Stimulation (Audit)Stimulation (Audit)

General Review of Stimulation.General Review of Stimulation.– First draft on March 24, general First draft on March 24, general

stimulation techniques, not specific for stimulation techniques, not specific for PWRIPWRI

Review of Damage Mechanisms.Review of Damage Mechanisms. Perkins & Gonzalez ModelPerkins & Gonzalez Model

– Excel spreadsheet tool, very unstable.Excel spreadsheet tool, very unstable. Injection Data Analysis ToolInjection Data Analysis Tool

– BHP standard.xls.BHP standard.xls.

Injection Data Analysis Injection Data Analysis ToolTool

A well written tool.A well written tool. Can take both SI and oil Field Units.Can take both SI and oil Field Units. Can compute tubular and perf Can compute tubular and perf

frictions.frictions. Plots many different curves with Plots many different curves with

stimulation shown on plots.stimulation shown on plots.– Rate, pressures, inj. Index versus time.Rate, pressures, inj. Index versus time.– Modified Hall Plot.Modified Hall Plot.– Rate versus BHP, ...Rate versus BHP, ...

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Injectivity index: v (v)Injection pressure v v v v (v) v v v v (v)Reservoir Pressure (v)Rate v v v v (v) v v v v (v)Completion information (fortubing friction and wellorientation)

v v v (v) (v) v v v

Perforation information (forperforation friction)

v v v v v v

Fracture pressure:Step rate test (SRT) – or – v (v) v v vGeomechanical informationMaximum (undamagedinjectivity)K and h or kh vFluid viscosityDrainage radius (re) (v) (v) (v)Water QualitySuspended solids (TSS orTFS): single value – or –

v v v (v)

Regular monitoring (v)Oil in water (OIW): Singlevalue – or –

v SWI v SW\I

Daily monitoring (v) v v vTemperature (T): single value –or –

vBHT

Daily monitoring (v) v vWell history:Drilling information, e.g.Under / overbalance:

v

Stimulation history: v (v) v v v vProduction Chemicals: v (v)

Injectivity Alert SystemInjectivity Alert System

When is the Hall Plot Changing?When is the Hall Plot Changing?

– How early can we identify real changes? How early can we identify real changes?

– Injectivity data is usually very scatteredInjectivity data is usually very scattered

– ‘‘Eyeballing’ slope changes on Hall Plot + Eyeballing’ slope changes on Hall Plot + noise in data leads to ‘wait till we’re sure’ noise in data leads to ‘wait till we’re sure’ attitudeattitude

– Early detection provides opportunity for Early detection provides opportunity for early remedial work and can therefore early remedial work and can therefore potentially save a lot of money!potentially save a lot of money!

Shawhart Control ChartsShawhart Control Charts

Big “random” variations in PWRI injection data occur leading to delayed detection Big “random” variations in PWRI injection data occur leading to delayed detection Cumulative Sum Control Charts are more appropriateCumulative Sum Control Charts are more appropriate

Cumulative Sum Method Cumulative Sum Method ProcedureProcedure1. Set target value - we use “running average” over 1. Set target value - we use “running average” over

15-25 data points15-25 data points

2. 2. Calculate difference between target and data pointCalculate difference between target and data point

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3. Sum the difference3. Sum the difference

4. Plot the sum 4. Plot the sum against observation against observation numbernumber

V-Mask for CuSum ChartsV-Mask for CuSum Charts The objective is to highlight critical shifts The objective is to highlight critical shifts

The angle of the mask represents number The angle of the mask represents number of standard deviations acceptable variations of standard deviations acceptable variations areare

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Narrow mask will Narrow mask will give early indication give early indication of change but is of change but is more likely to give more likely to give false alarmsfalse alarms

Wide mask gives Wide mask gives few false alarms few false alarms but takes longer to but takes longer to note real changesnote real changes

The V-Mask method works very well on The V-Mask method works very well on “perfectly random” synthetic datasets“perfectly random” synthetic datasets

Actual datasets are not perfectly random - Actual datasets are not perfectly random - “normal” fluctuations are big so the V-mask “normal” fluctuations are big so the V-mask either indicates many false alarms or takes either indicates many false alarms or takes long to notice actual changeslong to notice actual changes

They vary with time and from well to well and They vary with time and from well to well and therefore it is difficult to automate the designtherefore it is difficult to automate the design

- Custom design is necessary- Custom design is necessary Design of V-mask may only be practical when Design of V-mask may only be practical when

“normal” fluctuations are small“normal” fluctuations are small

Why Why difficult to design effective V-difficult to design effective V-Mask?Mask?

ObservationsObservations CuSum plots are a powerful tool for CuSum plots are a powerful tool for

early detection of early detection of slow gradual slow gradual changeschanges in injectivity (e.g. plugging) in injectivity (e.g. plugging)

This is important as there are many This is important as there are many cases where early detection followed cases where early detection followed by (cheap) remedial action will reduce by (cheap) remedial action will reduce costcost

– e.g. readjustment of process facilities e.g. readjustment of process facilities

rather than carry out well stimulationrather than carry out well stimulation

ConclusionsConclusions CuSum method is effective for sudden CuSum method is effective for sudden

injectivity changes e.g. fracturing, injectivity changes e.g. fracturing, stimulation,stimulation, butbut not not necessarily more effectivenecessarily more effective than the original than the original Hall-Plot method where such changes are Hall-Plot method where such changes are usually quite clearusually quite clear

Bottomhole and current reservoir pressure Bottomhole and current reservoir pressure are required to are required to quantifyquantify injectivity changes injectivity changes (skin, permeability)(skin, permeability)

Use of tubing head pressure alone Use of tubing head pressure alone can givecan give false alarms if either rate or reservoir false alarms if either rate or reservoir pressure is changingpressure is changing

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Flow Rate vs. Pressure