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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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