spe distinguished lecturer series spe foundation · why is cementing important ? prevention of flow...

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CEMENTINGPlanning for success to ensure isolation for the life of the well

Daryl Kellingray, BP Exploration

Wherever we construct a well, we use cement

Why is cementing important ?

Prevention of flow to surfaceMMS identified 19 cement related well control incidents 1992 -2002.

Sustained Casing Pressure• 25-30% of wells are estimated to

have annular pressure problems, cementing is one of the primary route causes.

Why is cement important ?

Mechanical well integrity during drilling• Isolation of weak formation &

structural support

Reservoir Isolation and Protection• Isolating production from other fluids

Mud Wt (ppg)

Dep

th (f

eet)

PP FG

Mud Wt (ppg)

Dep

th (f

eet)

PP FG

Production OptimisationOptimising stimulation treatments

Cementing Planning For Success

Drilling Related Problems and Learning

Gas migrationDeep water

Long Term IsolationIsolation breakdownCement bondingMechanical Issues

Integrated Mud and Cement Design

Balancing drilling fluids and cementing requirementsSpacersCement placement

Classification of annular flows

Annular flow after cementing falls under three general classifications:

• Percolation through unset cement

• Influx via mud channels (poor mud displacement)

• Flow and/or pressure transmission through set cement (microannuli, stress cracks, etc.)

In the MMS reporting system nearly all post cementing flows occurred 3 - 8 hours after the job

Drilling Related Problems

Gel strength terminology

• Static Gel Strength - rigidity in the matrix which resists forces placed on it

• Critical Gel Strength - the gel strength which the hydrostatic column (when accompanied by volume reduction)

• Zero Gel Time or Delayed Gel Time - time to start of critical gel strength (approx. 100 lb/100 ft2)

• Transition Time (nothing to do with thickening time) - time from zero gel time (usually 100 lb/100sqft) to 500 lb/100 ft2

Drilling Related Problems

Pressure decay

Fully Liquid

Hydration

Set Cement

Early Gelation

Full hydrostatic transmission < 100 lb/100sqft.Migration risk from insufficient slurry density.

Cement static with gels between 100–500 lb/100sqft.Volume reduction by fluid loss reduces hydrostatic.High risk of migration, mitigated by <30 min transition times and compressible cements.

Cement is no longer deformable with rapid development of strength, pore pressure in cement dropping. Migration risk if high permeability.

Cement now has high compressive strength and low permeability.

SETT

ING

Drilling Related Problems

Effect of gel strength on pressure transmission

Gel Strength to support overbalance = (OBP)*(300)(L/Deff),

whereOBP = overbalance pressure (psi)300 = conversion factor (lb/in.)L = length of the cement column (ft)Deff = effective diameter (in.) = Dc - DOHDc = diameter of the casing (in.)DOH = diameter of the open hole (in.)

E.g.For 26”OH and 20” casing with a 50 psi overbalance 1000 ft beneath seabed

Gel Strength = 300 * 50 = 9 lb/100sqft !!!!1000/6

Drilling Related Problems

Prevention of annular flow

• Quantify the risk (overbalance and cement column height dependent)

• Design mud displacement and cement placement

• Determine rate of static gel strength development

• Determine fluid loss requirements

Drilling Related Problems

Issues impacting deepwater slurry selection

Common cementing questions ?

Deepwater Considerations

Should cement be tested at seabed temperatures?

Ignores impact of cement heat of hydration.

Will WOC always be extended in deepwater surface strengths?

Actual strength requirements are low (<100psi for pipe support)

Is foam cement the only option?

API RP 65 favours foam but not the only solution.

Low fracture gradients require low density cements?

In deep water the seawater column reduces cementing ECD’s and can permit use of 1.92 SG slurries

Drilling Related Problems

Cementing temperatures in deepwater wells

Computer simulated cement circulating temperatures v API circulating temperatures for a deepwater 13 3/8” casing string 9000 ft below seabed.

Water depth 3000 ft 3000 ft 6000 ft 6000 ftBHST (oF) Simulated

cementing temperature

(oF)

API Spec 10 cementing

temperature

(oF)

Simulated cementing

temperature

(oF)

API Spec 10 cementing

temperature

(oF)

106 84 87 80 87146 100 128 100 128

Drilling Related Problems

Cement setting at low temperature

Are industry compressive strength tests representative when cementing surface strings in deep water ?

Large scale experiment in temperature controlled room at 43oF using simple 16 ppg class G cement.

020406080

100120140

1 2 3 4 5 6 7 8 9 10 11 12 13

2” cube

Centre of annulus

4” cube

Tem

pera

ture

o F

Time (hours)

Drilling Related Problems

Crossflow in a micro annulus

1

10

100

1000

10000

0 50 100 150 200 250 300 350 400 450 500Microannulus Hydraulic Aperture (microns)

Wat

er F

low

rate

(bpd

)

5,000psi across 65m 10,000psi across 65m 5,000psi across 20 m 5,000psi across 10m 10,000 psi across 10m

AssumptionsWater viscosity 0.3 cPPressure drop only in microannulusMicroannulus all around the liner

Long-Term Isolation

When does cement provide a seal?

Only good primary cement jobs were included in the analysis

Long Term Isolation

Cemented standoff between zones (ft))

No Breakdown Field A Isolation BreakdownNo Breakdown field B

0150 200 300250 3500

2000

50 100

6000

4000

10000

8000

12000

14000

400

Diff

eren

tial p

ress

ure

betw

een

zone

s (p

si)

Cemented standoff between zones (ft))

No Breakdown Field A Isolation BreakdownNo Breakdown field B

0150 200 300250 3500

2000

50 100

6000

4000

10000

8000

12000

14000

400

Diff

eren

tial p

ress

ure

betw

een

zone

s (p

si) High risk zone

Design steps for selecting a slurry

PRO

JEC

TED

IMPA

CT

MANAGEABILITYHigh Low0 Moderate

A

BC

D E

F

G

H I J

A Reservoir isolation B Poor hole conditions for

cementing C Failure to optimise slurry

designD Failure to execute job

according to planE Shallow FlowsF Inadequate Long Term

integrityG Downhole pressure

management during cementing

H Failure to keep the Quality Plan up-dated and alive

I Inadequate short term integrity

J Environmental Impact

Develop an assurance processReview analogous developments

Identify options and risk assessincluding assessment of flow potential

Determine mechanical loadings and complete modelling

Can robust pumpable slurries be designed which can be effectively placed?

Are the benefits real ? Warranting cost and complexity?

Long Term Isolation

Cement expansion

0 5 1 0 15

4.54

3.53

2.52

1.51

0.50

Age Days

5% MgO3% MgO

1% MgO

% li

near

expa

nsio

n

Does expansion occur when you need it ?

Long Term Isolation

Internal and external volume changes

Slurry Internal Shrinkage

Bulk Volume Change

16 ppg neat G -4.1% -1.7%14 ppg Pozmix -2.6% 0.5%

1.7 SG Pozmix

1.9 SG Neat Class G

1.9 SG Salt saturated class G

1.7 SG Pozmix

1.9 SG Neat Class G

1.9 SG Salt saturated class G

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

% in

tern

al s

hrin

kage

-4.0

-4.5

0 5 10 15 20 25 30 35 40 45 50hours

Long Term Isolation

Cement mechanical properties

To impact the mechanical resistance novel cements must decouple stiffness (Young's modulus) from tensile strength. Determination of mechanical properties under triaxial conditions is critical for modelling.

Long Term Isolation

Bond variability across sands and shale

Commonly, cement evaluation indicatessuperior bonds are obtained againstshale's comparedto sandstones.

gamma rayattenuation

Long Term Isolation

Possible explanations for lithology effect

The log is wrong!

Cement shrinkage due to fluid loss or bulk shrinkage

Formation fluids entering cement impacting acoustic properties

Shale squeezing onto the pipe

Mud filter cake

Long Term Isolation

OBM and WBM filter Cakes

OBM @ 280 deg F

24 hour fluid loss = 9.9 ml

“Cake” height = 6/32”

WBM @ 280 deg F

24 hour fluid loss = 40.2 ml

Cake height = 5/32”

Long Term Isolation

Areas of concern

Contamination of fluidsSpacer DesignCement Bonding Cement PlacementExecution / LogisticsAnnular Pressure Build UpCompletion / Productivity

Integrated Mud and Cement Design

Mud Contamination of the cement spacer

Barite Sag due to surfactants impacting oil based fluidsUnpumpable mixtures formed in surface lines or downhole

Spacer compatibility with mineral oil based mud(some values extrapolated as

reading off scale).

Note: Problem notreally evident at 25% mud contamination !

Rot

atio

nal v

isco

met

er 1

00 rp

m re

adin

g

% spacer in mixture

0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50 60 70 80 90 100

Integrated Mud and Cement Design

Temperature effect on compatibility

0

20

40

60

80

100

120

140

160

180

% Mud / % Spacer

Rot

atio

nal v

isco

met

er, 1

00 R

PM, l

b/10

0ft s

q

70 deg F120 deg F185 deg F

100 / 0 75 / 25 50 / 50 25 / 75 0 / 100

Surfactants are Temperature Dependent,Very Critical in Deep Water

Integrated Mud and Cement Design

Mud contamination of cement

Acceleration of cement due to brine phase in cement (cemented in liner running tools)Retardation by WBM (lignins/HEC/citrates/borates)Impact on strength/acoustic impedance and ability to quantify cement bonding

Mud Contamination Compressive Strength

0

500

1000

1500

2000

2500

3000

3500

0% 10% 20% 30% 40% 50% 60%

Mud Contamination [%]

Com

pres

sive

Str

engt

h [p

si]

SBM

WBM

Integrated Mud and Cement Design

Forces dictating efficient fluid displacement

Pressure ForceDue to rheology hierarchy and increases with increasing flow rate.

Buoyancy ForceFrom the density contrast between fluids, the buoyancy force reduces with increasing hole angle.

Resistance ForceThe resistance to movement of the gelled mud in the annulus, increases as mud rheology increases and casing centralisation decreases.

Simplistically for mud removal Pressure + Buoyancy > 1

Resistance

Integrated Mud and Cement Design

Displacement variables

VERY POORDISPLACEMENTVERY EFFECTIVE

DISPLACEMENT

POORDISPLACEMENT

Thin muds with goodpipe centralisation and high flow rates

Thicker gelled muds displaced with ineffective

spacers and poor centralisation

Thicker gelled mudsdisplaced with lighter

fluids with poorcentralisation

Variables to Improve Displacement

* Density Difference * Rheology of Pill and Mud * Pipe movement* Flow Rate * Volume / Contact Time

Integrated Mud and Cement Design

Predicted effect of rotation on annular velocity

7” liner in 8.5” holeStand Off = 40%

No Rotation

10 RPM

25 RPM

Axial Velocity(m/s)

2.21.91.61.31.00.70.5

0.350.20.1

0.050.025

0.0

GQS37586_30Integrated Mud and Cement Design

Does pipe rotation help ?

Computer modelling of flow during liner rotation

5 bbl/min1 bbl/min

1.0

0.8

0.6

0.4

0.2

00.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0

RPM

P V / Y P 5 0 / 2 1 , 7 0 % S t a n d o f f

No channelling = 1

Integrated Mud and Cement Design

Extended gel strength development for gel mud

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30

Gel

str

engt

h lb

/100

sqft

hours

Typical timeframe to break

circulation for offshore surface

pipe

Integrated Mud and Cement Design

Conclusions

There is a large industry problem related to the integrity of the cement sheath providing Long Term Isolation.

Cement mechanical properties are important, but are highly dependent on confining forces.

Temperatures in deepwater have a big impact on cementing temperatures and surfactant efficiency in cement spacer.

Mud displacement and subsequent cement placement is the main cause of poor zonal isolation.

END