The Real Causes of Borehole Misbehavior

and Operational Practices to Eliminate Them

Fred E. Dupriest, P.E.

Professor of Engineering Practices

Texas A&M University

Society of Petroleum Engineers

East Texas Chapter, Tyler

December 9, 2014

2

Three major performance limiters (cause of reduced ft/day)

• Whirl-induced borehole patterns

• Instability

• Hole cleaning

Borehole Misbehaviors That Limit Performance (ft/day)

3

Gulf of Mexico Sakhalin Australia South Texas

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MSEadj MSEadj

MSEadj

MSEadj

Control Drilling (Efficient Bit, non-bit limiters)

Whirl and

Stickslip

Whirl is Common, no Matter What Your Downhole Tools Say

SPE 102210

Blue Curve: Mechanical Specific Energy (MSE)

SPE 67818

Borehole appears 1.5” undergauge every 4 ft

All BHAs that whirl attempt to create

patterns, even those with low levels of whirl.

The result is:

• Inability to transfer weight to steer bent motors

• Effects of reduce weight transfer on ROP

• Reaming on connections and trips for tight

hole

• Incorrect belief that short trips are required due

to drag

• Misdiagnosed as inadequate hole cleaning,

which leads to circulating time and control

drilling

• Inability to properly centralizer casing

• Cement channels and poor zone isolation

• Poor log evaluation

• Stuck BHA or casing

Tight Hole is Caused by “Borehole Patterns” Created by Whirl

3D

Image

5

Whirl is best thought of as a sine wave

that develops in the BHA when we rotate

The bit is part of the wave

All BHAs have some amplitude of wave,

so all bits are effected by whirl, no matter

what the LWD tools say

- Reduce ROP

- Reduce Bit life

- Borehole Patterns

LWD tools tell you what vibrations are

doing to the BHA. MSE tells you want

they are doing to the bit

Whirl Is a Sine Wave in the BHA with a Bit Screwed On the End

We eliminate patterns by

eliminating whirl

SPE 134580

Maximize WOB to Constrain Lateral Bit Motion

SPE 14ATCE 2328

Run step tests to maximize WOB. Increased rock exposure from

depth of cut increases resistance to the lateral force from BHA wave

Run RPM Step Tests to Identify Low-Whirl Rotating Speeds

“Fingerprint” whirl vs RPM as soon as you get your BHA out of the shoe. Use

MSE to avoid RPM ranges with higher whirl

Extend the Gauge Length

Gauge Length

Increased gauge length limits the rate of side-cutting. Patterns become

more likely to decay than gain in amplitude

If the gauge area does not touch the curvature of the wellbore, it

cannot be interfering with the build rate.

Run 4-6” gauge in wells with 0-5 deg/100ft of build curvature

Above 5 deg/100ft, work with vendor on specific gauge design. It

depends on the type of profile

SPE 119625

If the gauge does

not make contact

at the planned

curvature, the

gauge does not

effect steering

9

Engineering Redesign: MSE Declines with Gauge Length

0

200

11250 10750 11000 10500

MSEadj

ksi

Dept (ft)

0

200

MSEadj

ksi

11250 10750 11000 10500 Depth MD (ft)

2 inch gauge

6 inch gauge

0

250

MSEadj

ksi

0

250

MSEadj

ksi

6500 ft MD 9500 ft MD

4 inch gauge

6 inch gauge

MSE pattern is similar, but reduced

Less effect in softer formations where amplitude is lower to start with

Softer Harder

SPE 119625

Comparison of well with

different gauge lengths

Lower MSE means longer bit

life, significantly higher ROP

for the same WOB, and

reduced amplitude in spiral

patterns

Minimize AKO in Bent Motors, or Use RSS

0.78 AKO

Straight Motor

IPTC 10706

Sliding Rotating

Borehole Instability Sequence of Events

1. Hole is cut in stressed rock. Stress

attempts to close the hole in the

radial direction (reduce the diameter

a few thousandths)

2. Circumference is compressed

tangentially as the radius declines

3. Hole diameter stops declining when

the tangential stress (Hoop Stress)

balances the radial

4. If the Hoop Stress exceeds the rock

strength before the stresses come

to equilibrium, the rock breaks and

the hole enlarges

5. If the Hoop Stress does not exceed

the rock strength, there is no

breakout and the hole remains

gauge

6. We reduce hoop stress by

increasing the internal pressure

(MW), which expands the

circumference

Resisting Pressure

Increased Circumferential Compression

Length of circumference for bit size

12

Borehole Contraction is Not the Cause of Tight Hole

How much does the borehole shrink due to stress changes?

Dia x P = Dia Change

E

ECD Example:

8.5 x 500 psi = 0.00212 in

2,000,000

Extremely low FCS Example:

8.5 x 2500 psi = 0.0106 in

2,000,000

Stress Contraction?

Stress contraction may occur during many operations. The magnitude

is usually very small • ECD lost on connection after each stand down

• MW cut

• BHP falls to equal FCS after losses

• Very ductile evaporates may creep over time (salt, anhydrite)

• Shale hydration results in small shale volume change (but also reduced

strength and enlargement)

13 13

Stress Contraction is Limited by Failure

Slight contraction of the ID will occur as stress tries to close the hole. But this deformation is not enough to cause tight hole in most shales or sands. The amount of stress and contraction that would cause tight hole is almost always sufficient to break the rock. The hole gets larger not smaller

Hole Shrinkage Prior to Enlargement

x Ductile or “Plastic” Rock

Brittle Rock

x

Failure and Breakout

Str

ess

Deformation

Result of

moderate stress

Instability limits

stress contraction

14

Angular or Splintery

Raise MW 0.2 - 0.5 ppg

Photo-document and Analyze Your Shaker Material

Tabular or Blocky

Raise MW 0.5 - 1.0 ppg

σ1 From the side of the hole with the

lowest stress anisotropy

(i.e., top of high angle hole)

From side of the hole with

the highest anisotropy

(i.e., side of high angle hole)

Higher

Anisotropy

Lower

Anisotropy

Third stress (into the page) is

the same for both side

15

Higher MW Does Not Destabilize Fracture Shales

Original Hole

MW = 9.2 ppg OBM

Sidetrack

MW = 11.7 ppg OBM

0 10 20 30 40

2750

2800

2850

2900

2950

3000

BGT - Kaliber Log

Bohrung: Walsrode Z5 1.Loch

Kaliber (in)

B

o

h

r

t

e

u

f

e

(

m

)

0 10 20 30 40

2750

2800

2850

2900

2950

3000

EMM - Kaliber Log

Bohrung: Walsrode Z5 2.Loch

Kaliber (in)

B

o

h

r

t

e

u

f

e

(

m

)

Naturally fractured shale

confirmed with core

The 11.7 ppg fully stabilized the non-fractured shales and reduced the enlargement in the

fractured shales. Full stabilization of the fractured shale would have required 13.0 ppg,

but integrity in shallower zones was inadequate to use this MW. Instability may also be

time dependent in fractured shale, even with a NAF.

Hole Diameter (inch)

Dep

th (

m M

D)

Hole Diameter (inch) D

ep

th (

m M

D)

SPE 128728

9.2 ppg 11.7 ppg

Caliper 2,4 -------

Caliper 1,3 -------

Non-fractured shale

16

High Angle Physics

Equilibrium Bed Height

(Fluid Force > Force required to roll)

Dynamic

fluid force

Gravity Buoyancy

Dynamic

fluid force

Gravity

Buoyancy

No net lifting force,

falls rapidly

Once it hits bottom, how

does it get off? It doesn’t.

It rolls or skips.

SPE 28306

Fluid Shear

Rolling

High Angle (60-90) deg

Resistive force

17

Enlargements Dominate Safe ROP for Hole Cleaning

If drill rate is limited by hole cleaning, you probably have enlargements. The safe

ROP in gauge hole may be 3x that in even moderately enlarged hole

High angle boreholes fill to the same flow area as gauge hole, not the same bed

height. If this large stored mass is mobilized, it will not fit in the gauge hole

above

A1

A1 A2

Equilibrium

Bed Height

A2

SPE 134580

Manage transient events that mobilize the stored mass: 1) rapid changes in

pump rate, 2) rapid string movement, 3) pulling/washing BHA into stored mass

18

Redesign What is Preventing You from Raising MW

Effect of MW on ROP

Higher MW increases the effective stress in the rock exposed beneath the bit, which

causes the rock’s compressive strength to increase slightly. The resulting small loss of

depth of cut can be offset by raising WOB

Differential Sticking

Differential sticking can be eliminated, regardless of MW used. The industry continues

to have sticking events because they do not consistently use the practices required to

eliminate them. See SPE 128129

Lost Circulation

Effective practices exist for preventing or responding to lost circulation. In general,

losses can be stop, while instability is irreversible. See SPE 163481

Formation Damage

If filtrate is damaging to well productivity, you simply have the wrong fluid. If whole

mud is entering and particles are plugging pore throat use properly engineered

blocking solids to prevent fines migration.

19

1. Tight hole is caused primarily by whirl-induced patterns.

Reduce whirl

2. Hole enlargement is caused primarily by inadequate MW.

Raise the MW

3. Poor hole cleaning is caused by enlarged hole.

Raise the MW

Key Take-Aways