Lesson 4 Drilling Cost

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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
Assignments
Read ADE, Ch. 1 (All)Learn Rig Components
- Definitions
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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
DEPTH ft
TD
DAYS or DOLLARS
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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
Eq. 1.16
footage drilled with bit, ft/bit
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
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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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Example 1.5 contd

• Assume that each of the bits was operated at near
  the minimum cost per foot attainable for that
  bit.
• Mean
• Bit Rotating Connection Penetration
• Cost 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

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

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Solution
Similarly, for Bit B,
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Solution, contd

• Finally, for Bit C,

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Solution, contd

• 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, contd
C aebD

• 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.

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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.

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

Integrating and solving for td yields
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Drilling Time contd

• 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
80
70
60
Minimum Cost
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
Variables

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

• Rock hardness
• Formation pore pressure

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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40,000 lbf
Consider 10 hole (dont overdo!!)
Drilling Rate, ft/hr
Bit Weight x 1,000 lb/in
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Dont overdo! Casing wear, bit life ...
Drilling Rate, ft/hr
Rotary Speed, RPM
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EFFECT OF BACK PRESSURE Keep DP_bit const. 550
psi
Drilling Rate, ft/hr
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EFFECT OF BACK PRESSURE 0 - 5,000 psi
Drilling Rate, ft/hr
Hydrostatic Pressure, 1,000s of psi
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EFFECT OF DRILLING FLUID mud vs. gas
Drilled with mud
Depth, ft
Drilled with gas
Drilling Time, days
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EFFECT OF DRILLING FLUID water vs. air
Depth, ft
Rotating Time, hours
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Relative Drilling Rate,
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

0.052 14.0 12,000 8,736 psig

(5,196 psi with water)
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Hydrostatic Pressure Required

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

MW 16.03 lb/gal