Causes of Low Well Productivity:
WellboreAnd/Or
Reservoir Issues
Iraj Ershaghi, USC
Outline
• Increasing Number of Marginal Wells• Causes for low Productivity• Solution Approaches• Question of Economic Feasibility
Source EIA
More Than 60% of California Wells produce less than 15 Barrels per day
Energy to Produce Oil
Rate of production=(Productivity index)*(Pressure Drawdown)
Reservoir Pressure, P
Wellbore
Pressure, pwf
Pressure
drawdown=p-pwf
Drop in Pressure Drawdown
• Loss of Reservoir Pressure• High Wellbore Pressure
Productivity Index
+S)
Drop in Productivity Index
• Decrease in Oil and Increase in Water or Gas Saturation• Increase in Wellbore Skin (Paraffin/ Asphaltene/scale)• Increase in Oil Viscosity (Loss of Light Components)• Collapsed Tubing• Collapsed Perforation• Poor Isolation of Zones
Need to Understand the Pore Structure
What Plugs the flow Channels?
Open Hole Cased Hole Perforation Gravel Pack Hole
Damaged Interval
Damage Affecting all Types of Completions
Major Causes of Formation Damage•Mechanical
•Chemical
•Biological
•Thermal
Mechanical Damage•Fines Migration
•External Solids Entrainment
•Relative Permeability Reduction
•Geomechnical Stresses-Collapsed Around the Well
•Fracture/Propant Plugging
•Propant Embedment
•Glazing and Mashing
Collapsed Formation
Extraneous Solids Entrainment•Overbalanced Drilling, Completion, Kill and Stimulation Fluids
•Solids From Overbalanced Fluid Losses
Solid Types•Weighting Agents (barite, hematite, CaCO3)
•Fluid Loss Agents
•Bridging Agents (Cellulosic fibers, Oil soluble resins..etc)
Adverse Relative Permeability Effects•Trapping of Water based Filtrate
•Trapping of Oil Based Filtrate
•Entrapment of water of Condensation
Kro
Krw
Sw
Chemical Damage Mechanisms•Rock-Fluid Interactions
•Fluid-Fluid Interactions
•Wettability Alteration
Fluid-Fluid Damage Mechanism•Precipitates (organic and inorganic)
•Scales
•Paraffins
•Asphaltenes
•Hydrates
•Diamondoids
Chemical Adsorption
Emulsions
Clay Swelling
Substitution of Water into a Clay Matrix
Near Wellbore WettabilityAlteration
OilWater
Kro KroKrwKrw
Biological Damage Mechanisms•Plugging
•Corrosion
•H2S Generation
Thermal Damage Mechanism•Mineral Transformations
•Dissolutions and Re-precipitation
•Fines Migration
•Wettability Alteration
Diagnostic Techniques•Proper Analysis of –In-situ Fluids and Properties
•Framework Composition Study of Rock,
Wettability, Capillary Pressure and Relative Permeability Character
•Displacement Testing to Evaluate Different
Damage Mechanism because of past Drilling, Completion,and production practices
Formation Damage
•Can be a very complex process and is generally
reservoir and application specific
•Several Mechanisms may be operative
•Various Lab Screening or Down hole Surveys
May be necessary
Scale Buildup
• Completion/Workover Fluids• Fluid Production (change of pressure, temperature,
pH)
Suitable for Acidizing?
Suitable for Frac Pac?
Corrective Actions Can work
Case Study Data
Another Example
Example from a Service Company
Another Example
Well Testing-To Diagnose Damage
Evidence of Skin from Well Test Data
Diagnostic Plots Indicating Skin
Production Data Diagnostic Plots•Well Operation history-Gross Vs. Net
•Offset Well Comparison
•Produced Fluid/Solid Samples
Check the HistoryOne time Pressure Transient Test may not be diagnostic of the source of the problem
Historical Perspectives•Use of Dirty Completion Fluids
•Lack of Good Perforating Practice
•Failure to Recognize Precipitating Oil Mixtures or Incompatible Waters
•Failure to Inhibit Fluids Against Clay Swelling in Sandstone Formation
•Use of Painted Tools and Rusty or Scaled Pipes
•Excess Use of Thread Dope
•Use of Impure Additives
•Use of Fresh Water Buffer
Historical…Use of Corrosion Inhibitors
Use of Scale Inhibitors
Deposition of Asphalt
Plugging of Gravel Packs
Damage When Cutting Paraffin or Asphaltenes
Damage From Acidizing or Fracturing Fluids
Damage During Perforation
Damage During Casing or Cementing Operations
Damage During Drilling
Summary- Formation Damage• Drilling Damage-clays, Weighting Agents, Lost Circulation Agents• Drilling Fluid Filtrate• Cementing Damage• Perforation Damage• Completion and Workover Fluids Damage• Damage in Gravel Packs• Damage During Production• Fines Migration• Loss of Rock Compressive Strength• Precipitation of Organic or Inorganic Materials• Change in Wettability• Damage From Stimulation Fluids• Damage From Water Control Polymers
Formation Damage Implications
Unrealized Production
Unrealized Stimulation Potential
Attitudes About a Marginal WellNegative
– Technologically Beyond Repair– Not Economical to Fix– Keep on With Reasonable Prices-Shut Down Otherwise
• Positive– Still Much Oil Left to Be Produced– Explore Ideas – Seek Cost Share to Fix Wells
Drop in Pressure Drawdown
• Loss of Solution Gas• Los of Gas Cap• Small Size or No Aquifer
Drawdown Corrective Measures
• Pressure Maintenance• Water Injection• Gas Injection• Steam Injection
Blessings of Gravity Drainage
Solution StrategyUse Material Balance to Roughly Estimate What may be left in the ground
(Do not use Decline Curve of Marginal Wells)
If If some fractional recovery of remaining oil justifies investment, focus on both wellborecleanup and pressure maintenance
Investment in Risky Unexplored Areas vs. Investment in a Secure Proven Development Areas
Study the history of the wells to assess the extent of the damage
Partners
• Producers• Government Support• Service Companies• Student Interns