Formation Evaluation

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Formation Evaluation

ObjectivesFormation Evaluation

• Describe the information that can be obtained from a mud log and from three types of core samples.

• List six conventional openhole well logs and describe the information that can be obtained from each.

• List six conventional cased hole well logs and describe the information that can be obtained from each.

• Calculate water saturation using Archie’s equation, given well log readings.

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OutlineFormation Evaluation

• Mudlogging• Cores • Logging while drilling (LWD)• Openhole Wireline logs• Cased-Hole Wireline logs

Introduction

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The Life of a Reservoir

Gather All Data

Operating Company

Logs BoreholeSeismic

2D/3DSurfaceSeismic

RegionalDataGeological

Data

CoreData

Informationfrom nearby

wells

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Subsurface Petrophysical Procedures

Mudlog Example

Visu

al P

oros

ity

Drilling RateM. per Hr.

20 15 10 5

Dep

th (m

)

CuttingsLithology

Continuous TotalGas in air %

1 10 PPM 1 K 10 K

Hydrocarbon AnalysisChromotograph PPM

AnalysisMethane--- Ethane---Propane--- Butane---

Pentane---

Remarks

Inte

rpre

ted

Lith

olog

y

LS: wht, dk, br, vf, xin, cin, hdw tr foss

SH: dk gy, gy, frm, occ sft,occ sity

SS: lt gy, cir, xin, sb ang, sb, rnd, m grn, oil stn, bri yelflour, bri gid stng cut

SH: dk gy, gy, frm, occ sft,occ sity

SS: lt gy, cir, xin, sb ang, sb,rnd, m grn, oil stn, bri yelflour, bri gid stng cut

SH: dk gy, gy, frm, occ sft,occ sity

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Cores• Allow direct measurement of reservoir

properties

• Used to correlate indirect measurements, such as wireline/LWD logs

• Used to test compatibility of injection fluids

• Used to predict borehole stability

• Used to estimate probability of formation failure and sand production

Information from Cores

• Porosity• Horizontal

permeability to air• Grain density

• Vertical permeability to air

• Relative permeability• Capillary pressure• Cementation exponent

(m) and saturation exponent (n)

Standard Analysis Special Core Analysis

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PDC Cutters

Fluidvent

Drill collarconnection

Inner barrel

Outer barrel

Thrust bearing

Core retainingring

Core bit

Coring Assembly and Core Bit

(Whole Core Photograph, MisoaSandstone, Venezuela)

Whole Core

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Sidewall Sampling Gun

Core bullets

Core sample

Formation rock

Sidewall Coring Tool

Coring bit

Samples

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Well Log Analysis

• Openhole logs• Cased-hole logs

Well LogSP Resistivity

Openhole Log Evaluation

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Gammaray

Resisitivity Porosity

Increasingradioactivity

Increasingresistivity

Increasingporosity

Shale

Oil sand

Shale

Openhole Well Logs• Passive measurements

• Gamma ray: Indicates lithology• Spontaneous potential: Indicates

lithology• Caliper: Hole condition

• Active measurements• Resistivity: Fluid saturation, fluid type• Porosity: Rock properties, quantity of

hydrocarbon• Density: Rock properties, seismic

response• Sonic log: Rock properties, seismic

response

Cap rock

Reservoir rock Source rock

Oil

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Well Log InterpretationLogs provide detailed essential information on wells and reservoirs

Well

Depth

• Rock type• Porosity• Permeability• Fluid type• Fluid volume• Formation tops• Fractures

Granite

Sandstone

Limestone

Shale

Claystone

Common Types of Logs

• Gamma (GR)• Spontaneous Potential (SP)• Sonic (DT)• Density (RHOB)• Neutron (CNT)• Resistivity (LLD)

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MotorSurface-adjustable

bent housing

Inclination RPMgravity toolface

Stabilizer and bearings

Measurementantenna

Azimuthal resistivity(depth of investigation

12 in. or less)Gamma ray

detector

3/4º fixed bent housing

Transmitter forwireless telemetryand measurement

of current

Logging While Drilling

Logging While Drilling (LWD)

• Provides:– Real time correlation for picking coring

and casing points– Real time overpressure detection in

exploration wells– Real time logging to minimize “out of

target” sections (geosteering)– Real time formation evaluation to

facilitate “stop drilling” decisions

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Cased-Hole Logs

Formation Properties From Cased-Hole Logs

• Porosity

• Saturation

• Permeability

• Reservoir geometry

• Temperature and pressure

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Why Log Cased Wells?

• To monitor production:– Fluid contact detection and movement – Other water/gas movement– Injection front advance– Water salinity

Common Cased-Hole Logs

• Gamma ray log• Natural gamma ray spectrometry log• Neutron log• Sonic log• Thermal decay time log• Gamma ray spectrometry log