1 PETE 411 Well Drilling Lesson 31 Plugback Cementing

2 Plugback Cementing  Case I: No Spacer  Case II: Equal Height Spacers  Case III: Spacer Ahead of Cmt. (only)  Case IV: Two Unequal Spacers  Mixtures and Solutions

3 Read: Applied Drilling Engineering Ch. 3. Cementing HW #16 Due November 22, 2002

4 Balanced Cement Plug Fig Placement technique used for setting cement plug.

5 Cementing (Open-Hole Plugging) 1. Plug-back for abandonment 2. Plug-back for fishing or hole deviation  Open-hole plugging is usually performed with “slick” drillpipe or tubing.  In some cases, reciprocating scratchers may be run to enchance cement bonding.

6 Types of Balanced Plugs Case I: No water or other fluid of different density from that in the hole is run ahead or behind the cement slurry. Case II: Water or other fluid of different density from that hole is run ahead and behind cement slurry. The volume of fluid ahead and behind slurry is calculated so that height in casing is same as height inside the string.

7 Displacement Case III: Water or other fluid of different density from that in the hole is run ahead of cement slurry and hole fluid only is used as displacing fluid. Case IV: Water or other fluid of different density from that in the hole is run ahead and behind cement slurry. In this case, the heights of fluid in annulus and drill string are not equal.

8 Case I C Height of plug after pulling pipe Height of plug with pipe in place T

9 Case I CT HFinal Height

10 Example Balanced Plug - Case I Set a balanced cmt. plug from 8,500-9,000 ft, with no fluid spacers. 1. Open hole diameter = 10 3/4” 2. Assume no washout 3. Use 5”, #/ft DP, open ended 4. Use class H cement, 15.6 #/gal

11 Example - Case I (a) Calculate volume of cement slurry required: DHDH L

12 Example - Case I (b) Calculate actual height of plug when DP is in place at 9,000 ft. If then CT = Height of Plug, with Pipe in place

13 Example - Case I (b) cont’d In this case, ( Halliburton Book )

14 Example - Case I (b) cont’d

15 Example - Case I (c) Determine the quantity of mud displacement inside the DP that will ensure a balanced plug. Balance requires that the pressures be equal inside the DP and in the annulus, at 9,000’. P D P A h MD = h MA

16 Example - Case I

17 Example - Case I V Displ = bbl (of mud) Volume of mud displacement (behind the cement slurry) = 8,469 ft * ft 3 /ft

18 Example - Case I Also required: Class H cement req’d Mix water req’d

19 mud water cement water mud Case II hWhW Height of plug after pulling pipe h WD = h WA Height of plug with pipe in place

20 Example, Balanced Plug - Case II Set a balanced plug, 500 ft high, with its bottom at 9,000 ft. Use water spacers of equal height inside DP and in annulus. Volume of annular water spacer = 10 bbl Open hole diameter = 10 3/4”. No washouts 5” DP, #/ft, open ended. Use class H cement, 15.6 #/gal

21 Example - Case II (a) & (b) From previous example:

22 Example - Case II (c) Calculate height (length) of water spacer in DP: In annulus,

23 Example - Case II (d) Volume of water spacer inside DP V W,DP … for spacers of equal height V W,DP = 2.02 bbls

24 Example - Case II (e) A balanced plug requires that P D P A

25 Example - Case II (e) cont’d

26 Example - Case II Volume of mud required to displace cement and spacers = ft 3 V Displ = bbls

27 Check

28 Pumping Sequence: 1. Water spacer for annulus: 10 bbls 2. Cement Slurry for Plug: 3. Water spacer behind cement: 2.0 bbls

29 Pumping Sequence 4. Mud displacement behind second water spacer: bbls Total fluid pumped = = bbls (at 10 bbl/min this would require ~22 min)

30 Case III Hole fluid density > density of water Hydrostatic heads in DS and annulus must balance at top of cement slurry with DS in hole. hWhW Height of plug after pulling pipe Height of plug with pipe in place 0

31 Case IV - General Case Hole fluid density is greater than water density. Hydrostatic heads in DS and annulus must balance at top of cement slurry with DS in hole.

32 Procedure in setting balanced plug 1. Run drillpipe in to depth where plug is to be set; in this case 9,000 ft. (open ended). 2. Circulate and condition mud one complete circulation to make sure system is balanced. 3. Pump spacers and cement per calculations and displace w/proper amount of fluid

33 Procedure in setting balanced plug 4. Stop pumps; break connection at surface. A. If standing full, plug is balanced. B. If flowing back, a mistake in calculations has been made. Stab inside BOP, or have a slug of heavy mud ready to pump.

34 Procedure in setting balanced plug 5. Once the end of the drillpipe clears the plug, there is a good chance the pipe will pull wet. This is because pressures have gone back into a completely balanced mud system. 6. If pulling wet, slug pipe and pull out of hole.

35 Procedure in setting balanced plug 7. Even if plug is severely out-of-balance, never try to reverse cement out of hole. 8. Tag plug with DP at end of 8 hours. If too high, plug may have to be drilled out and another plug spotted. If too low, spot another plug to required height with DP just above top of first plug.

36 Calculations to Design a Balanced Open Hole Cement Plug 1. Calculate cu. ft. of slurry required for plug in open hole. 2. Multiply this volume by excess factor (50% excess factor = 1.50)

37 When dealing with a washed-out hole, where an excess factor is required, it is usually easier to calculate a new, effective hole size, and use that instead of the excess factor. Calculations for balanced plug - HINT If 50% excess is required Use d 2 for calculations This is the effective dia.

38 Calculations for balanced plug 3. Find height (h, ft) cement will occupy when drillpipe is at bottom of plug during pumping:

39 Calculations for balanced plug - cont’d 4. Find height (ft) water spacer ahead of cement will occupy in annulus. Use d 2 to calculate this (to account for the excess factor). 5.Find height (ft) water spacer behind cement will occupy in DP. Do not use excess factor. 6. Pressures must balance at bottom of plug

Convert this mud to feet inside DP. Calculations for balanced plug - cont’d

41 9. Convert this footage to bbls inside DP for proper displacement. 10. To find sks cmt required, divide volume, V 2, by yield/sk. This yield, Y sk, may be in the Halliburton tables (or may not…). Number of sx req’d, Calculations for balanced plug - cont’d

If yield not shown, calculate from formula for mixtures. Solve for in this formula. Add the V’s for yield. 12. Total mix water will be times number of sacks. V W total = ( V W / sk ) * N Calculations for balanced plug - cont’d