sonic log / Acoustic logging tools and its interpretation
- 2. Sonic tool Sonic tool is one of convention porosity logs
- 3. porosity !
- 4. Types of porosity Primary porosity The porosity of the rock that formed at the time of its deposition. Secondary porosity Develops after deposition of the rock. For example: Fracture spaces formed in fractured reservoirs
- 5. Primary porosity Effective Porosity the pores are connected, Effective Porosity liquid can easily flow through (sponge) Ineffective Porosity the pores are not connected. Liquid cannot find a path through; it just gets stuck in the holes. (cork) Total Porosity (ΦT) = Effective Porosity (Φe) + Ineffective Porosity (Φi) The ratio of the entire pore space in a rock to its bulk volume
- 6. Note ! Effective Porosity the pores are connected Permeability Is the ability of the rock to transmit fluid
- 8. Acoustic wave P-WAVES S-WAVES STONELEY WAVES
- 9. P-WAVES is the fastest kind of seismic wave, and, the first to 'arrive' at a seismic station. The P wave can move through solid rock and fluids P wave, particles move in the same direction that the the wave is moving in, which is the direction that the energy is traveling in
- 10. S-WAVES The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium S waves move rock particles up and down, or side-to-side--perpendicular to the direction that the wave is traveling in
- 11. Stoneley wave When generated at low frequency, Stoneley waves travel as a tube wave They loose amplitude at the contact of permeable intervals, and are reflected by fractures. The loss of amplitude can be related to formation permeability
- 12. Wave propagation in a borehole
- 13. Acoustic wave
- 14. Acoustic wave
- 15. sonic tools
- 16. sonic tools The sonic tools create an acoustic signal and measure how long it takes to pass through a rock. Delay time / slowness
- 17. sonic tools types Early sonic tools Dual receiver sonic tools Borehole Compensated Sonic Long spacing sonic tools Array sonic tools
- 18. Early sonic tools • Early tools had one Tx and one Rx. • The body of the tool was made from rubber (low velocity and high attenuation material) to stop waves travelling preferentially down the tool to the Rx.
- 19. Early sonic tool problems The measured travel time was always too long because the time taken for the elastic waves to pass through the mud was included in the measurement. he measured time was A+B+C rather than just B. (ii) The length of the formation through which the elastic wave traveled (B) was not constant because changes to the velocity of the wave depending upon the formation altered the critical refraction angle
- 20. Dual receiver sonic tools These tools were designed to overcome the problems in the early tools. They use two receivers a few feet apart, and measure the difference in times of arrival of elastic waves at each Rx from a given pulse from the Tx This time is called the sonic interval transit time (Dt) and is the time taken for the elastic wave to travel through the interval D (i.e., the distance between the receivers).
- 21. Dual receiver sonic tools The time taken for elastic wave to reach Rx1: TRx1= A+B+C The time taken for elastic wave to reach Rx2: TRx2 = A+B+D+E · The sonic interval transit time: DT = (TRx2 - TRx1) = A+B+D+E – (A+B+C) = D+E-C. · If tool is axial in borehole: C = E, so DT = (TRx2 - TRx1) = D The problem with this arrangement is that if the tool is tilted in the hole, or the hole size changes
- 22. Dual receiver sonic tool problem The problem with this arrangement is that if the tool is tilted in the hole, or the hole size changes
- 23. Borehole Compensated Sonic (BHC) It has two transmitters and four receivers, arranged in two dual receiver sets, but with one set inverted (i.e., in the opposite direction). Each of the transmitters is pulsed alternately, and Dt values are measured from alternate pairs of These two values of Dt are then averaged to compensate for tool misalignment, at to some extent for changes in the borehole size.
- 24. Long Spacing Sonic (LSS) Tool It was recognized that in some logging conditions a longer Tx-Rx distance could help. Hence Schlumberger developed the long spacing sonic (LSS), which has two Tx two feet apart, and two Tx also two feet apart but separated from the Tx by 8 feet. This tool gives two readings; a near reading with a 8-10 ft. spacing, and a far reading with a 10-12 ft. spacing
- 25. Array sonic tools Multi-spacing digital tool. First to use STC processing. Able to measure shear waves and Stoneley waves in hard formations. Used for: Porosity. Lithology. Seismic tie in / time-to-depth conversion. Mechanical properties (from shear and compressional). racture identification (from shear and Stoneley). Permeability (from Stoneley)
- 26. Tool Calibration The tool is calibrated inside the borehole opposite beds of pure and known lithology, such as anhydrite (50.0 ms/ft.), salt (66.7 ms/ft.), or inside the casing (57.1 ms/ft.).
- 27. Time record the sonic log record the time T that required for a sound wave to travel in giving distance on formation time record in sonic tool depend on lithology and pore fluid porosity decrease velocity increase
- 29. Factor affecting in sonic log Lithology ! gas Mud type Pore hole rugosity Secondary porosity Compaction Overpressure
- 30. Secondary and Fracture Porosity The sonic log is sensitive only to the primary intergranular porosity The sonic pulse will follow the fastest path to the receiver and this will avoid fractures Comparing sonic porosity to a global porosity (density log, neutron log)should indicate zone of fracture.
- 31. Compaction As a sediment becomes compacted, the velocity of elastic waves through it increases If one plots the interval transit time on a logarithmic scale against depth on a linear scale, a straight line relationship emerges Compaction trends are constructed for single lithologies, comparing the same stratigraphic interval at different depths Compaction is generally accompanied by diagenetic changes which do not alter after uplift Amount of erosion at unconformities or the amount of uplift from these trends can be estimated
- 32. Overpressure An increase in pore pressures is shown on the sonic log by a drop in sonic velocity or an increase in sonic travel time
- 33. Applications Porosity Determination Stratigraphic Correlation Well to seismic tie Identification of Lithology
- 34. Porosity Determination /interpretation The basic equation for sonic porosity
- 35. Stratigraphic Correlation The sonic log is sensitive to small changes in grain size, texture, mineralogy, carbonate content, quartz content as well as porosity This makes it a very useful log for using for correlation and facies analysis
- 36. Well to seismic tie Synthetic Seismograms Represents the seismic trace that should be observed with the seismic method at the well location Improve the picking of seismic horizons Improve the accuracy and resolution of formations of intere
- 38. questions !