gas measurement
TRANSCRIPT
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
Unsaturated Hydrocarbons or Aromatics
Structure Name Formula
Benzene C6H6
Toluene C6H5 CH3
P14
Cn H2n-6
Unsaturated Hydrocarbons or Aromatics
• 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
Chromatographic Analysis
• 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
Chromatographic Analysis
• 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