FE 02-16D Gas Logging

Nội dung text: FE 02-16D Gas Logging

1. Equipment Needed To Remove and Detect Gas Entrained In the Mud • Gas Trap • Vacuum system • Gas detectors (THA, Chromatograph, Cuttings Gas Analyzer)
2. Functions of a Gas Trap • Extract gases contained in the mud • Sample consistently
3. Gas Unit Definition • 50 units = 1% gas methane-equivalent
4. Gas Chromatography • Gas chromatography is the physical separation of gases into its components.
5. Gas Chromatograph • Separates and analyzes hydrocarbons in the ditch gas sample to determine how much of each hydrocarbon is contained in the sample • Data output to an integrator
6. Chromatogram: Definition • Recorded gas-air mixture by the chromatograph /integrator system
7. Gas Show Evaluation • What is a gas show? • Sources of Gas Shows • Factors affecting Gas Shows
8. Definitions • True Zero Gas – The value seen by the THA when pure air is passed through its detector, for calibration purposes • System Zero Gas – The value of gas seen by the THA when circulating off- bottom under normal conditions. This represents only contamination or recycled hydrocarbons in the mud. This value is the baseline above which all gas readings are taken for drafting on the mudlog.
9. Gas Shows Type: What is a Good Gas Show? • To decide whether a gas show is “good” or “poor” requires a total evaluation of all mud logging parameters plus a consideration of many other variables
10. Liberated or Gas From Drilling • This is the gas released from the formation as the bit crushes it.
11. Recycled Gas • Drilling fluid gas that is pumped back into the hole. • Recycled gas will return at one complete cycle (down time + lag time + time to travel through the surface equipment) after the initial show. • It is similar in appearance to the initial gas show; but less than the original. It will have a larger proportion of heavier components than the original gas show.
12. Contamination Gas: Kelly-cut Gas • Kelly-cut gas is caused by air trapped in the kelly or TDS. It arrives one lag time plus a downtime after circulation commences. • It does not reflect increased gas concentration, though some gas may enter the borehole when the cut mud is pumped out of the bit, but rather greater gas trap efficiency when the air-rich mud reaches the surface.
13. Flushing • Mud pushes back gas from formation due to: – Positive differential pressure – Presence of permeability and porosity of formation – High impact force of jet nozzles (at bottom of hole)
14. Fluid Incursion: Causes • Mainly due to underbalance • Kick – formation has good porosity and permeability • Feed in – formation lacks permeability to sustain a massive influx • Caving or sloughing – formation has very little or no permeability (clay or shales)
15. Factors: ROP • A formation identical in all ways will produce higher mud gas readings if drilled at a higher ROP
16. Factors: Flow Rate • Mud flowrate increases -> the volume of gas and cuttings in a fixed volume of mud will decrease -> greater volume of mud passes through the gas trap -> net effect is zero • Overall effect is probably not great
17. Factors: Flowline • A high degree of degassing occurs at the bell nipple and flowline • Loss of gas in the flowline will be esp. high where: – The flowline is not filled with mud – Changes in slope promote turbulence – Sections of the flowline are open to the atmosphere – The flowline enters the possum belly above the mud level • Gas extraction efficiency is affected ditch geometry, location of flowline entry, direction of flow and degree of turbulence
18. Summary of Factors Affecting Gas Shows • Formation characteristics – porosity, permeability, fluid saturations • Flushing effects – overbalance, water loss, formation porosity/permeability • Volume of formation cut – controlled by ROP and bit diameter • Size and nature of cuttings – controlled by bit design • Flowrate – controlled by pump output and nozzle size • Produced, recycled and contamination gases in mud • Loss of mud and gas at the surface – flowline, ditch characteristics • Gas Trap efficiency • Vacuum system and gas detection efficiency and calibration