SPWLA 31st Annual Logging Symposium. June 24-27, 1990

CHARACTRRISTICS OF DRILL BIT GRNRRATRD NOISE

E. Iversen Naklcen and 0. Baltzersen IKU - Continental Shelf and Petroleum

Technology Research Institute

and

b. Kristensen Statoil

ABSTRACT

The sonic log is still missing in the suit of new MWD formation evaluation tools. This paper focuses on the drill bit induced noise as a possible acoustic source for passive MWD sonic applications.

The noise signatures of drill bits obviously play an important role in evaluating the feasibility of performing acoustic waveform measurements while drilling. The full scale drilling bench at the Petroleum Technology Center in Trondheim was applied in an experimental study on acoustic noise produced during drilling. 842” hybrid PDC and roller cone (IADC 517) bits were tested in chalk-dolomite samples. A triaxial accelerometer attached directly on the rock sample detected noise signals within a bandwidth of 30 kHz. Spectral analysis of bit noise concludes that both PDC bits and roller cone bits are broadband random sources. Significant noise components are produced in the frequency-band (l-30 kHz) applied in conventional sonic logging. A characteristic difference between the two sources is the distinct impulsive nature of the acoustic noise emitted from a roller cone bit, in contrast to the continuous character of PDC noise.

The laboratory measurements indicate that roller cone induced noise may be used as a downhole acoustic source for sonic logging. Recent cross- hole measurements conducted while drilling onshore support this assumption. Acoustic waveform measurements based on active transmitter pulses are, however, likely to interfere with bit noise and should therefore be restricted to periods where drilling is interrupted.

INTRODUCTION

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Sonic array tools for full waveform logging measure compressional-, shear- (when present) and Stoneley-wave (when present) velocity and attenuation. From processed interpretations of data, one can provide information on rock properties and mechanical characteristics; fluid saturations for porosity evaluation; lithology and seismic correlation.

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SPWLA 31st Annual Logging Symposium. June 24-27, 1990

The capabilities of recently developed MWD formation evaluation sensors (electromagnetic wave resistivity, neutron porosity and gamma density) allow more quantitative evaluation of rock and replacement of wireline logging. The MWD sonic log is, however, still not available.

In this paper, we describe some characteristic features of drill bit noise signatures that address possible future applications within MWD sonic logging. Present sonic tools use piezoelectric transmitters with a typical acoustic bandwidth of 5-25 kHz. Acoustic laboratory measurements were performed to examine the drill bit induced noise produced in this frequency band. One particularly interesting application is to use the drill bit as an acoustic source and derive sonic velocity data from a passive sonic array. In horizontal drilling, it is important to know the bit position relative to adjacent bed-boundaries. A passive MWD borehole sonar, POSLOG [ 11, may provide on-line information on relative bit location.

Surface seismic measurements have confirmed that it is feasible to use the natural acoustic noise generated from a rotating drill bit as a means of continuously locating the drill bit position in the earth [1,2]. The recently introduced TOMEX Survey [3] technique uses the drill bit as a downhole energy source to produce Vertical Seismic Profiling (VSP) data. The acoustic noise emit ted from roller cone bits has been characterised as a series of impulsive events caused by the interaction between bit teeth and the rock. The events occurred randomly in time and amplitude. The above mentioned references report a typical low-frequency (< 35 Hz) nature of the signals. However, this feature seems to reflect transmission effects rather than a band-limited acoustic source.

The experimental setup is illustrated in Figure 1. The horizontal drilling bench at the Petroleum Technology Center in Trondheim, consists of a 5 m long drill collar section powered by a hydraulic swivel and hydraulic WOB. Drilling is performed under atmospheric conditions with relatively low pressure fresh water circulation to remove cuttings. Cylindrical samples of chalk-dolomite rock, 0.25 m OD. and about 1 m long, were mounted in the core-holder of the drilling bench. Acoustic noise produced while drilling was measured with a triaxial accelerometer attached to the front end of the samples. The signatures of 8%” drill bits, hybrid PDC and roller cone (IADC 517)) were recorded. Both bit types are frequently used in soft to medium hard formations. The two bit types remove rock differently: PDC bits by a cutting action and roller cone bits by crushing rock through the impact of numerous teeth. This fundamental difference should obviously provide noise signature variations.

Acceleration and drilling data were logged on an Enertech Euromag I (8 channels) tape recorder operated in FM-mode with a signal bandwidth of 40 kHz (+_ 1 dB).

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SPWLA 31st Annual Logging Symposium, June 24-27. 1990

Acoustic signals above 30 kHz are in general not of practical interest in sonic logging due to the high frequency dependent attenuation experienced in reservoir-like formations. Therefore, a 30 kHz lowpass cut-off (40 dB/decade) was applied on the acceleration signal amplification. Spectrum analysis of the acceleration signals was performed with a dual channel Fast Fourier Transform (FFT) analyzer. Weight on bit (WOB) and torque measurements were taken with force transducers attached to the drilling bench. Additional parameters such as rotary speed (RPM) and rate of penetration (ROP), were logged on a separate data acquisition system sampled at 0.5 second intervals.

RESULTS - NOISE CHARACTERISTICS

Figures 2a and 2b show time function plots (8 ms and 2.048 s time windows respectively) of the hybrid PDC bit axial accelerations. These data were acquired while drilling at constant WOB (85 k.N) and rotary speed (120 rpm). The penetration rate was logged to 4.9 m/h. In an 8 ms time frame, acoustical emission from the PDC bit can be described as a continuous non-stationary stochastic process. High frequency noise signals are superimposed on low frequency vibration signals. Similar roller cone noise data captured between two subsequent transients are shown in Figure 3.

The power spectral density (PSD) plots presented in Figure 4 show the ensemble average based on 8 ms PSDs taken over a period of 2.048 s. The two peaks observed at approximately 4 kHz and 8 kHz in plot (a), at 10 kHz and less dominant at 20 kHz in plot (b), correspond to ‘h- and one- wavelength resonances in the rock sample. The spectra show that both bit types generate broadband noise with the major fractions of energy located at frequencies above 25 kHz, the roller cone PSD-level being about 10 dB higher than for the hybrid PDC bit. Harmonics related to rotary speed are as expected, less prominent for the PDC bit compared to the roller cone bit.

Ensembles of roller cone acceleration measurements taken over a period of 2.048 s are shown in Figure 5. Data have been captured at three different rotary speeds, (a) 120 rpm, (b) 80 rpm and (c) 60 rpm. Numerous impulsive events are observed in the roller cone data. These high amplitude acceleration peaks are characteristic of roller cone signatures. No time correlation between successive events has been observed. Peak amplitudes are also randomly distributed. However, the number of events and amplitude-levels increases rapidly with increasing rotary speeds and WOBs.

A single event captured from the data presented in Figure 5a is shown in Figure 6. The interaction of impacting roller cone teeth and a chalk- dolomite sample creates distinct oscillating transients. For the particular transient shown here, the average RMS-level is approximately 35 dB higher than the acceleration level measured prior to the transient. Figure 7 illustrates the change in power spectral density (PSD) due to the onset of a transient. The PSD reveals major spectral components at 25-40 kHz.

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SPWLA 31st Annual Logging Symposium, June 24-27, 1990

Extraneous noise and vibration signals due to radiation and resonances in the drilling bench structures, physical dimensions of the rock sample and low-pass filtering with cut-off above 30 kHz, obviously have a spectral shaping effect on the acquired data. Nevertheless, the observed spectra and the onset of typical transients do reflect the broadband nature of bit signatures.

CONCLUSIONS

Acoustic measurements performed while drilling in a chalk-dolomite sample have shown that both PDC and roller cone bits can be regarded as broadband sources with major spectral components at sonic frequencies.

The high level of acoustic noise experienced during laboratory drilling seems to indicate that MWD sonic logging based on the use of active transmitters will only be feasible if drilling is interrupted. On the other hand, the unique transient nature of the roller cone signature makes it a suitable downhole source in a passive MWD sonic logging tool. Another feature of importance is the extremely good acoustic coupling to the formation, provided by direct contact between bit and formation.

The feasibility of extracting accurate compressional and shear wave velocity data from roller cone generated noise has been confirmed through recent cross-hole acoustic measurements performed during onshore drilling in mica-silt formations.

ACKNOVLEDGNENTS

We thank Statoil R&D for permission to publish this paper and the Department of Petroleum Technology and Applied Geophysics for use of the drilling bench. Thanks to Geir Tandberg for giving valuable assistance.

[ll “MWD applications expand as data quality improves,” Offshore incorporating The Oilman, Vol. 50, Number 2, page 45-46, February, 1990.

[2] Katz,L.J.: “Drill bit location , guidance by seismic seen feasible,” Oil and Gas Journal, July 28, 1980.

[3] Rector, J.W. and Marion, B.P.: “MWD VSP and checkshot surveys using the drillbit as a downhole energy source,” paper OTC 6024 presented at the 21st Annual Offshore Technology Conference, Houston, May l- 4, 1989.

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SPWLA 31st Armual Logging,Symposium. June 24-21,199O ,

ABOUT THE AUTHORS

Erik Iversen Nakken is a research engineer with IKU' s Logging Technology Group. He is manager of the Position Logging Research Project (POSLOG) which deals with new’ borehole radar and sonar technology for MWD applications. He holds a master’s degree in physics from the Norwegian Institute of Technology in Trondheim.

Oystein Baltzersen is an electronics engineer with IKU’s Logging Technology Group. His work at IKU has been associated with data acquisition and signal processing for various offshore applications. He holds a master’s degree in electronics engineering from the Norwegian Institute of Technology in Trondheim.

Aage Kristensen is currently working as a senior scientist at SACLANT Undersea Research Center in La Spezia, Italy. He has a 3-year leave from Statoil, where he has been the Department Manager of Drilling and Production Technology R&D. Previous work has been within sonar technology, hydroacoustic transducers, digital and analog electronics design and application of acoustic methods in the fisheries research at the Underwater Acoustics group, Electronics Research Laboratory (ELAB), Trondheim. Since 1983, he has been working for the Petroleum Technology Research Institute (1983-84), SwRI (1984), IKU (1985-86) and Statoil within borehole seismics, well logging instrumentation and mathematical modelling of acoustic and electromagnetic wave propagation in boreholes. He holds a Ph.D. degree in physical electronics from the Norwegian Institute of Technology, Trondheim.

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SPWLA 31st Annual Logging Symposium, June 24-27. 1990

3P RPM ROP

Tape recorder

TORP . WOE dccx

Signal conditioning

nccy -4 amplifier ACCZ I

1.

2. 3. 4. 5. 6. 7. 8. 9. 10 11

12

Hydraulic cylinder Axial force transducer (WOE) Triaxial accelerometer (Acc,_~) Core-holder with sample Drill bit Force transducer (TORQ) Displacement transducer (ROP) Hydraulic cylinder Tachometer Power swivel Mud pressure transducer (AP) Mud inlet

Dual channel

FFT-analyzer

Figure 1: Experimental setup

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SPWLA 31st Annual Logging Symposium, June 24-27.1990

316

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0.0 Time (0.8 ms/div) 8.0

316 1 I

0.0 Time (0.2 s/div) 2.0

Figure 2 : Axial accelerations measured while drilling with 8 l/2” hybrid PDC bit in chalk-dolomite sample, (a) 8 ms and (b) 2.0 s time-window. (RPM=1 20, WOB=85 kN and ROP=4.9 m/h)

(b)

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SPWL.A 31st Annual Logging Symposium, June 24-27.1990

316

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0.0 Time (0.8 ms/div) 8.0

Figure 3 : Axial acceleration measured while drilling with 8 l/2” roller cone bit in chalk-dolomite sample. Noise signal captured between two subsequent transients. (RPM=1 20, WOB=45 kN and ROB=52 m/h)

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SPWLA 31s.t Annual Logging Symposium, June 24-27, 1990

3

(a)

Frequency (5 kHz/div)

3

UN

Frequency (5 kHz/div)

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Figure 4 : Average power spectral density (PSD) of hybrid PDC bit (a) and roller cone bit (b) accelerations.

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SPWLA 31st Annual Logging Sympsium. June 24-27.1990

3160

-3160 0.0 Time (0.2 s/div) 2.0

1580

1

(W

0.0

200 -

Time (0.2 s/div) 2.0

-200 0.0 Time (0.2 s/div) 2.0

Figure 5 : Axial accelerations measured while drilling with 8 l/2” roller cone bit in chalk-dolomite sample at rotary speeds of (a) 120 rpm, (b) 80 rpm and (c) 60 rpm. (WOB=30-45 kN and ROP=l.2-5.2 m/h)

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SPWLA 31st Annual Logging Symposium, June 24-27. 1990

3160

-3160 Time (0.8 ms/div)

Figure 6 : Single transient emitted from roller cone bit.

8.0

30

Frequency (5 kHz/div) 50

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Figure 7 : Power spectral density (PSD) of transient (full line) and of 16 ms time-window prior to transient (dotted line)

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SPWLA 31st Annual Logging Symposium, June 24-27. 1990

MODELING TRANSPORT IN

GRANULAR POROUS MEDIA

Lawrence M. Schwartz

Schlumberger-Doll Research Y

ABSTRACT

This paper is concerned with two related problems: (1) the construction of

geometrical models of porous media relevant to reservoir sandstones and (2) the

description of transport processes in these model systems. We will show that a

variety of interesting porous media can be generated by the packing and subse-

quent modification of spherical grains. This modification may involve a change in

either the grain’s size, shape, or both. Steady state transport processes such as the

flow of electrical current or viscous fluids are controlled by the distribution of pore

throat sizes and, within the present framework, can be studied efficiently by random walk simulations of diffusion. The techniques developed here are also of interest in

connection with dynamic transport processes such as the filtration of fine grained particles into consolidated granular networks. The modeling of such processes will be discussed briefly, as will the interaction of steady state and dynamic transport.