FE 02-16B1 Lag Time

Nội dung text: FE 02-16B1 Lag Time

1. Lag Time Determination • What is Lag Time • What are the factors affecting Lag Time • How is Lag Time measured • How is Lag Time checked
2. Lag Time • Always exists • Changes continuously as the hole deepens • Must be frequently checked and corrected
3. Annulus: Definition • It is the space around a pipe in the wellbore, the outer wall being the wall of either the hole or casing • Sometimes called annular space
4. Mud Flow Rate • The faster the mud is pumped the quicker it returns to the surface and hence the lag time is reduced
5. Pump Output Calculations • Duplex pump Output (gals/stroke) = (0.0515 x Stroke 2 2 Length x ((LinerOD) – (RodOD) /2))) x 0.2642 x Efficiency • Triplex pump Output (gals/stroke) = (0.0386 x Stroke 2 Length x (LinerOD) ) x 0.2642 x Efficiency • Output in bbls/stroke = gals/stroke/42
6. Example: Duplex Pump • Output (gals/stroke) = (0.0515 x 10 x ((6)2 – (5)2/2))) x 0.2642 x 0.95 • Output (gals/stroke) = 3.03 gals/stroke • Output (bbls/stroke) = 0.072 bbls/stroke
7. Well Profile • Depth: 7750 ft • Hole size: 8.5” • Pump Output: 425 gpm at 100 spm • Casing: – shoe at 6000 ft – 9 5/8” OD; 9” ID • DP: – 6350 ft. – 5” OD; 4.276” ID • DC: – 1400 ft. – 7” OD; 3.5” ID
8. Example Section 1: 24.51 x 425 / (92 - 52) Annular Velocity = 186 ft/min Annular Section = 6000 feet Lag Time for Section = 32.26 minutes Section 2: 24.51 x 425 / (8.52 - 52) Annular Velocity = 220.46 ft/min Annular Section = 350 feet Lag Time for Section = 1.59 minutes
9. Lag Time measured using Volume method: in minutes Annular volume = (D2 – d2) x 0.000971 x Length of annular section or Annular volume = ((D2 – d2) / 1029.4) x Length of annular section Lag time ann. sec (mins) = Annular volume (bbls) / flow rate (bbls/min) Lag time (mins) =  Lag time ann. sec • Disadvantage: affected by flow rate changes
10. Example Section 1: (92 - 52) x 0.000971 x 6000 Annular volume = 326.25 bbls Section 2: (8.52 - 52 ) x 0.000971 x 350 Annular volume = 16.05 bbls Section 3: (8.52 - 72 ) x 0.000971 x 1400 Annular volume = 31.60 bbls
11. Example Using ((D2 – d2) / 1029.4) x L: Section 1: ((92 - 52) / 1029.4) x 6000 Annular volume = 326.40 bbls Section 2: ((8.52 - 52 ) / 1029.4) x 350 Annular volume = 16.07 bbls Section 3: ((8.52 - 72) / 1029.4) x 1400 Annular volume = 31.62 bbls
12. Actual Lag Determination • A tracer is used to obtain an “accurate” measurement of lag time • Common tracers used are: – Calcium carbide (most common) – Rice, wheat, cellophane – Propane • Carbide + water -> acetylene
13. Using A Carbide Tracer: Frequency • Under normal circumstances a check should be made every twenty-four hours or 400 ft, whichever comes first. However, if carbide information is required due to suspicions of incorrect lag or washout, then carbides should be run as required
14. Using A Carbide Tracer 1. Make a carbide “bomb” by wrapping carbide in an envelope of paper towels (usu. 2 paper towels) and securing it with scotch tape. Do not use a lot of tape. 2. Place the carbide bomb in a plastic envelope. 3. Inform the driller that you intend to drop a carbide during connection. 4. During connection, after the cessation of pumping activity, zero the stroke counters. 5. On the rig floor, ask the driller to hold the next joint above the pipe in the slips, while you push the bomb inside the pin. Take extra care while doing this. 6. In the unit the gas equipment will detect the upcoming carbide. Note down the strokes when the acetylene peaks.
15. Comparing Different Carbide Checks • In order to compare different carbide checks, it is important that the same amount of calcium carbide each time • Recommended amount is 100 ml (though not usually followed in the field)
16. Downtime • “Downtime” or surface-to-bit lag is time it takes (measured in minutes or strokes) for a mud packet to travel from the surface to the bit.
17. Example • Example: – Length of 19.5 lb/ft, 5" drill pipe = 6350 ft. – Length of 9" x 3.5" drill collars = 1400 ft. – Pump output (bbls/stroke) = 0.1012 – Pumping rate = 120 spm
18. Carbide Lag Determination 1. Calculate the downtime in strokes 2. Carbide lag strokes = Actual strokes (from carbide) – downtime in strokes 3. Calculate the theoretical lag in strokes 4. Theoretical Open hole volume (bbls) = Bit diam2 x 0.000971 x Open hole length 5. Excess Carbide volume (bbls) = (Carbide lag strokes – Theoretical lag in strokes) x Pump output 6. New open hole diam (ins) = ((Theo. OH vol + Carbide excess vol) / (Open hole length x 0.000971))0.5
19. Example • Example: – Actual strokes from carbide = 5128 strokes • Calculations: – From previous example the down strokes is 1278 strokes. – From previous example the lag is 3697 strokes – Pump output is 0.1012 bbls/stroke
20. Example • Excess Carbide volume (bbls) = (3850 – 3697) x 0.1012 • Excess Carbide volume (bbls) = 15.48 bbls • New open Hole diameter = ((122.77 + 15.48)/(1750 x 0.000971))0.5 • New open Hole diameter = (138.3/1.69925)0.5 • New open Hole diameter = 9.02 in2
21. Presentation of Carbide Results on the Mudlog • Carbide results are placed in the remarks column of the mudlog • Example: Carbide = 0.5% at Vis 45 Lag Time at 7750 ft = 4956 strokes
22. End of Topic