Monitoring and interpretation of seismic observations in hot dry rock geothermal energy systems

Geothermics, Vol. 16, No. 4, pp. 441-445, 1987 Printed in Great Britain

0375--6505/87 $3.00 + 0.00 Pergamon Journals Ltd. © 1987 CNR.

KONITORING AND INTERPRETATION OF BEISHIC OBSERVATIONS IN HOT DRY ROCK GEOTHERMAL ENERGY SYSTERS

J WOHLENBERG,

H KEPPLER

ABSTRACT

As the value of microseismlc monitoring during development of HDR reservoirs is already proven, the discussion group concentrated on assessing what further improvements might be made. They suggested improvements both to the receivers and to the methods of analysis, ulth particular reference to the derivation of different source mechanisms from the signals received.

INTRODUCTION

The concept of heat extraction from poorly permeable hot and usually deep formations (Hot Dry Rock) is based on the idea that water can be circulated through a system of cracks between two boreholes. Engineering such a system entails the stimulation of existing Joints and the opening of new fractures to provide sufficient heat exchange area and low flow resistance. Seismological observation and experiment has been important in all }{DR projects to date. There is no doubt that it was primarily the results from the seismic investigations that were responsible for the new insights into large-scale hydraulic fracturing and which supported new concepts of artificial fracture systems. During this period, seismological observation has evolved from an academic curiosity to become an indispensable tool for decision making during the creation, development and operation of an HDR system. In planning future projects, passive seismological observations as well as active seismic measurements will play a key role. The major questions covered by the seismologists' discussion group during the Workshop were: - what contribution can a seismic net make at present? what is the future potential of seismic data? - what is required for further development?

CURRENT

STATE

OF THE A R T

The original model for induced seismic events was s Jerky propagation of a pure tensile hydraulic fracture. Theoretical considerations (Buttkus et sl, 1978) and the initial seismic observations during small frac experiments at Fenton Hill (Albright & Hanold, 1976) and Falkenberg (Leydecker, 1981) implied very small source energies which could only be recorded by highly sensitive seismic tools close to the source. Consequently, several lines of temperature-hardened geophone, accelerometer and hydrophone borehole tools were developed for sustained deployment in deep hot wells during HDR fracturing operations and are now in routine use (Dennis et el, 1985; Batchelor, 1984). When seismic observations became available during larger hydraulic injections, evidence increased that the majority of signals were due to shear slip and a new mechanism for the induced seismicity was proposed (Pearson, 1981). The shear sllp hypothesis was corroborated by direct observation of double-couple fault plane solutions for single large events st Fenton Hill (cash et el, 1983; House et el, 1985). It was also shown to be the mechanism responsible fop the unexpected downward growth of the reservoir at Rosemanoees (Batchelor, 1983). With increasing injection volumes, both the number and magnitude of induced microearthquakes increased. This allowed many events to be located by relatively uidespaced but shallow subsurface networks. Such three-dimenslonal networks allowed the location of seismic sources to be determined from inversion of arrival times (of P- and/or S-waves). This method is now being used

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Hicroselsmic group discussion

443

as the standard location technique in all }{DR projects, and several case histories describe the results of such "seismic mapping" (e.g. Batchelor et al, 1983; Keppler et al, 1983; Haria et al, 1985a; House et al, 1985). Figure I shows examples of seismic maps of fracture zones during stimulation experiments at Rosemanowes and Fenton Hill.

FENTON HILL EXPERIMENT 2862 MICROSEISMIC EVENT LOCRTIONS '

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The accuracy of the locations depends on a knowledge of the velocity distribution within the network. Some velocity information could be obtained from detonator calibration experiments. A typical uncertainty in absolute location is 50 m; the possible error on source depth can become much greater, however, for hypocentres located well below the deepest station of the network (fop a detailed analysis, see Saris et al., 1985b, p. 203ff). While there can be good control on the location of event origins, determination of other characteristics of the microearthguakes is much more uncertain at present. Calculation of any quantity involving seismic wavefo~s, such as spectral content, energy, source displacement, source size, moment, stress drop or polarization, requires the measurement of the vector of true ground motion. This means that it must be possible to determine the transfer function of the complete locked-in borehole tool and to recalibrate it in-situ, and to measure precisely the sensor orientation. These conditions are not satisfied with any of the existing temperature hardened tools.

Fig. 2 : Cross-section (viewed from the east) of seismic activity developing during an injection experiment at Fenton Hill. The openhole segment of the injection well EE-3^ is shown as a bold line. The h~a>ocentres of events are sho~m as a function of time: Filled circles First 12 hours Open circles 12 to 24 hours Triangles 24 to 36 hours Crosses 36 to 48 hours

Fig. I : Cross sections of h~q>ocentre distributions for two injection experiments : a and b sho~ results of Rosemanowes test 2052, c a~d d of Fenton Hill experiment 2032. Horizontal and vertical scales are equal and the same for all plots, a a n d c show approximately a face-on view of the seismically active zone (looking from N210oE in s, and from east to west in c). b and d represent an "edge-on- view (looking along the strike from NI43°E in b, and from south in d). These views show the t~ue dip of the structure. The openhole segment of the respective injection well is marked with s bold line. Adapted from Baria et al. (1985a) and House et al. (1985).

444

J Mohlenberg, H Keppler

FUTURE CO~rlIRIBWIONS AND DEVELOPNENTS The sequence of located seismic events allows the position, size, shape and growth of the reservoir to he inferred. Figure 2 shows an example of temporal change of the seismically active zone. To allow management decisions on the operational control of the system to be aided by such monitoring it is necessary to instal on-line data acquisition, event detection, arrival picking and location of seismic events. To obtain similar information without the need for additional seismic observation boreholes, multiple tool hodogram location methods deserve further development. Besides geometrical monitoring, seismological investigations are potentially the only means of inferring the Pock mechanical and hydraulic processes during the creation, development, and exploitation stages of an HDR reservoir. In particular, seismic data can contribute in the following ways to a future comprehensive HDR reservoir model : - Structural trends in the seiamicity may p r o v i d e information on pre-existing Joints. M. Fehler (unpublished results, 1986) sho~ed that such trends can be resolved by statistical methods. "Emission tomography": simultaneously inverting large sets of seismic events for velocity structure and hypocentre coordinates might provide an additional parameter for the reservoir model (e.g. induced porosity) as a function of space coordinates and time. -

Determination of the seismic quality factor Q would not only relate to an additional material property, but is also a prerequisite for determining source parameters from the recorded signals. Anisotropy effects (in particular from S-wave splitting) supply a parameter of the propagation path which can be related to the ratio of principal stresses or to structural effects (e.g. microcrack distribution). Characteristics of mlcroearthquake source models can be computed from the seismic uaveforms after correcting them for propagation effects. Individual source parameters (source dimension, moment, displacement, stress drop, rupture velocity) and cumulative parameters (energy release, magnitude frequency distributions) should be relatable to variables (such as local friction, strength, fluid pressure, Joint spacing, fracture aperture etc.) of the general reservoir model which, however, has yet to be established.

It should be noted that the last three topics involve seismic uaveform interpretation and therefore first require improved receivers. Improved data would also provide a better base for the interpretation of some "atyPical" signals, as discovered by Fehler & Bame (1985).

HARDMARE DEVELOPMENT NEEDS Continuation and further progress of seismological work within the HDR concept entails a programme of hardware development. The new tools would differ from standard logging units in two respects: they have to be deployed for weeks or even months in the hot wellbores, and the bandwidth and dynamic range of the acquired data is much more demanding. Specifications and tasks for the development of s new three-component dounhole seismic tool may look as follows: - smooth transfer function (e.g. flat velocity response) between 10 Hz (or 50 Hz?) and 10 kHz, calibration of transducers and resonance experiments with housing; - design and testing of a io~ mass transducer housing and a stiff borehole wall coupling; -

-

-

design and test of an in-hole recalibration device;

p r e c i s e (0.1o?) and complete h i g h t e m p e r a t u r e o r i e n t a t i o n

part;

In-situ tests of (In)dependence of tool response with respect to azimuth and angle of inclination;

- data transmission through the cable for a broad frequency band, high amplitude dynamic range, with absence of cross talk ( F ~ or PCM).

Microseismlc group discussion

445

A string of tools is required for implementing a multiple tool h o d o ~ a m location technique in a single borehole (e.g. the future production hole). Only one of the elements would have to be a clamped three- component unit. For reliable calibration experiments the development of a (non-explosive) deep hole seismic source would also be highly desirable. From t h e t a s k s a t hand, i t i s e v i d e n t t h a t p a r a l l e l w o r k c o u l d be a v o i d e d o n l y b y an i n t e n s i v e cooperation and coordination of the different }{DR seismology g r o u p s .

REFERENCES

Albrtght

J N, H a n o l d R J (1976) - " S e i s m i c mapping o f h y d r a u l i c f r a c t u r e s made i n basement r o c k s " - P r o c e e d i n g s o f t h e E n e r g y Research and Development A d m i n i s t r a t i o n Symposium on Enhanced 0 i l and Gas R e c o v e r y , 9 - 1 0 S a p . , T u l s a , O k l . , ~ (Gas), C-8/1 - C - 8 / 1 3

S a r i a R, Hearn K C, B a t c h e l o r A S (1985a) - " I n d u c e d s e i s m i c i t y d u r i n g t h e h y d r a u l i c s t i m u l a t i o n of the potential Hot D W Rock geothermal reservoir" - submitted to the Fourth Conference on Acoustic Emisaion/Mlcroseismic Activity in Geologic Structures and Materials, Pennsylvania State University, 22-24 Oct., 26 pp. S a r i a R, Hearn K C, Lanyon G N, B a t c h e l o r o f Mines Geothermal E n e r g y P r o j e c t

A S (19855) - " M t c r o s e t s m i c R e s u l t s " Phase 2A r e p o r t , Group I I , P a r t 8, V o l .

Camborne S c h o o l 1, 228 p p .

Batchelor A S (1983) - "Hot DrY Rock reservoir stimulation in the UK: an extended summary" Proceedings of the Third International Seminar on the Results of EC Geothermal Energy Research, M u n i c h , 29 Nov - 1 Dec 1983, 681 - 711. Bstchelor A S (ed.) (1984) - "Microseismic System" - Cambor~e School of Mines Geothermal Ener
Dennis B R, Koczan S P, Stephani E L (1985) - "High-temperature borehole instrumentation" Los Alamos National Laborator~J report LA-10558-HDR, 46 pp. o f m t c r o e a r t h q u a k e s accompanying h y d r a u l i c Fehler M C, Same D A ( 1 9 8 5 ) - " C h a r a c t e r i s t i c s fracturing as determined from studies of spectra of seismic waveforms" - Geothermal Resources Council Transactions, 9, Part II, 11-16 House L, K e p p l e r H, K a t e d a H (1985) - " S e i s m i c s t u d i e s o f a m a s s i v e h y d r a u l i c f r a c t u r i n g e x p e r i m e n t " - Geothermal Resources C o u n c i l T r a n s a c t i o n s , ~, P a r t I I , 105-110

Keppler H, Pearson C F, Potter R M, Albright J N (1983) -_"Microearthquakes induced during hydraulic fracturing at the Fenton Hill HDR site : the 1982 experiments" - Geothermal Resources Council Transactions, L 429-433 L e y d e c k e r G (1981) - " S e i s m i s c h e O r t u n g h y d r a u l t s c h e r z e u g t e r BrdJche im GeothePmik F r a c P r o J e k t Falkenberg" - Bericht der Bundesanstalt fur Gec~issenschaften und Rohstoffe, Hannover, Archly NP.86549, 113 pp.

Pearson C F (1981) - "The relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks" - J.Geophys.Res., 86. 7855-7864