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Gas measurement
- 1. Dave Hawker DATALOG Hydrocarbon Evaluation andInterpretation
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- 3. Standard Safety &Consulting Service (1978) Ltd
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- 5. Aims of theCourse • 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.
- 6. Timetable - Day1 • 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
- 7. Timetable - Day1 • 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
- 8. Timetable - Day2 • 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
- 9. Hydrocarbon Compounds • SaturatedHydrocarbons – possessing single covalent bonds between the carbon atoms; all free bonds used by hydrogen • Unsaturated Hydrocarbons – possessing double bonds between the carbon atoms P9
- 10. 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
- 11. Straight Chain Paraffinsor 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
- 12. Paraffins - BranchedAlkanes Structure Name Abbreviation Formula Iso Butane iC4 C4H10 Iso Pentane iC5 C5H12 P12
- 13. Saturated Hydrocarbons ALKANES Paraffin straight orbranch 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
- 14. Naphthene - CyclicChained Alkanes Structure Name Formula Cyclopropane C3H6 Cyclobutane C4H8 Cyclopentane C5H10 P13 Cn H2n
- 15. Unsaturated Hydrocarbons orAromatics • Saturated Hydrocarbons – possessing single covalent bonds between the carbon atoms • Unsaturated Hydrocarbons – possessing double bonds between the carbon atoms P14
- 16. Unsaturated Hydrocarbons orAromatics Structure Name Formula Benzene C6H6 Toluene C6H5 CH3 P14 Cn H2n-6
- 17. Unsaturated Hydrocarbons orAromatics • 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
- 18. Wellsite Measurement • Gasanalysis 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)
- 19. 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
- 20. Agitator Trap mud flow electricor air motor gas released by impeller agitation and lifted by air flow mud in mud out air in Gas/air sample drawn to unit P20
- 21. Mud level ¶meters Flowline design & length Trap position Length of sample line
- 22. Limitations of theAgitator • 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
- 23. 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
- 24. Quantifying the GasMeasurement? • 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
- 25. Quantifying the GasMeasurement? • 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
- 26. GasWizardTM - QUANTITATIVE GAS INMUD MEASUREMENT P24
- 27. 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
- 28. 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
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- 30. GasWizardTM vs Gas Trap…..%By Volume 5 min 0% 10% Gas Trap Direct Gas in Mud
- 31. GasWizardTM versus Agitator Values typicallylower in water-based muds; higher in oil-based muds P27
- 33. 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
- 34. Catalytic Combustion Detector P29 Platinumfilament 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
- 35. CC Response • Detectorresponse 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 1.000 C2 1.478 C3 1.812 iC4 1.938 NC4 1.710 H2S 2.456 P30
- 36. CC Response Detector Response Concentration inAir C1C2C3 LEL P31 • For linear methane response, the gas mixture has to be diluted and kept below the LEL
- 37. 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
- 38. 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
- 39. Thermal Conductivity Detector Response (relativeto air) Air 1.00 C1 1.25 C2 0.75 C3 0.58 iC4 0.55 NC4 0.55 He 5.90 CO2 0.60 ActiveReference Sample P33
- 40. 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
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- 42. FID Circuit Ground A Hydrogen Ionization Cell(anode)Combustion Chamber (cathode) + air sample P35
- 43. FID Operation • Completecombustion 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
- 44. 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
- 45. The Value ofTotal 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
- 46. Limitations • Measurement isqualitative rather than quantitative • Can not distinguish hydrocarbon type, therefore can’t identify fluid type • Poor understanding of the differences between detector measurements
- 47. Difference in DetectorResponse P38
- 48. Total Gas MonitoringSystems • 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
- 49. Chromatographic Analysis • Absolutemeasurement 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
- 50. Chromatographic Analysis • Chromatographscan 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
- 51. Chromatographic Analysis • Sampleshave 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
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- 53. Capillary Column &Micro-Detector
- 54. Sample Chromatogram 10 2030 elution time (seconds) O2+N2 C1 CO2 C2 C3 iC4 nC4 iC5 nC5 composite Column A Column B P45
- 55. 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
- 56. 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
- 57. Summary • Careful considerationshould be given, as to the requirements of gas detection, when selecting the type of service.
- 58. What Type ofService? • 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
- 59. Conventional Fluorescence • Colourunder 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
- 60. Fluorescence Colour High APIgravity oil Medium API gravity oil Low API gravity oil Very low gravity, typically low intensity Condensate 10 15 35 45 P237 API degree
- 61. Solvent Cut • Solventtakes 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
- 62. Limitations to UVFluorescence • 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
- 63. 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
- 64. QFT™ vs Gas/Fluorescence ReservoirTop Reservoir Base Fluoresence QFT Total Gas P233
- 65. QFT™ - OperationalLimitations • 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