Cálculos de Cementación Primaria

CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 1 of 14

Primary Cement-Job Calculations

INTRODUCTION .............................................................................................................2 SLURRY AND PREFLUSH VOLUMES................................................................................2 CEMENT-SYSTEM QUANTITY..........................................................................................6 DISPLACEMENT VOLUME ...............................................................................................7 CASING CAPACITIES ............................................................................................................ 7 WATER REQUIREMENTS ................................................................................................8 MAXIMUM LIFTING FORCE ............................................................................................8 EXAMPLE WELL INFORMATION .............................................................................................. 10 CEMENT CALCULATIONS ..................................................................................................... 12 DISPLACEMENT VOLUME ..................................................................................................... 12 WATER REQUIREMENTS...................................................................................................... 12 MAXIMUM LIFTING FORCE ................................................................................................... 13

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 2 of 14

Introduction The following must be known before a primary cement job can be successfully completed: •

slurry and preflush volumes

•

cement-system quantity

•

displacement volume

•

water requirements

•

maximum lifting force (casing buoyancy)

The manual calculation of these values is presented in this manual section.

SLURRY AND PREFLUSH VOLUMES Casing/openhole annular volumes are calculated to determine •

the amount of slurry required for a desired fill-up

•

the preflush volumes to provide a desired annular-height coverage.

During the initial cement-job design, drilling is normally still in progress and the caliper log has not been run. The slurry and preflush volumes are estimated based on the bit size plus an excess volume (e.g., 30%) determined from field experience or based on government regulations. Vslurry = Cannulus x Lslurry where Vslurry = slurry volume (ft3) Lslurry = length of slurry column (ft) Cannulus = annular capacity (ft3/ft; from the Dowell Field Data Handbook). The job design is later finalized based on annular volumes determined from the caliper log. The type of caliper can affect the calculated amount of cement, and the resulting fill-up by the cement. Two- or three-arm calipers, with arms that operate together, may underestimate (or overestimate in the case of the two-arm caliper) the size of the hole. This is especially true for deviated wells which tend to have oval boreholes. For these situations, four-arm Hole calipers or six-arm calipers (with independently operating arms) are preferred.

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 3 of 14

Figure 1: Hole Calipers

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 4 of 14

For calculating the annular volume using a basic caliper log, the interval of interest is divided into increments, and the average hole diameters are estimated for each increment. Annular-Volume Calculations from Caliper Measurements Hole Diameter (in.)

Annular Capacity for 7-in. Casing (ft3/ft)

Annular Length (ft)

Annular Volume (ft3)

10.0

0.2782

30

8.346

10.5

0.3341

40

13.364

11.0

0.3927

10

3.927

13.5

0.7267

10

7.267

15.5

1.0431

10

10.431

TOTAL

43.335

Once the slurry volume has been calculated, an excess is added (normally 10 to 20%), based on field experience or government regulations, and then the cement (or blend) requirements are determined. Assuming a 43.335-ft3 total slurry volume (from Table 98), a 10% excess, and a slurry yield of 1.18 ft3/sk, the required cement is calculated as follows. Slurry Volume = 43.335 ft3 x 1.10 (10% excess) = 47.669 ft3 Most logging companies offer computerized annular-volume calculations which are presented on the basic caliper log (see figure below).

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 5 of 14

Figure 2: Borehole Geometry Log

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 6 of 14

The tick marks on the depth track represent the total hole volume (left) and the annular volume between the casing and openhole (right) in 10-ft3 increments. The long tick marks represent 100ft3 increments and therefore replace each tenth small tick mark. For metric logs, the small and long tick marks indicate the total volume in 1-m3 and 10-m3 increments, respectively. The total hole volume (VHOL) and cemented annulus (VCEM) are also shown in the header. Slurry excess is only calculated for the openhole portion to be cemented. This excess is to account for the inaccuracy of the caliper measurement, cement which may be lost into the formation, hole enlargement, or fluid loss from the cement into permeable zones. When slurry returns to the surface are desired or required, excess volumes may be used to ensure that they are achieved. The amount of excess must be carefully selected. If the well has a weak formation which is close to being fractured, then excess cement (which will raise the cement top) may cause the formation to be fractured because of the increased hydrostatic and friction pressures. The final slurry-volume calculation is the amount that will remain in the shoe joints (between the float collar and the shoe). This is simply the casing volume between those two points. This volume is added to the annular slurry volume and the slurry excess to equal the total slurry volume for the job.

CEMENT-SYSTEM QUANTITY Besides the class of cement and additive details, a cement-system description always includes •

slurry density (lbm/gal)

•

slurry yield (ft3/sk)

•

mix-water requirement (gal/sk).

The slurry yield is the volume occupied by one unit of cement or cement blend (e.g., sack, equivalent sack, tonne) plus additives and mix water. For cement measured in sacks, the yield is expressed in cubic feet per sack (ft3/sk) or cubic feet per equivalent sack (ft3/eq sk); for cement measured in tonnes, the yield is expressed in liters per tonne (liter/t) or cubic meters per tonne (m3/t). The term equivalent sack is used when the cementitious material is a blend of fly ash and cement. The amounts of fly ash and cement to equal an equivalent sack can be obtained from your laboratory. Once the total slurry volume has been determined, the total cement in sacks, equivalent sacks, or tonnes is calculated using the following equation: Total Cement = Total Slurry Volume / Slurry Yield

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 7 of 14

DISPLACEMENT VOLUME The displacement volume to land the plug equals the length of the pipe to the float collar times the pipe capacity. Vdisplacement = Lfloat collar x Cpipe where Vdisplacement = displacement volume (bbl/ft) Lfloat collar = float-collar depth (ft) Cpipe = casing capacity (bbl/ft; from the Dowell Field Data Handbook). During the displacement, the actual volume pumped may be greater than the calculated volume due to air entrainment in the slurry and pump inefficiency. Overdisplacement of the slurry past the shoe must be avoided. Therefore, the decision to pump a volume in excess of the calculated volume must be well thought out.

Casing Capacities The casing dimensions and weights used by CemCADE and presented in the Dowell Field Data Handbook are nominal values as defined in API Specification 5CT. Tolerances are associated with these nominal values. A 9-5/8-in., 36-lbm/ft casing with a nominal ID of 8.921 in. is used in this discussion to illustrate the possible effect of these tolerances. The casing OD tolerances are +1.0% with an absolute maximum of 0.125 in. and -0.5%. Therefore, the casing OD can vary from 9.577 to 9.721 in. The casing weight tolerances are +6.5% and -3.5%. Therefore, the casing weight can vary from 34.74 to 38.34 lbm/ft.API Specification 5CT does not define a tolerance on the casing ID, but derives it from the tolerances on the casing OD and weight. The maximum possible casing ID corresponds to the maximum casing OD and the minimum casing weight. The minimum possible casing ID corresponds to the minimum casing OD and the maximum casing weight. Assuming a steel density of 505 lbm/ft3 (value calculated from the nominal OD, ID and weight), the minimum and maximum ID for a 9-5/8-in., 36-lbm/ft casing are 8.820 and 9.049 in., respectively. The casing capacities for the different inside diameters are •

minimum ID: 0.07557 bbl/ft

•

nominal ID: 0.07731 bbl/ft

•

maximum ID: 0.07954 bbl/ft.

For a displacement length of 10,000 ft, the absolute errors on the displacement volume (from the nominal value) are +22 bbl and -17 bbl. All of the casing joints in a 10,000-ft well do not have their ID at the upper or lower limit. Statistical ID data from the casing manufacturers are required to calculate more reasonable error figures. However, this calculation exercise does show that the displacement volume for a given casing size and depth is not fixed but may vary significantly as a result of the tolerances in the casing OD and weight. CONFIDENTIALITY This manual section is a confidential document which must not be copied in whole or in any part or discussed with anyone outside the Schlumberger organization.

CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 8 of 14

WATER REQUIREMENTS The water requirements for a primary cement-job operation equals the sum of •

water required to fill the mixing/pumping units and the treating lines

•

water to pressure test the treating lines

•

mix water for the washes and spacers

•

mix water for the cement

•

displacement volume (if displacing the slurry with water)

•

water required for flushing the treating lines before the displacement

•

water needed for washing up the cementing equipment

•

tank bottoms (the tank volume from the tank bottom to about six inches above the suction valves).

The mix water for the cement is calculated as follows: Vmix water = REQmix water x AMTcement where Vmix water = volume of mix water (gal) REQmix water = mix-water requirement (gal/sk) AMTcement = amount of cement (sk). The mix-water volumes for the spacer and washes are calculated by following the instructions in their respective sections in the Cementing Materials Manual. The instructions for determining the displacement volume are discussed in Subsection 4 of this manual section. The volume for the tank bottoms can be calculated. The remaining water volumes must be estimated. Once the total water requirements have been determined, a safety factor (excess) should be included (e.g., an additional 50 bbl).

MAXIMUM LIFTING FORCE During some cementing treatments, there is a danger that the casing may be " pumped " out of the well. The conditions which favor such an occurrence are 1. lightweight pipe 2. short pipe length 3. large-diameter pipe 4. high-density cement slurries 5. low-density displacement fluids 6. high annular friction pressures 7. bridging in the annulus 8. backpressure. CONFIDENTIALITY This manual section is a confidential document which must not be copied in whole or in any part or discussed with anyone outside the Schlumberger organization.

CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 9 of 14

Conditions 2 through 5 are all met when cementing surface or conductor casings. The Fill Sequence module of the CemCADE program automatically calculates the maximum lifting force (MLF) based on the static conditions at the end of the job. The CemCADE calculation of the MLF is not performed for liner cement jobs. The MLF is manually calculated as follows: MLF = 0.785 x (Phyd(ann) Phyd(cas)) x Dcas2 where Phyd(ann) = annular hydrostatic pressure at end of job (psi) Phyd(cas) = casing hydrostatic pressure at end of job (psi) Dcas2 = casing outside diameter (in.). If the MLF value exceeds the total weight of the casing, then the casing can be pumped out of the hole and must therefore be chained down. The casing and annular hydrostatic pressures at the end of the job are calculated using the following equation: Phyd = 0.052 x H where ρ = fluid density (lbm/gal) H = height having fluid density (ft).

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 10 of 14

Example Well Information Surface casing:

13-3/8 in., 54.5 lbm/ft to 1700 ft

Openhole:

12-1/4 in. to 4950 ft

Casing:

9-5/8 in., 36.0 lbm/ft

Excess required:

25% (caliper log is not available)

Shoe joint:

42 ft

Top of cement:

300 ft inside 13-3/8-in. casing

Top of tail cement:

4450 ft

Lead system:

50:50, fly ash (Denver):Class A + 4% D20 + Additives

Tail system:

Preflush:

density:

12.9 lbm/gal

yield:

1.54 ft3/eq sk

mix water:

7.80 gal/eq sk

Class H + Additives density:

16.4 lbm/gal

yield:

1.05 ft3/sk

mix water:

4.29 gal/sk

40 bbl Chemical Wash 100 (41.5 gal/bbl water) density: 8.32 lbm/gal

Displacement fluid:

11.5 lbm/gal mud

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 11 of 14

This well information is illustrated in the figure below. Data is taken from the Dowell Field Data Handbook.

Figure 3: Example Well For Primary Cement-Job Calculations Casing Capacity and Annular Capacities Casing capacity:

0.4341 ft3/ft or 0.0773 bbl/ft (9-5/8 in.)

Annular capacities:

0.3627 ft3/ft (9-5/8-in. casing/13-3/8-in. casing) 0.3132 ft3/ft (9-5/8-in. casing/openhole)

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 12 of 14

Cement Calculations Lead slurry volume between 9-5/8-in. and 13-3/8-in. casings: V1 = 0.3627 ft3/ft x 300 ft = 108.8 ft3 Lead slurry volume between 9-5/8-in. casing and openhole: V2 = 0.3132 ft3/ft x (4450 - 1700) ft x 1.25 (25% excess) = 1076.6 ft3 Total lead slurry volume: VL = V1 + V2 = 1185.4 ft3 Total lead cement SacksL = 1165.4 ft3 / 1.54 ft3 / eq sk = 770 eq sk Tail slurry volume between 9-5/8-in. casing and openhole: V3= 0.3132 ft3/ft x (4950 - 4450) ft x 1.25 (25% excess) = 195.8 ft3 Tail slurry volume in shoe joint: V4= 0.4341 ft3/ft x 42 ft = 18.2 ft3 Total tail slurry volume: VT = V3 + V4 = 214.0 ft3 Total tail cement SacksT = 214.0 ft3 / 1.05 ft3 / sk = 204 eq sk

Displacement Volume The treating lines are to be flushed with water before commencing the displacement. Displacement volume: VD = 0.0773 bbl/ft x (4950 - 42) ft = 379.4 bbl

Water Requirements Mix water for the cement: VMIX = 7.80 gal/eq sk x 770 eq sk + 4.29 gal/sk x 204 sk = 6882 gal = 164 bbl Mix water for the preflush Vpreflush = 41.5 gal/bbl x 40 bbl = 1660 gal = 39.5 bbl 40 bbl The rig and two 100-bbl water trucks will supply Dowell with fresh water. Therefore, tank bottoms are not a concern.

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 13 of 14

Water Requirements Purpose

Water Volume (bbl)

Mix water for preflush

40

Mix water for cement

164

Fill mixing/pumping units and treating lines

5

(estimate)

Pressure test treating lines

2

(estimate)

Flush treating lines

5

(estimate)

Wash up the cementing equipment

10

(estimate)

Additional water available (safety factor)

50

(estimate)

Total Water Requirement

276

Maximum Lifting Force To calculate the maximum lifting force, the annular height that the 40-bbl preflush occupies must be determined before the casing and annular hydrostatic pressures at the end of the job are computed. The annular height of the preflush: Hwash = (40 bbl x 5.6146 ft 3 / bbl) / (0.3627) ft3 = 619 ft Because the top of the lead cement is at 1400 ft, the height of the mud: Hmud= 1400 ft - 619 ft = 781 ft The hydrostatic pressure of each fluid segment is calculated using the following equation and the results are summarized in Table 102. Phyd = 0.052 x x H where ρ = fluid density (lbm/gal) H = height having fluid density (ft). The maximum lifting force: MLF = 0.785 x (3207 - 2971) x 9.6252= 17,163 lbm The casing weight: Wcas = 36.0 lbm/ft x 4950 ft = 178,200 lbm Since Wcas is greater than the MLF, the casing will not be pumped out of the hole and does not need to be chained down.

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CEMENTING ENGINEERING MANUAL Section 8.E.1 Primary Cement-Job Calculations (Revised Sept. 1999) Page 14 of 14

Hydrostatic Pressure of Fluid Segments Fluid Segment Drilling mud

Casing Calculations Hydrostatic Pressure (psi)

Interval (ft)

Hydrostatic Pressure (psi) 467

Preflush

781 to 1400

268

Lead slurry

1400 to 4450 2046

TOTAL

2935

Interval (ft) 0 to 781

Tail slurry

0 to 4908

Annular Calculations

4908 to 4950 36

4908 to 4950 426

2971

3207

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