PETROPHYSICAL APPLICATIONS OF NEW SEISMIC-WHILE-DRILLING TECHNOLOGY IN DEEP WATER

Transcription

1 PETROPHYSICAL APPLICATIONS OF NEW SEISMIC-WHILE-DRILLING TECHNOLOGY IN DEEP WATER Greg Myers*, Dave Goldberg*, Alex Meltser* Erich Scholz**, and the Ocean Drilling Program Leg 179 Shipboard Science party *Lamont-Doherty Earth Observatory of Columbia University ** Downhole Systems, Inc. Abstract During the Ocean Drilling Program's Leg 179, we recorded drill string vibration data to investigate the sub-seafloor environment as part of two Seismic-While- Drilling experiments in May, The Ocean Drilling Program's drill ship the JOIDES Resolution drilled two holes in 714 m and 1660 m water depth in the Indian Ocean to conduct these experiments. To our knowledge, such measurements have never before been recorded in the Ocean Drilling Program or on other deep-water drilling rigs. By comparison of vertical and horizontal drill string acceleration with wireline logs and core data, variations in the petrophysical properties are correlated to drilling parameters. Drill string acceleration varies inversely with porosity from logs and core data which correlate to fracturing and lithologic changes in these examples. Determining petrophysical properties while drilling, therefore, may assist both log analysts and drillers in identifying lithologic contacts and sediment/rock interfaces even in cases where core and log data are not available. Introduction In the course of everyday operations on board the D/V JOIDES Resolution, the drill string vibrates continuously. These vibrations are characteristic and can be acquired at the rig floor or at the drill bit, although the latter provides a more accurate signature of the environment being drilled. Our goal is to evaluate whether these reverberations generated from the formation and environment encountered at the bit, by ship heave motions, and from other extraneous noise sources can be used to improve operations and provide some understanding of the petrophysics of the rock encountered while drilling. Drill string acceleration data was acquired at two sites reoccupied during Ocean Drilling Program (ODP) Leg 179. The sites are located in Figure 1. Our primary location, Site 1105, is situated in the south-western Indian Ocean on the eastern flank of the Atlantis II fracture zone (Leg 179 Shipboard Party, 1998). The rocks encountered at this site are characterized as anisotropic meta-gabbro containing alternating layers of olivine and metal oxide. Numerous intervals of secondary fracture porosity are observed in the core and logs. These are associated with intense regional uplift and deformation along the fracture zone (Leg 118 shipboard party, 1987). Core recovery at this site was exceptional, on average greater than 87%, and the structure of these metamorphic rocks may be readily observed. These features were oriented using borehole image logs (Goldberg et al, 1991). The foliation and fracturing that cross-cuts these cores dip steeply and strike NNW. Site 1107 is located on the Ninety-East Ridge in the eastern Indian Ocean, near ODP Site 757 drilled 10 years earlier. Sub-aerially emplaced basaltic lava flows are overlain by more than 350m of unconsolidated silt, clay, volcanic breccia and tuff (Leg 121 Shipboard Scientific party, 1988). In this paper, we describe the drill string acceleration recorded during Leg 179 and investigate their correlation with the available log and core data at these sites. Methods In typical Seismic-While-Drilling (SWD) experiments, the force of a roller cone drill bit impacting the formation generates energy which radiates axially and is received by sensors located on the seafloor (Rector and Marion, 1991). A pilot sensor is used to record the axial vibrations that travel up the drill string, representing the drill bit source signal (Meehan et al., 1998). This type of experiment can be described as an reverse vertical seismic profile where the source is usually at the surface and the receivers are placed at various levels in a borehole. 1

2 A pilot sensor system was developed and manufactured at the Lamont-Doherty Earth Observatory specifically for the experiments conducted during Leg 179 (fig 2.). The system includes an acceleration measurement module (Fig 3.) attached to the drill string using alloy wedge clamps (Fig.4), and a PC based Data Acquisition System (DAS) located in a lab adjacent to the rig floor. The pilot sensor system records drill string vibrations at the rig floor using a three-axis accelerometer with the measurement range of ±1g to ±10. A ±5g unit was utilized at both sites during Leg 179. The accelerometer signals in the Hz frequency range were digitized in the acceleration measurement module with 16-bit resolution at 400 samples/s sampling rate. Data blocks were transmitted once per second through a wireless telemetry link to the DAS where they were time-stamped with GPS time to ensure correlation with other data. A LabVIEW data acquisition program stored recorded data on a hard drive, provided real-time monitoring of signal quality and spectral content, and allowed the operator to control channel gain and sampling rate during acquisition. The pilot sensor battery set is sufficient for continuous 72-hr. operation. The wireless data link greatly simplified system deployment and provided reliable data transmission at 57K baud rate despite high level electro-magnetic interference on the rig floor. The water depths at Sites 1105 and 1107 are 714 m and 1659 m, respectively. Both are considerably greater than conventional SWD experiments for industry applications. A four roller-cone bit was used to drill gabbro at Site 1105 and a three roller-cone bit was used in sediments and basalt at Site Drilling rotation and pump rates were relatively constant during both of these experiments. The weight on bit steadily increased from 10,000 to 12,000 lbs. during drilling and the weight of the drill string was approximately 220,000 lbs. As for all ODP drilling operations, dynamic positioning thrusters are utilized to position the ship over a site. Sea states during this experiment were not exceptionally large therefore thruster activity was moderate. The thruster activity did not appear to have an effect on drill string acceleration. Results Four key data sets are used in this paper: (1) Stacked spectra of drill string acceleration data. (The sampling interval of the acceleration amplitude and frequency is 0.1 m). (2) The RMS value of the acceleration versus depth at Sites 1105 and (Depth smoothing of 1.5 m was applied). (3) Drilling depth. (Depth resolution of the driller's log is roughly 0.5 m). (4) Logs acquired at Site (Vertical log resolution is approximately 0.15 m). At Site 1105, the pilot sensor recorded data from 33 to 80 m below the sea floor (mbsf) and from 91 to 107 mbsf. The 11-m break in the deployment was the result of an operational requirement to remove the unit from the drill string during a wiper trip. At Site 1107, the pilot sensor recorded data from 170 to 422 mbsf. In Figure 5, three spectra are displayed illustrating the relative power spectra of the components of acceleration at one depth in basalt at Site 1107 and in gabbro at Site The rate of rotation of the drill pipe is identified by the small peak in the spectra at Hz, corresponding to rev/min. The large peaks between 2 and 3 Hz are associated with the rotation rate of each roller-cone as the bit turns one revolution. The four rollers used in the gabbro rotate faster than the three in the basalt. The vertical acceleration in the gabbro also contains significant energy at high frequencies, which is absent in basalt, and considerably more energy on the horizontal than on the vertical axis component. The ratio of signal amplitude between horizontal and vertical axes is approximately 6:1 at this depth. The signal amplitude in sediments (not shown) is roughly half that in either basalt or gabbro. These differences show important characteristics of the advancing drill bit significant energy radiates through the seafloor and differently through various formations; thus, it may be a useful evaluation of drilling conditions and petrophysical conditions encountered at the bit. Other signals are also present in these spectra. Every rig floor operation, as well as other ship vibrations, creates noise. Spectral signatures are observed, for example, when the iron roughneck is used to connect sequential lengths of pipe, when the main motors rev to rack or pickup pipe joints, or when the elevators are clamped on a drill collar. Rig floor and ship operations punctuated the spectra. Of these, the most important sources of noise appear to be the ship s motors at Hz and the lab fans at 41 Hz, both of which overlap with the formation signals generated during drilling. These noise sources increased as loads were added (e.g. motors picking up pipe), but their overall amplitude 2

3 was measured to be 7-10 times lower than that generated during drilling and is likely of minor importance in the drill bit signal for this deep water setting. Other noise may be generated from non-periodic sources. The hydraulic pumps labor when the top drive is rotating and when the iron roughneck is making or breaking pipe connections, thruster activity increases substantially in rough seas, the drill string bangs the rig floor equipment, and residual low-frequency heave may not be completely removed by signal conditioning. Such noise sources have not been isolated or removed in this analysis. The variation in vertical and horizontal accelerations versus depth is shown in Figure 6. The RMS amplitude for all three accelerometers is computed at each depth over the 0 to 50Hz frequency band, as illustrated in Figure 5. The horizontal axes similarly show between 4 to 6-fold higher amplitude than the vertical axis over the entire depth interval. The horizontal and vertical amplitudes also tend to vary inversely over 2 to 5 meter intervals, with the horizontal components ranging considerably. This may indicate that more noise is generated by the lateral action of the cutting surfaces at the bit than by the downward weight on the drill string in these rocks. Rig floor operations, length of the pipe stands used and the hole depth may also influence these observed variations in acceleration with depth. The variation in the horizontal acceleration versus depth in three separate frequency bands is shown in Figure 7. RMS amplitudes are computed over 0-5 Hz, 5-25 Hz, and Hz frequency bands. The RMS amplitude profiles have similar characteristics over all three bands, although greater variation is observed in the 0-5 Hz and Hz frequency band. The highfrequency band (25-50Hz) consistently has the largest amplitudes, likely resulting from high noise associated with rig operations and ship s motors. The lowfrequency band (0-5Hz) has intermediate-high amplitudes that are generated by the resultant thrust and torque generated by the roller cone bit (Eustes, et al, 1995). The intermediate-frequency band (5-25 Hz) has the lowest overall amplitudes and the least variation versus depth. Although the entire spectral response depends on the rocks encountered at the bit, this intermediate-frequency band is often used to isolate the formation response in SWD experiments (e.g. Rector and Marion, 1991). To illustrate the broadband effect of the formation properties on the acceleration data, a spectral image is computed over a 10-m interval at Site 1105 (Fig. 8). The spectra are stacked over 5-minute recording intervals, corresponding to approximately 0.10 m depth at the average drilling rate. A comparison of these computed depth to driller s depth indicates errors no greater than 0.5 m. The image shows a large decrease in amplitude and frequency shifts over the 0-5 Hz band near 100 mbsf. The low amplitudes are associated with fracturing and hole enlargement which, of course, reduce the thrust and torque generated at the bit. The drill string acceleration therefore appears to encounter broadband effects due to the formation. Assuming that this kind of relationship will be robust when drilling in relatively consistent rock types, such as in these examples, we extend this analysis using the petrophysical and core data acquired at Site 1105, and then using core data alone at Site Discussion: Site 1105 A comparison of the acceleration data with the log and core information at these sites illustrates that the drill string vibrations are related to the porosity and hole conditions encountered at the bit. Intuitively, fractured or softer rocks drill easily and transmit less energy back into the drill string; strong thrust and torque at the bit is needed to break competent rock, generating large signals transmitted up the drill string. In Figure 9, RMS amplitude of the horizontal and vertical acceleration is compared to the caliper, gamma ray, and porosity logs at Site The amplitude curves were smoothed and resampled at a 0.15 m interval in order to merge depths with log depth. Over this depth interval, the amplitude and porosity curves, in particular, correlate with the enlarged zones observed in the caliper log near 102 mbsf. An increase in porosity and amplitude at 65 mbsf shows no caliper enlargement and also corresponds with an increase in natural gamma ray. This may indicate a compositional change in the gabbro which causes an apparent increase in porosity as well as a reduction in drilling noise at the bit. Although such subtle variations may be difficult to quantify, they indicate a consistent relationship between drill string acceleration and formation properties in these rocks where an inverse relationship with porosity is most apparent. This qualitative relationship is illustrated by the crossplot in Figure 10. The porosity log and logarithm of drill string acceleration are plotted over the interval from mbsf. The general trend in this figure 3

4 shows a logarithmic and inverse relationship. The unique aspects of ODP operations, such as heave motion on the ship and deep water drilling operations, make further inferences from these data difficult. However, longer recording times with the pilot sensor and complete log and core data sets acquired in a wide variety of geologic settings may help to further quantify this relationship. Site 1107 Using a similar approach at Site 1107, spectral amplitudes of the drill string acceleration data are correlated to downhole information. Although the drill string signals were recorded continuously, an extremely limited set of petrophyscial information is available at this site. The drill string acceleration data are in fact the only continuous vertical profiles available since no core or log data were collected. We used core porosity data from nearby ODP Site 757 located on the Ninety-East ridge in approximately the same water depth and rock types (Leg 121 Shipboard Science Party, 1988). Both holes drilled through approximately 365 m of sediment and into basalt basement rocks. The amplitude of horizontal acceleration at Site 1107 and the core data from Site 757 are shown in Figure 11. Like Site 1105, increases in acceleration are associated with decreases in porosity, such as near 235 m and 365 mbsf. The sharp porosity decrease at 365 mbsf at Site 757 is due to the sediment/basalt contact. The drill string data can be used to infer the existense of this contact at approximately the same depth at Site The anomaly at 235 mbsf is less pronounced, but may perhaps represent a hard or compacted sediment layer also present at both sites. At Site 1107, like Site 1105, an inverse relationship between drill string acceleration and porosity is indicated. In addition, these results suggest that the drill string data may be used to infer lithological and petrophysical boundaries when limited or no core or petrophysical data are available. Conclusions The use of drill string acceleration for acquiring petrophysical data has been demonstrated in two deepwater drilling environments. A qualitative correlation between drilling and log data was made in low-porosity gabbro and basalt rocks. Although the results of this experiment are dependent on drilling conditions, our results suggest that an inverse relationship observed between the amplitude of the drill string acceleration and porosity may be useful in identifing lithologic contacts and fracturing. The results can be improved by addressing the current limitations of the system. Improvements in depth control and digital recording of other drilling parameters will certainly aid in reducing uncertainties in the data. Future deployment of this system will hopefully occur with many of these improvements in place. This approach may potentially serve to assist both drillers and petrophysicist's to understand the insitu conditions encountered at the bit. Acknowledgements Special thanks to Gilles Guerin, Hartley Hoskins, Ullyses Ninnemann and Jim Murray. References Cited Von Herzen, R. P., Robinson, P.T., et al., Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 118: College Station, TX. Leg 121 Shipboard Science party, 1988, Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 121: College Station, TX. Leg 179 Shipboard Science party, 1998, Proceedings of the Ocean Drilling Program, Preliminary Reports, Vol 1/1: College Station, TX. Goldberg, D., Broglia, C., Becker, K., 1991 Fracturing, Alteration, and Permeability: In-Situ Properties in Hole 735B. Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 118 :College Station, TX. Eustes, A.W., Long, R.C., Mitchell, B.J., 1995, Core Bit Frequency Signatures, ASME Energy-sources Technology Conference & Exhibition, Houston, TX. Goldberg, D., Myers, G., Meltzer, A., Scholz, E., the Lamont-Doherty Borehole Research Group and the Leg 179 Shipboard Scientific Party, 1998, Measurement- While-Coring in ODP Success During Leg 179 and the Road Ahead, JOIDES Journal. Meehan, R., Nutt, L., Dutta, N. and Menziesm, J., Seismic information helps predict drilling hazards, choose casing point, Oil & Gas Journal, 96(19):

5 Rector, J. W. and Marion, B. P., 1991, The use of drill bit energy as a downhole seismic source, Geophysics, 56: About the Authors Greg Myers is presently serving as the Manager of Technical Services for the Borehole Research Group at the Lamont-Doherty Earth Observatory of Columbia University. Greg graduated from Rutgers University and has since contributed to the field of borehole geophysics on many levels including, field service, data analysis, tool research and development and new technology integration. He has participated in three Ocean Drilling Program Legs and is an active member of the Society of Professional Well Log Analysts. Dave Goldberg is a Research Scientist at the Lamont- Doherty Earth Observatory and Director of its Borehole Research Group, which manages logging services for the Ocean Drilling Program. He received B.S. and M.S. degrees in marine geophysics from MIT in 1981 and a Ph.D degree in borehole geophysics from Columbia University in He has authored more than 100 scientific publications on topical research in borehole and marine geophysics and is an active member of the Society of Professional Well Log Analysts, the Society of Exploration Geophysicists, and the American Geophysical Union. Alex Meltser joined the Borehole Research Group at LDEO in During his 17 years of industry experience in the former USSR he has been involved in development of versatile logging tools and systems, participated in Kola Super deep Borehole (SG-3) Project, authored numerous patents and technical publications. His responsibilities in BRG include design and development of special logging tools, telemetry and data acquisition systems, real time and embedded software design. Alex participated has in several ODP Legs, testing new logging equipment. He holds MS degree in Radiophysics and Electronics from Byelorussian University, Minsk, USSR. Erich Scholz is the President and Lead Engineer of Downhole Systems Inc. He has 18 years experience designing, manufacturing, and fielding a wide variety of Geophysical instrumentation and systems including borehole logging tools, In-Situ Stress hydrofracturing systems, sub-sea oceanographic instruments, as well as various machines for Rock Mechanics laboratories. He has participated in numerous scientific cruises and completed extensive fieldwork on 7 continents. 5

6 Figure 1. Site locations for two SWD experiments conducted during ODP Leg 179. GPS antenne to satellite Pilot Sensor Wireless Modem 3-axis accelerometer Data Acquisition Figure 2. Schematic drawing of the pilot sensor components used in SWD experiments conducted during ODP Leg 179.

7 Clamp Battery Electronics and modem Figure 3. Pilot sensor with protective cover removed. Figure 4. Wedge clamps on the top and bottom of the pilot sensor compress to rigidly fasten the unit to drill pipe. The pilot sensor is deployed out of harms way approximately 1 m below the top drive.

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9 RMS Data for 3-Axes at Site Depth (mbsf) Vertical Horizontal Horizontal 120 Figure 6. RMS amplitude profiles for 3-axes of acceleration at Site 1105.

10 Broad Band Pass Filtered RMS Data for Site Depth (mbsf) Figure 7. Computed RMS amplitudes over three frequencies (0-5Hz, 5-25Hz, and 25-50Hz) for the vertical-axis pilot sensor data at Site 1105.

11 Frequency (Hz) Depth (mbsf) Figure 8. Stacked vertical-axis spectra between 0-5 Hz showing the effect of high porosity on drill string acceleration.

12 40 60 DEPTH (MBSF) Horizontal Acceleration (Relative) Vertical Acceleration (Relative) Core Porosity (%) APLC Porosity (%) Gamma (API) Figure 9. Horizontal and vertical axis acceleration with log and core data at Site Caliper (inches) X

13 Vertical Axis RMS vs. APLC Porosity Vertical Axis RMS APLC Porosity % Figure 10. Crossplot of drill string acceleration and porosity log data from mbsf at Site 1105.

14 150 Horizontal Axis RMS Core Porosity (%) DEPTH (MBSF) Figure 11. Horizontal axis RMS amplitude data from Site 1107 with core porosity from adjacent Site 757.