AN INTRODUCTION TO PRESSURE TRANSIENT TESTING OF INJECTION ... 2017_02_22... · an introduction to...

AN INTRODUCTION TO PRESSURE TRANSIENT TESTING OF INJECTION WELLS AND USE FOR UIC PERMIT AND LANDBAN PETITION COMPLIANCE

GROUNDWATER PROTECTION COUNCIL ANNUALUNDERGROUND INJECTION CONTROL CONFERENCEFebruary 2017; Austin, Texas

Ken CooperLewis Wandke & Aaron Payne

PETROTEK ENGINEERING CORPORATION

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OBJECTIVES

Overview of Pressure Transient and Fall Off Testing

Why Conduct Testing?

How Does Fall Off Testing Work?

Fall Off Analysis and What Can it Tell Us?

Critical Issues for Analysis and Compliance Use

Suggestions for Optimizing Fall Off Tests

Summary and Questions

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2017 GWPC UIC Conference

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WHAT IS PRESSURE TRANSIENT TESTING?

A well test that involves recording pressure versus time to determine how flow rates influence pressure behavior measured in a well

Mathematical relationships between flow rate, pressure and time are applied to data to infer properties and conditions of the well and reservoir

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MAIN TYPES OF PRESSURE TRANSIENT TESTS Fall-off or Build-upstable flow period followed by pressure recovery period after the shut-in of a tested well

Drawdown or Injection (Single or Multi-rate) pressure decrease/increase during stable test well flow periods

Step Rateinjection pressure increase versus time for multiple, consecutive, constant rate, equal duration steps

Interference (Standard or Pulse)observation (test) well pressure response due to rate changes in offset active well

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WHY PERFORM WELL TESTING? Reservoir characterization over a larger scale around a well

than logs or cores can investigate

Real-world field confirmation of well capacity and pump pressure requirements

Assessment of current well condition (completion efficiency from borehole into disposal reservoir, aka skin factor)

Evaluate fracture pressure

Determine reservoir continuity - pressure interference between wells and inter-well/directional properties

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WHY PERFORM WELL TESTING?(REGULATORY)

Ambient monitoring requirements (40 CFR 146.13 & 146.68)

Investigate permeability-thickness and reservoir extent assumptions used for cone-of-influence calculations

Verify that well conditions remain consistent over time with values used as the basis for regulatory approvals

Provide insight regarding reservoir pressure trends for comparison to model projections in permits and/or no-migration demonstrations

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WHY PERFORM WELL TESTING?(TECHNICAL)

Ongoing understanding of realistic well capacity, operating limitations, changing conditions, and well life expectancy

Aid with differentiation of reservoir limitation or wellbore conditions as potential reasons for decreasing capacity

Provide insight into wellbore plugging for the evaluation of treatment options (timing and near wellbore or deep damage)

Provide insight regarding reservoir pressure trends for comparison to projections that might impact migration (manage liability)

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HOW DOES FALL OFF TESTING WORK? Pressure propagates in a reservoir as a log function of time and

distance from the source term (well)

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HOW DOES FALL OFF TESTING WORK? A rate change at a well will cause a pressure change in the reservoir

that can be measured in a well

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HOW DOES FALL OFF TESTING WORK? The pressure change is represented by the superposition of new shut-in

rate, q = 0, on the prior injection rate

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2017 GWPC UIC Conference Courtesy KAPPA (modified), 2007

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HOW DOES FALL OFF TESTING WORK? Must account for changes from surface to bottom hole and for the

transition from borehole to porous media

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HOW DOES FALL OFF TESTING WORK?

Exponential Integral (Ei) solution to Diffusivity Equation

Log approximation

Assumes radial flow with homogenous & isotropic conditions

More complicated scenarios require more complex treatments and introduce analysis uncertainty

Differential equations used to represent pressure behavior in a porous media also used to evaluate heat transfer and electricity

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BASIC FALL OFF ANALYSIS EQUATIONS

Δp = -162.6 q µ B [ log k t – 3.2275 + 0.869 s ]k h Φ μcrw

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m = -162.6 q µ Bk h

s = 1.1513 [ P1hr – Pwf – log ( k ) + 3.2275 ]m Φ μcrw

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Earlougher (1977), Matthews and Russell (1967)

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WHAT IS SKIN FACTOR?

A mathematical convenience related to equivalent wellbore radius used to represent wellbore performance as compared with an ideal completion

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2017 GWPC UIC Conference Courtesy Schlumberger (modified), 2006

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FALL OFF TEST ANALYSIS

Plotting and processing these changes as various functions or history-matches of observed data to idealized predictions allows us to infer well and reservoir properties

Diagnostic dp/derivative log-log plot

Semi-log plots

Complex functions and superposition

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WHAT DOES AN IDEAL FALL OFF TEST LOOK LIKE? 16


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WHAT DOES AN IDEAL FALL OFF TEST LOOK LIKE? 17


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FLOW REGIMES – PATTERN RECOGNITION Near wellbore, reservoir or boundary conditions?

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2017 GWPC UIC Conference Courtesy Fekete (modified), 2009

Early Time Middle Time Late Time

Log

Pres

sure

Cha

nge

and

Der

ivat

ive

(rate

of C

hang

e)

Log Time

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ASSUMPTIONS MATTER:FALLOFF TESTING UNCERTAINTY Total Historical Injection Volume (Pseudo Time, Tp)

Rates and Pre-test Flow Period Duration

Year to Year Operating Changes

Offset Injection

Mobility (k/u)inner/(k/u)outer or Transmissivity (kh/u) with Distance

Reaching Heterogeneity or Boundary

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WHAT ASSUMPTIONS MATTER?FALLOFF TESTING: HISTORICAL VOLUME

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could be confused with changing pressure, skin, or permeability

Base CaseFix Pi andattempt analysis

1/10 of historicalvolume, match Pi

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WHAT ASSUMPTIONS MATTER ?FALLOFF TESTING: MOBILITY, RADIAL FLOW?

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2017 GWPC UIC Conferencecould significantly change interpreted permeability, skin and pressure

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WHAT ASSUMPTIONS MATTER ?FALLOFF TESTING: OFFSET INJECTION

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2017 GWPC UIC Conferenceeasily confused with altered permeability (5000 mdft) or pressure (Pi = 2000)

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PRIMARY REGULATORY USE AND LIMITATIONS OF FALL OFF TEST RESULTS

Changing kh or skin factor with time? Operating pressure increase Reduced injection capacity Is it different operating conditions? Did the effective thickness or viscosity change?

Accelerated reservoir pressure increase? Not P*, not Pwf, but final shut-in pressure or Pave

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FALL OFF TEST ANALYSIS – COMPLIANCE USE

Differences between Pi, Pwf, P*, Pave, P1hr

Pwf is “flowing” pressure impacted by near wellbore effects, skin, well geometry, friction, density, viscosity

P* aka “false pressure” only = Pi in an ideal infinite acting reservoir, limited use and requires corrections for reservoir geometry

P1hr is extrapolation from radial-flow slope “m”, used for calculations and is not 1-hour gauge value

P* ≠ Pwf ≠ P1hr ≠ Pave ≠ Pi

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FALL OFF TEST RESULTS – COMPLIANCE USE

History Match“Static” BHPRise at End ofFall Off Tests < Simulation

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History Match…..What aboutYears of BHP from Fall OffPwf > Model ?

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History Match….. After WellStimulation,BHP of Fall OffPwf < Model

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Skin and kh/uWith Static dPExplain Pwf ..…Near WellborePlugging thatwas Reversible

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FALL OFF TEST RESULTS – COMPLIANCE USE Together with other operating and well data, fall off trends should

be examined together, in context

An isolated viscosity increase one year might yield higher Pwf and apparent skin, without changing deep reservoir kh/u

Early in a well life, fall off testing could “see” a boundary in a late time fall off test slope before significant deviation from simulation pressure predictions

Offset well operation in a reservoir could cause Pwf and Pavemeasured in a fall off test to rise without any associated skin factor increase or decreasing transmissivity

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DESIGN ISSUES - OPTIMIZE FALL OFF TESTING Larger skin, smaller permeability, changing storage will extend the test

duration needed to see useful radial flow period

Increasing rate = increased semi-log slope (m), important in high permeability systems or with lower gauge resolution

Constant injection rates and periods help but are not critical using newer simulators and superposition techniques

Pay attention to offset well rates, it can be easy to “see” reservoir issues or artificial slopes that are actually interference effects

It is hard to differentiate between rock and fluid heterogeneity (kh/u) so understand fluid viscosity, density and temperature

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FIELD PRACTICE - OPTIMIZE FALL OFF TESTING Use BHP gauges for highly variable density, significant tubing friction or

when positive wellhead pressure will not be maintained

If valves do not seal, injection continues after “shut-in”

Gauge position close to completion top is often better

Run gradient surveys in/before and out/after testing

Pressure without depth is of limited use, verify depths in & out

Specify KB or GL

psig or psia (get gauge calibrations)

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ANALYSIS ISSUES - OPTIMIZE FALL OFF TESTING Assuming “good” pressure data is obtained, useful pressure transient

analysis is not possible without good rate data; garbage q in = garbage kh/u out

If Pi, Pwf and P* are unrealistic in a graphical or simulation match; the interpretation could be misleading

Wellbore transients typically dominate reservoir transients

Simple models that acknowledge uncertainty can sometimes provide more insight than overly complicated approaches that are not justified nor unique

Fall off tests must be used in context with all available information

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Contact Questions and Discussion33

Ken Cooper, MS, PEPrincipal, Engineering ManagerLewis Wandke, PE; Aaron Payne, MS, PG

Petrotek Engineering Corporation5935 South Zang Street, Suite 200Littleton, Colorado 80127(303) 290-9414 x 415

www.petrotek.comall rights reserved


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