DAMAGE ISSUES IMPACTING THE PRODUCTIVITY OF TIGHT GAS PRODUCING FORMATIONS; Formation Damage; Fracturing/Refracturing; Hydraulically Fractured; Tight Gas Reservoir; Economic Tight Gas Reservoir Production

1. FORMATION DAMAGE ISSUES
IMPACTING THE PRODUCTIVITY OF
TIGHT GAS PRODUCING
FORMATIONS
Prof. Dr. Hassan Z. Harraz
Faculty of Sciences, Tanta University, Egypt
October 9, 2019
hharraz2006@yahoo.com

2. Keywards:
 Formation Damage; Fracturing/Refracturing;
Hydraulically Fractured; Tight Gas Reservoir;
Economic Tight Gas Reservoir Production

3. Outline of lecture
 What is formation Damage
 Definition of a tight gas reservoir
 Conditions generally required for economic
tight gas reservoir production
 Common formation damage types occurring in
tight gas reservoirs
 Reducing formation damage in tight gas
reservoirs

4. What is Formation Damage?
 Any process causing a reduction in the inherent
natural permeability of an oil or gas producing
formation.
 In Many cases the Exact cause of the damage is
difficult to define-Another Good Definition of the
Formation damage is:
 The Impairment of the Unseen by the
Inevitable, Resulting in an unknown
reduction in the unquantifiable..!

5. What is a Tight Gas Reservoir?
 Somewhat arbitrary classification
 Often defined as a gas bearing sandstone or
carbonate matrix (which may or may not
contain natural fractures) which exhibits an in-
situ permeability to gas of less than 0.10 mD
 Many ‘ultra tight’ gas reservoirs may have in-
situ permeability down to 0.001 mD

6. What Controls the Ability to Economically
Produce Tight Gas Reserves?
 Effective permeability
 Initial saturation conditions
 Size of effective sand face drainage area
accessed by the completion
 Reservoir pressure
 Degree of liquid dropout from gas (rich vs. dry
gas)

7. Capillary Equilibrium in Gas Reservoirs –
High Perm
Water Saturation Water Saturation
CapillaryPressure-Psi
RelativePermeability
FWC

8. Capillary Equilibrium in Gas Reservoirs –
LOW Perm
Water Saturation Water Saturation
CapillaryPressure-Psi
RelativePermeability
FWC

9. Generally if a Tight Gas Matrix is in
Equilibrium With a Free Water
Contact, Unless Very Large Vertical
Relief is Present, Equilibrium Water
Saturation Reduces Reserves and
Permeability to Gas Below the
Economic Limit for Production

10. Non - Capillary Equilibrium in Gas
Reservoirs – LOW Perm
Water Saturation Water Saturation
CapillaryPressure-Psi
RelativePermeability
NO FWC

11. For Significant Reserves and Mobile Gas
Production in Very Low Perm. Gas Reservoirs, a
CAPILLARY SUBNORMAL Water Saturation
Condition Usually Must Exist
Water Gauge

12. Subnormally Water Saturated Tight
Gas Reservoirs – What Are They
 A gas reservoir in which the initial water
saturation is less than that which would be
achieved on a conventional drainage capillary
pressure curve at the effective capillary gradient
of the reservoir

13. Postulated Mechanism for Establishment
of the Low Swi Condition
Low Perm matrix
Initially 100%
Saturated with
Water
Gas Migration commences
Pore system displaced to
Capillary equilibrium swirr
Pore System is isolated from
Dynamic capillary contact
With active recharge water
Source (faulting, etc)
Long Term Migration of Gas
Slightly out of equilibrium with
Reservoir results in Desiccation
Of water saturation to subnormal
value

14. Postulated Mechanism for Establishment
of the Low Swi Condition
 Low Perm. matrix Initially 100% Saturated with Water.
 Gas Migration commences Pore system displaced to
Capillary equilibrium Swirr
 Pore System is isolated from Dynamic capillary contact
With active recharge water Source (faulting, ..etc).
 Long Term Migration of Gas Slightly out of equilibrium
with Reservoir results in Desiccation of water
saturation to subnormal value.

15. Postulated Mechanism for Establishment
of the Low Swi Condition
Results in unique combination of low
perm. and low Swi

16. Subnormal Saturation Conditions
 Generally a pre-requisite for an economic
gas reservoir in ultra tight rock (<0.1 mD)
 Increases reserves and gas permeability
 Increases apparent salinity and
suppresses Rw (proven by case studies)
 Swi often difficult to precisely measure
using conventional logging
 Direct measurement via traced coring
program common method used

17. Common Subnormally Saturated
Formations in Western Canada
 Deep basin area:
 Paddy
 Cadomin
 Cadotte
 Jean Marie
 Montney
 Rock Creek
 Ostracod
 Gething
 Bluesky
 Halfway
 Doig
 Cardium
 Viking

18. Subnormal Initial Water Saturation Gas
Reservoirs
 USA
 Powder River Basin
 Green River Basin
 DJ Basin
 Permian Basin
 Also documented in South America,
Europe, Asia, Africa and Australia

19. Dominant Formation Damage Mechanisms
in Tight Gas
 Unless natural microfractures are present,
almost all tight reservoirs must be fracture
stimulated to obtain economic production rates.
 In the case where fracture stimulation is
required, classic formation damage associated
with drilling is not normally problematic due to
the radius of penetration of the fracture
treatment

20. Exceptions – Tight Matrix With Enhanced
Natural Permeability Conduits
 Natural fractures
 Interconnected
vugular porosity
 Possible deep
invasion of whole
drilling fluids
 Possible application
of UBD

21. Hydraulically Fractured Tight Gas Systems
 High fracture conductivity essential (proppant
crushing, embedment, residual gel/fracture fluid
entrainment are issues).
 Water or hydrocarbon based phase trapping a
major source of matrix damage in the near
fracture face area

22. Highway Analogy

23. Highway Analogy

24. Highway Analogy

25. Water Based Phase Trapping
Water Saturation
Water Saturation
CapillaryPressureRelativePerm

26. Water Based Phase Trapping
Water Saturation
Water Saturation
CapillaryPressureRelativePerm

27. Water Based Phase Trapping
Water Saturation
Water Saturation
CapillaryPressureRelativePerm

28. Reducing Water Based Phase Trap Potential
 Avoid use of water based fluids (OB, pure gas, etc).
 Use surface tension reducing agents to reduce capillary
pressure and trapping potential (mutual solvents,
alcohols, etc).
 Low fluid loss systems with rapid recovery times to
minimize imbibition.
 May also be an issue in some underbalanced
operations.

29. Using Hydrocarbon Based Fluids in
Reservoir Prone to Water Trapping
 May still be the preferred method as relative
volume of non wetting phase hydrocarbon
which is trapped is often much less than water
 Resulting damage is far less than if water based
fluid had been used in the same situation in
many cases

30. Hydrocarbon vs. Water Based Fluids in Low
Perm., Low Swi Gas Reservoirs
Total fluid saturation
Relativepermeability

31. Hydrocarbon vs. Water Based Fluids in
Low Perm, Low Swi Gas ReservoirsRelative
permeability
Total fluid saturation

32. Common Stimulation Treatments
for Water Blocks
 Dry gas injection (natural gas, CO2)
 Mutual Solvent Injection (methanol, CO2)
 Extended shut in time
 Formation heat treatment
 Direct penetration (Fracturing/Refracturing)
 High drawdowns normally not effective

33. DJ Basin –Colorado
Reservoir Parameters
 Very fine grained sandstone
 Depth – 2400 m
 BHP = 20 MPa
 kh = 1 – 4 mD-ft (0.3 – 1.2 mD-m)
 Typical treatment
 550,000 lbs (250 tonnes) in X-linked water
 Post-frac production
 50 mcf/day – 500 mcf/day Slide Courtesy of Calfrac

34. DJ Basin – J Sand
Two Production Cycles
Slide Courtesy of Calfrac

35. Common Stimulation Treatments
for Hydrocarbon Blocks
 High pressure lean gas injection (natural gas,
nitrogen).
 Lower pressure rich gas injection (CO2, ethane,
propane, butane).
 Mutual solvent (heavy alcohol) treatments.

36. Diagnosis of Problems and Evaluation of Most
Effective Prevention or Stimulation Treatments
 A variety of lab/core evaluation techniques exist
to evaluate:
 Water and hydrocarbon phase trap potential.
 Interplay of reservoir pressure, invasion and
drawdown effects.
 Evaluation of optimum stimulation methods
for existing damaged wells.
 Evaluation of optimum drilling and completion
methods in naturally fractured formations.

37. Lab Regain Perm Test Equipment

38. Conclusions
 Tight gas reservoirs have a huge future potential
for production.
 Generally to be economic tight gas reservoirs
are normally in a subnormal water saturation
condition.
 Fluid trapping tends to be a dominant damage
mechanism for tight gas reservoirs.
 Techniques exist to evaluate and minimize
phase trapping problems and to stimulate
existing damaged wells.