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PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 24-26, 2014

SGP-TR-202

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Testing the Application of Low Amplitude Seismic Emission Analysis for Detection of

Microseisms Generated by Geothermal Fluid Flow Through Fractures Underlying the

Western Flank of Newberry Volcano, Oregon

Albert F. Waibel1 and Zachary S. Frone

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[email protected], [email protected]

Keywords: Newberry Volcano, seismic

ABSTRACT

Davenport Resources was awarded a DOE grant 109 in 2010 to conduct a combination of traditional and innovative geothermal

exploration tools on Newberry Volcano. An important component of this grant was the testing the application of Low Amplitude

Seismic Emission Analysis (LASEA) for detecting and locating low amplitude noise generated by geothermal fluid flowing

through formation fractures. Davenport Resources engaged Apex HiPoint (now Sigma3) to deploy two arrays of seismometers in

cased shallow wells on the western flank of Newberry Volcano. The first array was deployed in December of 2011. The second

array was deployed in September, 2013, and was coordinated with flowing well NWG 46-16. Final analysis is not available, as

processing of data is still on-going. Results to date, however, show signals from the vicinity of NWG 46-16. Additional analysis of

the large data base should show a correlation or lack of correlation between the signals and the timing of the well discharge.

1. INTRODUCTION

Davenport Resources (Davenport) was awarded a DOE grant 109 to test a combination of traditional and innovative exploration

tools for the identification of blind geothermal targets in a volcanic terrain. The test location was the western flank of Newberry

Volcano in central Oregon (Figure 1). Data from drill holes had identified a large thermal anomaly underlying the west flank. Four

deep exploration test wells had been drilled in the northern portion of the western flank. Three of the four holes intersected rock

with measured temperatures from 288°C to greater than 315°C, though no evidence of fracture interconnectivity (wells CE 86-21

and CE 23-22, drilled by California Energy Company and well NWG 55-29, drilled by Davenport). The fourth well, NWG 46-15,

drilled by Davenport, intersected geothermal fluid-bearing fractures with comparable temperature/depth values as the three other

deep wells. This well was not fully tested due to formation problems near the 5,000 ft. depth, below the casing (Waibel et a., 2012;

Waibel, 2013). However this well flowed on its own without stimulation when opened in September 2013.

Zucca and Evans (1992) identified areas within the Newberry Volcano caldera and under the western flank they inferred to host

two-phase geothermal fluid (Figure 2). The inference of boiling geothermal fluid was based on interpretations of seismic velocity

and attenuation. Davenport's geothermal discovery well, 46-16, complements the conclusions of Zucca and Evans.

A critical question for the Davenport scientific team was: What is the geometry of the hydrothermal fracture system identified by

this one-well-point discovery? Results of the MT survey provided no insight into the resolution of this question, as the location of

this hydrothermal cell intersected by well 46-16 was not identified by the MT data. The efforts by Zucca and Evans lead the team to

consider seismic techniques as a possible solution. As part of the Davenport DOE Grant 109 program the team proposed a trial

adaptation of a passive seismic tool used in the oil and gas industry for the location and geometry imagery of fractures hosting fluid

flow. Apex HiPoint (now Sigma3) worked with the Davenport team to design a test program using their newly patented Low

Amplitude Seismic Emission Analysis (LASEA) program. The low-amplitude seismic emission array had been successfully

deployed by Apex Highpoint/Sigma3 in the oil and gas industry to identify the location and geometry of fluid flow within natural

and induced fractures. This experimental program was designed as a test effort to adapt technology from the oil and gas industry to

hydrothermal exploration. Initial questions regarding transferring this tool to a volcanic geothermal environment were: (1) would

the array be able to pick up deeper signals at reasonable distances from each monitoring hole to be useful, and (2) would surface

microseismic noise drowned out any deeper low amplitude signal sources?

Waibel and Frone

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Figure 1: Location Map showing Newberry Volcano in central Oregon. Also shown are the locations of major volcanoes of

the Cascade Range.

Figure 2: Zucca and Evans inferred two-phase hydrothermal location under the caldera and the west flank of Newberry

Volcano (Figure from Zucca and Evans, 1992).

2. METHODOLOGY

Davenport rotary-drilled the upper 700 ft. of proposed temperature gradient wells and cemented casing in each. The location of

these wells were originally picked and permitted to resolve subsurface temperature anomaly boundary questions, and were adapted

for the seismic monitoring test. One additional well site, to the north of NWG 46-16 was proposed and permitted for seismic

monitoring. Funding for this hole was never provided. The Sigma3 LASEA survey deployed three-component 4.5 Hz digital

geophone sondes within the cases wells. Each observation well contained 11, 12, or 13 3-component digital geophone sondes

spaced at 50 ft. The geophones sondes (Figure 3) were manufactured by GeoSpace of Houston, Texas, and are the digital

instruments used by Sigma3 for passive microseismic work in the oil and gas industry. The survey data were continuously

recorded every 0.5 milliseconds but were broken up into records of 10-second units. Each 10-second unit contains all data from

each of the geophones for that time period.

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The field execution of the LASEA test was divided into two arrays in order to accommodate other activities in the area and to

accommodate equipment availability. The southern array with four monitoring wells is located in the southern portion of the

western flank (Figure 4). This is a "blind test" area with no evidence of subsurface fluid flow. Only one temperature gradient well

had been drilled to depth in the area prior to Davenport's work. Equipment deployment and monitoring in the southern array was

conducted in December of 2011. The northern array with five monitoring wells is located in the northern portion of the western

flank (Figure 4). The northern array is located in the vicinity of the two deep exploration wells drilled by Davenport in 2008 (Figure

4, NWG 46-16 and NWG 55-29). It was anticipated by the Davenport scientific team that the northern array would be a true

controlled test of the LASEA microseismic experiment. NWG 46-16 had intersected geothermal fractures (Waibel et al., 2012) and

had a closed-in well-head pressure of 600 psi. During deployment of the geophones in the northern array NWG 46-16 would be

opened at specific intervals to create controlled-timing fluid flow within formation fractures intersected by the well and fluid flow

up the well and through the venturi created by the formation bridge at 5,000 ft. within the well. The fluid flow into and up the well

bore, and the fluid pressure and velocity changes at the venturi would provide subsurface fluid flow signals from known source

points at known times.

Figure 3: One of the high-grade digital geophones, manufactured by GeoSpace of Houston, Texas, deployed by the Sigma3

field team in the monitoring wells.

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Figure 4: Map of the upper western flank of Newberry Volcano, with seismic monitoring holes and deep exploration holes

identified.

3. RECORDING AND ANALYSIS

Microseismic monitoring within the four wells of the southern array (Figure 4) began on the 23rd of December 2011 and continued

through the 30th of December 2011. The data were processed over the following three months with a grid spacing of 100 m in both

north-south and east-west directions. Concerns regarding the ability of the array to receive signals from more than one or two km

were alleviated by the data set showing cultural industrial noise from sources at least 10 km distance (Figure 5). The data set also

showed that processing was dealing with an extremely large volume of signals rather than too few signals. The dominant

microseismic signals that were identified on a recurring basis, with a duration period of near 27 hours, clustered around a north-

northwest strike (Figure 6).

Figure 5: The four highest-amplitude energy clusters are outlined here with start and end times of 0900 to 1600. Given the

regularity of the start and end times of these periods they are almost certainly man-made cultural noise related to daily

business operating heavy equipment somewhere in the area. (from Apex Highpoint/Sigma3 report to Davenport)

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Figure 6: Southern array shows a dominant trend of episodic micro-seismic signals observed in the processed data. The

four larger dots identify the four monitoring well locations. The smaller dots represent resolved signal sites. The grid

spacing is 100 m. Figure from Apex HiPoint, 2012.

Microseismic monitoring of the wells in the northern array occurred from the 8th to the 15th of September 2013. The geophones

were deployed on the 7th and the morning of the 8th of September. Cyclic flowing of NWG 46-16 occurred on the 8th, 9th and

10th. Monitoring continued until the afternoon of the 15 to watch background signals from the NWG 46-16 area, and to monitor

testing that Alta Rock was conducting for their EGS efforts in well NWG 55-29. NWG 46-16 was expected to flow non-

condensable gas during the test. The team was pleasantly impressed that the well also flowed liquid, dominated by drilling mud that

had been left in the hole (Figure 7).

Table 1, 46-16 flow cycle timing:

Date Time Comments

8 September 2013 1410 opened well, flowing gas phase

1605 well started flowing liquid phase

1835 shut well in

9 September 2013 0845 Opened well, flowing gas phase

1122 mixed liquid and gas flow

1144 liquid phase flow

1700 shut well in

10 September 2013 0936 Opened well, flowing gas phase

1017 Oscillating gas and liquid phase flow

1515 Shut well in.

The initial seismic energy computations were made for a single horizontal plane at an elevation of -1250 m (-4101 ft) relative to

mean sea level. This corresponds to approximately 10,000 ft below the mean surface elevation in the area of interest. The size of

the grid spacing for the northern array data processing is 200m in both north-south and east-west directions (the spacing for the

southern array was 100m). The grid plane was 6 km by 6 km centered near the middle of the 5 observation wells, placing well

NWG 46-16 at the northern boundary of the grid.

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Figure 7: Fluid flowing from well 46-16 through a four-inch bleed line, approximately 45 minutes after the liquid-phase

flow commenced. (8 September 2013).

Figures 8, 9 and 10 have been prepared by Sigma3 for an interim report. Figure 8 shows high amplitude signals identified during an

8 hour slice prior to opening well NWG 46-16 the first time. Figure 9 shows the distribution of low amplitude signals during a 12

hour slice which includes the 4 hour 25 minute flowing of well NWG 46-16. Figure 10 shows the distribution of very low

amplitude signals during a 14 hour slice, 5 days after well NWG 46-16 flowed.

Figure 8: Large amplitude signals detected, with a bandpass filter of 15 to 52 HZ, by the northern array prior to opening

well NWG 46-16. The activity detected are likely related to EGS water injection tests run by Alta Rock. Figure from Sigma3

interim report.

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18© SIGMA3 Integrated Reservoir, LLC. All Rights Reserved.

Lowest Amplitudes: September 09 – General activity increases throughout the area and in particular near observation well nn07 which had previously been a particularly quiet location.

nn07

nn09 nn21nn24

nn19

55-29

46-16

Sept 9: 12 AM

Sept 9: 12 PM

Figure 9: Low amplitude signals detected, with a bandpass filter of 15 to 52 HZ, by the northern array during and

subsequent to flowing of well NWG 46-16. Figure from Sigma3 interim report.

Figure 10: Very low amplitude signals, with a bandpass filter of 15 to 52 HZ, detected by the northern array 5 days after the

last flowing of well NWG 46-16. Figure from Sigma3 interim report.

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Figure 11: Grid nodes for this calculation were within the red boundaries shown with a 100 m grid. Seismic data had a

bandpass filter of 40 to 135 Hz. Please note signal pattern in figure 9 which includes this time frame.

4. CONCLUSIONS

The results of the southern array are inconclusive with regard to fluid flow. A dominant recurring pattern of signals with a north-

northwest strike were recorded. The interpretation of this pattern, however, is open speculation. This strike and location do not

match any observed physical surface trends, and does not show up on the gravity, aeromagnetic or MT surveys. There is no obvious

unique interpretation of this signal pattern that is supported by corroborating evidence. Fluid movement along a permeable plane is

possible. A more favored interpretation is that of a planer structure which is capable of reflecting signals from unknown sources.

This structural interpretation is sympathetic to the strike of a volcanic vent trend which passes through the caldera and extends well

to the NNW on the flank of the volcano. The strike is also similar to the strike of the structural boundary between the La Pine

graben along the western edge of the volcano (Waibel et al., 2012).

The northern array should have been a good controlled source test for evaluation of LASEA as a potential hydrothermal exploration

tool. Key to the test was to have monitoring wells located to provide good geometric coverage around well NWG 46-16. Important

monitoring to the north of the well did not happen. All of the completed monitoring wells are to the south of the well, providing

coverage for only a 120 degree arc, leaving a 240 degree arc around the well uncovered. Seismic signals in the vicinity of well

NWG 55-29, particularly the high amplitude signals, are clearly identified and are likely related to EGS fluid injection efforts

designed to induce formation shearing (Figure 8).

The products of data processing to date do not show unique evidence that the array has been able to identify unambiguous signals

generated by movement of hydrothermal fluid within the well bore or in fractures feeding fluid to the well bore. Figure 11 shows

the signal distribution during the second flow episode of well NWG 46-16, though processing data from only the area immediately

in the vicinity of NWG 46-16. Figure 9, which includes the time frame of figure 11, shows signals in the vicinity of NWG 46-16

indistinguishable from a more regional pattern. The high amplitude results prior to flowing well NWG 46-16 shown in figure 8

identify no signals in the vicinity of well NWG 46-16. Low amplitude signals from 5 days after the well had been flowed (Figure

10) show similar signal patterns covering broad areas, including in the vicinity of well NWG 46-16. Comparative processing of

data from non-well-flowing periods, or of the other two flow events, has yet to be completed, leaving the empirical evaluation of

this test incomplete.

A number of issues cannot be addressed until the complete data processing is available. The preliminary results shown in figures 8,

9 and 10 are not encouraging. The results shown in Figure 11 may be encouraging, though the uniqueness of this pattern to NWG

46-16 flow events has yet to be demonstrated. Additional data processing by Sigma3 will cover a depth range from 5,000 ft. to

15,000 ft. with more constrained timing blocks. For now the scientific team eagerly awaits the final data processing results.

Waibel and Frone

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REFERENCES

Apex HiPoint, 2012, Data processing Results: Davenport Newberry Volcano Project. Unpublished report submitted to Davenport

Newberry.

Sigma3, 2014, An Interim Report for the 2013 Newberry Low-Amplitude Passive Seismic Monitoring Project. Unpublished report

to Alta Rock Energy.

Waibel, A., Frone, Z and Jaffe, T.: Geothermal Exploration at Newberry Volcano, Central Oregon, Geothermal Resources Council

Transactions (2012).

Waibel, A., Beard, L., and Oppliger, G.: The Evolving Role of MT in Geothermal Exploration at Newberry Volcano, Oregon,

Proceedings, 38th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2013).

Zucca, J. and Evans,J.: Active High-Resolution Compressional Wave Attenuation Tomography at Newberry Volcano, Central

Cascade Range, Jour. Geophys. Res., vol. 97, no. B7, (1992), 11047-11055.