DE2.1.5A Gas Principlesv1-Red 2

Gas Detection PrinciplesJan 2010

Support for test DE2.1.5

2

Gas DetectionGas Detection

Expression of Gas Values Measured at Surface

Gases most frequently measured:

Light alkanes (C1, C2, C3, iC4, nC4, iC5, nC5)

H2S

CO2

Units of Measurement:

% Gas in Air

Parts per million (ppm); 10,000ppm = 1%

Units (varies; usually 50 units = 1%)

3

Hydrocarbon Reservoir Fluid Types

Gas Detection

Black Oil: gas-oil ratio of 2000 standard cu. ft. (SCF)/ bbl or less

Volatile Oil: gas-oil ratio of 2000-3300 SCF/bbl

Retrograde Gas: gas-oil ratio of 3300 SCF/bbl or more, up to approx. 150K SCF/bbl

Wet Gas: >0.3 gallons of condensable liquids per 1000 SCF

Dry gas:<0.3 gallons of condensable liquid per 1000 SCF

Fluids recovered from zones of retrograde and wet gases are commonlycalled ‘condensates’.

A saturated reservoir contains sufficient gas to result in a ‘cap’ of free gas.

4

Saturated Hydrocarbons

Gas Detection

ParaffinsAlso known as alkanes, these are the most common types of hydrocarbon). Alkanes form straight or branched (isomer) chains of atoms with the general arrangement: Cn H(2n + 2)

Saturated hydrocarbons have single bonds between carbon atoms.

CycloparaffinsAlso known as napthenes, these form ring structures. Each component is given the name of the corresponding paraffin, with the prefix cyclo-. They have the general arrangement:

Cn H2n

6

Structure of Light Alkanes

Gas Detection

The different arrangements (and thus physical sizes) of C4-C5 alkanes of the same composition explains why we must measure each arrangement separately in chromatography.

In practice, neo pentane does not occur in amounts measurable by conventional equipment.

7

Boiling points of Alkanes

Gas Detection

C1: -161.52°C -258.73°F

C2: -88.51°C -127.49°F

C3: -42.08°C -43.75°F

iC4: -11.81°C -10.75°F

nC4: -0.51°C 31.08°F

iC5: 27.84°C 82.12°F

nC5: 36.07°C 96.82°F

neoC5: 9.5°C 49.1°F

8

Hydrocarbon (reservoir and source) data is obtained from analysis of drilled gases and mud hydrocarbon content


Gas Extraction from Drilling Mud

Gas transport to Mud Logging Unit

Hydrocarbon Analysis Total Gas Chromatograph C1 - nC5

Data Recording & Correlation

Gas Line

Spare Gas Line

DryingAgent

Total GasDetector

Chromato-graph

CompressedAir Supply

Vent

Vent

CalibrationSystem

RecordingEquipment

Degasser

Mud LoggingUnit

Return Mud FlowFrom Well

(Auxiliary Gas Detection Equipment Not Shown)

In-lineH S Sensor2

Pump

Pump

9

Standard Colours Used for Gas Charts

Gas Detection

Total gas: Black

C1: Red

C2: Green

C3: Medium Blue

iC4: Magenta (reddish-purple)

nC4: Cyan (light blue)

iC5/nC5: Not defined, usually black

10

Total Gas vs. Chromatography

Gas Detection

Total Gas Measurement: Continuous curve of aggregate hydrocarbons C1-nC5 (plus occasional heavier HCs)

Chromatography: batch separation and individual measurement of light hydrocarbons (usually C1-nC5)

C1

Time

TG

Gas %in air

Gas %in air

Time

11

Chromatogram

Gas Detection

The arrival of the separated gases at the detector results in a signal thatdescribes a series of peaks vs. time. This chromatogram is normallyonly displayed for calibration and troubleshooting purposes. Final outputis a histogram plotted for each gas, after computer processing.

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Gas sampling should take place as close to the well bore as possible


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1. Conventional degasser with standard chromatograph (cycle >200 sec)

2. Conventional degasser with high speed chromatograph (cycle <50 sec)

3. Reserval degasser, installed near bell nipple, with Reserval analyser

Imaging Reservoir Zones

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Flame Ionization Detector (FID)

Min. Sensitivity: 1-5 ppm

Measurement Range: 0-30% (0-100% with dilution)

Advantages:

Very low threshold of detection

High repeatability

Linear signal response

Disadvantages:

Detects alkane HC only

Requires continuous supply of H2

Increased complexity

'Collector'

IonisingFlame

Signal to flame jet

Carrier + gas

Hydrogen

Combustion air

+

-Polarising voltage

from regulated power supply

Signalground

Electrometer(signal amplifier)

Resistance

Signal out


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FCP / FGP and Pump Unit


C1-C5 AnalysisChromato cycle: 240 secondsMin. Resolution 5-10 ppm

C1-C5 AnalysisChromato cycle: 42 secondsMin. Resolution 5 ppm

GFF (Geoservices Fast FID)

Standard or QGMGas Trap+

Standard Gas Detection

or

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Reserval Gas Analyser

Reserval Advanced Gas Evaluation

C1-C5 AnalysisChromato cycle: 42 secondsMin. Resolution 1 ppmGas In-Gas Out measurement (optional)

GZG Gas Trap+


19

Methane Equivalence

Gas Detection

FID response is very specific to the detection of alkanes.

The FID Total Hydrocarbon Detector, which is calibrated to respond in equivalent methane in air, thus provides a quantitative gas richness indicator.

• Response will be specific to petroleum hydrocarbons only.

• Response will be proportional to the carbon content.

Methane equivalence is determined by applying the following equation to the chromatographic response:

C1 + 2C2 + 3C3 + 4(iC4 + nC4) + 5(iC5 + nC5)

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Gas Chromatography-Mass Spectrometry (GC-MS)

Min. Sensitivity: 0.1 ppm

Measurement Range: 0-100%

Measures hydrocarbon and non-hydrocarbon gases

Advantages:

Extremely low threshold of detection

‘Universal’ gas detector

C1/C2 ratio higher than 8500


Mass Chromatograms

Mass Spectra

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Gas Detection

FLAIR Gas Analysis (GC-MS)

Gas Chromatograph-Mass Spectrometer

C1-C8 (plus non-hydrocarbon gases)Chromato cycle: 60-90 secondsMin. Resolution 0.1 ppmGas In-Gas Out Measurement (optional)

FLEX Gas Trap+

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Min. Sensitivity: 1 ppm

Measurement Range: 0-100ppm

Advantages:

Simplicity (solid state)

Good reliability

Disadvantages:

Highly sensitive to moisture

Requires periodic activation with H2S

Must be capped when unpowered to prevent damage

Semiconductor H2S Sensor

HH22S/COS/CO22/Methane Detection/Methane Detection

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Infrared Absorption (IFR)

Min. Sensitivity: 100ppm

Measurement Range: 0-10%(0-100% w/reduced low-endsensistivity)

Advantages:

Simplicity (solid state)

High reliability

Disadvantages:

Lower sensitivity than FID

Only single gases can be measured

Source

SourceSpacer

AnalysisCell withW indow

DetectorFront

Housing

Thin FilmFilter

RubberR ings

DetectorAssem bly

Therm istor(Tem p.

m onitor)

HH22S/COS/CO22/Methane Detection/Methane Detection

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Sources of Gas Recovered at Surface

Gas Detection

1. Gas liberated from drilled cylinder

2. Produced gas

3. Recycled gas

4. Contamination gas.

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Gas Nomenclature

Liberated Gas: Gas liberated from cuttings only

Produced gas: gas entering borehole from adjacent, undrilled strata (related to pressure imbalance)

Recycled gas: residual gas in mud, recirculated through mud system and back into borehole

Contamination gas: gas entering mud stream from source other than formation or recycling

Background gas: average or ‘baseline’ liberated gas values

Connection gas/trip gas: gas produced by swabbing effects during a pipe connection or trip

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Factors Affecting Gas Shows

Gas Detection

Accountable: (Possibility of Correction)

• Rate of Penetration• Hole Size• Flow Rate• Degasser Efficiency• Recycled fluids

Unaccountable: (Not Possible to Correct)

• Differential Pressure ( Mud weight ) Petrophysical • Type of Mud & viscosity• Surface Losses * Saturation• Swabbing * Permeability • Surging * Porosity • Caving * GOR• Diffusion * Density of fluids• Mud Loss / Mud gain• Mud temperature at flow line.

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Sources of Gas Shows

Gas Detection

HYDROSTATIC PRESSURE OF MUD = FORMATION PRESSUREBalanced drilling

BG DUE TO CAVING, SWELLLING ANDGAS DIFUSION FROM DIFFERENT SOURCES, SUCH AS SHALE GAS, OIL AND GAS ZONES, FRACTURES etc.

Gas diffusion from shale

Diffusion from Gas Zone Diffusion from Oil Zone

Caving and Swelling

Gas seepage from Fractures

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Effect of Differential Pressure

Gas Detection

DIFFERENTIAL PRESSURE is the main parameter that affects the gas data.

The amount of gas recovered at the surface is only a fraction of the actual gas per unit volume in the reservoir drilled.

In addition, the proportion of gas components recovered is not the same as the actual in-situ composition.

The extracted proportion of the gas components depends largely on the differential pressure.

Higher differential pressure reduces mud gas content; in particular, the heavier components will be reduced or absent

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Effect of Differential Pressure

Gas Detection

Variations in differential pressure will affect the gas recovered at surface.

A B C Effect of differential pressure where differential is:

A: much greater than zero

B: slightly greater than zero

C: less than zero

Gas

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Liberated Gas

0 20 40 60

ROCKTYPE

BOREHOLE

WALLCAKE

LAG TIME

TOTALCIRCULATION

TIME

PENETRATION RATECURVE mn/m

GAS DETECTOR RESPONSEDRILLING FLUID %

L

R

BG

BG

BG: BACKGROUND GASL: LIBERATED GASR: RECYCLED GAS

NORMAL CONDITIONS

HYDROSTATIC PRESSURE (MUD) > FORMATION PRESSURE

THE GAS RELEASED TO MUD IS THE CONTENTS OFDRILLED CYLINDER OF FORMATION (BIT-SIZED)

DE

PT

H

TIM

E

FILTRATE INVASION ZONE

0 5 10 15 20 25 30

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Effect of ROP

Gas Detection

0 20 40 60

ROCKTYPE

BOREHOLE

LAG TIME



L

BG

BGBG: BACKGROUND GASL: LIBERATED GASR: RECYCLED GAS

NORMAL CONDITIONSHYDROSTATIC PRESSURE DUE TO MUD > FORMATION PRESSURE

DECREASE IN ROP PROLONGS GAS PEAKS ON THE CHART. THE NET RESULT WILL BE TO GET LOW PEAK ON THE MASTERLOG,OFTEN OCCURS IF BIT IS WORN OUT OR DUE TO SOME OTHER REASONS SUCH AS - DECREASE IN FLOW RATE, CONTROLED DRILLING, DIFFERENT TYPE OF BIT USED ON OFFSET WELLS, ETC.

DE

PT

H

TIM

E

0 5 10 15 20 25 30

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Detection of Fluid Contact

Gas Detection

0 20 40 60

ROCKTYPE

BOREHOLE

LAG TIME



L

BG

BG

BG: BACKGROUND GASL: LIBERATED GASR: RECYCLED GAS

NORMAL CONDITIONS

HYDROSTATIC PRESSURE DUE TO MUD > FORMATION PRESSURED

EP

TH

TIM

E

0 5 10 15 20 25 30

POROUS

NON POROUS

GAS

WATER

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Flushing of Formation

Gas Detection

HYDROSTATIC PRESSURE DUE TO MUDEXCESSIVELY HIGH( MW / SG) COMPARED

TO FORMATION PRESSURE.

INVASION OF PERMEABLE ZONES WILL STOP AS MUD CAKE IS BUILT UP.

IF VERTICAL PERMEABILITY OF ZONE IS GOOD, MUD WILL FLUSH GAS OR

FORMATION FLUIDS AWAY FROM BORE HOLE.

SOME PART OF FLUIDS MAY COME TOBORE HOLE AS BIT PENETRATES

LOWER & LOWER IN OPENED ZONE.

Excessive mud density results in little or no recovery of C4/C5 hydrocarbons, making show identification very difficult.

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Produced Gas

0 20 40 60

ROCKTYPE

BOREHOLE

LAG TIME

TOTALCIRCULATION

TIME

Penetration RateCurve mn/m

Gas Detector ResponseTotal Gas %

L

R

BG

BG

BG: Background GasL : Liberated GasR : Recycled Gas P : Produced Gas

Formation Pressure > Hydrostatic Pressure due to Mud.

EXTRA GAS compared to gas liberated by cylinder of formation drilled is due to PRODUCTION OF GAS FROM ADJACENT FORMATION.

DE

PT

H

TIM

E

0 5 10 15 20 25 30

P

P

P

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Swabbing : Produced gas that enters hole because of suction. This can occur due to: 1. High viscosity of mud. 2. Balled up bit. 3. Fast rate of pulling out. 4. Collar size too large for the hole. 5. Swelling of clays 6. Insufficient cutting transport.

Surging : Injection effect – mud is pushed into the formation. This can occur due to fast rate of running in, and other aspects as above.

Before each trip we are supposed to provide a Swab and Surge report to the drilling personnel.

Gas Detection

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Drilled Gases

Gas Detection

Gas

CG

CG

CG

CG

CG

CG

CG

CG

CG

CG

A B C

A good indicator of an increase of Pf is gas swabbed from the formation during a trip or a pipe connection.

Effect of differentialpressure on connection gas:

A. Positive, stable DP

B. Positive, decreasing DP

C. Negative DP

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COMPARISON BETWEEN A GAS KICK AND A LARGE GAS SHOW

INCREASED FLOW ATFLOWLINE AND FOAMAT BELL NIPPLE DUETO GAS CONTENT

INCREASED FLOW ATFLOWLINE AND MUD OVERFLOWS BELL NIPPLE BEFORE INTRUDING FLUIDREACHES SURFACE

DRILLING FLUID ISDISPLACED BY INTRUDING FORMATION FLUID

GAS, OIL OR WATERFLOWS INTO BOREHOLEFROM FORMATION

GAS FROM CUTTINGSEXPANDS IN ANNULUSINCREASING TOTALOUTFLOW VOLUME

GAS - BEARINGCUTTINGS

WELL BALANCED WELL UNDERBALANCED

Gas Detection

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Recycled Gas

Gas Detection

Amount of recycled fluids depends on:

1. Efficiency of rig degasser.

2. Mud Properties, such as Visc., Temp. etc.

3. Type of mud: WBM or OBM (synthetic).

4. Oil / diesel in mud.

Reserval can measure Gas in and Gas out. Recycled fluids can be detected.

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0

5

10

15

20

25

30

35

40

6:00

:14

6:33

:34

7:06

:54

7:40

:14

8:13

:34

8:46

:54

9:20

:14

9:53

:34

10:2

6:54

11:0

0:14

11:3

3:34

12:0

6:54

12:4

0:14

13:1

3:34

13:4

6:54

14:2

0:14

14:5

3:34

15:2

6:54

16:0

0:14

16:3

3:34

17:0

6:54

17:4

0:14

18:1

3:34

18:4

6:54

iC5 OutiC5 In

Important gas influx

(Trip gas)

Re-injection of the

recycled gas

Detection of the recycled gas,

at the flow line

17 February 2000

Recycled Gas

Gas Detection

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Influence of Hole Size on Gas Shows

Gas Detection

17 1/2”

Same amount of gas per unit volume

of rock.

12 1/4” But different bit size changes the Total Gas.

8 1/2” Even large gas show in 17 1/2 “ hole may be water-bearing.

6”

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Influence of Flow Rate on Gas Shows

Gas Detection

•

Cylinder of rock mixes with small volume of mud.

Slower flow rate

Same cylinder of rock mixes with larger volume of mud.

Faster flow rate

Increase in Mud Flow Rate decreases Gas recorded.

42

Influence of Measurement Frequency

Gas Detection

Reserval and ConventionalFID running simultaneously.

Curves show the difference in data acquired .

Conventional FID is not able to keep pace with fast rate of drilling.

This results in chromatographic curve with step-like appearance.

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Influence of Thresholdof Measurement

Gas Detection

Difference between Reserval and ordinary FID out-put. Reserval has

greater consistency and higher resolution so that heavier hydrocarbons

are better represented.

44

Influence of GasTrap Mud Level

Gas Detection

Conventional gas trap may sometimes misrepresentgas out put. Trap starvation

(decrease in mud level)as in this case shows decrease in gas level as against actual

zone shown in bottom of the figure.

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Effect of Mud Type

Gas Detection

The TYPE OF MUD also affects the composition of the gas recorded.

Water-base mud is probably the best for gas recovery, whereas contamination of the mud with crude oil increases retention of gas in mud and thus increases recycled gas.

Recycled gas makes evaluation of the gas data very difficult.

Thank you for your attention.

DE2.1.5A Gas Principlesv1-Red 2

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