Seismic imaging in the Krafla high-temperature geothermal field, NE Iceland, using zero- and far-offset vertical seismic profiling (VSP) data

https://doi.org/10.1016/j.jvolgeores.2018.02.016Get rights and content

Highlights

  • First VSP experiment successfully applied in a high-temperature geothermal field, Iceland

  • Zero-offset air gun and far-offset explosives provide signals up to 40 Hz down to 2 km

  • Key volcanic sequences are imaged using a one-component corridor stack and multicomponent Kirchhoff depth and Fresnel volume migration

Abstract

Among geothermal exploration methods, active surface seismic methods have played only a minor role to date. Especially in high-temperature volcanic systems, reflection seismic data often reveal poor delineation of volcanic features, due to the internal heterogeneity of volcanic sequences. To enhance the vertical resolution, one possibility is the application of downhole seismic methods like vertical seismic profiling (VSP). A test experiment was carried out in the Krafla high-temperature geothermal field, NE-Iceland, to assess the ability of VSP to image subsurface structures, such as fractures, zones of high permeability, magmatic bodies, and zones of supercritical fluids and steam. Logging in such hostile environments is technical challenging in many aspects, but mainly due to the high temperature impact on the downhole electronic components of the measuring equipment. This requires a thorough pre-examination and implementation of the measurement, especially to avoid delays and tool failures. This paper presents results of zero- and far-offset VSP data from the K-18 borehole from within the Krafla caldera, which reveal good correlation with the surrounding lithology. The raw three-component seismic data display a good signal-to-noise ratio and dominant signal frequencies between 20 and 40 Hz, down to c. 2200 m depth, for air gun and explosive sources, respectively. A zero-offset source comparison was also conducted to assess the use of different impulsive sources for future VSP surveys in similar settings. By applying a standard VSP processing, we identified stratigraphic boundaries between lavas, hyaloclastites, and intrusions, which are in good agreement with existing well data. For the zero-offset VSP, both P- and S-wave velocity models were calculated and a depth-converted corridor stack was determined. In addition, multicomponent Kirchhoff depth migration and Fresnel volume migration were tested around the borehole. The 3D results are promising, but the specific shape and lateral extent of the reflectors could not be determined due to the restriction to only two sources and the insufficient spatial coverage (aperture). Our study demonstrates that vertical seismic profiles can clearly detect variations in the subsurface volcanic stratigraphy in high-temperature geothermal fields. A more detailed reservoir characterization can be achieved by further data integration, enhanced survey design including more source positions, and improved processing and imaging techniques, such as full-waveform inversion.

Introduction

Although seismic applications for geothermal exploration have already been carried out (e.g., Hloušek et al., 2015; Riedel et al., 2015; Schmelzbach et al., 2016, and references therein), active seismic exploration methods still play a minor part, in particular, in high-temperature (HT) volcanic geothermal systems, which are often found along active tectonic rift zones exhibiting temperatures of 200 to 300 °C in 1 to 2 km depth (Wohletz and Heiken, 1992). Seismic experiments within similar volcanic sequences have shown that seismic wave energy suffers from strong attenuation and scattering effects due to the inhomogeneous nature of volcanic formations which often include a complex combination of primary fragmentation, emplacement brecciation and cooling contraction jointing, along with secondary reworking, alteration and fracturing (Planke et al., 2000; Planke and Cambray, 1998; Planke and Flóvenz, 1996).

Mapping fractures and permeable zones is crucial in geothermal reservoir evaluation and for ensuring future production rates (e.g., Sausse et al., 2010). Surface exploration methods, such as magnetotelluric and wide-angle refraction profiling, despite their good lateral coverage, provide only a low vertical resolution. Geophysical logging, instead, has a high vertical resolution but only a limited depth of investigation, commonly of an order of a few decimeters. In addition, invasion and near-wellbore damage caused by the drilling process or heterogeneous volcanic facies may adversely affect well logs in volcanic settings (Millett et al., 2016). Vertical seismic profiles combine active seismic sources on the surface with seismic receivers lowered in a borehole, within the reservoir (Hardage, 2000). VSP therefore serves as a tie between surface methods (good lateral coverage, weak vertical resolution) and borehole logs (high vertical resolution, low lateral extent) (e.g., Christie et al., 1995; Hackert and Parra, 2002; Stewart, 2001). It can be used to image structures on reservoir scale, away from and below the borehole, which, for example, has successfully be shown in the Soultz-sous-Forêts geothermal system, France (Place et al., 2010, Place et al., 2011).

In VSP but also in borehole geophysical logging in general, tools including sensitive sensors and electronics are constrained to a temperature and pressure range exposed over a certain period of time. A downhole geophone, as for instance used in this experiment, may withstand a maximum tool temperature of 150 °C, over 6 to 8 h. Higher temperatures or longer time periods can cause data transmission and recording errors, or even tool damages. Latest developments also provide tools up to 200 °C. Aggressive borehole fluid constituents (e.g., H2S, CO2) may also damage the O-rings which prevent fluid break-ins into the tool. Especially in geothermal context, this requires a sufficiently well-known temperature profile (log) at depth and in time. Temperatures have to be monitored and acquisition has to be interrupted if temperatures are too high, which result in delays and, ultimately, additional costs. Alternatively, the increasing development of the distributed acoustic sensing technology has also shown promising results in the field of geothermal exploration (e.g., Reinsch et al., 2015). However, for hanging cables which are not cemented behind the casing, these methods are not favorable due to low signal-to-noise ratios.

The applicability of VSP as a method for subsurface mapping in geothermal volcanic environments is investigated by the European Union's project IMAGE1 (Integrated Methods for Advanced Geothermal Exploration). In this regard, the Krafla high-temperature geothermal field (Fig. 1) provides a suitable test site because of fairly good knowledge about the subsurface geology including potential, shallow zones of steam and magma. Among information from >40 wells and surface exploration work, it also offers viable conditions (well access, cooling of the wells, personnel infrastructure) for a VSP experiment within the time and cost constraints of the project. The geothermal field is located directly on top of the onshore active-volcanic rift system of the Mid-Atlantic Ridge, in northeastern Iceland (Thordarson and Höskuldsson, 2002, chap. 7). The VSP experiment that forms the basis of this study comprises the first of its kind carried out in a high-temperature geothermal field on Iceland. Planke and Flóvenz (1996), however, describe a previous VSP survey in a low-temperature field in Eyjafjardar where they studied the seismic velocity and anisotropy from two low-temperature wells. Other VSP surveys successfully carried out in geothermal fields, outside of Iceland, are documented, for example, by Cameli et al. (1995), Lorenzo et al. (2015), Nakagome et al. (1998), Place et al. (2011), and Riedel et al. (2015). In a more recent study, Reiser et al. (2017) gained valuable insights about the design of VSP experiments to image fracture zones over hard-rock basement geothermal environments.

One important objective of the IMAGE project was to develop and test exploration methods such as the VSP method as a complementary, active seismic exploration method, in magmatic environments. Herein, we investigate the acquisition of VSP data with an air gun and explosive source, the expected signal quality, and its ability to get proper reflections from the subsurface. The investigated data are based on the VSP-test experiment carried out at Krafla in May and June 2014. More precisely, we discuss three-component zero- and far-offset data from the K-18 borehole and, furthermore, test the ability to image reflections away and ahead from the borehole using Kirchhoff depth migration (KDM) and Fresnel volume migration (FVM). VSP surveys in general require repeatable sources which can be excited several times, without changing the signal phase and amplitude. Active seismic for geothermal exploration may also face logistical problems such as the proximity to plant facilities or surface conditions (e.g., hot springs, borehole cellar, snow etc.). Therefore, two additional zero-offset sources (detonating cord, shot-hole explosive) were tested in a source comparison, in order to assess their potential for future VSP surveys.

The here presented data and resulting processing involving 3-D migration serve as a first imaging test experiment as part of the IMAGE project. With respect to available costs and the difficult area, the experiment was limited to two major zero-offset shot surveys and several low-priority offset profiles having only a fraction of the receiver coverage. Therefore, to provide a first feasibility study, we selected the two most representative shots based on the highest depth coverage along the borehole and the best signal-to-noise ratio of the raw data. Our results focus on the basic imaging potential in the scope of this particular geological and experimental setting.

Section snippets

Geological setting

The study area is part of the Krafla volcanic system, located in the neovolcanic zone in northeast Iceland (Fig. 1). Being part of the onshore extension of the Mid-Atlantic Ridge, this active spreading zone is characterized by several N-S elongated volcanic systems, each associated with a central volcano and swarms of linear volcanic fissures composed of tensional cracks, normal faults, and graben structures (Thordarson and Höskuldsson, 2002). The Krafla volcanic system is characterized by a

Methods and data acquisition

Vertical seismic profiling uses active seismic sources on the surface and receivers placed in a borehole to explore and image structures of the subsurface. Having a long tradition in the oil and gas industry, VSP results are often compared with surface reflection seismic profiles and geological logs (Chopra et al., 2002; Poletto et al., 2013; Stewart, 2001). Due to its geometry, with receivers at depth, smaller Fresnel zone, and generally wider frequency bandwidth, VSP provides a better

Results

Three-component vertical seismic profiling data were successfully recorded in well K-18 of the Krafla high-temperature field. In terms of signal-to-noise ratio, zero- and far-offset data show a similar, good overall quality, with clear and coherent first arrivals at almost all depths.

The zero-offset seismograms (before wavefield decomposition) show strong direct P-wave arrivals on the vertical component and direct S-wave arrivals on the two horizontal components (Fig. 4). Some P-wave energy,

Discussion

Our results show that VSP surveying offers the chance to image subsurface structures in high-attenuating and scattering media, such as volcanic rocks where surface-based seismic methods (2D/3D profiling) often produce poor results. At Krafla, the signal-to-noise ratio of surface-to-well recordings lies in the 40 dB range. For small offsets, the application of an air gun close to the wellhead displays a good option in such geological backgrounds. For larger offsets (up to 2 km), however, a

Conclusion

We have presented results from a VSP experiment in the K-18 high temperature geothermal well at Krafla. This study comprises the first high-temperature VSP study of its kind on Iceland. The recorded borehole seismic data show an overall good data quality providing coherent first arrivals within a frequency range of 5–40 Hz using a single air gun for the zero-offset and dynamite explosives for the far-offset shots. By comparing different impulsive sources, we have shown that for the zero-offset

Acknowledgments

The research leading to these results has received funding from the European Community's Seventh Framework Program under grant agreement No. 608553 (Project IMAGE). We thank Landsvirkjun, the operator of the Krafla geothermal field, for technical and logistical support during the survey. We also thank the Operational Support Group of the International Continental Scientific Drilling Program (ICDP) for their technical support. We further acknowledge the support from the Research Council of Norway

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