research article

Microseismicity Ð so what?John Danger®eld1, Robert Paul Young2 & Shawn Maxwell3

Introduction

Phillips licence 018 group is testing the use of microseismicactivity to understand compaction and subsidence. Monitoringin 1997 in the C11a well bore showed activity rates of 5 eventsper hour. If part of this activity is in association with waterfloodfronts, then such a high event rate, combined with good eventpositioning, shows the potential to define the waterflood frontsin the field. At least 16 oil and gas fields have been describedwith microseismicity which may be associated with hydro-carbon production and waterflooding, so this phenomenoncould be more widespread than expected. At Ekofisk,positioning the microseismic activity has the strong addedbenefit that the faults interpreted in the crestal area areobscured from normal seismic view by gas in the overburden.

Microseismic events have been monitored at Ekofisk withdownhole geophones on a wireline in four separate short-termperiods over the last 10 years. In all cases, rates of activity ofbetween 5 and 15 events an hour were observed. Subsidence ofthe sea bottom has been continuing throughout that period atthe average rate of c. 37 cm per year. It is significant that therewas no microseismicity in the upper overburden because itindicated the absence there of seismically active faults. The1997 work was the first to provide accurate event positioning.This was due to two elements: the tool was largely resonancefree and the analysis, performed by the team from KeeleUniversity, used more sophisticated analysis tools.

Acquisition and processing

The geophone system was the CGG SST 500 tool: it wasmodified to hold six 30 Hz, fixed-element, geophones. Thetool string was first tested at a borehole test site for accuracy inits hodograms (first break directions) and for absence of toolresonances below 350 Hz. The results demonstrated the highquality of the seismograms, which would allow detailed analysisfor accurate locations of the events.

This tool string was deployed at Ekofisk for 20 days in April1997: it was positioned in the upper portion of the reservoir atthe C11a observation well at the centre of the field. The 6geophones were spread over 100 m in the upper reservoir

layers EA to ED. Recording was effectively continuous. A seriesof airgun shots was used to determine the geophoneorientations to an accuracy of 5 8. These and the subsequentlyrecorded microseismic events were extracted from the data byCambourne School of Mines Associates and sent to KeeleUniversity for processing.

The event analysis began with visual determination of thearrival times of P- and S-waves at each geophone, andpolarization analysis of the initial P-wave to constrain raypathorientations. A 3D velocity model was constructed for thefield. A Stratamodel was used to interpolate sonic velocitiesbetween the logged wells, along all the reservoir layers. The 3Dvelocity model also included the structural dips of the layers,which are up to 8 8s in this part of the field. Raytracing wasperformed using a finite-difference solution to the Eikonalequation, to determine travel times and raypath orientations. Agrid search technique was then used to determine the locationwhich best matched the observed arrival times and rayorientations from the hodogram results.

Magnitudes were also determined using a local magnitudescale based on peak amplitudes, which was calibrated on asubset of the data using the moment magnitude. Magnitudesof events fell in the range 73.5±0.

The accuracies of the computed locations were assessed byexamining the variations in locations resulting from variationsin the arrival time and hodogram data equivalent in magnitudeto the estimated data accuracy. Depth estimations weregenerally found to have an accuracy better than 20 m.Uncertainty in the velocity model resulted in an additionalerror of up to 15 m. Errors in the horizontal component wererelatively larger, corresponding to a 58 error in azimuth or adistance error of 70 m at an offset of 800 m.

Accuracies of event locations were further improved with aclustering technique. Clustering attempts to minimize thescattering effect resulting from location uncertainties. Figure 1shows the map view of locations out to 400 m from thewellbore where the event density is strong enough to clearlydefine faults. Further details of the data analysis can be found inMaxwell et al. (1998a).

Two thousand events were located within 800 m of the wellbore: they were almost all within the reservoir and largelyrestricted to the slightly stiffer reservoir layers overlying thedeeper waterflooded zones (Maxwell et al. 1998b). Figures 2and 3 illustrate the event distribution with distance from thewellbore and with depth. More events are received as distanceincreases from the wellbore out to 150 m because a largerreservoir volume is being inspected. Beyond that distance the

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1Formerly at Phillips Petroleum, Norway; present address: Eltervag,

4311 HommersaÊ k, Norway, 2Department of Earth Sciences, Keele

University, Staffs ST5 BG, UK, 3ESG (Engineering Science Group, 1

Hyperion Court, Kingston, Ontario, Canada K7K 7BG3

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numbers of events falls with distance, because attenuationallows only larger events to arrive. The magnitudes of eventsdetected were between 73.5 and 70.5, corresponding to faultthrows of between 0.1 and 0.005 mm, along fault planesbetween 0.5 and 3 m long.

Events within 400 m of C11a clearly delineated the activefault pattern and the trends are parallel to the those in the restof the field. The faults found are in the same locations as thosethat needed to be put into the reservoir model to explain thepresence of water in those locations.

Events from further away were too sparse to create a clearfault pattern. The data suggest that monitoring of about 6

months would have been required to establish a clear faultpattern out to about 800 m. Such a distance suggests thatmicroseismicity should provide an opportunity for monitoringthe whole of the seismically obscured area at Ekofisk withpermanent geophones in only a few well locations.

Lack of certainty about the positions of the waterflood frontsaround the C11a wellbore, and the short duration of the test,precluded deducing any relationships of microseismicity towaterflood fronts. To show that relationship requires long-term monitoring of a new waterflood injector and this requiresdevelopment of a new permanent system. If emplaced, we hopeto see increased microseismic activity when injection starts andexpect that the compaction activity associated with the sweepshould be identifiable as a moving zone as the front moves. AtEkofisk the fronts generally move a few hundred meters a year,unless they intersect major flow conduits, such as an open fault.Using time-lapse microseismicity might allow the flood frontto be followed and breakthrough in adjacent wells predicted.

The answer to the question: `microseismicity Ð so what?' isclear in these special cases of fields with high porosity chalksthat undergo compaction associated with waterflood, and thatare obscured from normal seismic view. However, a moregeneral application is also likely.

Monitoring in other fields has generally been performed bygeophones in shallow boreholes or by surface networks. Noneof the events detected at Ekofisk would have penetrated tosuch monitors, yet microseismicity has been reported in manyproducing fields, including 16 fields undergoing waterflood.

It would not be surprising if a large number, perhaps eventhe majority of producing fields are undergoing microseismicactivity associated with production. Microseismicity shoulddetect the small-scale active fault systems and their changingactivity with time. The changes in pore pressures and stressescould also provide a more direct link to fluid flow thanmonitoring changes in acoustic impedance. These relationshipsdeserve much more study.

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Figure 2 Histogram of the number of events vs. distance from the

geophone tool.

Figure 1 Map view of clustered event locations within 800 m of

the monitoring well. Blue dots indicate neighbouring wells, the

monitoring well is in the centre.

Figure 3 Histogram of the number of events vs. depth, with the

major reservoir layers indicated.

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Summary

Twenty days of passive seismic recording at Ekofisk allowedlocation of 2000 microseismic events from within 800 m of theobservation well. The events out to 400 m radius show a veryclearly defined set of faults in the Ekofisk layers. This area is notvisible with normal surface seismic so the previously mappedfault pattern there was highly speculative. The microseismicfaults correspond very closely in position to faults that hadalready been added to the reservoir simulator, although theirexistence was only inferred by the need to help move water intothe area. The microseismic events, corresponding to displace-ments of the order of 1/100th of a mm, are attributed tomovements of pre-existing faults induced by the reservoircompacting at an annual rate of the order of 38 cm. Laboratorystudies indicate that water flooding causes localized increases incompaction rates due to a `water-weakening' effect, creatingthe possibility that the waterflood will produce bands ofmicroseismic activity at the fronts.

Acknowledgements

The authors acknowledge permission to publish this paperfrom Phillips Petroleum Company, Norway, and Co-venturers,including Fina Exploration Norway S.C.A., Norsk Agip A/S,Elf Petroleum Norge AS, Norsk Hydro Produksjon AS, andSaga Petroleum. The view in this paper are those of the authorsand do not necessarily represent those of the PhillipsPetroleum Company, Norway, or its coventurers.

References

Maxwell, S.C., Young, R.P., Bossu, R., Jupe, A. and Dangerfield, J.[1998a] Microseismic logging of the Ekofisk Reservoir (SPE#47276).Proceedings of EUROCK''98 pp. 387±394.

Maxwell, S.C., Bossu, R., Young, R.P. and Dangerfield, J. [1998b]Processing of induced microseismicity recorded in the EkofiskReservoir. 68th Annual Meeting, Society of Exploration Geophysicist,paper 793.

MS received October 1998, accepted March 1999

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