WP5 - Reservoir Modelling
RWTH AACHEN, EGP, CNR
The objective of this work package is to accompany the drilling efforts with reliable predictions of the physical state (temperature, natural flow, rock properties) of the reservoir on regional and local scale.
Any commercial exploitation of the natural heat in rock layers below the reservoirs currently exploited at Larderello, requires a thorough understanding of the physical conditions of solids and fluids coexisting at these depths. Drilling in such high-temperature and pressure geothermal system poses a challenge to current drilling technologies and demands a reliable identification, characterization, and estimation of the physical conditions and the magnitude of the accessible resources. Numerical modelling is a tool for analysing and predicting major changes in the fluid-dynamics and the behaviour of the fluids and rocks that might be encountered during drilling.
The dynamic performance of a hydrothermal system in fractured bedrock is described by a set of coupled, nonlinear partial differential equations for mass and heat transport and the related state equations. Solving these equations numerically allows predicting the amount of heat that can be extracted from the reservoir over time and thus evaluating the commercial viability of the reservoir. Moreover, such modelling predictions have the potential to forecast possibility critical conditions when drilling close to supercritical fluids and thus are also important for safety considerations.
The simulations will be accomplished in three dimensions for the reservoir in natural conditions and for geothermal fluid production. Model simulations and predictions are updated whenever new information from seismic data interpretation, drilling logs and reports, and petrophysical rock measurements are available. The regional model will cover an area of about 15 km by 15 km to access the natural fluid circulation and pressure variations in the reservoir, which is an important information for ensuring sustainability of the reservoir. The local model will focus on the drilling site and including the hydrothermal well in full production and the rock physical conditions in its vicinity. The regional model will be linked to the local one by providing the necessary boundary conditions to the local model area.
The main challenge regarding numerical modelling of water in supercritical condition occurs near and at the saturation curve separating the two phase domains of liquid water and water vapour. Recent modelling techniques and increased computational power combine the increased complexity in reservoir geometry and heterogeneity with more accurate state equations for the reservoir fluid. Simulators capable of modelling sub- and super-critical conditions with both the desired level of thermodynamic detail and computational and gridding flexibility/efficiency will be applied in this work package. This work package will provide a progressively updated thermo-hydraulic model of the super-critical reservoir taking into account the estimated petrophysical properties of rocks at depth (temperature, pressure, porosity, permeability, compressibility, phase saturation, capillary pressure), the reservoir geometry, and discharge of the well field while considering the uncertainties of these parameters.
Task 5.1 Simulations of the regional hydrothermal system (RWTH, CNR, EGP)
The regional hydrothermal model provides information on the general fluid flow and on the recharge system in the area and predicts the regional temperature field. The regional model will cover a surface area of about 10 km to 10 km centred on Venelle 2. For this area data from 16 seismic profiles are available to resolve the subsurface structure. Modelling will be accomplished with the software SHEMAT-Suite (developed at the RWTH). This code is designed for hydrothermal mass and heat transport and is highly parallelized and can handle large models and stochastic reservoir properties. Most of the reservoir is in sub critical conditions; therefore a first reference model will be based on sub critical equations.
After including equations for the water-vapour phase transition supercritical conditions will be studied in particular in regard to the heat transport and pressure variations in the deep reservoir.
The regional model will be refined and updated whenever new information from the drilling progress and the reservoir characterization team (WP4; seismic interpretations, petrophysical measurements etc.) is available.
The calibrated regional reservoir model: the model will be calibrated with temperature and well pressure information from nearby wells, and it provides an estimate of the regional flow system and the temperature field.
Task 5.2 Simulations of the surrounding of the Venelle drilling (RWTH, CNR, EGP)
The software SHEMAT-Suite has a build-in well-function which gives the opportunity to study the physical effects of a pressure-test in the well on its surrounding host rocks. Based on a section of the regional model we will study the particular conditions at the drilling site including the supercritical conditions in high resolution using the results of the regional model as boundary conditions. The recordings of the high pressure/high temperature logging tool developed in Task 3.1 and applied in Task 3.4 will be used as constraints to calibrate the model.
The Local model of the Venelle 2 production well will be an high resolution model of geological structure, temperature field and water phase conditions (vapour/water/supercritical) at the drill site (information of the regional model are used as boundary conditions). The model provides a prediction of the conditions to be expected during drilling.
Task 5.3 Inverse modelling to infer permeability at the well bottom based on pressure tests in the Venelle drilling (RWTH, CNR, EGP)
At the end of the drilling phase productions test will be performed (WP 2 Task 2.9). Based on the time series of productions tests, inverse modelling will be used to infer the permeability around the bottom of the well. The inverse modelling is done with the Ensemble Kalman Filter (already implemented in SHEMAT-Suite) which sequentially assimilates observations data into a numerical simulation of a transient system (which is the local model). Each assimilation step contains an update of (selected) reservoir states and properties and leads to a minimum variance estimate of (selected) reservoir properties (e.g. permeability distribution at a certain depth).
The permeability around the Venelle 2 well inverted from pressure tests; after the completion of the well, time series of pressure test data are used to invert the permeability distribution in the local model. First delivery two months after the first pressure test, possibly updated with following-up pressure test data.
Task 5.4 Simulation with TOUGH2 (CNR)
For the simulation of natural state and exploitation scenarios a 3D model will be built using TOUGH2 V.2.0 numerical reservoir simulator. Among the Equation of State (EOS) modules available within TOUGH2 V.2.0, considering the nonnegligible NCG content in Larderello field (represented mainly by CO2), and the expected high salt content the EWASG module for mixtures of water, NaCl and CO2 will be employed under sub-critical conditions, with the improvement in the EOS and the implementation in the code to be implemented during the IMAGE project.
With respect to supercritical conditions, the dataset for pure water are available (NIST data) and as pure water were implemented in the code by the Auckland University (AUTOUGH2, is a a modification of the TOUGH2). The proposed activity is to develop a version of TOUGH2v2 that takes in to account the supercritical phase of pure water, at first, and in a second stage to allow the presence of NaCl and handle the change of the critical point due to salinity, in order to obtain a reliable evaluation of heat and mass transport into the reservoir outside of the standard range of validity of the commercial TOUGH2 code.
Task 5.5 Integration and Testing (EGP, RWTH, CNR)
A series of integrated tests of performances of the simulators will be applied at the Venelle 2 conditions. The codes developed in Task 5.2, 5.3 and 5.4 will be compared, tested and improved to be used for future deep wells.
A commercial PDE solver (e.g. COMSOL , MAPLE), will be also applied to evaluate some basic cases in order to check the performance of the modelling tools in simple or literature reference case. As an example, a simple model of a well with a skin zone could be used for the interpretation of the hydraulic tests (injectivity and pressure relaxation) that will be carried out.