow \ 319\ ft\ \quad[/latex] also required in tubing [latex]319\ ft\ \times 0.00579\ \frac{bbl}{ft}\ =\ \textbf{1.85 bbl post-flush}[/latex] C. Volume of mud needed to displace cement to top of plug depth: Top of cement = 2020 ft 1.85 bb of post-flush in annulus ⇒ = 319 ft Displace of mud to (2020 + 319) = 2339 ft Bbl of mud to displace [latex]=\ 2339\ \times\ .00579\ \tfrac{bbl}{ft}\ =\ \textbf{13.5 bbl}[/latex] Assessment Plug Cementing (adsbygoogle = window.adsbygoogle || []).push({}); 1. What is the main advantage of the two-plug method over the balanced plug method? A .Does not require the use of additional equipment or toolsB .Reduces the likelihood of cement contamination CorrectC .Reduces bottomhole pressureD .Does not require circulating out excess cement 2. Which of the following are characteristics of the dump bailer method? (Select all that apply.) A .The dump bailer tool is typically run on wireline. CorrectB .The dump bailer method uses dual wiper darts that displace cement down a work string.C .The dump bailer method is generally used for setting relatively small volumes of cement slurry at shallow depths. Correct 3. Identify key slurry design considerations for plug cementing. (Select all that apply.) A .A spacer fluid should be pumped ahead of and behind the cement. CorrectB .Fluid loss control should be tailored to the formation permeability.C .The slurry should be batch mixed. CorrectD .All cement plugs should use a densified cement. 4. What is the most common method for placing a cement plug? A .Dump bailer methodB .Two-plug methodC .Balanced plug method Correct 5. Identify reasons why a cement plug can fail. (Select all that apply.) A .Fluid loss to formationB .Plug sinks in the wellbore fluid CorrectC .Debris in the perforationsD .Improper setting depth Correct 6. Which of the following are reasons for setting a cement plug in a well? (Select all that apply.) A .Repair casing leaks from split or corroded casing.B .Abandon a depleted zone. CorrectC .Complete well suspension or abandonment. CorrectD .Repair mud channels left behind the pipe.E .Seal off lower intervals in a well so that the well can be sidetracked or recompleted in a shallower zone. CorrectF .Protect weak formations from high pressure operations that may take place in shallower intervals. Correct 7. When applying the balanced plug method, which of the following is a common practice to prevent mud flowing back on the rig floor? A .Pump the displacement fluid 2 or 3 barrels over full displacement.B .Pump the displacement fluid 2 or 3 barrels under full displacement. CorrectC .Pump the calculated volume of displacement fluid so that the top of the cement in the drill pipe or tubing is at the desired cement top. Recommended for You (adsbygoogle = window.adsbygoogle || []).push({}); Introduction to Well Cementing Chemistry and Classification of Oil well Cements Cement Additives in Drilling Cement Testing Procedures Cement Slurry Flow Properties and Mud Displacement Cementing Equipment Primary Cementing Squeeze Cementing (adsbygoogle = window.adsbygoogle || []).push({});