chapter 6 underbalanced drilling

Chapter 6

Underbalanced Drilling

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PCB – 4213 ADVANCED DRILLING ENGINEERING Semester- JANUARY 2016

BY . ASIF ZAMIR

UNIVERSITI TEKNOLOGI PETRONAS

PETROLEUM ENGINEERING DEPARTMENT

Underbalanced Drilling

Drilling Contractor,

Jul/Aug 2003)

1. Underbalanced drilling

requires rock strong enough

to remain open under the

additional forces from fluid

influx.

2. UB drilling is much faster:

time-dependent strength

losses in shale are low.

3. Fluid flux is inward, thus

shale strength is not

affected by mud filtrate.

4. Formation damage reduced

(e.g. capillary blockage),

giving better production.Open

Underbalanced

drilling zone

Drilling Below the Pore Pressure

• In the upper zone of many basins, po is roughly hydrostatic

• If it is an eroded basin, the shales are strong

• It is possible to drill with MW < po for substantial depths: i.e.: underbalanced

• Much faster drilling

• But: care and monitoring are essential

stress (or pressure)

depth

4 kmmild

overpressure

po

h

v

First

large

gas sand

MW

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Advantages of Underbalanced Drlg.

• Faster drilling• Lower rig time costs

• Shales are exposed for a shorter time

• Occasionally, saving of one casing string

• Shales are not exposed to mud filtrates• Swelling and deterioration reduced

• Less damage in the reservoir• If it is possible to enter the reservoir just at the balance, far less damage

takes place

• This has resulted in many “unproductive” gas zones in Alberta becoming economical zones

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BY . ASIF ZAMIR

When to Drill Underbalanced…

• If no significant gas sands will be encountered (small ones are feasible)

• If water influx is manageable

• When the shales are intact & strong enough that sloughing is manageable

• If faster drilling is saving you money

• When the rig capabilities allow UB drilling:• Top drive is certainly best

• Special return line equipment is installed

• Mud system can take foam, air…

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BY . ASIF ZAMIR

Wellbore Stresses and Pressures

• Effect on borehole wall stresses:• Stresses are higher in UB

• This pushes the rock closer to failure• Shown on the next figure, a strength plot

• po > pw, so flow is out of, not into the shale• Creating a small additional destabilizing force

• Because we are not supporting the shale with positive pressure, we have to watch carefully for excessive sloughing!

• Monitor and estimate volume of cavings…

• Examine them for shape, stratigraphic position

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BY . ASIF ZAMIR

UB Drilling and Shale Strength

n - normal stress

t - shear stress

Shale failure line

Stresses in underbalanced drilling

Overbalanced, good shale support (OBM?)

r can be slightly negative q is increased in UB case

Shear stresses are always

higher in UB drilling case

Close to

failure

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Effect on Shale Deterioration

• When mud filtrates invade shales:

• Swelling

• Deterioration

• Loss of support

• In UB drilling:• Flow is toward the hole

• Shales not exposed to drilling mud filtrate

• Diffusion is suppressed

• Geochemical stability!

• Shales don’t suffer filtrate deterioration!

pw

po

Damaged

shale

po > pw: inward gradient

This eliminates outward

seepage that can further

damage shalesOpen

Some Additional Comments

• Smectitic shales are usually extremely geochemically sensitive (swelling…)

• OBM is a solution, but…• OBM is expensive

• Cuttings are environmentally difficult

• OBM can block gas zones

• UB drilling: another solution for smectites• Providing strength is sufficient…

• UB drilling avoids swelling

• Chips, mud, are easier to dispose of

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BY . ASIF ZAMIR

Why Faster Drilling?

• High MW pastes chips against hole bottom

• Also increases the strength of the shale

• UB condition actually helps the shale drill chips to spall off

• Mud rings, bit balling are reduced

• Typically, time to drill the interval is thus reduced by 50-60%

pw = static p

+ dynamic p

po = pore pressure

The “plastering”

effect is counter-

acted by po > pw

co

llars

bitshale - po

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Approaches to UB Drilling

• Drilling with air• Carrying capacity may be limited

• Very cheap

• Low pressure in the hole, trip issues

• Drilling with inexpensive foams• Higher pressure in the hole = +stability

• Foams have beeter hole cleaning capacities

• Chips and foam all rejected to a pit

• Drilling with stable foams• Recycling chemicals

• More expensive

• Cost-benefit analysis and experience…

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BY . ASIF ZAMIR

Underbalanced Drilling - USA

Atlantic

coastal plain

and offshore

basinsOffshore and

near-coast

California

The basins marked are likely to be poorer candidates for

UB drilling; the entire remainder of the Lower 48 may be

considered because the shales are mechanically strong

However, local stratigraphy, gas

zones, etc., must be investigated

in UB drilling assessment.

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When Not to Use UB Drilling

• When shales are weak and will slough (i.e. a non-tectonic basin with no erosion history)

• In areas of very high tectonic stress• i.e.: when HMAX is much higher than hmin

• When significant gas sands generate risks to drilling that cannot be managed

• When there are intensely fractured shales

• When massive water influx cannot be handled in an underbalanced mode

• When you don’t have the right equipment

• In horizontal or high angle wells

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BY . ASIF ZAMIR

Typical UB Drilling Profile4

50

0 m

Soft,

swelling

shales

Overburden

Strong

shales and

sandstones

Fractured

shale

Target

Gas, water

sands

No large gas,

water sands

Aquifers

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Typical UB Drilling Profile

• Set surface casing

• Drill w air/foam through swelling shale intervals to the first serious gas/water sand, one bit only if possible

• Set casing to isolate the sensitive shales

• Drill w WBM (best for fractured shale) to TD

• Set production casing

45

00

m

Soft,

swelling

shales

Overburden

Strong

shales and

sandstones

Fractured

shale

Target

Gas, water

sands

No large gas,

water sands

Aquifers

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Recent Developments

• UB drilling for tight gas occurrences• 0.25 to 1 MPa below the reservoir pressure

• To avoid formation damage

• Is the formation stable enough?

• UB drilling even in softer and weaker shale• Slightly underbalanced, aerated mud

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BY . ASIF ZAMIR

Lessons Learned – UB Drilling

• Faster drilling, less formation damage, reduction of clay-filtrate effects

• Only suitable if shale strength is high, minimal gas, and no massive water zones

• Rock properties, sedimentary basin history, lithostratigraphy… all help in screening

• Careful surveillance and chip morphology assessment is recommended

• Less likely to be used offshore in relaxed basins that have not experienced erosion

• MORE EXPENSIVE, COMPLICATED

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BY . ASIF ZAMIR

Closed Loop Circulation System

SEPARATOR

OIL TANKS

WATER TANKS

NITROGENPUMPERS

MUD TANKS

RIG

RIG

PUMPS

R-BOP

TOP DRIVE

SYSTEM

CATCHERSSAMPLE

CHOKEMANIFOLD

FLARESTACK

N2 / FLUID MIX

CUTTINGRETURN

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BY . ASIF ZAMIR

TECHNICAL IMPROVEMENTS

• Compressor Evolution

• Nitrogen• Increased availability

• Reduced cost

• On site generation

• Recyclable foam systems

• Better gas separators

• Closed loop circulation

• Hydraulics models

• Improved rotating heads

• Wire line wet connect

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BY . ASIF ZAMIR

• Improved MWD

• Rig assist snubbing units

• Coiled tubing equipment

• Non-damaging drilling fluids

• Top drive

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• Gasified Fluid Operations (aerated fluid operated)• Operations intentionally undertaken with a two-phase drilling fluid

containing some form of gas mixed with a liquid phase.

• Gasified fluids normally do not contain a surfactant

• Foam Operations• Operations intentionally undertaken with a two-phase drilling fluid

containing some form of gas mixed with a liquid phase and tied together with a surfactant

• The liquid phase is continuous

• Mist Operations• Intentionally drilling with a two-phase fluid having a gas as the continuous

phase

• The liquid in this fluid system is suspended in the mixture as droplets

• Air Operations• Intentionally drilling using a pure gas as the drilling fluid.

• The gas can be air, nitrogen, natural gas, or any combination of gases

Types of Fluid in UBD


BY . ASIF ZAMIR

MISTFOAMLIQUID GASAERATED

LIQUID

Types of Fluid Flow

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BY . ASIF ZAMIR

Generalized“Fluid” Systems

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BY . ASIF ZAMIR

Closed Loop Circulation System

SEPARATOR

OIL TANKS

WATER TANKS

NITROGENPUMPERS

MUD TANKS

RIG

RIG

PUMPS

R-BOP

TOP DRIVE

SYSTEM

CATCHERSSAMPLE

CHOKEMANIFOLD

FLARESTACK

N2 / FLUID MIX

CUTTINGRETURN

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BY . ASIF ZAMIR

Stainless Steel

Liquid Nitrogen(-320OF)

Carbon Steel

Gaseous nitrogen to well

80OF, 0-10,000 psi

Pump

Vaporizer

6.11.3

Equipment - Gas Source

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BY . ASIF ZAMIR

AIR DRILLING

A brief summary

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BY . ASIF ZAMIR

Air Drilling

•Air/Gas drilling (“dust”) is a technique used in areas where the formations are “Dry” i.e., there is no influx of water or liquid hydrocarbons.

•This medium requires significant compressed gas volumes to clean the well with average velocities of over 3,000 ft per minute.

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BY . ASIF ZAMIR

Air Drilling Dusting

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BY . ASIF ZAMIR

Air Drilling Benefits

•Increased Rate of Penetration•Reduced Formation Damage•Improved Bit Performance•Lost Circulation Control•Continuous Drill Stem Test

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BY . ASIF ZAMIR

Air/Dust Drilling Layout

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Misting

• Addition of 6 to 30 bbl/hr of fluid to the air stream.

• Cleans and lubricates the bit

• Carries the cuttings to the surface as a mist or more normally in a modified two phase flow.

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BY . ASIF ZAMIR

FOAM DRILLING

TT96-86 31Open


BY . ASIF ZAMIR

Foam Drilling

•The most versatile of the gas-generated systems.

•Effective operating range from 0.2 to 0.6 s.g.

•Mixture of gas phase and foaming solution.

•Foam flow varies with depth in the hole.

•Adjustable effective BHP.

•Enhanced lifting and well bore cleaning.

•A “displacing medium”, not a propelling medium.

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BY . ASIF ZAMIR

Improved Hole Cleaning

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BY . ASIF ZAMIR

Foam Drilling Benefits

• Faster Penetration rate

• Low Air requirements

• Low fluid requirements

• Low Hydrostatic head

• No damage to formation

• Continuous Drill Stem test

• Best for large holes

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BY . ASIF ZAMIR

Mist or Foam Drilling Layout

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AERATED

DRILLING

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BY . ASIF ZAMIR

Gaseated

Mist

TransitionWater

Gas

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Aerated Fluid

•Gasification of Primary Drilling Fluid.

•Initially designed as a technique to lighten mud to reduce lost circulation.

•As an UB fluid, it is easiest to control in small holes.

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BY . ASIF ZAMIR

Aerated Fluid Layout

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Compressor/N2

CostRotating BOP’s

Solid/Liquid/GasSeparation

Corrosion

Vibration

High Torque/Drag

Borehole StabilityMWD Transmission

Hydraulic Calculations

Cuttings Lifting

Fluid Influx

Fire/Explosions

UnderbalancedCompletion

Aerated Drilling Problems

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BY . ASIF ZAMIR

chapter 6 underbalanced drilling

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