04- Ershaghi

Causes of Low Well Productivity: Wellbore And/Or Reservoir Issues Iraj Ershaghi, USC

Outline • • • •

Increasing Number of Marginal Wells Causes for low Productivity Solution Approaches Question of Economic Feasibility

More Than 60% of California Wells produce less than 15 Barrels per day

Source EIA

Energy to Produce Oil Rate of production=(Productivity index)*(Pressure Drawdown)

Reservoir Pressure, P Pressure drawdown=p-pwf Wellbore Pressure, pwf

Drop in Pressure Drawdown • Loss of Reservoir Pressure • High Wellbore Pressure

Productivity Index

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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

Damaged Interval

Gravel Pack Hole

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

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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 Wettability Alteration Kro

Kro

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Oil Water

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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 History One 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 Well Negative – 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

Investment in Risky Unexplored Areas vs. Investment in a Secure Proven Development Areas

Solution Strategy Use 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 wellbore cleanup and pressure maintenance Study the history of the wells to assess the extent of the damage

Partners • • • •

Producers Government Support Service Companies Student Interns