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Hydraulic Fracture GeomechanicsHydraulic Fracture Geomechanics

Maurice Dusseault

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csHydraulic Fracturing UsesHydraulic Fracturing Uses

� To enhance well productivity (drainage area)�Propped fractures in reservoirs, geothermal well

fracs, access to naturally fractured zones ....

� To introduce thermal energy (steam fractures)

� To measure stress (Minifrac, LOT, XLOT)

� For drill cuttings annular reinjection - CRI

� For massive solid waste disposal

� For acidizing, for “choking” rates

� Other uses

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csE.g.: A Fracture Choking ProductionE.g.: A Fracture Choking Production

Poor recovery from lower

sand bodies

Propped fracture chokes off the high-k zone, allowing a larger

production proportion from lower zones, increasing recovery ratios

high k sand body

medium k sand

shales

medium k sand

medium k sand

Concept developed by Statoil – Arthur Bale

Problem Solutio n

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csHF ModelingHF Modeling

Courtesy Natchiq Corp

Stress assumptions for analysis

growth

Pressures vs time

Equipment issues

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csConventional AssumptionsConventional Assumptions

� Fractures propagate as a planar surface through a solid material

� The material is assumed to have an intrinsic resistance to fracture (eg: KIC)

� The far-field stresses stay sensibly constant, material properties as well

� Fractures are approximately symmetric

� Other similar assumptions are common, and these assumptions are used in developing models that are used in analysis

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csE.g.: E.g.: FracturesFracturesare Symmetricare Symmetric

saltdome

gasoil

sulphur

fracture

A A´salt

salt domeFractures reflect the local stress field, and tend to elongate asymmetrically

Close wells

More distant wells

Clearly, not all fractures are symmetric! Local stress fields are important! Are other HF modeling assumptions

robust?

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csTypical Model AttributesTypical Model Attributes

� Rock behaves as a L-E material

� Fracture orientation remains constant

� Constant fracture tip toughness (KIC)

� Mode I (opening mode) fracture, no shear Bleed-off with a 1-D flow model = ƒ(1/√t)

� Fluid buoyancy effects often ignored

� Constant permeability assumed

� Simplifications are necessaryfor modeling, but they must be robust!

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csImpact of AssumptionsImpact of Assumptions

� In soft, weak sandstones, the various assumptions made in fracture modeling lead to a number of problems�Length in SWR greatly over-predicted

�Aperture predictions are invalid

� Injection pressures rise with time

�Fracture orientation changes occur

�Fractures rise, even horizontal ones

�Shearing along flanks ignored

�Non-linear bleed-off ignored (L, ∆t)

� Other effects as well…

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csSo What Do We Do??So What Do We Do??

� We behave as responsible engineers:�Recognize that models are simplifications

�Learn more about stresses, geomechanics

�Calibrate models in real field cases

�Understand that production changes stresses

�Take measurements when it is feasible

� Design on “expected” behavior, but…�Understand the physics behind fracturing

�Learn from the data, analyze the unexpected

� This is what engineering is all about

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Behavior of Hydraulically Behavior of Hydraulically Induced FracturesInduced Fractures

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csFracture Growth is Complex!Fracture Growth is Complex!

Pay

Pay

“Perfect”fracture

Multiple fracturesdipping from vertical

T-shaped fractures

Twisting fractures

Out-of-zone

growth

Poor fluid diversion

Upward fracture growth

Horizontal fractures

?

?

?

? ?

?

?

Pinnacle Tech. Ltd.

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csControls Controls on on Fracture DirectionFracture Direction

� In situ stresses are the major control!!!

� Fractures propagate normal to σ3

� Local fracture propagation direction may be affected by joints, fractures, bedding, but for short distances only

� Stresses may also be changed by processes!

�By massive injection processes (+∆p)�By thermal effects (∆T)

�By production (depletion) effects (-∆p)�By solid waste injection (∆V)

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csLocal Fabric and FracturingLocal Fabric and Fracturing

σ3

σ3 Joint system in the rock

Locally, fracture follows fabric;

globally, fractures follow stress fields

Local stress field around the borehole (10 D max)

Local fabric will affect fracture direction, but at a large-scale, σ3 direction governs the fracture orientation.

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csGoals and RealityGoals and Reality

What we getWhat we want

Pay zone

500 ft

Or Or

1200 ft

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csMultiple Zone StimulationMultiple Zone Stimulation

What we getWhat we want

Pay zone

Pay zone

Pay zone

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csWhy Vertical Fractures RiseWhy Vertical Fractures Rise

� Fracture fluid gradient is almost always less than the σ3 gradient = excess ∆p is generated at the top of the fracture

� Rise rate can be affected by fluid density

� Rise rate can be affected by leak-off rates (more leak-off = less rise)

� Rise rate can be affected by stresses and stiffness of overlying strata

� Rock strength is largely irrelevant in stopping large vertical fractures rising!!

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csWhy Fractures RiseWhy Fractures Rise

� Fracture fluid has a density of < ~1.2

� The gradient of lateral stress (dσh/dz) is much more than this value

� Thus, there is an extra driving pressure at top

� Deficiency in driving pressure at bottom

� Fracture tends to rise

pressure (stress)lateral

stress

positivedrivingforce

injectionpoint

verticalfracture

injection point

stress gradient is typically

17-23 kPa/m

fracture fluidgradient is

10-13 kPa/m

pressure and stressare about the sameat the injection point

fluid pressure

σ3

pressure deficiency

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csFractures Rising Out of ZoneFractures Rising Out of Zone

injectionwellbore

reservoir

shale overburden

t1

t2

t3

t4

perforations

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cs““ HorizontalHorizontal”” FracturesFractures

� At shallow depth, heated or large ∆V cases, tectonic stress cases (σ3 = σv, thrust regime)

� Tend to climb away from injection point� Tend to be highly asymmetric in shape� Propagation of a shear band well in advance of

the parting fracture plane is common� Shallow rising fracs tend to “pan-out” under

stiff, competent strata (eg: cemented zones)� Almost impossible to numerically model in a

physically rigorous manner

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csHorizontal Fractures in SWR*Horizontal Fractures in SWR*

� Horizontal fractures do not grow ⊥ σ3

least principal stress = σv

boreholeinjection

fractures tend to rise gently in this case

pinj > σv

*SWR = Soft, weak rock such as unconsolidated sandstones

fracturepans-outunder shaleσ3

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csDifferent Stresses in StrataDifferent Stresses in Strata

� Often, fractures do not rise out of the zone, they stay in the zone and propagate laterally. Why?

� This usually means that σhmin in the upper zone is larger than in the lower zone

� This is a barrier to upward propagation

� It is easier to grow laterally that to grow upward, which needs a higher fracturing pressure

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csBlunting Upward GrowthBlunting Upward Growth

stress

σhminσv

High lateral stress “blunts” vertical growth

Fracture grows in the zone of lower

σhmin

depth

This is the “ideal”fracture, only attained when a higher stress

gradient in the overburden blunts rise

Key!!

Is this common? Yes. For example, in the Gulf of Mexico

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csNatural Natural σσhminhmin (P(PFF) Variations) Variations

stress

σhmin

depth

σv

salt

hydrostatic po

Pore pressure distribution

limestone

shale

sandstone

shale

shale

(po is undefined in salt beds)

depth

σhmin

zσv

z

Absolute stress values Stress gradient plot

Frac gradient, PF in ppg, is fracture pressure/depth = σhmin/z

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csGoM CaseGoM Case

� In the GoM, it is typical that the shales have higher lateral stresses than the sands

� In other words, PF (shales) > PF (sands)� This provides a stress “barrier” to upward

propagation of hydraulic fractures� It is the common case in all gravitational

basins, also common in normal fault basins� However, this may not apply at great depth

�Shales have undergone diagenesis, changes�Lateral stresses in shales now lower than sands

� Also, not in tectonic basins, near salt…

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csLower Overburden Lower Overburden σσ33 CaseCase

stress

depth

σhmin

Fracture retreat

Initial fracture growth phase

Preferential propagation in the zone of lower σhmin

σv

Normal σ case

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csCase of Low Overburden PCase of Low Overburden PFF

� In this case, for tectonic reasons or diagenetic alterations:�The overlying cap rocks (shales or siltstones)

have a lower PF than the reservoir rocks

� It doesn’t matter if the overlying rocks are impermeable (shale), strong (limestone) or of low porosity (anhydrite):

� Fractures will tend to rise through them, rather than propagate laterally

� In some parts of the world, deep gas fractures can rise 4000 m to the sea floor!

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csInduced Changes in Stress FieldsInduced Changes in Stress Fields

� Near-field stresses are altered by fracture

fracture tipσ +∆σ3 3

∆p

primary fracture

secondary fracture

dilated zone

high pressure zonep > σ (original)

3

pressure increasecauses stresses to

increase as well

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csFracture Direction ChangesFracture Direction Changes

� A fracture pushes the rock apart, and the pressures are higher than σ3

� As the fracture L grows, the aperture also grows, and this increases the stress normal to the fracture

� Near the well, it now becomes easiest to propagate in a different direction

� This is done deliberately in “”Frac’n Pack”� Also, the injection plane may flip back and

forth between the two directions� This has been measured in real frac jobs…

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csSpanish PeaksSpanish Peaks

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csSpanish PeaksSpanish Peaks

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csFracture Orientation ChangesFracture Orientation Changes

Courtesy Pinnacle Technologies

Limited further growth of N80°E fracture

Wellbore

Fracture geometry after first 2/3 of main treatment

vertical frac

Probable fracture geometry at end of pumping

Creation of new vertical frac ⊥ to

original vertical frac

horizontal frac

Tiltmeter data during fracturing confirms multiple orientations and flipping of growth plane (California)

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csDepletion and FracturesDepletion and Fractures

� The well-known depletion effect changes the total stresses in the well influence region

� Not all wells are depleted evenly� There are other effects associated with:

�Proximity of no-flow boundaries�Lithological differences (stratification)�Reservoir heterogeneity, plus ∆φ, ∆k with ∆p�Compaction and stress redistribution

� Combined, these give an “uncertainty” as to fracture direction after the depletion of a field

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csDepletion Effect HeterogeneityDepletion Effect Heterogeneity

Concept: SPE 29625 by Wright et al.

Original fracture orientation, virgin reservoir conditions

Fracture orientation in a mature field with infill wells, altered p, refracs…

σhmin

initial

Local effects have overridden initial stress orientations, so that the second EOR generation had fractures at

different and generally unpredictable orientations

production

injection

Differential depletion, natural heterogeneity, thickness

differences…

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csDepletion and PressurizationDepletion and Pressurization

� Suppose in situ stresses are similar (±5-8%)� If fractures originally horizontal, σ3 = σv

�Depletion can reduce σhmin to below σv

�This means refracs will be vertical!

� If fractures originally vertical, σhmin = σ3

�Pressurization can increase σh to above σv

�This means fracs may become “horizontal”during an injection process! (Especially heating)

� Be careful, ∆p can change frac orientations!� Re-determine your fracture directions in

wells if this is critical to the process

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csIncrease in Increase in σσ33 →→∆∆OrientationOrientation

pBD, breakdown pressure

Bot

tom

Hol

e P

ress

ure

Time (or V if constant injection rate used)

Sudden propagation

σ3

σ3

Large-scale stress change with continued injection

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csDo Fractures Initiate Suddenly?Do Fractures Initiate Suddenly?

� In intact rock, yes, because the value of σθ is the highest at the borehole wall

� However in many cases, σθ is reduced in a zone near the well (T, damage, etc…)

� The fracture can initiate before breakthrough� It grows slowly and gives a non-linear response� When it passes the peak σθ, it then “shoots” out

suddenly� The p-t response is quite non-linear� Very common in unconsolidated sandstones…

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csNonNon--Linear ResponseLinear Response

botto

mho

le p

ress

ure

virgin reservoir pore pressurep

o

fracture initiation occurs very early

stable fracture propagation

breakthrough

propagation

non-linear responsetime (constant pumping rate)

Fracture is initiated and grows well before it breaks through and extends

This is usually the case in weak rocks like tar sands…

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csPermeability EffectsPermeability Effects

� High k stratum generates massive blunting

� Propagation potential reduced if a new high-k stratum encountered (loss of hydraulic E)

� In extremely low-k strata (shales), no bleed-off, distant propagation, high p generated

� Bleed-off changes with time as the pressure gradients change with inflow

� Fluid-loss control agents can be used wisely in such cases, but understand the role of stresses as well, or your risks will increase

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csBlunting in a HighBlunting in a High--k Zonek Zone

High k stratum

Low k stratum

Low k stratum

A

A′

Section A-A′

Fracture retreats after high k zone intersected

“Blunting” through high k zone effect

Fluid flow

Fracture before intersection

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csRock Strength EffectsRock Strength Effects

� Rocks are jointed, fissures, bedded, flawed

� Fracture will “find” these flaws immediately

� Resistance of such materials to propagation is minimal with a a large fracture length

� If strength is correlated to another property (k, E, σ3), it may “appear” to be important

� In general, strength (fracture toughness) is largely irrelevant for large fracs

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csLocal Fabric and FractureLocal Fabric and Fracture

σ3

σ3 Joint system in the rock

Locally, fracture follows fabric;

globally, fractures follow stress

The strength of the intact rock is not relevant in this case

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csCoolingCooling--Induced FracturesInduced Fractures

Water displacement

front

σhmin

σHMAX

∆T front

∆T

∆T

∆TTo

To

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csGeothermal FracturingGeothermal Fracturing

Cold water inHot fluids out

Cross-section Large propped fracture

Massive cooling by conduction

“Daughter” fractures propagate at 90° to the

mother fracture, heat exchange becomes better.

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csThermal Cooling Effect on SRTThermal Cooling Effect on SRT**

*Step-Rate Testpressure

rate

before injection

after injection

�lowered pfrac near wellbore�higher pfrac far from wellbore

pfrac

pfrac

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csMonitoring FracturesMonitoring Fractures

� Precision real-time tilt monitoring (<3000m)

� Microseismic monitoring using geophones at depth relatively near the fracture site

� Pressure-time response in the injection well

≈ Impedance tests in a propped fracture

≈ Borehole geophysical logging (T, tracers)

� Other methods are problematic at best

≈ Implies a “poorer” method of monitoring

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csHydraulic Fracture MappingHydraulic Fracture Mapping

� Characteristic deformation pattern makes it easy to

distinguish fracture dip, horizontal and vertical fractures� Gradual “bulging” of

earth’s surface for horizontal fractures

� Trough along fracture azimuth for vertical fractures

� Dipping fracture yields very

asymmetrical bulges

Dip = 80°Maximum Displacement:

0.00045 inches

Dip =90°Maximum Displacement:

0.00026 inches

Dip = 0°Maximum Displacement:

0.0020 inches

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csTiltmeter Fracture MappingTiltmeter Fracture Mapping

� Tilts measured

� Mathematical sol’n

� If depth > 3 km, tilt measurements are quite difficult

� One solution is use of borehole tiltmeters

� Mapping has recently been achieved at > 3km

Dep

th

Surface tiltmeters

Downhole tiltmeters in offset well

Fracture

Courtesy Pinnacle Technologies

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csFracture and Tilt VectorsFracture and Tilt Vectors

1000 feet

Measured Tilt -- 250 nanoradians

Theoretical Tilt -- 250 nanoradians

Frac: Vertical Azimuth: N39°E Dip: 87° W Depth: 2300 ft

North

Tiltmeter Site

1000 feet

Measured Tilt -- 500 nanoradians

Theoretical Tilt -- 500 nanoradians

Frac: Horizontal Azimuth: N/A Dip: 6° N Depth: 2900 ft

North

Tiltmeter Site

Wellhead

Courtesy Pinnacle Technologies

Vertical

Horizontal

Azimuth

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csLessons LearnedLessons Learned

� HF behavior is complex, but understandable

� Stress fields dominate fracture propagation behavior, strength is almost irrelevant

� Almost all fractures rise from buoyancy, except if there is a stress barrier that prevents it

� Permeability, stiffness, etc. are important, but they are second-order effects

� Fractures change directions over time!

� Monitoring fracture behavior is feasible, useful

� Geomechanics concepts are essential for HF