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UCSS UCSS -- Unconsolidated SandstonesUnconsolidated Sandstones

Maurice Dusseault

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sUnconsolidated Sandstones Unconsolidated Sandstones -- UCSSUCSS

� The major factor is lack of cementation� Some distinguishing characteristics:

-These all depend on depth of burial-High porosity (>30%@1000m, >25%@3000m … )-Little to no CaCO3 or other mineral cements -Usually the cleaner sands are more cement-free-Usually of lower velocity than surrounding strata-Lack of large tectonic compressive loading-Quartzose sands are more likely to be UCSS

� But, UCSS can be stiff, soft, quartzose or not, young or old, etc…

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sUnconsolidated SandstonesUnconsolidated Sandstones

� Geology has a vital role in helping engineers understand the behavior of uncemented and lightly cemented sandstone

� Sedimentary environment affects fabric, clay content, mineralogy, sorting…

� Diagenesis increases strength, stiffness, cohesion, and in some cases can lead to formation of neogenic (new) clays

� Geological history of burial & erosion affects present-day stress fields, stiffness, etc…

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sWill this Become a UCSS?Will this Become a UCSS?

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sA Consolidated Sandstone (Sydney)A Consolidated Sandstone (Sydney)

This is the famous Hawkesbury Sandstone in the Sydney Basin, Australia (sorry, no oil!). In areas it is SiO2 cemented, making

good building stone, in many areas it is unconsolidated.

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sWhy are UCSS of Interest?Why are UCSS of Interest?

� Huge fresh water aquifers in US, Libya, etc.

� Most new discoveries offshore around the world are found in UCSS at depth

� Perhaps ~6 Tb of viscous oil is found in UCSS �Canadian Oil Sands - > 350 × 109m3

�Venezuela Faja del Orinoco - > 250 × 109m3

� Russia + China + Middle East... �200 × 109m3 HO in UCSS

� Rest of World, perhaps another 200 × 109m3

� So… about 6 Tb

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50 m of Cretaceous Athabasca Oilsands underlain by Devonian age carbonates, North of Fort McMurray, Al ta

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sTechnical Challenges in UCSSTechnical Challenges in UCSS

� Sand production problems are extremely common during fluid production

� Non-linear behavior (shearing) makes good hydraulic fracture predictions challenging

� Stiffness is a function of confining stress (σ′)� Thermal processes (∆T) cause extensive

shearing and dilation (properties change)

� Extremely difficult to get undisturbed samples in some cases (heavy oil, deep strata…)

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sSampling Difficulties in UCSSSampling Difficulties in UCSS

� The lack of cohesion means that UCSS are difficult to sample without disturbance

� A very small amount of disturbance is enough to give a spurious compressibility

� For example, using damaged cores, for years Venezuelan oil company persons assumed good compaction drive in the Faja del Orinoco�From lab on bad core: 20-100 × 10-6 kPa-1

�From analogues, other sources: 1-5 × 10-6 kPa-1

�Clearly, damage can lead to bad decisions…

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sHeavy Oil Cores, MR

Scans

Courtesy of Glen Brook, Nexen and

Apostolos Kantzas, U of

Calgary

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CT-Scan Evidence of Damage in Heavy Oil CoresCourtesy of Glen Brook, Nexen and Apostolos Kantzas, U of Calgary

decr

easi

ng d

ensi

ty

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sRecommendations for UCSSRecommendations for UCSS

� Sample very carefully, preserve core carefully

� Use log-derived porosity based on the γ-γ log (density) to calculate porosity, compare to the core-derived porosity for quality control

� If too much damage…�Test an analogue that is easier to sample

�Examine coring methods carefully

�Use data banks and correlations for properties

�Use UCSS core for other purposes (fluids, etc.)

� Damage, in general, increases Cc, φ, and k

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sEffect of DiagenesisEffect of Diagenesis

timetemperature

chemistry

cementation,25-32%

pressuresolution,25-32%

initially, 35% porosityfor a typical sand

porosity reduction

stresses

+

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sThe Effect of DiagenesisThe Effect of Diagenesis

� Pressure solution at grain contacts is a major reason for porosity loss and E increase, but…

� If this does not involve cementation, no true tensile strength develops – friable or UCSS

Ap – pore throat

f′′′′ + ∆∆∆∆f′′′′

E,νννν-∆p = +∆σ′

reduced Ap

f′k+ ∆f′

f′i + ∆f′

f′k

f′iIncreased

contact area

small porosity

reduction

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sEffect of Diagenetic DensificationEffect of Diagenetic Densification

porosity

log(σ′v)

diageneticporosity loss

@ constant σ′

stiffresponse

“virgin”compression

curvepresent state

apparent threshold ∆σ′

For UCSS, the stress history and diagenetic fabric are extremely important indicators of probable behavior

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sDiagenesis and StrengthDiagenesis and Strength

shear stress

normalstress

chemical cementation

densification(more interlock)

originalsediment

diageneticstrengthincrease

σ′1σ′3

diagenesiseffects on theM-C envelope

cohesion

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sUCSS GuidelinesUCSS Guidelines……

� Quartz arenite UCSS: lower Cc, higher shear strength, more dilation than arkoses or greywackes (feldspars and lithic particles)

� UCSS is stronger and stiffer if…�Subjected to burial, then erosion (100’s of m)

�Subjected to intense diagenesis (grain fabric)

�Subjected to compression (close to mountains)

� The true tensile strength may be zero, but during testing there is still an apparent cohesion because of curve fitting…

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sTrue Cohesion or Not?True Cohesion or Not?

shear stress

normalstress

σ′1σ′3

curvatureeffects on theM-C envelope

apparent cohesion

The cohesion intercept is affected by how we draw our MC Y-line. An apparent cohesion can be seen here.

Y from a linear fitY - curvilinear fit

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sQuartz Sandstone FabricQuartz Sandstone Fabric

Overgrowths on a Q grain

Examples of quartzose unconsolidated sandstones

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sUCSS UCSS -- Extreme Diagenesis CaseExtreme Diagenesis Case

SiO2 grains

Highly soluble grain

Crystal overgrowths

Interpenetrating fabric

An intense diagenetic fabric, but no true cement!

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sDiagenesis and UCSS PropertiesDiagenesis and UCSS Properties

� Cohesion (c′) may be increased� Tensile strength (To) low to zero

� Friction angle (φ′) increased considerably� Stiffness (E) is increased greatly� Poisson’s ratio probably unaffected� Weak anisotropy may be generated in

sandstones if there is tectonic stress as well� Permeability drops (pore throat dia., fractures)� Thermal properties are altered only slightly…

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sStability of Perforation TunnelsStability of Perforation Tunnels

� In a UCSS, there is little cement to resist disaggre-gation & sand production

� In fact, as the tunnel is depressurized, shearing takes place around wall

� And this destroys any small amount of cohesion

� Which leads to sand entering the borehole under high flow gradients

Courtesy, IKU-Sintef, Norway

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sCorrelations for UCSS SandingCorrelations for UCSS Sanding……

� Developed for GoM

� “Predicts” sanding or no sanding

� Uncertainty range is very large

� Most data fall in the blue area…

� A very rough indicator of strength only

� There are other factors200

225

250

275

300

325

350

400

450

500

525

375

425

475

3.02.82.62.42.01.8 2.21.6

70

80

90

100

110

120

130

140

150

1603.02.82.62.42.01.8 2.21.6

Bulk Density, ρb (g/cm3) x 1000

Com

pres

sion

al T

rans

it T

ime,

∆t c

(µse

c/ft)l =

1.0

x 10

6

l = 1.

2

l = 1.

4

Sand Production

Usually Sand-Free Production

l = 3.0

l = 3.5

l = 4.0

l = 5.0

l = 6.0

Bulk Density, ρb (g/cm3)I is the combined modulus of strength.

I = K + m = 1.34 x 10104

3 ∆tc2

ρb

Com

pres

sion

al T

rans

it T

ime,

∆t c

(µse

c/ft)

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sE.g. E.g. -- West Cameron 587 (UNOCAL)West Cameron 587 (UNOCAL)

In the zones of interest, data plotted mainly in the centre zone (risk difficult to assess), but also some plotted above and some plotted below the zone.

In all correlation methods, there are uncertainties. In fact, in all petroleum rock mechanics analysis, uncertainty is an issue.

Quantitative risk calcs. are difficult!

LIKELY SANDING

SANDING UNLIKELY

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sE.g. West White Lake (UNOCAL) E.g. West White Lake (UNOCAL)

Here, the data all plotted below the sensitive (uncertain) range. One would assume that the sand production risk is low.

What will you do? Install expensive control systems?

A viable option in these poorly consolidated sandstone cases is to assume no sand, but be prepared to live with a little bit if sand influx, and handle it rationally.

Poorly consolidated sandstone

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sPoorly Consolidated SandstonesPoorly Consolidated Sandstones

You can brush sand off the face of this outcrop with a stiff paintbrush: i.e.: friable or unconsolidated or poorly consolidated

sandstone. There is no rigid definition, but I would suggest that an unconsolidated sandstone is one that truly has no grain-

to-grain cement and can be easily disaggregated when in a saturated state.

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sFaja del Orinoco, VenezuelaFaja del Orinoco, Venezuela

� A classic unconsolidated sandstone reservoir

� 60-80 MYBP, >85% quartz, 26-32% porosity

� Mainly estuarine blanket and channel

� In a sand-silt-shale (arenaceous) sequence

� No CaCO3 or SiO2 cement between grains

� Quartz arenites common, diagenesis

� No lateral tectonic compression or structure

� Maximum burial depth (below today) +1000 m

� Stiff, relatively strong, will not compact

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sLocation of the Faja del Orinoco Location of the Faja del Orinoco

� Group of coalescing deltas

� Quartz arenite reservoirs generally (more arkosic to the west)

� >1.4 Tb of viscous oil (10% of world’s oil)

Venezuela

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sChannels Channels –– Lateral VariabilityLateral Variability

OESTE ESTE

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sIn a Thick Channel In a Thick Channel –– Oil Saturated!Oil Saturated!

� One of the World’s wonders 11 km

Over 150 m of net pay in places, + high porosity, permeability and

saturation, viscosity 1000-4000 cP