Consequences of sand production:
• Wellbore fill.
• Equipment problems due to sand fill.
• Sand erosion of downhole and surface equipment.
• Sand accumulation on surface and Sand Disposal.
• Production Loss.
• Casing / Liner Collapse.
Forces Aiding in Resisting Sand Production:
• Intergranular frictional forces and bonding (cementing) help resist grain movement
• Capillary Pressure adds further grain-to-grain bonding.
Forces Contributing to Sand Production:
• Pore pressure relieves frictional forces.
• Production of wetting phase reduces capillary pressure forces.
What causes sand production?
• Totally or weakly unconsolidated formations.
• Changing stresses in the near wellbore area.
• Water Production.
Sand Production Mechanism
Primer:
– Rock shear collapse owing to in situ stress forming “shear band”.
Sand Production:
– Erosion of perforation tunnel around “shear band” by the produced fluids.
Detecting sand production:
• Erosion of Equipment.
• Acoustic Detectors.
• Sand Traps/Desanders.
Predicting sand production:
• Experience in the area.
• Offset well production data.
• Drilling data.
• Core sample evaluation.
• Sand Flow Test.
• Logs.
• Computer modeling.
What can be done about sand production?
• Rate Exclusion:
– Drawdown Control.
– Selective Perforations.
• Mechanical Methods – “Downhole Filter” :
– Slotted Liner and Screens.
– Gravel Pack.
• Chemical Methods:
– Consolidation of the formation.
– Resin-Coated Sand.
Drawdown control:
• Reduction in production rate will reduce drag forces and drawdown to provide reduced risk of sand production
• Procedure:
– Slowly increase rate until sand production begins to increase.
– Sequentially reduce flow rate until the sand production declines to
an acceptable level.
• Attempting to establish maximum flow rate in conjunction with stable arch.
Selective Perforating Practices:
For high rate wells this will require a high shot density to prevent additional pressure drop and associated sand
production.
However, high shot density lead to perforation interaction which also promotes sand production.
read also Chemical Used in Oil Well Drilling
Gravel Pack:
Consists of sized particles placed in the annular space between an unconsolidated formation and a centralized
screen. Open or cased hole.
Information for designing a sand control
FORMATION CHARACTERISTICS
– Types of Clays, Mineralogy & water-sensitivity.
– Permeability, Heterogeneity, Height, SBHT & SBHP.
FORMATION GRAIN SIZING
– Sieve Analysis for gravel size selection.
SLOT SIZING
– Gravel Pack vs. FracPack.
– Slotted Liners, Screen only (wire wrapped, prepacked, etc.)
TYPE OF WELL
– Producer, Injector , Deviation & Size.
– Off-shore, On-shore, sub-sea.
Screen Selection:
Gravel pack and Prepack Screen
The screen opening is typically between 50-70% of the smallest gravel diameter
– For 20/40 mesh sand the smallest gravel is the 40 mesh which has a D50 of 0.0165” .
– 0.0165” x 70% = 0.01155” or 0.012” opening (12 gauge)
Screen Only and Slotted Liner
– Screen Opening is approximately equal to the D10.
Screen Diameter
– Fishing (7” csg: IDmax= 4” screen; 9 5/8” csg: IDmax= 6” screen …).
– ID for logging and or selective equipment.
Screen Length – Centralizers
• Length normally 5’ above and below the perfs.
• Length needs to be considered for shipments.
• Centralizer spacing to avoid any casing contact:
– Open hole, Bow-Spring type.
– Cased hole Weld-on Lug type.
• Recommended centralizer OD clearance from casing ID to be 1/8” to 1/4”.
Gravel and Screen Summary
• Representative formation samples are required for gravel pack sand size design.
• Gravel pack sand is typically designed to be six times larger than the formation sand at the median grain size.
• Only gravel pack sand meeting API RP58 specifications should be used.
• Gravel pack sand substitutes are available for special applications.
• In a gravel pack the screen serves only to retain the gravel pack sand.
• Prepack screens offer “second line of defense”.
Cased Hole Gravel Pack:
1. Slurry is injected down the workstring.
2. The slurry crosses over to the annulus below the packer, depositing sand
adjacent to the screen.
3. The carrier fluid leaks off through the screen and is carried back to the
surface via the washpipe and crossover tool, crossing back over to the casingworkstring annulus just above the packer.
ICGP – General Procedure:
• Well site preparation.
• Casing clean-up.
• Fluid filtration.
• Casing perforation.
• Killing and fluid loss control.
• Buttomhole clean-up.
• Running and setting GP assembly.
• Gravel placement.
• Running and setting completion string.
• Well clean-up.
Casing Cleaning:
• Prior to gravel packing the casing should be thoroughly cleaned with a bit and a scraper to remove any dirt or scale
• Circulation of completion fluid, mud solvents, scouring material, acid, and caustic solutions may be required to fully remove these material
• The casing should also be pressure tested for leaks on older wells.
read also Oil Well Casing
Clean Perforations
Limit damage both within and surrounding perforations:
– Low debris/carrot free charge.
– Underbalanced perforating.
– Crushed zone removal.
– Limit use of fluid-loss control material in empty perforations, Damage Prevention is Also Critical.
Perforation Cleaning
• Underbalanced Pressure
– selected on formation permeability and Oil Wells , Gas Wells
• Backflow
– When perforating is performed overbalanced a period of backflow is sometimes used to clean up the perforations
with a dedicated string.
Perforating for Gravel Packs & Frac-Packs
• Number of perforations should be sufficient to allow unimpaired (flow maximize inflow area).
• Large diameter charges recommended when gravel placed in tunnels:
– Enhance gravel placement in tunnels.
– Enhance flow of produced fluids.
Depth of penetration not critical
Various studies indicate that cavity is often not formed when perforating in unconsolidated formations. “Disturbed” zone is filled with dilated formation material.
Need to concentrate on tunnel through casing and cement.
• Entry holes less than 0.6 inches result in high pressure drops.
• Benefits associated with maximized inflow area leads to need for high shot density.
• Perforations must be fully packed to prevent excessive pressure drop, and screen plugging and/or erosion.
Use of Fluid Loss Control Material in Empty Perforations:
This should be avoided whenever possible.
• Increased viscosity will decrease leakoff rate without halting it.
• If leakoff completely stopped, perforations will not be able to be packed and it will be difficult to remove
FLCM prior to prepacking.
• Spearheading acid in front of prepack treatment may help re-establish initial leakoff rates.
Important Factors for Gravel Placement:
• Washpipe size
– Must keep area outside the screen larger than area inside the screen; especially in deviated wells where is much
easier to create sand bridge.
• Return Flow Rate
– Sufficient to keep an annular velocity of at least 1 ft/sec at end of screen (turbulence helps to suspend sand
particles, annular velocity must kept less than 2.77 ft/sec to minimize turbulent effects)
• Low-viscosity carrier fluids result in high-quality gravel pack in nearly all situations.
• For cased-hole completions, high fluid loss rates and prepacking both have positive effect.
Gravel Pack Techniques:
• Slurry Pack
– Cased or open hole completions.
– Typically used in very high permeability formations.
– The carrier fluid is viscosifed.
– Sand concentrations can range from 3 to 15 ppg.
• Water Pack
– Cased or open hole completions, horizontal wells.
– Typically used in low permeability formations.
– Brine or slightly viscosifed brine.
– 2 bpm of Returns required.
– Lower sand concentrations required.
– Very good annular packing technique.
• High Rate Water Pack
– Water or slightly viscosifed water.
– Higher rates required to fill perforations and create numerous mini-fracs into the formation in high permeability
wells
• Typically 5-10 bpm or 1 bpm per 10 ft of perforations.
– Sand concentrations 0.5-4 ppg.
– Additional Surface Equipment is required.
