Sonic Log 40o17

Principles and Interpretation of Sonic Logs PETE 3036 Well Logging

Fall 2015

Objectives 1. To understand the physical principles behind the operation of Sonic logging tools, 2. To learn how to interpret sonic/acoustic logs in of lithology and types of fluids, and some other petrophysical properties 3. To understand the importance of environmental and interpretation corrections applied to sonic logs.

Reference for Reading Chapter 16, Openhole Log Analysis and Formation Evaluation by Bateman, Second Edition Chapters 3 and 10 of the book Theory, Measurement, and Interpretation of Well Logs by Z. Bassiouni, Z.,1994, SPE Textbook Series Vol. 4, Richardson, Texas.

Acoustic waves • Acoustic waves are pressure waves that propagate (move) through the earth in a manner and velocity that is dependent upon the characteristics and geometry of the formations. These waves are recorded and analyzed to estimate many properties of interest in the oil field. • Acoustic waves move through a medium as wavefronts. The wavefronts are classified by how they move in relation to the particle movement.

Acoustic Classifications term

Definition

Seismic

Seismic means to shake or to move violently. Seismic applications in the oil field originated from the study of seismic events in the earth (i.e., earthquakes). Seismic applications can be distinguished from other acoustic applications as the energy wave can be felt as it moves through the earth. Seismic measurements cover very large areas.

sonic

Sonic applications fall into a higher range of frequencies than seismic. These are the frequencies that can be heard by most human beings. The area of investigation of sonic waves is much smaller than that of seismic waves. Sonic measurements can be thought of as microseismic measurements.

Ultrasonic Ultrasonic waves can neither be heard nor felt by people. The area of investigation of ultrasonic waves is very small and is normally directly in front of the energy source.

Analyzing Sound The analysis of the sonic waveform falls into two categories: • Velocity measurements • Amplitude measurements

Velocity Measurements The measurement of the speed at which each of the wave fronts (compressional and shear) travel along the borehole has several applications: • Time to depth conversion can be accomplished by using the compressional velocity. • Porosity can be estimated by using the compressional velocity. • Elastic properties can be determined by the relationship between compressional and shear velocities which can be used to estimate properties such as: – Poisson's ratio – Young's Modulus of elasticity

Sonic Log: Emitters and Receivers • Sonic logging tools utilize devices called transducers to generate and detect acoustic energy. • A transducer can convert electrical energy into mechanical energy or mechanical energy into electrical energy • Most commonly used in sonic tools fall into three categories: – Magnetostrictive – Piezoelectric – Electromagnetic

Amplitude Measurements • The measurement of the amplitudes of the received waveforms is primarily a cased hole application. The primary application of this measurement is to determine the density of the material that is behind the casing. • Sonic amplitudes are measured with the same equipment used for the open hole velocity measurements. Since slowness is not being measured only one receiver is needed. Because of different applications, most amplitude measuring tools consist of a minimum of one transmitter and two receivers.

SONIC/ ACOUSTIC LOGS Applications: • Porosity & Lithology • Mechanical Properties: Fracture Pressure, Sanding Potential • Detection of Natural Fractures/ gas zones

• Calibration of Seismic- synthetic seismograms

Sonic (or Acoustic) Logs t = tf  + tma (1- ) tf = 189 sec/ft tma = 55.5 sec/ft – sandstone

More generally:

t = A + B  Single-receiver system

Basics of Sonic/Seismic Waves • Sound velocity ranges from 5300 ft/sec (water) to 23000ft/sec • Corresponding DT (transit time) is 189 μ sec/ft (water) to 43.5 μsec/ft.(Dolomite) DTc = 106/ Vc

VC 



b

E (1  ) (1  )(1  2 )

VS 

G



b

where, G is Rigidity of medium; ρb is bulk density • Usually when porosity increases rigidity (G) decreases faster than density decrease • Therefore with porosity increasing velocity of sound decreases

Porosity Determination Wyllie time Average Method for Determining Porosity

tlog = tf  + tma (1- )

Units of sec/ft (inverse of velocity)

 t log  t ma         t  t f ma   Compressional Wave Transit Time (tc) (for use in Wyllie Time-average equation) In general Sandstone: 51-55.5 sec/ft Limestone: 43.5-47.5 ,sec/ft Dolomite: 43.5 sec/ft Anhydrite: 50.0 sec/ft Fresh water: 189 sec/ft Salt water: 185 sec/ft

Common Lithology Matrix Travel Times Used

Lithology Sandstone Limestone Dolomite Anydridte Salt

Typical Matrix Travel Time, tma, sec/ft 55.5 47.5 43.5 50.0 66.7

Wyllie time Average Method for Determining Porosity • Found to over predict porosity in under compacted formations. • Under compacted formations can be identified by tsh > 100 sec/ft. • In under compacted formations, a correction factor can be calculated B = tsh / 100 • Modified Wylie equation becomes:

 t log  t ma  1 .     B   t  t f ma  

Problems: • Transit time includes path through mud. • Logging tool must be centralized. • “Cycle skipping”

Single-receiver system

Compressional waves

E1

E3

E2

T0 50 sec

Rayleigh waves

Mud waves

ACOUSTIC (SONIC) LOG • Tool usually consists of one sound transmitter (above) and two receivers (below)

Upper transmitter R1 R2 R3 R4 Lower transmitter

• Sound is generated, travels through formation • Elapsed time between sound wave at receiver 1 vs receiver 2 is dependent upon density of medium through which the sound traveled

term

Definition

Compressional The compressional arrival is used to estimate the velocity of the formation and the energy transferred at the boundaries between materials in a well. The compressional wave has the highest velocity.

Shear

The shear arrival is used to estimate the shear velocity of the formation. The shear wave is slower than the compressional wave. It ca be as much as one half the velocity of the compressional wave.

Stoneley

The Stoneley wave is the lowest wave. It exists only on the boundary between the borehole and the formation. The Stoneley wave has ramifications in fracture identification.

SONIC LOGS Types of Sound Waves: • Compressional/ Pressure/ longitudinal • Shear/ transverse • Stonely / guided

Any point on the interface excited by the advancing wave front produces pressure wavelets in the mud. These wavelets expand at the velocity of mud forming a head-wave. The head-wave velocity measured parallel to the borehole is equal to the wave front that formed it. The process shown is for a compressional wave. The same process applies for shear waves.

The of the material behind the casing is low. Therefore, the acoustic impedance is low and Z1/Z2 is high.

The of the material behind the casing is high. Therefore, the acoustic impedance is low and Z1/Z2 is low.

FACTORS AFFECTING SONIC LOG RESPONSE • Unconsolidated formations • Naturally fractured formations • Hydrocarbons (especially gas) • Rugose salt sections

Mud-path correction time as a function of formation compressional velocity.

Comparison between single- and dualreceiver system response

Example of a sonic log

3- and 1-ft-spacing sonic logs recoded in a west Texas well.

Two-receiver system wave path in case of (a) tilted tool and (b) irregular borehole.

Log example of effect of borehole enlargement in thick and thin beds.

BHC tool in a tilted position with respect to hole axis.

Comparison of a BHC log run with a centered sonde to one run with a deliberately tilted sonde.

Comparison of 1-ft-span conventional sonic log and 2-ft-span BHC sonic log

ISF/sonic combination log showing the Wilcox formation of Louisiana.

 t log  t ma  1 .     B   t  t f ma  

Electrical and sonic logs

Example Shale velocity is 120 ft/section: undercompacted B = 120/100 = 1.2 Travel time in the wet sand is 113 µsec/foot Travel time for sandstone is 55.5 µsec/foot Travel time for water is 189 µsec/foot Φ = [ (113-55.5)/(189-55.5)]/ 1.2 = 36% Gas – slower velocity than water – other effects True Porosity in Gas Zone = .7 calculated from time average method assuming fluid is water Crude Oil Travel time = 238 µsec/foot

Two or more receivers eliminates path through mud in borehole.

Provided wellbore is uniform diameter.

Schematic of the sonic tool incorporating the dual-receiver system concept.

Spacing vs. transmitter-to-receiver time.

Minimum critical transmitter/receiver spacing, (LS)c, required to ensure that first arrivals are compressional waves.

DEPTH OF INVESTIGATION • Depth of investigation varies with wavelength – λ = velocity/frequency velocity = 5000-25000 ft/sec frequency of 20kHz = 20000 cycles per second λ varies from 0.25 to 1.25 ft • Thickness of 3λ is required to a compressional Wave • Therefore, depth of investigation will vary with the velocity in the formation – higher velocity, greater depth • 0.75 ft to 3.75 feet

Enlarged Boreholes • Minimum critical transmitter to receiver distance may exceed tool design in enlarged boreholes and slow formations (high cmf) • Washouts in unconsolidated sandstone formations may result in anamalously slow travel time readings • looks like cycle skipping • subjectively edited out – replaced with data from regional trend curves for seismic calibration • cannot be used for porosity or lithology determination

Sonic log shown cycle skipping caused by slow gas formations.

Sonic logs obtained in fractured and fissured Edwards limestone, south Texas.

“Cycle skipping” occurs when compressional wave is attenuated due to gas and fractures.

Schematic of the waveform at the receiver showing time measured by the single-receiver system.

Cycle Skipping • Indicated by anomalously slow travel times • Thin beds, gas sands, poorly consolidated formations • Identification: comparison with expected results from adjacent sands and trend curves • Subjectively edited with trend data or data from other logs • OK for synthetic seismic, time to depth conversions • Porosity or lithology determination not possible

Homework • What is the compressional and shear waves travel time at 6600, 6650, 6918, and 9950 feet? • What is the sonic velocity at these depths? • What is the sonic porosity in the sandstones? • If the sandstones grains were calcite instead of quartz, what would be the porosity? • If a neutron log showed 28% porosity in a carbonate rock, and a sonic log showed 20% porosity, what is the percentage of vuggy porosity? • Problem 10.3 from the textbook

Sonic Logging • • • • •

Applications of the sonic log Application of the Wylie Time Average Equation Correction for undercompacted sands Units- velocity, travel time Concepts of mud velocity, critical transmitter/receiver spacing • Recognizing effect of enlarged boreholes, cycle skipping

Log Evaluation • QUESTION: what zones are potential economic hydrocarbon reservoirs • REQUIRED: porosity, permeability and hydrocarbons • SANDSTONES – – – –

STEP 1 – Identify sands STEP 2 – Identify resistive sands STEP 3 – Confirm porosity STEP 4 – More detailed analysis for SW, Φ, H, k

CARBONATES – STEP 1 – Identify porous zones – STEP 2 – Identify resistive zones – STEP 3 – More detailed analysis

Gas Effect

• Density -  is too high • Neutron -  is too low

• Sonic -  is not significantly affected by gas

Terminology • Pay – zone that contains economic hydrocarbons • Gross sand – true vertical thickness of total sand • Net sand – gross sand thickness minus thickness of interbedded shale or tight streaks • Net pay – net sand that contains hydrocarbons

ACOUSTIC (SONIC) LOG Working equation

t L   S xo t mf   1  S xo  t hc  Vsh t sh  1    Vsh  t ma tL

= Recorded parameter, travel time read from log

 Sxo tmf

= Mud filtrate portion

 (1 - Sxo) thc = Hydrocarbon portion Vsh tsh = Shale portion (1 -  - Vsh) tma = Matrix portion

ACOUSTIC (SONIC) LOG • If Vsh = 0 and if hydrocarbon is liquid (i.e. tmf  tf), then • tL =  tf + (1 - ) tma or

tL  t ma s    t f  t ma s = Porosity calculated from sonic log reading, fraction

tL = Travel time reading from log, microseconds/ft tma = Travel time in matrix, microseconds/ft tf = Travel time in fluid, microseconds/ ft

ACOUSTIC (SONIC) LOG 0

GR API

6

CALIX IN

DT 200

16

140

USFT

40

30

SPHI %

10

4100 Sonic travel time

Gamma Ray Sonic porosity

4200

Caliper

SONIC LOG 001) BONANZA 1 GRC 0 150 SPC -160 MV 40 ACAL 6 16

0.2 0.2

0.2

ILDC SNC MLLCF

200 200

RHOC 1.95 2.95 CNLLC 0.45 -0.15

DT 150 us/f 50

200

10700

150

10800

Sonic Log 10900

DT us/f

50

EXAMPLE Calculating Rock Porosity Using an Acoustic Log Calculate the porosity for the following intervals. The measured travel times from the log are summarized in the following table.

At depth of 10,820’, acoustic log reads travel time of 65 s/ft. Calculate porosity. Does this value agree with density and neutron logs? Assume a matrix travel time, tm = 51.6 sec/ft. In addition, assume the formation is saturated with water having a tf = 189.0 sec/ft.

EXAMPLE SOLUTION SONIC LOG 001) BONANZA 1 GRC 0 150 SPC -160 MV 40 ACAL 6 16

0.2 0.2

0.2

ILDC SNC MLLCF

200 200

RHOC 1.95 2.95 CNLLC 0.45 -0.15

DT 150 us/f 50 SPHI 45 ss -15

200

10700

10800

SPHI

10900