drilling through shale: instabilities, fallouts…. lavrov - drilling through shale... · drilling...

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DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov SINTEF Petroleum Research, Trondheim, Norway

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Page 1: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

DRILLING THROUGH SHALE:

INSTABILITIES, FALLOUTS, AND LOST CIRCULATION

Alexandre Lavrov

SINTEF Petroleum Research, Trondheim, Norway

Page 2: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

How wells are drilled

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Drilling fluid:• cleans the hole (transports

cuttings to surface)• keeps cuttings in suspension

when the pump is stopped• creates some pressure on the

borehole walls to support therock

• prevents influx of formationfluids

• lubricates and cools the drill bit

Casing Drilling mud flows up the annulus

Drill bit

Drilling fluid flowsdown the pipe

Page 3: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

Bottomhole pressure

3Bottomhole pressure (BHP), Equivalent circulating density (ECD)

BHP = ECD * mudweight

BHP = static weight of mud column+ APL

An

nu

lar

pre

ssu

relo

ss (

AP

L)

Page 4: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

Two key facts about gas-bearing shales:

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• Gas-bearing shales have very low matrix permeability.

• Gas-bearing shales are naturally fractured.

Page 5: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

"Mudweight window":

BHP is too low

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Breakouts

Borehole failure(breakouts or collapse)

BHP /mudweight should beabove the lower limit toprevent formation fluid influxand borehole instabilities.

Exception: underbalanceddrilling (incl. air drilling).

Influxof formation fluids

Page 6: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

"Mudweight window":

BHP is too high

6Induced fractures

Drilling fluid may escapeinto natural fractures

BHP /mudweight should bebelow the upper limit toprevent formation fluid influxand borehole instabilities.

Exceptions: in practice, theremay be.

Drilling fluid may escapeinto induced fractures

Page 7: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

Erosion & fallouts are bad for drilling

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Example of a really bad scenario:

Fallout / erosion of borehole wall

Too many solids accumulatedin the annulus

Packoff near the bottomhole assembly

Bottomhole pressure surges

Formation breaks down

Fluid escapes into the fracture

Mud colum in the annulus drops

Static mud pressure in the annulus drops

Formation fluid influx or kick

Page 8: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

The upper mud weight window in naturally-

fractured shales is not well-defined

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• Losses in naturally-fractured rocks start when natural

fractures reopen to accept mud/cement

• Lost-circulation pressure Fracture Reopening Pressure (FRP)

σv

σv

σhσh

σn

Normal stresses:v: verticalh: horizontaln: normal to fracture

Page 9: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

Fracturing pressure ("fracture gradient") in naturally-fractured shale:It's not a single number!

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vs.

Single value of lost-circulation pressure

Spectrum of lost-circulation pressures

Page 10: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

Lost-circulation pressure spectrumin a naturally-fractured rock

Discrete fracture networks (DFN) were generated using the FracSim3D software developed at University of Adelaide.

Fracture network with 183 fractures

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Spectrum of lost-circulation pressure gradient

σ σ (25 MPa/km)

σ σ σ (15 MPa/km)

z v

x y h

LCP gradient spectrum covers the entire range from horizontal stress gradient (15 MPa/km) to vertical stress gradient (25 MPa/km).

Page 11: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

Cementing in naturally-fractured shale

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Washouts are bad for cementing: cement cannot fillthe washout → mud pocket is left → issues with

zonal isolation and mechanical stability afterwards.

Incomplete cementing of washouts Cement losses into fractures

Losses → Targeted height of cement sheath cannotbe reached

Page 12: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

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Cementing in a naturally-fractured rock:

Lost circulation during a cement job

Natural or induced fracture

Page 13: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

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Cementing in a naturally-fractured rock:

Lost circulation during a cement job

Radius of cement bank in fracture vs time

Rad

ius

of

cem

ent

ban

k, m

Time, s

Mud losses (black curve) vs. cement losses (blue curve)

Effect of cement rheology on cement losses

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20

Rad

ius

of

cem

en

t b

ank

in t

he

fra

ctu

re, m

Time, min

Cement with yield stress 10 Pa, plastic viscosity 0.02 Pa∙sCement with yield stress 20 Pa, plastic viscosity 0.01 Pa∙sCement reference case (yield stress 20 Pa, plastic viscosity 0.02 Pa∙s)

Lower YP of cement → higher cumulative lossesLower PV of cement → higher initial and cumulative losses

0

2

4

6

8

10

12

0 5 10 15 20

Rad

ius

of

cem

en

t b

ank

in t

he

fr

actu

re, m

Time, min

0.15 mm wide fracture

0.1 mm wide fracture (reference case)

Effect of fracture width on cement losses

Propagation distance fracture aperture

Page 14: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

The good news is:

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Worldwide, 25-30% of all hydrocarbons are producedfrom naturally-fractured reservoirs (carbonates, chalks, tight gas, tight sand, etc).

It means it is still somehow possible to drill and cement such wells.

Page 15: DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS…. Lavrov - Drilling through shale... · DRILLING THROUGH SHALE: INSTABILITIES, FALLOUTS, AND LOST CIRCULATION Alexandre Lavrov

Example: Wellbore strengthening

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Leakoff Large particles deposited (bridging) Small-particles deposited (sealing)

Concept: Wellbore strengthening: short & widefractures are induced and filled with loss-preventionmaterial. The interval can subsequently be drilledwithout losses.

SPE 87130, 90493, 168041

Challenges in gas-bearing shales:• Low permeability → Difficult to build a stable bridge• Natural fractures will accept particles and will open

up when BHP is raised

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Conclusions

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• Drilling and cementing challenges in gas-bearing shales are a combination of thosein shales and those in conventional naturally-fractured rocks.

• This creates some unique challenges, the reason why some conventionaltechniques might not work well here at first.

• Thorough characterization of fracture networks (connectivity, aperture, density, etc) will help design drilling operations and cementing jobs (mud weight, fluid rheologies, lost-circulation materials, pumping schedules, etc).

• "The importance of acquiring a borehole image log in fractured reservoirs to identify the natural fractures directly cannot be overemphasized." (Ehlig-Economides et al., 2000, SPE 59057)

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Thank you

This presentation is part of a project that received funding by the European Union’s Horizon 2020 research and innovation programme under grant agreement number 640715.

The content of this presentation reflects only the authors’ view. The Innovation and Networks Executive Agency (INEA) is not responsible for any use that may be made of the information it contains.

Disclaimer