Gas measurement

Dave Hawker

DATALOG

Hydrocarbon Evaluation and Interpretation

W. Wylie ERCB

Standard Safety & Consulting Service (1978) Ltd

W.Wylie ERCB

Aims of the Course• Study mechanisms by which gas enters the borehole.• Identify factors controlling final gas measurements.• Determine the importance of extraction methods, Total

Gas and Chromatographic analysis.• Correct interpretation of real-time/depth-based logs.• Evaluation of formation pressures.• Reservoir evaluation from gas responses, including

porosity, saturation and permeability changes.• Using gas ratios to determine fluid type and contacts.• Further applications and benefits of gas analysis.

Timetable - Day 1

• What gases are being measured?– Common hydrocarbon groups, API classification

• Extraction and measurement; What service to select?– Gas Traps; Quantitative Measurement; Total Gas

Detectors; Chromatography• Quantitative Fluorescence Technique™• Phase and solubility considerations• Classification of gas sources

– recognition from surface measurements

Timetable - Day 1

• Controls on quantity and composition– Formation, drilling and external influences

• Show evaluation– Real-time and depth based logs– Determination of Background gas – Show evaluation to determine porosity and gas

saturation changes– Gas normalization

Timetable - Day 2

• Recognition and evaluation of produced gas– Background trend analysis; Evaluation of connection

and trip gases• Chromatographic Analysis

– Evaluating fluid type and contact points using gas ratio analysis; limitations to gas ratios

• Case Studies and Applications– productive or non-productive? Wireline correlation and

non-correlation; geosteering; fracture identification

Hydrocarbon Compounds

• Saturated Hydrocarbons– possessing single covalent bonds between the

carbon atoms; all free bonds used by hydrogen

• Unsaturated Hydrocarbons– possessing double bonds between the carbon

atoms

P9

Saturated Hydrocarbons• ALKANES

– short carbon chains with every bond occupied by hydrogen atoms

• Paraffin Group - most common hydrocarbons– straight chained - termed the normal Alkanes– branch chained - isomers with 4+ carbon atoms

• Naphthene– cyclic chained group

P10

Straight Chain Paraffins or Normal Alkanes Structure Name Abbreviation Formula

Methane C1 CH4

Ethane C2 C2H6

Propane C3 C3H8

Normal Butane nC4 C4H10

Normal Pentane nC5 C5H12

P11

Cn H2n+2

Paraffins - Branched Alkanes

Structure Name Abbreviation Formula

Iso Butane iC4 C4H10

Iso Pentane iC5 C5H12

P12

Saturated Hydrocarbons

ALKANES

Paraffinstraight or branch chained

Naphthene closed, cyclic chain Paraffin names prefixed with cyclo- Molecularly lighter than paraffins but analyzed as if the same Associated with higher density crude oil

P12

Naphthene - Cyclic Chained Alkanes

Structure Name Formula

Cyclopropane C3H6

Cyclobutane C4H8

Cyclopentane C5H10

P13

Cn H2n

Unsaturated Hydrocarbons or Aromatics

• Saturated Hydrocarbons– possessing single covalent bonds between the

carbon atoms

• Unsaturated Hydrocarbons– possessing double bonds between the carbon

atoms

P14


Structure Name Formula

Benzene C6H6

Toluene C6H5 CH3

P14

Cn H2n-6


• Closed chained but not saturated with hydrogen

• Minor component to crude oils• Highly soluble, difficult to detect• Benzene

– most common aromatic, present in most crude oils; proximity to source indicator

Wellsite Measurement

• Gas analysis is typically restricted to the lighter, common hydrocarbons due to analysis time and heavier hydrocarbons not being present as a gas at surface– Saturated Hydrocarbons

• Normal Alkanes and isomers (Paraffins)• Methane (C1) through Pentane (C5)• Cyclo-Alkanes (Naphthenes)

Extraction and Measurement

• Agitator Trap– operational limitations; quantification

• Direct Gas in Mud - The GasWizardTM

• Total Gas Detectors– combustion, thermal conductivity, flame ionization,

stand alone detectors• Gas Chromatographs

– advantages, thermal conductivity, flame ionization

Agitator Trap

mud flow

electric or air motor

gas released by impeller agitation and lifted by air flow

mud in

mud outair in

Gas/air sample drawn to unit

P20

Mud level & parameters

Flowline design & lengthTrap position

Length of sample line

Limitations of the Agitator• Changes in mud flowrate

– inconsistent sampling and measurement• Extracted gas expelled with mud• Air dilution of gas sample

– causing delay and reduced definition• Trap loading or saturation

– erroneously high measurement• Extraction efficiency

– rheological, mechanical, gas composition

P18

Location and Positioning

• Directly over flowline entry?• Correct depth for maximum efficiency?• Away from cuttings obstruction so that flow

of mud is not restricted?• Direction of exit port?

– downstream so not recycling degassed mud– avoiding wind fluctuations

Quantifying the Gas Measurement?

• Texaco QGMTM system– patented trap design reduces the limitation of flowrate

change and expelled gas; eliminates wind fluctuations• Calibrate gas-in-air measurement against gas-in-

mud measurement– using steam or microwave stills– accounting for losses to the atmosphere?– poor sample quality if mud is gas cut?– frequency of mud gas sampling?

P22

Quantifying the Gas Measurement?

• Equate to formation gas volume (apparent gas porosity) by comparing cuttings to mud volume ratio and allowing for gas expansion– changes in liberated gas volume due to the

effects of flushing, influxes, washouts• The system is only accurate for low gas

volumes and small bubble size

EVALUATION OF RELATIVE CHANGES

P23

GasWizardTM - QUANTITATIVE GAS IN MUD MEASUREMENT

P24

DIRECT TOTAL GAS MEASUREMENT - GasWizardTM

• Patented quantitative gas-in-mud measurement

• Flow line, bell nipple or suction line mountable

• Oil, water, air / foam drilling systems• Automatic calibration, zeroing,

thermostat & ranging• No moving parts• No agitator; No sample line

P25

GasWizardTM - Evaluation Advantages

• Quantitative extraction from mud, of all gas components, dissolved or free

• Response 6x faster than agitator traps• No ‘trap loading’ leading to erroneously high

gas values• Minimal dilution - better defined shows• Excellent depth resolution• Not affected by mud density/viscosity• Sample heated - no condensing of gas

GasWizard Test Response

GasWizardTM vs Gas Trap…..% By Volume

5 min

0% 10%

Gas Trap

Direct Gas in Mud

GasWizardTM versus Agitator

Values typically lower in water-based muds; higher in oil-based muds

P27

Total Gas Detectors• How do the different types of gas detector vary in

their operation, response and resulting evaluation?• What is the value of Total gas measurement?• What are the limitations to Total gas measurement?

– Catalytic Combustion or “Hotwire”– Thermal Conductivity– Flame Ionization

Catalytic Combustion Detector

P29

Platinum filament

catalyst

Alumina bead

• A filament combusts a fraction of the gas sample; it’s temperature increases resulting in a change of electrical resistance and potential difference which is calibrated in terms of gas concentration

CC Response• Detector response increases with molecular

weight; An increase can therefore be caused by a change in quantity or in composition

• Non-linear measurement of EMA

Response (relative to C1)

C1 1.000C2 1.478C3 1.812iC4 1.938NC4 1.710H2S 2.456

P30

CC Response

Detector Response

Concentration in Air

C1C2C3

LELP31

• For linear methane response, the gas mixture has to be diluted and kept below the LEL

Catalytic Combustion

• Advantages– Industry standard for

30 years– Simple, reliable, cheap– Good sensitivity– Response is

proportional to heat energy of gas

• Disadvantages– Gas mixture has to be

below LEL– Sensor can be poisoned– Sensor deteriorates

over time– non linear

measurement of EMA

Thermal Conductivity Detector

• Measures the cooling effect that the gas/air mixture has on a filament; A larger response is given by molecularly lighter gases

• Methane/Air has a linear response from 0 to 100%• All other hydrocarbons give a lower response• Other gases also register; eg CO2 and H2S have a

lower cooling effect; H2 and He, very light, give a large positive response

P32

Thermal Conductivity Detector

Response (relative to air)Air 1.00C1 1.25C2 0.75C3 0.58iC4 0.55NC4 0.55He 5.90CO2 0.60

ActiveReference

Sample

P33

Thermal Conductivity

• Advantages– Cheap, reliable– Long Life– Range to 100% C1,

linear measurement

• Disadvantages– Poor sensitivity <0.1%– C2+ lowers reading– Poor zero stability– non linear

measurement of EMA– interference from other

gases

Flame Ionization Detector

P34

FID Circuit

Ground

A

Hydrogen

Ionization Cell (anode)Combustion Chamber (cathode)

+

air sample

P35

FID Operation

• Complete combustion of gas sample in a hydrogen flame

• Detects the ionization process when combustion breaks down the carbon-hydrogen bonds, releasing electrons that change the electrical current

• Gives a linear measurement of Equivalent Methane in Air

Flame ionization

• Advantages– Excellent sensitivity

and range– Stable– Response equal to

number of carbon atoms, linear measurement of EMA

• Disadvantages– Expensive– Complicated– USE OF HYDROGEN

The Value of Total Gas Measurement

• Continuous gas monitoring, instantaneous response

• Effective when zone is well known or only one fluid type or gas will be encountered

• Assists the wellsite geologist in core point selection and formation tops

• Backup to chromatographic analysis• Safety• Stand-alone monitoring systems

P39

Limitations

• Measurement is qualitative rather than quantitative

• Can not distinguish hydrocarbon type, therefore can’t identify fluid type

• Poor understanding of the differences between detector measurements

Difference in Detector Response

P38

Total Gas Monitoring Systems

• Used independently by wellsite geologist• Automated with lagged gas, ROP and basic

logging information, optional H2S• Continual printout and data storage; LAS

output, compatible with strip-log software• Well safety• Insurance against wireline data not being run or

being of poor quality due to invasion

P40

Chromatographic Analysis

• Absolute measurement of individual gases and hydrocarbon compounds– Separation occurs as sample passed through

columns containing separating medium• Different retention rates for gases of varying

chemical or physical properties• Individual components passed to detector

where they are analyzed and measured

P42


• Chromatographs can work on the principle of any of the previous detectors

• Particular gases analyzed dependent on:-

• separating medium• carrier gas • column temperature and pressure• separation time allowed


• Samples have to be separated and analyzed before the following sample can be taken

• Chromatographs can be limited by this sample cycle

• Short sample time allows for: -• effective analysis with fast ROP’s• detection of fractures, thin beds• identifying formation tops• identifying fluid contacts

P43

The Portable Micro-Chromatograph

Capillary Column & Micro-Detector

Sample Chromatogram

10 20 30elution time (seconds)

O2+N2

C1

CO2

C2

C3

iC4 nC4iC5 nC5

composite Column AColumn B

P45

Advantages/Benefits of Chromatography

• Quantitative measurement of all selected hydrocarbon components

• Non-hydrocarbon analysis with TCD’s• Determination of reservoir fluid type• Determination of fluid contacts• Applications such as geo-steering

TCD versus FID?

• TCD variable response due to air flow and gas type is not a factor due to auto-zeroing and gas separation

• Micro-detector provides fast response ensuring linearity comparable to FID

• Both subject to non-linearity as a result of gas viscosity and entry into columns

• Both subject to amplifier and column saturation• FID’s requirement of hydrogen supply• Measurement of non-hydrocarbons with TCD• TCD lower sensitivity is 10ppm, FID to the ppb.

P44

Summary

• Careful consideration should be given, as to the requirements of gas detection, when selecting the type of service.

What Type of Service?

• Total Gas Detection is effective when….– drilling gas wells– identification of relative changes is sufficient to

determine zones of interest– users understand the different responses from the

different types of detectors• Gas Chromatography should be used….

– in exploratory wells with minimal offset data– when fluid type/changes is to be evaluated

Conventional Fluorescence

• Colour under ultra-violet light being an indication of the density of the petroleum fluid

• The intensity of the fluorescence being an indication of the presence of water

• Solvent cut as an indication of density and mobility

P225/228

Fluorescence Colour

High API gravity oil

Medium API gravity oil

Low API gravity oil

Very low gravity, typically low intensity

Condensate

10

15

35

45

P237

API degree

Solvent Cut

• Solvent takes the fluid into solution and leaches it out of the cutting

• Speed and nature of the ‘cut’ reflects fluid density, viscosity, solubility and permeability

• The better the permeability, the faster the cut

• The lower the viscosity, the faster the cut

• Uniform blooming indicates good permeability and mobility

• Streaming cut indicates reduced permeability and/or high viscosity

P230

Limitations to UV Fluorescence

• Subjective colour descriptions• Presence of contaminants• Much of the fluorescence emissions fall in

the ultra-violet range of the spectrum– any fluorescence visible is only a fraction of the

total emission– Some emissions may go completely undetected

Quantitative Fluorescence Technique™

• Patented and licensed by Texaco• Quantitative measurement of the fluorescence

intensity which is proportional to the quantity of oil– removes subjective descriptions– removes error through fluorescence in the ultra-violet range

• Uses crushed dried drilled cuttings, solvent such as heptane and a portable fluorometer

P232

QFT™ vs Gas/Fluorescence

Reservoir Top

Reservoir Base

Fluoresence

QFT Total Gas

P233

QFT™ - Operational Limitations• For a given oil, QFT response relates to oil concentration,

however:• Response is not linear across changing oil gravity - heavier

oils generate a larger response• How representative is the cuttings sample to the producing

formation?• Less accurate with flushed zones or very good permeability• Responses can be seen from coals and other solid

hydrocarbons that possess the fluorescing aromatics• Mud contamination, OBM systems, recycled hydrocarbons

P234

Gas measurement

Engineering