Acoustic Borehole Research Articles

Uranium anomaly in groundwater of the hard rock aquifer system in southeast Brazil

This study presents a hydrogeological characterization that fills knowledge gaps in an area where a complex geological scenario connects two fractured (granite/metamorphic rocks and diabase) and one porous (sedimentary rocks) aquifer systems. Previous groundwater studies from the four local deep supply wells suggested the sedimentary aquifer as the source of uranium (∼60 μg/L) in one of the wells. Despite the water extraction interruption from this well, this finding needed to be supported by evidence provided by the geological and structural characteristics of the aquifers. Data collected through video inspection, caliper, optical (OPTV), and acoustic borehole televiewers (ATV) revealed the predominance of the crystalline aquifer in the area. As recorded by video inspection, the well's borehole with high U intersects the crystalline rocks at several open fractures and presents the highest gamma radiation values. Therefore, we infer that the origin of dissolved U is the weathering of primary minerals locally occurring in those rocks. The U leaching and complexation as calcium-uranyl-carbonates favors its solubility in that specific aqueous medium, as shown by hydrogeochemical modeling using PHREEQC. Additionally, the major ions and trace elements concentrations allow the division of the wells into two groups: a) two with higher U concentrations, influenced by the granite/metamorphic aquifer and possibly longer residence times, and b) the other two wells with lower U concentrations, higher nitrate concentrations, influenced by the diabase aquifer, possibly have younger residence times.

A Universal Data Format for Wellbore Logs

This paper describes a proposed high-specification standard format that is ideally suited for the data management of definitive records of wellbore logs. For this reason, it is a good standard for data exchange between applications. The format is suitable for complex three-dimensional (3D) data, including those generated by deep azimuthal resistivity (DAR) and ultradeep azimuthal resistivity (UDAR) tools, acoustic borehole reflection images, vertical seismic profiles (VSP), borehole imaging tools, multifingered caliper logs, and array data with multiple depths of investigation. It is applicable for use with logging-while-drilling (LWD) and wireline-conveyed logging tools. The format also naturally collapses down when utilized to store simple conventional logs that contain one value per depth in the wellbore. The proposed format provides spatial details of every data point collected by or interpreted from a wellbore-logging tool. The position of each data point is defined by reference back to the measure point of the sonde, which in turn is defined by the wellbore deviation survey and its coordinate reference system (CRS). Each data point in space may have an unrestricted number of parameters. An example might be most likely horizontal and vertical resistivity, maximum value based on uncertainty, minimum value based on uncertainty, and flags indicating the data position with respect to depth of detection (DOD). The new proposed format is so versatile. It is suitable as an Open Group Open Subsurface Data UniverseTM (OSDUTM) standard to store and exchange all data measured by logging tools in a wellbore and can possibly be extended to include all well data (for example, core, cuttings, and more). The proposed format requires a detailed definition so that computer scientists can implement it in applications used for subsurface modeling. The OSDU will also require this detailed definition in order to adopt it as a standard.

Effects of a filled discontinuity in a rock mass on transmission losses of compressional and shear wave of full-waveform sonic log data

The presence of discontinuities in a rock mass affects the propagation of compressional (P) and shear (S) waves recorded by an acoustic borehole logger. The purpose of this study is to develop a new approach for characterizing such discontinuities using the full-waveform (FW) sonic tool. The approach focuses on evaluating the sensitivity of P-wave and S-wave transmission losses to the mechanical and geometrical properties of the discontinuities intersecting the borehole. Numerical simulations enabled us to quantify the impact of a discontinuity on full-waveform sonic log data using two transmission loss factors—velocity variation and amplitude attenuation—associated with P- and S-waves. These factors were determined by comparing the arrival times and amplitudes of waves propagating across discontinuous and intact media. We performed a sensitivity analysis for a wide range of discontinuity widths, lengths, P- and S-wave velocity, and density of the infilling material, and for different mechanical properties of the embedded rock. S-waves were found to be generally more sensitive to the presence of a discontinuity than P-waves. The discontinuity length influences both the compressional and shear wave responses depending on their relation to the prevailing wavelength. We demonstrate that velocity variation is more indicative of the discontinuity width than amplitude attenuation. Overall, the analysis of our dataset indicates that the P- and S-wave transmission loss factors provide complementary information, as they tend to be more sensitive to a different discontinuity property. This finding, in turn, may lead to the development of integrated algorithms for robustly characterizing discontinuities.

Using geophysical log data to predict the fracture density in a claystone host rock for storing high-level nuclear waste

Previously drilled boreholes of a host rock for a potential nuclear waste repository in Hungary revealed a highly fractured claystone rock body. A crucial step for characterizing the hydrodynamic behavior of such a fractured reservoir is fracture identification and accurate calculation of the fracture density. Although acoustic borehole televiewers provide a reliable base for determining the fracture density, older boreholes usually lack such data. However, conventional borehole geophysical measurements are often accessible in such cases. The aim of this study was to identify any correlations between well log data and fracture density. Multiple linear regression analysis was performed on data from two boreholes penetrating the Boda Claystone Formation in southwest Hungary. The upper section of the BAF-4 borehole was used for training, where the fracture density was estimated with a fit of R2 = 0.767. The computed regression function predicted the fracture density with high accuracy in both boreholes for all intervals with typical lithological features. However, in some sections where anomalous well log data indicated changes in the lithology, the prediction accuracy decreased. For example, the function underestimated the fracture density in sandy intervals.

The characteristics of natural open fractures in acoustic borehole image logs from the pre-salt Barra Velha formation, Santos Basin, Brazil

Natural open fractures have been reported in the Barra Velha Formation (BVE) in the pre-salt lacustrine carbonate reservoirs in the Santos Basin offshore Brazil. These fractures contribute to excess permeability i.e., well test permeability is significantly larger than core permeability or log derived permeability, and they are therefore important for the fluid flow in the reservoirs during production and injection. In this paper we summarize the characteristics of natural open fractures in acoustic borehole image logs (BHI) in 19 wells from the BVE reservoirs and discuss their distribution, occurrences and formation. The data quality is overall good and consistent so that results can be compared between wells. The P21 fracture intensity, which is fracture trace length per area (bore hole wall) was used, instead of Terzaghi corrected P10 fracture intensity which do not distinguish between minor and major fractures. Mean P21 fracture density shows a variation of 2 orders of magnitude between interpreted wells. Data binning was used in the analysis of P21 fracture intensity vs petrophysical parameters to reduce the effect of sampling volume being much smaller than the representative elementary volume (REV) of the fracture pattern. P21 fracture intensity shows a positive correlation with silica content, which varies in the carbonates, whereas the porosity is less important for fracture intensity. The occurrence of fractures is also associated with vug-fraction estimated from the acoustic BHIs. Fractures are also frequent in fault damage zones, above and below cavities. The main orientation of the fractures is NNW-SSE, that is in the same direction as the dominating fault trend, and this indicates a structural control on fracture formation. Natural open fractures are rare in high porosity rocks with a shrubby or grainy texture, and also in intervals with a conglomeratic appearance in the BHI image logs.

Water flux profiling in fractured rock boreholes with an In-Well Point Velocity Probe (IWPVP)

• First field measurements with the fracture adapted IWPVP reached 38 m below ground surface. • Depth profile of water fluxes compares well with PFM, ORP, and FLUTe liner profiles. • Demonstrated calibration of probe measurements with acoustic borehole televiewer data. • IWPVP shown to be a simple, cost-effective tool for measuring flow in fractures. Flow in a fractured rock aquifer beneath the Edwards Air Force Base in California was characterized by depth profiling two wells with In-Well Point Velocity Probes (IWPVPs). The probes, which were originally designed for use in porous media wells, were optimized for use in fractured rock wells and to meet several challenges including: sampling depths up to 38 m; high background site-water salinity; and variable well construction (screened vs open borehole, and well diameters of 7.62 cm and 15.8 cm). The IWPVP measures water flux inside the probe (internal flux), which is converted to fluxes in the aquifer through calibration. At this site, the internal fluxes ranged from 3 to 53 m/d. The channeled internal design of the probe inherently provides information about general flow directions and allows further interpretation of specific flow direction within ±15°, which at this site was generally consistent with the expected regional flow direction. Notably, a significant shift in the flow system was observed following a rain event (up to 180° shift in the flow direction). The IWPVP identified highly transmissive zones in the fractured rock, which were independently confirmed by passive flux meters and oxidation-reduction potential sensors. Transmissivity profiles determined during a FLUTe (Flexible Liner Underground Technologies™) liner deployment also showed similarities in the depths of potentially high flow rates, particularly in the shallower portion of the well (<20 m). With additional information on fracture apertures provided by an acoustic televiewer, the internal fluxes were converted to water fluxes in the observed fractures, indicating seepage velocities in the rock aquifer between 3.7 m/d and 22.4 m/d.

Heat flux estimation from borehole temperatures acquired during logging while tripping: a case study with the sea floor drill rig MARUM-MeBo

During trip out of the drill string at the end of a drilling operation (logging while tripping) borehole temperatures can be measured without the need for additional operational time. A simple interpretation of the measured borehole temperatures is difficult due to the interfering influences of the drilling operations, mainly due to flushing the borehole during drilling. In this study, we present borehole temperature data from drilling campaigns with the sea floor drill rig MARUM-MeBo200 at the Danube Deep Sea Fan (Black Sea) and west of Taiwan (South China Sea). The temperature measurements were conducted with a PT1000 temperature sensor which is integrated in a memory acoustic borehole logging tool. We developed a modeling approach in order to simulate the drilling perturbations and subsequent evolution of the temperature field within the borehole. By fitting the model data to the measured time dependent temperature depth profiles, we estimated the undisturbed heat flux at the drill sites. This study shows that knowledge of the pattern of drilling operations with alternating phases of drilling/flushing and drill string handling is crucial for comparing temperatures measured during logging while tripping and simulated temperatures.

Elastic least-squares migration for quantitative reflection imaging of fracture compliances

To quantitatively image fractures with high resolution, we have developed an elastic least-squares migration (LSM) algorithm coupled with linear-slip theory, which accurately addresses seismic wave interaction with thin structures. We derive a linearized waveform inversion using the Born approximation to the boundary integral equation for scattered waves, including linear-slip interfaces for P-SV and SH wavefields. Numerical modeling tests assuming a laboratory-scale fracture where a 20 cm long fracture is illuminated by waves with a 50 kHz center frequency show that our LSM successfully estimates fracture compliances. Furthermore, due to the presence of coupling compliances at the fracture, the results using our LSM show better images than those using the conventional LSM estimating the Lamé constants. We also numerically illustrate that our LSM can be successfully applied to dipole acoustic borehole logging data with 3 kHz center frequency for single-well reflection imaging of a 10 m long, dipping fracture embedded in a random background. Finally, we apply LSM to laboratory experimental data, measuring PP reflections from a fluid-filled fracture. We confirm that the estimated fracture compliances correspond well to those estimated by earlier amplitude variation with offset inversion. Furthermore, the LSM resolves the spatially varying fracture compliances due to local filling of water in the fracture. Because the linear-slip theory can be applied to thin structures in a wide range of scales, high-resolution imaging results and estimated fracture compliance distributions will be crucial to further address small-scale properties at fractures, joints, and geologic faults.

Az ImaGeo magszkennelés módszerei egy Mecseki fúrás nagyfelbontású értelmezésének példáján

A saját fejlesztésű ImaGeo rendszer terepen, fúrásokban és bányatérségekben is nagy felbontású, digitális földtani dokumentációt és 3D földtani-tektonikai adatfelvételt tesz lehetővé, normál és UV fény megvilágításban egyaránt. A fúrásokban kapott adatok mélyfúrás-geofizikai akusztikus (Borehole televiewer, BHTV) vagy ellenállás (pl. Formation MicroImager, FMI) adatsorok felhasználásával újra orientálhatóak és ezzel részletes földtani elemzések megvalósítása válik lehetővé a valós térben. A rendszer részei a Magszkenner, a Fotórobot és a LIPS (Lézer gerjesztésű plazma spektrométer). Jelen cikk az ImaGeo rendszer módszereinek bemutatása mellett esettanulmányt közöl az Ibafa, Ib–4 fúrás mezozoos rétegsorának magszkenneléses eredményeiről különös tekintettel a Jakabhegyi Homokkő Formáció elemzésére. A Formáció vizsgálatát az adatok 45/13°-os (dőlésirány/dőlésszög) tektonikusan kibillentett helyzetből történt visszabillentés után valósítottuk meg. A magszkennelésből származó szemcseméret, rétegvastagság, dőlésirány és dőlésszög eloszlások vizsgálata alapján a Jakabhegyi Homokkő Formáció harántolt rétegsorát 5 szakaszra lehetett bontani. A szakaszhatárok nem korrelálnak a földtani dokumentáció szakaszainak határaival. A visszabillentett dőlések DDK, D és DNy felé mutatnak, de bizonyos mélységszakaszokban a tisztán Ny-i és K-i irányok is jelentősek. Mindezeket üledékszállítási főirányként értelmezzük. A szállítási irányok a formáció egészét tekintve is széles spektrumon oszlanak el. A rétegvastagság, dőlésirány és dőlésszög adatok ciklicitás elemzését vizuálisan, mintázatok felismerése útján és geomatematikai periodicitás elemzéssel vizsgáltuk. Ezek alapján több, különböző periódushosszúságú (deciméteres, 1, 3 és 8 méteres) ciklust lehetett meghatározni. A ciklusosság megállapítható a lemezvastagságban, a dőlésszögek és a dőlésirányok eloszlásában is. A hosszabb ciklusok leginkább a dőlésirányok eloszlásában mutatkoznak. A dőlésszögekben a vizuális, mintázatokon alapuló és a geomatematikai módszer is a 0,5 m körüli ciklust mutatta ki. A geomatematikai elemzés 2 párhuzamos ciklushosszt mutatott ki a dőlésirányokban és a dőlésszögekben. Ezek 1,3 és ~4,5 m ciklushosszúságú periódusok. A vizuális elemzés feltárt egy mintegy 50 m-es ciklust is, ezt geomatematikai úton nem lehetett igazolni.

Determining fracture properties from acoustic borehole waves: theory and experimental results

A borehole in the subsurface may penetrate rocks which are porous, permeable and fractured. Pressure transients in the borehole will therefore cause viscous fluid to flow into and out of the wall of the borehole. This forced flow consumes some energy and affects the phase velocity and amplitude of the waves traveling in the borehole fluid column. These effects can be evaluated to extract information about the rocks adjacent to the borehole, which is of paramount importance for the oil and gas industry. This work discusses laboratory wave experiments in a 7.5 m long vertical shock tube. Rock samples having a vertical borehole and horizontal fractures are installed in the shock tube and filled with water. The shock tube generates broadband pressure transients in the borehole. These are measured in the borehole at variable depth by means of a sliding pressure probe. Repetitive experiments are combined into borehole microseismograms. In this way borehole wave reflection and transmission coefficients over the fractures are determined. The results show good agreement with low-frequency theory, where it is assumed that the wavelengths are much larger than the tube diameter. Inversely, it is shown that the aperture and the length of the fracture can directly be inferred from acoustic borehole experiments.

Towards the multi-scale characterization of braided fluvial geobodies from outcrop, core, ground-penetrating radar and well log data

Abstract The integrated application of advanced visualization techniques – validated against outcrop, core and gamma ray log data – was found to be crucial in characterizing the spatial distribution of fluvial facies and their inherent permeability baffles to a centimetre-scale vertical resolution. An outcrop/behind outcrop workflow was used, combining the sedimentological analysis of a perennial deep braided outcrop with ground-penetrating radar profiles, behind outcrop optical and acoustic borehole imaging, and the analyses of dip tadpoles, core and gamma ray logs. Data from both the surface and subsurface allowed the recognition of two main architectural elements – channels and compound bars – and within the latter to distinguish between the bar head and tail and the cross-bar channel. On the basis of a well-constrained sedimentological framework, a detailed characterization of the gamma ray log pattern in the compound bar allowed several differences between the architectural elements to be identified, despite a general cylindrical trend. A high-resolution tadpole analysis showed that a random pattern prevailed in the channel, whereas in the bar head and tail the tadpoles displayed characteristic patterns that allowed differentiation. The ground-penetrating radar profiles aided the 3D reconstruction of each architectural element. Thus the application of this outcrop/behind outcrop workflow provided a solid database for the characterization of reservoir rock properties from outcrop analogues.

Study on Structural Plane of Deep Rock Mass in Xinchang Preferred Site for Underground Research Laboratory on Geological Disposal of High-Level Radioactive Waste in China

Structural plane is an important influence factor for the rock mass quality, 3D modelling, borehole hydraulic test and engineering design of underground research laboratory. Xinchang rock mass in Beishan area of Gansu province has been confirmed the preferred site for URL on geological disposal of high-level radioactive waste in China. The characteristics of structural plane in Xinchang rock mass are obtained based on acoustic borehole televiewer and other geological information. The results indicate that the granite rock mass with a high integrity is beneficial for construction of URL. The width of faults around preferred site and incidence in horizontal and vertical decrease as the depth increases. Characteristics of dip and strike of structural plane is the same as other structure in this area. All research results provide meaning information for design of URL and further geological research.

Direct Inversion Algorithm for Shear Velocity Profiling in Dipole Acoustic Borehole Measurements

The evaluation of radial heterogeneities within formations plays an important role in oil and gas exploration and exploitation. Dipole acoustic well logging data are widely used to invert radial shear velocity profiles, for which efficient dispersion analysis algorithms and radial profile inversion algorithms constitute fundamental issues. A direct parametric inversion method is, therefore, proposed in this letter, and a phase-based dispersion method is employed to calculate the dispersion curves of measured dipole acoustic data. The accuracy and computational efficiency of the inversion can be simultaneously guaranteed by using the proposed dispersion analysis method. An exponential radial profile function with three parameters, namely, the relative change in the shear modulus, the decaying parameter of the exponential function, and the radial extent of the perturbation zone, is used to describe alterations in formation shear wave velocities. Formations with different alteration characteristics can be expressed effectively using this proposed exponential function. The cost function is then calculated in terms of the three exponential function parameters and the real data dispersion curve. After the three parameters are inverted by finding the minimum value of the cost function, the radial shear wave velocity profile can be expressed using the exponential function. Finally, processing results using synthetic data confirm the efficiency of the proposed algorithm.

Acoustic reflection log in transversely isotropic formations

We have calculated the waveforms of sonic reflection logging for a fluid-filled borehole located in a transversely isotropic rock. Calculations have been performed for an acoustic impulse source with the characteristic frequency of tens of kilohertz that is considerably less than the frequencies of acoustic borehole imaging tools.It is assumed that the borehole axis coincides with the axis of symmetry of the transversely isotropic rock. It was shown that the reflected wave was excited most efficiently at resonant frequencies. These frequencies are close to the frequencies of oscillations of a fluid column located in an absolutely rigid hollow cylinder. We have shown that the acoustic reverberation is controlled by the acoustic impedance of the rock Z=Vphρs for fixed parameters of the borehole fluid, where Vph is the velocity of horizontally propagating P-wave; ρs is the rock density. The methods of waveform processing to determine the parameters characterizing the reflected wave have been discussed.

In-situ stress and fracture characterization of a candidate repository for spent nuclear fuel in Gansu, northwestern China

Hydraulic fracturing in-situ stress measurements and acoustic image logging were used to characterize the southern Beishan region of Gansu Province for China's first spent nuclear fuel repository. A total of 47 hydraulic fracturing measurements and 18 impression tests were performed in four boreholes at two sites to estimate magnitudes and orientations of horizontal principal stresses. The stress regime is mainly characterized by SHmax>Shmin≥Sv, indicating that the regional stress field is dominated by the maximum horizontal stress. This stress regime is prone to thrust and strike-slip faulting, according to Anderson's faulting theory. Measured results indicate that magnitude orders and stress gradients of horizontal principal stresses are larger than those previously obtained from other boreholes in the Beishan region, implying that the current stress field of candidate sites might be influenced by regional tectonic and geological controls. Using acoustic borehole logging data, geometric features of natural fractures of the four boreholes were mapped. We also found that wide fractures or faults penetrated in both boreholes at one site changed the orientations of the maximum horizontal principal stresses. Fractures with optimal orientations can be permeable owing to their narrow angles with in-situ principal stress orientations. Rock mass permeability is a priority for future investigation. Adopting the Coulomb frictional-failure criteria, we evaluated the possibility of reactivation of the wide fractures and faults and concluded that the crust in Jinta field should be stable.

Dispersion correction for acoustic borehole logging data

Compressional and shear formation velocities are key to the prediction of petrophysical properties from seismic attributes. In fast formations shear velocity may be obtained from monopole source, but in slow formations, it is commonly determined from the flexural mode associated with dipole excitation, which is a dispersive borehole-guided mode who’s low frequency and high frequency asymptote to the formation S-velocity, and to the Scholte-velocity, respectively. The shear slowness is commonly computed from well log dipole flexural mode data using Semblance Time Coherence (STC) processing. Dispersion is handled by restricting the waveforms spectral content to the low frequencies that travel close to the formation’s shear velocity. This restricting may not eliminate the need for a residual dispersion correction. Inversion addresses this difficulty by computing shear slowness directly from observed dispersion characteristics. In order to make the inversion efficient the iterative steps which compare observed and forward modeled dispersion curves are replaced with a neural net trained on a large number of pre-modeled curves generated with known formation and borehole properties. Automated mode frequency detection constrains the bandwidth over which dispersion curves are matched. Results from 127,000 modelled and field data points show improved accuracy and precision relative to STC processing.

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

AbstractAnalysis of fracture orientation, spacing, and thickness from acoustic borehole televiewer (BHTV) logs and cores in the andesite‐hosted Rotokawa geothermal reservoir (New Zealand) highlights potential controls on the geometry of the fracture system. Cluster analysis of fracture orientations indicates four fracture sets. Probability distributions of fracture spacing and thickness measured on BHTV logs are estimated for each fracture set, using maximum likelihood estimations applied to truncated size distributions to account for sampling bias. Fracture spacing is dominantly lognormal, though two subordinate fracture sets have a power law spacing. This difference in spacing distributions may reflect the influence of the andesitic sequence stratification (lognormal) and tectonic faults (power law). Fracture thicknesses of 9–30 mm observed in BHTV logs, and 1–3 mm in cores, are interpreted to follow a power law. Fractures in thin sections (∼5 μm thick) do not fit this power law distribution, which, together with their orientation, reflect a change of controls on fracture thickness from uniform (such as thermal) controls at thin section scale to anisotropic (tectonic) at core and BHTV scales of observation. However, the ∼5% volumetric percentage of fractures within the rock at all three scales suggests a self‐similar behavior in 3‐D. Power law thickness distributions potentially associated with power law fluid flow rates, and increased connectivity where fracture sets intersect, may cause the large permeability variations that occur at hundred meter scales in the reservoir. The described fracture geometries can be incorporated into fracture and flow models to explore the roles of fracture connectivity, stress, and mineral precipitation/dissolution on permeability in such andesite‐hosted geothermal systems.

Analysis of the Thermal and Hydraulic Stimulation Program at Raft River, Idaho

The Raft River geothermal field, located in southern Idaho, roughly 100 miles northwest of Salt Lake City, is the site of a Department of Energy Enhanced Geothermal System project designed to develop new techniques for enhancing the permeability of geothermal wells. RRG-9 ST1, the target stimulation well, was drilled to a measured depth of 5962 ft. and cased to 5551 ft. The open-hole section of the well penetrates Precambrian quartzite and quartz monzonite. The well encountered a temperature of 282 °F at its base. Thermal and hydraulic stimulation was initiated in June 2013. Several injection strategies have been employed. These strategies have included the continuous injection of water at temperatures ranging from 53 to 115 °F at wellhead pressures of approximately 275 psi and three short-term hydraulic stimulations at pressures up to approximately 1150 psi. Flow rates, wellhead and line pressures and fluid temperatures are measured continuously. These data are being utilized to assess the effectiveness of the stimulation program. As of August 2014, nearly 90 million gallons have been injected. A modified Hall plot has been used to characterize the relationships between the bottom-hole flowing pressure and the cumulative injection fluid volume. The data indicate that the skin factor is decreased, and/or the permeability around the wellbore has increased since the stimulation program was initiated. The injectivity index also indicates a positive improvement with values ranging from 0.15 gal/min psi in July 2013 to 1.73 gal/min psi in February 2015. Absolute flow rates have increased from approximately 20 to 475 gpm by February 2 2015. Geologic, downhole temperature and seismic data suggest the injected fluid enters a fracture zone at 5650 ft and then travels upward to a permeable horizon at the contact between the Precambrian rocks and the overlying Tertiary sedimentary and volcanic deposits. The reservoir simulation program FALCON developed at the Idaho National Laboratory is being used to simulate and visualize the effects of the injection. The simulation model uses a discrete fracture network generated for RRG-9 using acoustic borehole imaging and analysis of microseismic activity. By adjusting the permeability of the fractures, a pressure history match for the first part of the stimulation program was obtained. The results of this model indicate that hydraulic fracturing is the dominant mechanism for permeability improvement for this part of the stimulation program.

Heterogeneous bedrock investigation for a closed-loop geothermal system: A case study

This paper investigates bedrock heterogeneity by applying three different geophysical approaches, in order to study the long-term behaviour and the interaction between closed-loop geothermal systems. The investigated site consists of four boreholes equipped with geothermal pipes on the campus of University of Liege, Belgium. The first approach includes acoustic borehole imaging, gamma-ray logging and cuttings observation and results to a detailed fracture characterisation, rock identification and layer dip angle determination. The second approach consists of measuring the thermal conductivity of cuttings at the laboratory. Study of cuttings thermal conductivity measurements can contribute to bedrock heterogeneity knowledge concerning the transition of one formation to another and the layer dipping. The third approach is based on high-resolution temperature profiles, measured during the hardening of the grouting material and the recovery phase of a Distributed Thermal Response Test. Through this approach a correlation of the temperature profiles to the geological characteristics of the surrounding bedrock is identified. The analysis of this correlation can provide information on fractured zones, alternation of different rock types and layering dipping. This latter approach can be easily applied on closed-loop geothermal systems to characterise the bedrock and investigate its heterogeneity as well as contribute to the their long-term behaviour prediction and to the optimisation of their efficiency.

Heterogeneity of structure and stress in the Rotokawa Geothermal Field, New Zealand

AbstractGeometric characterization of a geothermal reservoir's structures, and their relation to stress field orientation, is vital for resource development. Subsurface structure and stress field orientations of the Rotokawa Geothermal Field, New Zealand, have been studied, for the first time, using observations obtained from analysis of three acoustic borehole televiewer logs. While an overall NE‐SW fracture strike exists, heterogeneity in fracture dip orientation is evident. Dominant dip direction changes from well to well due to proximity to variously oriented, graben‐bounding faults. Fracture orientation heterogeneity also occurs within individual wells, where fractures clusters within certain depth intervals have antithetic dip directions to the well's dominant fracture dip direction. These patterns are consistent with expected antithetic faulting in extensional environments. A general SHmax orientation of NE‐SW is determined from induced features on borehole walls. However, numerous localized azimuthal variations from this trend are evident, constituting stress field orientation heterogeneity. These variations are attributed to slip on fracture planes evidenced by changes in the azimuth of drilling‐induced tensile fractures either side of a natural fracture. Correlation of observed fracture properties and patterns to well permeability indicators reveal that fractures play a role in fluid flow in the Rotokawa geothermal reservoir. Permeable zones commonly contain wide aperture fractures and high fracture densities which have a dominant NE‐SW strike orientation and NW dip direction. Studies of this kind, which show strong interdependency of structure and stress field properties, are essential to understand fluid flow in geothermal reservoirs where structural permeability dominates.