1. Oil-Field Hydraulics Chapter 4 Completion and Workover Fluids
PTRT 1321
Oil-Field Hydraulics
Chapter 4
Completion and Workover Fluids

2. Fluid Characteristics
Density
High enough to control well pressures
Low enough to avoid formation fracture
Cost-effective
Expensive sometimes required to avoid formation damage
Less expensive alternatives can be selected with sufficient expertise
Free from solid particles
Non-corrosive
Stable
Avoid precipitation of solids in formation (filtered or cleaned)(matched to formation properties)

3. Fluid Spacers Separate incompatible fluids from one another
Salt water
Special muds
Injected as a slug in between the two incompatible fluids
Acids
Cement slurries

4. Fluid Functions Typical fluids are not unusual or exotic
Several important functions
Transport of materials into and out of the well
Suspension of materials in static conditions
Pressure control
Absorption of heat plus lubrication
Delivery of hydraulic energy
Energy transfer medium for logging or perforating tools
Avoiding damage of
Formations
Downhole and surface equipment
Personel and the environment

5. Transport of Materials
Materials circulated into and out of the well
Acid
Cement
Gelled pills
Plastic
Gravel
Frac sand
Sealers
Potentially damaging materials
Dry cement
Corrosive fluids
Cuttings
Debris
Gas
Metals
Contaminated mud

6. Suspension of Materials
Thixotropic behavior (gel when static, fluid when flowing)
Gel properties prevent settling when static
High gel strength can produce excessive swab and surge pressures (think of friction)
Cause formation kick or breakdown
Control steps for either can damage formation
Reverse circulation is used

7. Pressure Control Most work requires killing the well
Formation pressure balanced by hydrostatic fluid pressure
Weighted materials
Clay
Barite
Excess pressure can damage formation

8. Other Items Fluid mass absorbs heat of friction
Only means of delivering hydraulic energy (pressure) from the rig pumps down hole
Optimum thickness, viscosity, solids content, etc allow easy transport and operation of downhole tools

9. Avoiding Damage Formation damage
Silts, fines, sludges, or gums
No change in wettability of the rock
Most are water-wet (salt water)
Some are oil-wet
Fresh water can cause emulsions to form and reduce permeability
Formation swelling or scale formation can also reduce permeability
Corrosion and environmental hazards

10. Types of Completion and Workover Fluids
Oil Fluids
7 ppg – excellent for low pressure oilwells
Disadvantages
Wax
Sand, solids, or asphalt
Corrosive if H2S or CO2 present
Potential fire hazard
Diesel or kerosene sometimes used
Environmental hazard

11. Types of Completion and Workover Fluids
Oil-emulsion Fluids
Oil-in-water emulsion most common
Water phase either fresh or salt water
Emulsifying agents for stability
Starch, soap, organic colloids
Diesel most common oil used
Cost effective
Less damaging to formation

12. Types of Completion and Workover Fluids
Gas Fluids
Can be used in some low pressure reservoirs
Flow controlled using surface back-pressure
Natural gas commonly used
Readily available
Cheap
Hazardous (flammable)
Nitrogen – inert (non-reactive)
Foams added to assist with suspension of trash and other solids

13. Types of Completion and Workover Fluids
Water-based Fluids
Fresh water (seldom used)
Salt water (brine)
Water-based muds
Low-salinity water
Plentiful
Inexpensive
Requires little treatment
Filtration can remove suspended solids

14. Brines Salt dissolves in water
Density increases
Increased hydrostatic head
No suspended solids (as long as all the salt is dissolved)
Usually inhibits clay hydration (swelling)
In cases where hydration occurs when NaCl is used, CaCl2 or KCl can be used
Single-salt brines using CaBr2 and ZnBr2 can be used to form a high-density brine

16. Saturation
Salt can be added to increase density only until a saturation point at a given temperature is reached
Beyond saturation the salt drops out of solution by crystallization or by settling out as undissolved solids
If saturation is reached with a low density solution a higher density salt may be added to increase the density

18. K = temperature correction factor
Density Changes
Density decreases with increasing temperature – density quoted at 60 ⁰F
Temperature increases with well depth (up to 400 ⁰F in some cases) – can cause dramatic density decreases
D60 = Dwb + (Tav – 60)(k/100)
K = temperature correction factor

20. Example Bottomhole temperature = 220 ⁰F Require a 9.9 ppg brine
With a surface temp of 60 ⁰F what density of brine is required?
= (140 – 60)(0.2/100) = 10.1 ppg

21. Water-base Muds Combination of water, clays and chemicals
Sometimes used in workover and completion
Laden with solids that can damage formations through water loss and blocking pore spaces
Used because they are low cost and often easier to work with

22. Packer Fluids
Fluid in the annular space between the tubing and casing. This packer fluid:
Provides formation pressure control
Prevents casing collapse
Prevents production string from bursting
The packer fluid must be:
Noncorrosive
Stable with time and temperature
Economical
Pumpable when placed and remain pumpable
Sufficient density
Cause no damage to packer seals
Able to keep solids suspended
Many good packer fluids are currently available

23. Plugs (Pills)
Plugs or Pills are used in similar ways as mechanical plugs
Seal casing leaks
Correct injection profile in water-injection or disposal wells
Stop lost circulation
Divert acid during well cleanup
Shut off saltwater flows
Plugs can also be used:
stabilize unconsolidated gravel zones
Seal fractures

24. Types of Fluids Soft and pumpable fluid available
Neat cement
Thickened oil-base mud
Diesel oil-cement
Diesel oil-bentonite
Bentonite-cement
Silica-clay
Polymers
Plastics
Acids
Other lost circulation chemicals
Weighting materials and viscosifiers can also be added to make then dense and highly viscous
Retarder or accelerator can also be used as needed

25. Other intricacies Time-delayed, self-complexing plug
A breaker can be used to provide a predictable breakdown of the plug
Polymer pill with an enzyme added; e.g. break down complex polysaccharide polymer (cooked starch) into low-molecular-weight polymers (digested starch) plus simple sugar
Amylase is one such enzyme

26. Example
Dual completion well in which one of the two producing zones requires a relatively high density fluid to kill it.
This will cause lost circulation in the other producing zone (permeability is higher or formation pressure is much lower)
Spot a plug in the weak zone to seal it with a breaker added so it dissolves when the zone is ready to be produced

27. General Fluid Safety HAZCOM PPE
Goggles
Splash protection
Gloves
Boots
Mixing chemical into water NEVER the other way around