4. drilling cost and drilling rate

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PETE 411 Well Drilling

Lesson 4

Drilling Cost & Drilling Rate

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Contents

The AFE Drilling Cost and Bit Change Factors Affecting Drilling Rate Bit Weight, Rotary Speed Bottom-hole Cleaning Mud Properties, Solids Content Hydrostatics

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HW #2. ADE 1.12, 1.13, 1.14 Due Friday, Sept. 13, 2002

Read: ADE, Ch. 1 (All)

Learn: Rig Components - Definitions

Assignments

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Before getting approval to drill a well the Drilling Engineer must prepare an AFE- a detailed cost estimate for the well

DRY COMPLETED

HOLE

INTANGIBLE COSTS $ $

TANGIBLE COSTS $ $

TOTAL COST $ $

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AUTHORIZATION FOR EXPENDITURE (AFE)

EXPENDITURE DRY HOLE COMPLETED (24.5 DAYS) (32.5 DAYS)

INTANGIBLE COSTS LOCATION PREPARATION 30,000 65,000 DRILLING RIG AND TOOLS 298,185 366,613 DRILLING FLUIDS 113,543 116,976 RENTAL EQUIPMENT 77,896 133,785 CEMENTING 49,535 54,369 SUPPORT SERVICES 152,285 275,648 TRANSPORTATION 70,200 83,400 SUPERVISION AND ADMIN. 23,282 30,791 SUB-TOTAL 814,928 1,126,581

TANGIBLE COSTS TUBULAR EQUIPMENT 406,101 846,529 WELL HEAD EQUIPMENT 16,864 156,201 COMPLETION EQUIPMENT 0 15,717

SUB-TOTAL 422,965 1,018,447

SUB-TOTAL 1,237,893 2,145,028+ CONTINGENCY (15% ??) 1,423,577 2,466,782

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Drilling Cost vs. Time

DEPTHft

DAYS or DOLLARS

TD

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Drilling Cost Analysis

The Drilling Engineer:

Recommends drilling procedures that will safely drill and complete the well at the lowest cost possible

Makes recommendations concerning routine rig operations:

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The Drilling Engineer

Examples of routine rig operations

drilling fluid treatment

pump operation

bit selection

handling problems during the drilling process

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The Drilling Cost Equation:

Cf = drilling cost, $/ft

Cb= cost of bit, $/bit

Cr = fixed operating cost of rig, $/hr

tb = total rotating time, hrs

tc = total non-rotating time, hrs

tt = total trip time (round trip), hrs

D

ft

$

D

)ttt(CCC

tcbrbf

Eq. 1.16

= footage drilled with bit, ft/bit

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Example 1.5

A recommended bit program is being prepared for a new well using bit performance records from nearby wells.

Drilling performance records for three bits are shown for a thick limestone formation at 9,000 ft.

Determine which bit gives the lowest drilling cost if the operating cost of the rig is $400/hr, the trip time is 7 hours, and connection time is 1 minute per connection.

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Assume that each of the bits was operated at near the minimum cost per foot attainable for that bit.

MeanBit Rotating Connection PenetrationCost Time Time Rate

Bit ($) (hours) (hours) (ft/hr)

A 800 14.8 0.1 13.8 B 4,900 57.7 0.4 12.6 C 4,500 95.8 0.5 10.2

Example 1.5 cont’d

Which bit would you select?

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Solution:

The cost per foot drilled for each bit type can be computed using Eq. 1.16. For Bit A, the cost per foot is

/ft.81.46$)8.14(8.13

)71.08.14(400800Cf

ft

$

D

)ttt(CCC

tcbrbf

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Solution:

Similarly, for Bit B,

/ft.56.42$)7.57(6.12

)74.07.57(400900,4Cf

ft

$

D

)ttt(CCC

tcbrbf

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Solution, cont’d

Finally, for Bit C,

/ft.89.46$)8.95(2.10

)75.08.95(400500,4Cf

ft

$

D

)ttt(CCC

tcbrbf

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Solution, cont’d

Bit A: $46.81 /ft

Bit B: $42.56 /ft

Bit C: $46.89 /ft

The lowest drilling cost was obtained using Bit B. - Highest bit cost …but - intermediate bit life and ROP...

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Drilling Costs

Tend to increase exponentially with depth. Thus, when curve-fitting drilling cost data, it is often convenient to assume a relationship between total well cost, C, and depth, D, given by

C = aebD …………………..(1.17)

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Drilling Costs, cont’d

Constants a and b depend primarily on the well location.

Shown on the next page is a least-squares curve fit of the south Louisiana completed well data given in Table 1.7.

Depth range of 7,500 ft to 21,000 ft.

For these data,

a = 1 X 105 dollars

b = 2 X 10-4 ft -1.

C = aebD

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Fig. 1-65. Least-square curve fit of 1978 completed well costs for wells below 7,500 ft in the south Louisiana area.

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Penetration Rate

When major variations are not present in the subsurface lithology, the penetration rate usually decreases exponentially with depth. Under these conditions, the penetration rate can be related to depth, D, by

where K and a2 are constants.

)18.1,........(Kedt

dD Da303.2 2

WHY?

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Drilling Time

The drilling time, td , required to drill to a given depth can be obtained by separating variables and integrating. Separating variables gives

dDedtK Da303.2D

0

t

0

2d Integrating and solving for td yields

)19.1.....().........1e(Ka303.2

1t Da303.2

2d

2

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Drilling Time cont’d

Plotting depth vs. drilling time from past drilling operations:

A. Allows more accurate prediction of time and cost for drilling a new well

B. Is used in evaluating new drilling procedures (designed to reduce drilling

time to a given depth).

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EXAMPLE - Cost per ft

t R D Total Cost Cf

hr fph ft $ $/ft

5 90 475 36,950 77.80 10 80 900 47,800 53.10 20 60 1,600 69,200 43.30 25 50 1,875 79,750 42.50 30 40 2,100 90,200 43.00 35 30 2,275 100,550 44.20 40 20 2,400 110,800 46.20

These cost data are plotted below.

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Cost per ft for one entire bit run

Minimum Cost

80

70

60

50

40

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Economic Procedure in above Table Can pull bit after about 25 hr. ($42.50/ft)

- the precise pulling time is not critical

Note that the cost in dollars per foot was $43.00 after 30 hr.

Primarily applicable to tooth-type bits where wear rate is predictable.

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Economic Procedure in above Table

Also used with tungsten carbide insert bits when inserts are broken or pulled out of the matrix.

Unfortunately, wear rate with insert bits is unpredictable.

Economically, the insert bit should be pulled when the cost in $/ft begins to increase.

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Economic Procedure in Table

Bits pulled for economic reasons make it hard to obtain wear information.

Operator might pull bit after 120 hr of use but part of bit might get left in hole. Recovery is very difficult. Avoid!

75% of rock bits are pulled green or before the bit is worn out.

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An increase in TORQUE may

indicate that a bit should be pulled.

Experience often dictates when to

pull bit (footage or hours).

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Factors that affect Penetration Rate

Type of Drill bit Bit weight Rotary speed Bottom-hole cleaning Mud properties

Rock hardness

Formation pore pressure

Variables:

Fixed Factors:

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Bit Selection is based on

Past bit records

Geologic predictions of lithology

Drilling costs in $/bit...

Drilling cost in $/ft

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Bit Weight and Rotary Speed

Increasing bit weight and rotary speed boosts drilling rate

These increases accelerate bit wear

Field tests show that drilling rate increases more or less in direct proportion to bit weight

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Consider 10” hole

(don’t overdo!!)

Dri

llin

g R

ate,

ft/

hr

Bit Weight x 1,000 lb/in

40,000 lbf

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Dri

llin

g R

ate,

ft/

hr

Rotary Speed, RPM

Don’t overdo!Casing wear, bit life ...

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Dri

llin

g R

ate,

ft/

hr

EFFECT OF BACK PRESSUREKeep P_bit = const.= 550 psi

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Dri

llin

g R

ate,

ft/

hr

Hydrostatic Pressure, 1,000’s of psi

EFFECT OF BACK PRESSURE0 - 5,000 psi

35Drilling Time, days

Dep

th,

ft

Drilled with gas

Drilled with mud

EFFECT OF DRILLING FLUIDmud vs. gas

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Dep

th,

ft

Rotating Time, hours

EFFECT OF DRILLING FLUIDwater vs. air

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Rel

ativ

e D

rill

ing

Rat

e, %

Old style water course bits

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EFFECT OF SOLIDS IN THE MUD

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Hydrostatic Pressure Gradient

Fresh Water Pressure Gradient = 0.433 psi/ft

Density of Fresh Water = 8.33 lb/gal

Hydrostatic Pressure (at 12,000 ft depth):

with water: p = Gw * Depth (vertical depth)

= 0.433 psi/ft * 12,000 ft

= 5,196 psi

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Hydrostatic Pressure

with 14 lb/gal mud: p = GM * Depth

Depth*psi/ft 433.0*8.33

Mud

Depth* WeightMud*0.052Pressure

= 0.052 *14.0 *12,000

= 8,736 psig

(5,196 psi with water)

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Hydrostatic Pressure Required

Depth * 0.052

Pressure Weight Mud Required

What mud weight is required to balance a pore pressure of 10,000 psig at a vertical depth of 12,000 ft?

12,000 * 0.052

10,000 Weight Mud Required

MW = 16.03 lb/gal