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