Gas Migration: Causes And Cures

GAS MIGRATION CAUSES

AND CURES

Module CMT 102 June 2000

Gas migration: Causes and Cures  Consequences of gas migration  Gas migration paths  Root causes for gas migration  Schlumberger technology

Consequences of gas migration  Blow-out: surface or underground

 Danger to personnel  Lost rig  Less dramatic but important consequences

 Lost production  Treatment fluids injected in wrong zones  Annular pressure on surface  Damage to the environment  Repair required: prevention is better than cure

Gas Migration  A complex problem involving a hierarchy of potential

problems with a corresponding hierarchy of solutions Den Flui sity d Con tro l

al v o Rem d u M

Cemen t

s n t rt i e e em rope C t P Se ical an h c Me

Hydrat ion

C Slu eme nt rry De sig n

Gas migration paths  Mud channels  Mud cake  Channels due to free water development or

sedimentation in the cement slurry  Cement matrix during liquid/solid transition  Cement matrix fissures/fractures, cement/casing

or cement/formation interface once cement is set

Paths for gas migration

In the cement slurry before it sets or during setting process

In a mud channel or mud cake

Paths for gas migration FREE WATER CHANNEL

CEMENT

GAS ZONE

In a channel due to free water or sedimentation of the cement slurry (highly deviated wells)

Paths for gas migration

In the cement after it sets

At cement/casing or cement/formation interface once cement is set

Paths in the drilling fluid  Mud removal is the KEY  Remove the bulk of the mud:  Centralize the pipe  Pipe movement  Apply adequate displacement techniques: WELLCLEAN

 Remove at least the soft part of the mud cake

 Mud properties requirements  Low rheology/gel strength  Low fluid loss and thin impermeable mud cakes

Paths in the cement before it sets  Hydrostatic pressure transmission is the key  During the job

 cement density is the key  After placement cement loses its ability to fully

transmit hydrostatic pressure due to:  Gel strength development  Downhole volume variations: fluid loss, temperature and hydration volume reduction

 Consequence  pore pressure within the gelling cement is decreasing and may become smaller than formation pore pressure  Gas can now possibly invade the annulus

Cement slurry requirements  Appropriate density  Appropriate rheology for good mud/spacer removal  Good stability at downhole conditions  Very low fluid loss to minimize volume variations

downhole and hence slowdown hydrostatic pressure decline: API < 50 mL/30 min  Ideally right angle transition from liquid to solid  In practice during the transition period:  Minimum gel strength development  Low permeability and low hydration volume reduction  Short transition time from 30 to 100 Bc

Paths in the cement after it sets  Set cement has a very low permeability:

 it acts as a seal  But set cement may fail at providing a seal:

 The cement itself may crack due to variations downhole stresses:  changes in temperature, pressure, far field stresses

 The cement may debond from the casing and/or formation  These failure mechanisms provide paths for gas

from the formation to possibly invade and migrate up the annulus

Example of cement sheath failure rock

t

cement

P,T

casing

Radial crack: failure in tension (hoop stress) due to pressure and or temperature increase

Example of cement sheath failure

rock cement

r

r

casing

Cement debonding: failure in tension (radial stress) due to pressure and or temperature decrease

Set cement requirements  Ductility is the key:

 Requirement for a material that can better stand downhole stresses variations than most cement systems which are fairly brittle  Very important when large temperature or pressure variations are expected during the life of the well  Very important when large variations in far field stresses are expected during the life of the well e.g. formation subsidence  Good bonding to the casing and the formation  Low long term shrinkage

SLB technology  GASBLOK Service has been proved effective in

a wide range of conditions:  Slurry density from 10 to 24 lb/gal (1.20 to 2.88 kg/L)  Temperatures up to 400°F (204°C)  Depths greater than 20,000 ft (6,000 m)  GASBLOK additives are used all over the world

in particular in area where gas migration represents a significant hazard

Gas-migration-control slurries  GASBLOK (D600/D134) additives

 Suspension of submicron latex particles  Temperature range from 150 to 400°F (65 to 204°C)  Works in a wide density range 10 to 24 lb/gal (1.20 to 2.88 kg/L)  Easy to design and to mix  Synergetic effect with CemCRETE technology  Applications

 Gas migration in low (D500), medium (D600) to high (D134) temperature wells

Gas-migration-control slurries  GASBLOK D600

 Temperature range from 150 to 250°F (65 to 121°C)  Above typically 200°F (93°C) with D135 stabilizer

 Density range 13 to 24 lb/gal (1.56 to 2.88 kg/L)  Can be mixed with fresh or sea water  Salt tolerance:  Up to 6% NaCl or KCl BWOV without D135  Up to 18% NaCl BWOV with D135

 Concentration depends on BHST and slurry porosity  Stabilizer and dispersant: D65/D80/D604/D135

Gas-migration-control slurries  GASBLOK HT D134

 Temperature range from 200 to 400°F (93 to 204°C)  Density range 15.6 to 24 lb/gal (1.87 to 2.88 kg/L)  Can be mixed with freshwater only  Stabilizer D135  Dispersants: D121, D65, D080  Concentration depends on BHST and slurry porosity  Use D066 above 230°F (110 °C) BHST

D600/D134 GASBLOK slurries  Excellent slurry rheological properties  Very good stability  Very low API fluid loss (lower than 50 mL/30 min)

 Extremely low fluid loss rate  Thin and impermeable filter cakes  Low permeability during liquid/solid transition  Low hydration volume reduction in particular in

CemCRETE slurries  Short transition time from liquid to solid

D600/D134 GASBLOK cements Conventional cement

GASBLOK cement

 Improved bonding and mechanical properties (SPE 13176)  Improved set cement resistance to chemical attack

Example of D600 slurry design Density : 20.1 lb/gal API Class G cement 30% BWOC Silica flour 85% BWOC Hematite 0.8% BWOC D121 3.10 gal/sk D600 0.06 gal/sk D080 0.05 gal/sk D047 2.38 gal/sk Fresh water

Yield Thickening time Fluid loss Free water PV at 80 deg.F TY at 80 deg.F PV at 185 deg.F TY at 185 deg.F 24 hours CS

1.76 ft^3/sk 5 hr 20 min 34 mL/30 min Nil 260 cp 15.7 lbf/100ft^2 211 cp 34.8 lbf/100ft^2 > 6000 psi

Gas-migration-control slurries  GASBLOK LT (D500) additive

 A liquid microgel (submicron particles) with a density of 1.0 kg/L  Environmentally friendly  Temperature from 80 to 165°F (27 to 74°C)  Slurry density from10 to 16.5 lb/gal (1.20 to 1.98 kg/L)  Easy to design and to mix  Compatible with all Dowell accelerators and retarders  Non-retarding  Synergetic effect with CemCRETE technology  Non-damaging to formations

D500 GASBLOK LT slurries  Applications of D500 slurries  Shallow annular gas migration  Gas migration in cold wells  Properties of slurries and set cements using D500  Excellent slurry rheological properties  Excellent stability  Extremely low API fluid loss (lower than 40 mL/30 min)  Extremely low fluid loss rate  Thin and impermeable filter cakes

 Low permeability during liquid/solid transition  Low hydration volume reduction in CemCRETE slurries  Shorter transition times at low temperatures

Summary Slide  D500 Fluid-Loss Control Behavior

Examples of D500 slurry designs 15.8 ppg: 80F

16.4 ppg: 120F

15.8 ppg: 160F

Slurry design:

Slurry design:

Slurry design:

Dyckerhoff Class G +

Lonestar Class H +

Indocement Class G +

D047 (gps)

0.03

D047 (gps)

0.03

D047 (gps)

0.03

D080 (gps)

0.04

D080 (gps)

0.06

D145A (gps)

0.14

S001 (%BWOC)

1

D500 (gps)

1.00

D500 (gps)

1.20

D500 (gps)

0.80

Rheology @ BHCT 2

Rheology @ BHCT 2

Rheology @ BHCT 2

Ty (lbf/100 ft )

10

Ty (lbf/100 ft )

3

Ty (lbf/100 ft )

13

PV (cp)

49

PV (cp)

58

PV (cp)

28

10-min gel

24

10-min gel

24

10-min gel

25

API fluid loss (mL)

20

API fluid loss (mL)

26

API fluid loss (mL)

18

API free water (mL)

0.5

Free water (mL)

none

Free water (mL)

0.4

3:50

Thickening time @ BHCT (hr:min)

3:30

Thickening time @ BHCT (hr:min)

5:38

Thickening time @ BHCT (hr:min)

Examples of D500 slurry designs 12.5 ppg: 80F

12.5 ppg: 120F

12.5 ppg: 160F

Slurry design:

Slurry design:

Slurry design:

Dyckerhoff Class G +

Dyckerhoff Class G +

Dyckerhoff Class G +

D047 (gps)

0.03

D047 (gps)

0.03

D047 (gps)

0.03

D075 (gps)

0.50

D081 (gps)

0.10

D081 (gps)

0.10

S001 (%BWOC)

1

D075 (gps)

0.50

D075 (gps)

0.50

D500 (gps)

1.50

S001 (%BWOC)

1

D500 (gps)

1.50

D500 (gps)

1.00

Rheology @ BHCT 2

Rheology @ BHCT 2

Rheology @ BHCT 2

Ty (lbf/100 ft )

29

Ty (lbf/100 ft )

16

Ty (lbf/100 ft )

9

PV (cp)

17

PV (cp)

13

PV (cp)

11

10-min gel

31

10-min gel

19

10-min gel

194

API fluid loss (mL)

28

API fluid loss (mL)

30

API fluid loss (mL)

34

API free water (mL)

0.2

API free water (mL)

none

API free water (mL)

1

Thickening time @ BHCT (hr:min)

12:00

Thickening time @ BHCT (hr:min)

4:22

Thickening time @ BHCT (hr:min)

6:17

GASBLOK technology A complete technique covering all aspects of the gas migration problem  Encompasses WELLCLEAN

 Superior slurries with

mud removal as the first key element Mud Removal

Fluid Density Control

Cement Hydration Set Cement Mechanical Properties

GASBLOK additives

Cement Slurry Design

 Thin, but stable (no free water, no sedimentation),  Non gelling  Excellent fluid loss control  Short transition times  Impermeable to gas

Superior set cement characteristics for long term