WP4 - Reservoir Characterization

The acquisition of four new well seismic profiles (VSP) and the data recording will be planned before drilling, in order to

1. calibrate the dataset of the existing 2D and 3D surface seismic data acquired at the earth surface
2. improve the resolution of geological structure down to the reservoir prior to drilling, accurately determine the propagation velocities of compressional (P-) and shear (S-) waves in situ that are required for a profound petrophysical assessment and comparison with laboratory results (Task 4.7).

We expect to be able to investigate about 1500 m of well depth with approximately 50 recording levels for each survey.
A first zero-offset VSP will be acquired with air-gun as a source. Up to three further surveys will be offset- and or walkaway-VSPs with vibroseis as a source. The source locations will be chosen to optimize the detection of the seismic anisotropy of the seismic horizons at a suitable distance from the well.
Determination of seismic anisotropy in situ can help to identify preferably oriented microcracks and local horizontal stress directions that may have a direct influence on hydraulic permeability.
Acquisition and processing of these data will be carried out by a service company. For the supervision of these activities, the upgrade of the EGP software for the VSP data processing will be required; this activity will be outsourced.

Seismic measurements will be made to accompany the vertical seismic profiling with a short-term deployment of approximately 100 mobile three-component (3C) surface seismic stations. Since this sort of seismic measurements is a recording-only (“passive”) measurement that profits from the seismic energy applied in a different “active” seismic experiment (VSP in this case) it is called “piggy back” experiment. This “piggy-back” seismic network will cover an area of 10-20 km in diameter around the drilling site in order to determine seismic velocities by refraction measurements to illuminate parts of the K-horizon at oblique angles of wave propagation and perhaps to record converted shear waves from the target reservoir. These sorts of measurements will be essential,

1. for determining the seismic velocities of the depth interval between the bottom of the VSP and the target K-horizon;
2. thereby for setting up a migration-velocity model required for a precise imaging of the geological structure of the reservoir and the hanging-wall;
3. for improving the determination of the seismic parameter contrast at the K-horizon through elastic impedance inversion and waveform modelling and inversion.

The precise knowledge of reservoir structure and the related parameter contrast are needed for estimating the physical conditions of the reservoir, because pore pressure has a strong effect on seismic velocities. In analyzing the data of the seismic network we will focus on compressional-to-shear and shear-to-compressional converted waves in order to determine the Poisson ratios as an additional reservoir parameter.
The results of the seismic piggy back experiment will be gained iteratively and organized in two sections. Stage-1 results will consist of a preliminary seismic velocity model available after data acquisition and consider basically data of the VSP and piggy back measurements, stage-2 results consisting of the final model will be available after the completion of the data integration & interpretation task and will incorporate also results and constraints from the integrated interpretation.

The newly acquired data will be combined with all the existing data from previous surveys or acquired within the IMAGE project:.

• survey data in the Larderello/Venelle area, from previous survey of EGP (2 survey profiles) and CNR;
• in-hole and cross-hole electromagnetic data in the Venelle well.

Characterization of the super critical region: the K-horizon can be regarded as the exemplary case of a potential supercritical reservoir indicator. The aim of DESCRAMBLE is to use this excellent opportunity to apply the latest seismic processing, imaging and interpretation technology for exploring the super-critical reservoir prior to drilling and to evaluate and calibrate the seismic exploration through drilling results. The aims of the seismic investigation are:

• to map the geological structure at the highest possible resolution by applying the best available imaging software, especially 3D prestack Fresnel volume and coherency migration algorithms and Full-Waveform Inversion (FWI);
• to determine constraints on the petrological and physical conditions and on fracture distributions in situ. These constraints will be derived from FWI and from an analysis of reflection strength (elastic impedance inversion), coherency (attribute analysis) and numerical modelling.

Special emphasis will be put on estimating uncertainty bounds for the rock parameters derived. Both the determination of rock parameter uncertainty and the calibration of the seismic results will be based on numerically modelling the observed wave fields and on information regarding boreholes. Besides new insights into the nature of the K-horizon, the seismic study will provide general guidance for identifying deep super-critical conditions through seismic measurements at different sites. Reprocessing existing 2D and 3D seismic data will involve:

• first-arrival travel-time-tomography and reflection-tomography to derive shallow P-wave velocity models;
• 3D prestack migration (Fresnel-Volume migration and coherency migration) to image the geological structure at the highest possible resolution;
• determination of constraints on the petrological and physical conditions and on fracture distributions in situ and estimations of uncertainty bounds for the rock parameters combining observed wave fields and borehole information.

Resistivity values as inferred from magnetotelluric and in-hole, cross-hole EM data will be integrated to other information and re-interpreted.




The results of this task are three-fold:

1. a comprehensive subsurface model including geological layers and fault zones as well as geophysical and hydrothermal properties,
2. detailed studies of the K-Horizon and its physical state based on geophysical field and laboratory data, waveform inversion and numerical modelling,
3. a post-drilling assessment of these models leading to general guidance for identifying deep super-critical reservoirs through seismic measurements.

The results will be organized in 2 stages: items (1) and (2) after the seismic field measurements and laboratory investigations, item (3) after reaching the critical region by drilling.

• The high resolution microseismic monitoring of the drilling site area will be achieved integrating the Larderello Travale Microseismic Network (LTMN) with 4 additional Temporary Seismic Stations (TSS) placed at 1.5 to 2 km distance from the Venelle 2 well.
  Three of the TSS will be equipped with surface high resolution broad band seismometer and one with a downhole high resolution broad band seismometer placed into the nearby dry well.
  The 4 TSS with 11 Seismic Stations (SS) of the LTMN, in an area of 10 km in radius around the drilling site Venelle 2, will ensure the detection of microseismic events over a wide magnitude range, and robust hypocentral localization error reduction.
  Seismic signals of the 4 TSS, sampled at 250 sps, will be transmitted by radio-link and real-time acquired by the acquisition system (SAMES) placed in the seismic room at the Larderello laboratory. SAMES performs the real-time analysis of the seismic signals, the triggering, the automatic phase parameters detection and hypocentral locations. In addition, the continuous signals are stored in order to perform a detailed off-line analysis.
  The main target of these new stations will be to improve the resolution and quality of the monitoring of the natural seismicity of the area, in order to enforce environmental safety and to check the independence of seismic events from any drilling operations, even during the supercritical phase, which has never been reached before.

Besides pressure and temperature data gathered at temperatures above 400°C using a novel tool, as described in the WP3, logging at lower temperatures (with the well cooled down) will be performed using commercially available tools.
For the drilling phase when the well is cooler than 170°C, the commercially available logs will be considered.
The following list shows the applied tools and measured parameters.

• Spectral Gamma Ray: Natural formations radioactivity, volumes of potassium, thorium and uranium.
• Sonic: Vp and Vs velocity, amplitudes and waveforms.
• Density: Bulk density of formation, photoelectric effect.
• Neutron: Neutron porosity.
• Resistivity: Resistivity profiles of different depths of investigation.
• Structural Logs: Image logs (e.g. acoustic televiewer) or at least a dip logs for structural and geomechanical aspects.
• Formation Lithology eXplorer (FLeXSM) measurements. This new leading-edge technology applies the principles of gamma ray spectroscopy to provide accurate in-situ mineralogical characterization of the reservoir, thereby reducing uncertainties in interpretations that do not incorporate mineralogical data.

Since the measured surroundings of the borehole are at or close to the super-critical conditions, new log interpretation methods need to be developed, and appropriate work flows as well as quality control standards need to be set up. Based on experimental data on rock samples and theoretical approaches, we will develop petrophysical models in order to understand the effects of super-critical conditions on the physical properties (thermal conductivity, thermal capacity electrical resistivity, sonic velocity) of a fractured reservoir system. Once established, the models will be transferred into log interpretation methods and extended to the specific conditions of the logging environment, where the drilling fluids and formation fluids interact with each other. Effects of cooling and invasion at varying distances from the wellbore will also need to be considered.
Special focus will be on temperature logs in order to fully integrate them into the formation evaluation workflow. Since formation thermal conductivity significantly affects the thermal gradient, the interpretation of temperature data will provide important insights into reservoir properties. This is especially the case when they are repeatedly recorded to account for transient temperature variations (e.g. using the novel high temperature logging tool). The temperature data will be integrated with other logs, such as gamma-ray and resistivity, and the reservoir will thus be characterized in terms of its thermal and hydraulic properties. This will result in the definition of input parameters, which are important for numerical modelling.
The activity starts with project start for planning of logging campaigns. Prior to drilling and downhole measurement operations, methods will be adapted and developed for log interpretation at or close to supercritical conditions.
Interpretation concepts and workflows are delivered before logging operations is scheduled. Log data analysis will start immediately after logging operation and with integration of other data (core data, tests and seismic results). Results of data analysis will enter into reservoir characterization and forwarded to the groups of reservoir modelling whenever available.

When approaching the K-horizon depth, the monitoring of the active hydrothermal system will integrate thermal logging with fluid geochemistry (depending on sampled fluid types).
The drilling target will have extremely harsh temperature, pressure and chemistry conditions, but the need to acquire all the recoverable thermal and fluid data/samples must be stressed. Geochemical-isotopic monitoring of fluids during the new deep drilling will enable possible exchanges among the exploited reservoirs and the deep super-critical reservoir to be traced.
B, Sr, Li, O and H isotopes are useful to decipher water/rock interactions and constrain the fluid reservoir, while noble gases (e.g. He) can trace the gas input from the mantle. Noble gases play an important role in understanding the processes controlling fluid generation in geothermal systems, and in characterizing tectonics. The low solubility and wide variation in isotopic ratios of noble gases make these elements very sensitive for tracing the deep versus shallow (atmospheric) origin of the fluid, as well as the processes occurring in the geothermal reservoir. The main goal of this contribution is to monitor variations in temperature while tracing the physical-chemical characters of fluids approaching the supercritical zone. About 10 gas samples and few fluid samples from production tests will be analyzed.
Moreover, mud samples will be collected for additional laboratory determinations, in addition to the standard ones, performed on the drilling site.

Standard determination from EGP
On condensed vapour:

• Field determination: pH, T [°C] (at which pH is determined), Conductivity, Alkalinity and H₂S.
• In laboratory, chemical characterization of the condensable phase of the fluid: pH, Conductivity, Alkalinity, Alkalinity backtitration, Na, NH₄, Cl, H₃BO₃. and of the Gas/Steam ratio.

Gas phase:

• Field determination: G/S ratio.
• In laboratory: major and trace gases (CO₂, N₂, NH₃, H₂S, CH₄, C₂H₆, H₂, Ar, CO, He) composition by gaschromatography.

Determination from CNR:

• Noble Gases isotopes (He, Ne, Ar, Kr, Xe ) by mass-spectrometry.
• Stable and radiogenic isotopes (d²H-H₂O, d¹⁸O-H₂O, d¹³C-CO₂, d¹³C-CH4, ¹¹B/¹⁰B, ⁶Li/⁷Li, ⁸⁷Sr/⁸⁶Sr) by mass-spectrometry.
• Chemical analyses of selected parameters (Cl, B, Li) in liquid samples extracted from the drilling fluids (cement or mud).

The main goal of this task is to predict the characteristics of fluids and host rocks at the K-horizon depth in order to help drilling activities while approaching the super-critical target.
Since the Larderello geothermal field has been subject to exhumation and uplifting, rocks which are currently at a given depth and temperature in the past had to be at a greater depth and temperature. These rocks may contain information on the physical-chemical conditions of older super-critical reservoirs (paleo K-horizons?). Previous studies on samples cored above the present-day K-horizon documented the presence of paleo fluid of magmatic/metamorphic origin, with T > 400°C and fluctuating pressure conditions (between lithostatic and hydrostatic). Similar information was derived from the study of much older hydrothermal systems linked to granite intrusions (ca. 5.9 Ma) and now exposed at the surface in the east of the island of Elba (Tuscany, Italy).
This topic is strictly linked to the IMAGE project, and will produce the main bridge between the two projects. During drilling, the recovered cores and cuttings will offer the opportunity to perform several petrographic, geochemical, isotopic and fluid inclusion analyses aimed at defining the overall characters and the temporal evolution of this paleo K-horizon.
The characterization of the fossil hydrothermal systems will integrate the metamorphic petrology of basement rocks (thermal evolution of the contact aureole/s) with the petrographic-geochemical-isotopic study of hydrothermal veins/breccias and possible granite intrusions (fluid inclusion, mineral chemistry, geochemistry and isotopic composition of Sr, Nd, Pb, B, O and H).
Analyses of whole rocks and minerals (separated and in situ) will be performed on cores/cuttings from the Venelle well, as well as on available cores from nearby wells. Temporal evolution of the fossil magmatic-metamorphic-hydrothermal system will be constrained by isotope dating of suitable minerals for ⁴⁰Ar-³⁹Ar and high-resolution U-Pb methods.
Definition of pressure-temperature-chemical features and lifespan of past magmatic-hydrothermal events will provide fundamental insights for a predictive quantification of the potential supercritical hydrothermal system at depth. Petrological data will be also integrated with available geophysical (seismic and magnetotelluric) and petrophysical data.
In order to determine physical and petrological properties of the reservoir rocks we plan to take at least one or two core sections from the well at a safe depth, as well as using existing cores from nearby wells from metamorphic basement.
This will offer the opportunity to perform petrophysical laboratory analyses on core samples so as to define important physical characteristics of the reservoir host rocks, in particular seismic P- and S-wave velocities, thermal conductivity, electrical conductivity, as well as hydraulic and geomechanical properties. Some of the rock samples will be investigated under simulated in-situ conditions at the high-pressure, high-temperature by CAU. Here, P- and S-wave velocities and their anisotropy can be measured on 5 cm cubic samples up to 600 MPa and 750°C using a pulse transmission from 2 MHz transducers. P- and S-wave velocities and their anisotropy (incl. S-wave splitting) will be determined for three orthogonal directions of the cubic samples under the following HP/HT conditions:

1. Temperature: room temperature (20°C), Pressure: 0-600 MPa in steps of 50 MPa.
2. Temperature: reservoir temperature (~450°C), Pressure: 50-600 MPa in Steps of 50 MPa.
3. Temperature: 20-750°C, Pressure: effective pressure at reservoir level (~100-150 MPa).

The petrophysical studies carried out on rock samples under high P and T conditions will be complemented by measurements on rock cuttings under ambient pressure and high T conditions in the petrophysical lab at RWTH. Thermal conductivity will be measured with a thermal needle probe (TK04), matrix density with a pycnometer, and specific heat capacity with a calorimeter on approximately 20 cutting samples from the well Venelle 2 in particular for the depth range below 2 km. These measurements are also important for the calibration of log data.
These data, which will be derived from experiments under ambient and simulated in situ conditions, will serve as an input and calibration data set for rock physics modelling, seismic characterization and log interpretation studies.
Additional data from the present Venelle 2 well and other wells, already drilled in the same area, will be used for an extensive data collection before the drilling and a characterization of rock properties of the metamorphic basement.

Determination from CNR

• Petrographic-textural characterization by optical microscope, SEM and Qemscan.
• Geochemistry and isotopic composition (Sr, Nd, Pb, B, O and H) by ICP-MS and TIMS Mass Spectrometry.
• Mineral chemistry by EPMA and ICP-MS.
• Microthermometry and geochemistry of fluid inclusions.
• Dating by ⁴⁰Ar-³⁹Ar and U-Pb methods by Mass Spectrometry.

Determination from RWTH

• Thermal conductivity (for cuttings and cores in original and saturated condition).
• Density (for cuttings and cores, matrix and bulk density).
• Specific heat capacity (for cores in original and saturated condition).
• P-wave velocity (for cores in original and saturated condition).
• Magnetic susceptibility (for cores in original condition).
• Porosity (for cores).

Determination from CAU

• P and S-wave velocities as a function of pressure up to 600 MPa and temperature up to 750°C.
• Seismic anisotropy (P-, S1, S2-wave velocities; shear wave splitting) in the same pressure-temperature range.
• Volume strain and crack porosity of the sample deforming under increasing pressure.
• Compressional and shear moduli.
• Poisson's ratio.

The petrological data of the well for the prediction of the characteristics of fluids and host rocks at the K-horizon and petrophysical lab measurements on cuttings and core samples of the Venelle 2: the results of the petrophysical measurements on 20 cutting samples and core samples are delivered as statistical means and standard deviations in a spreadsheet file with an accompanying report describing the measurement procedures.