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

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  • DRILLING THROUGH SHALE:

    INSTABILITIES, FALLOUTS, AND LOST CIRCULATION

    Alexandre Lavrov

    SINTEF Petroleum Research, Trondheim, Norway

  • How wells are drilled

    2

    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

  • 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)

  • Two key facts about gas-bearing shales:

    4

    Gas-bearing shales have very low matrix permeability.

    Gas-bearing shales are naturally fractured.

  • "Mudweight window":

    BHP is too low

    5

    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

  • "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

  • Erosion & fallouts are bad for drilling

    7

    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

  • The upper mud weight window in naturally-

    fractured shales is not well-defined

    8

    Losses in naturally-fractured rocks start when natural

    fractures reopen to accept mud/cement

    Lost-circulation pressure Fracture Reopening Pressure (FRP)

    v

    v

    hh

    n

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

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

    9

    vs.

    Single value of lost-circulation pressure

    Spectrum of lost-circulation pressures

  • 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

    7

    Spectrum of lost-circulation pressure gradient

    (25 MPa/km)

    (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).

  • Cementing in naturally-fractured shale

    11

    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

  • 12

    Cementing in a naturally-fractured rock:

    Lost circulation during a cement job

    Natural or induced fracture

  • 13

    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 PasCement with yield stress 20 Pa, plastic viscosity 0.01 PasCement reference case (yield stress 20 Pa, plastic viscosity 0.02 Pas)

    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

  • The good news is:

    14

    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.

  • Example: Wellbore strengthening

    15

    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

  • Conclusions

    16

    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)

  • 17

    Thank you

    This presentation is part of a project that received funding by the European Unions 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

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