Numerical Modelling and Simulation Optimization of Geothermal Reservoirs Using the Tough2 Family of Codes

In order to improve the reservoir engineering activities and, in particular, to optimize numerical modelling and simulation of geothermal reservoirs using the TOUGH family of codes, it has been decided to use the software T2Well for the interpretation of well-tests, coupling T2Well with the equation of state module EWASG, which describes the typical thermodynamic condition in high enthalpy geothermal reservoirs. T2Well-EWASG has been coupled and tested through the typical process of verification and validation. The application of T2Well-EWASG for the interpretation of well-tests related to the slim hole WW-01 drilled in the Wotten Waven Field (Commonwealth of Dominica) proves that it can be used as a tool for integrated interpretation of surface and downhole measurements collected during the performance of production tests in geothermal wells. The strength of this tool is that it allows to reduce the different possible solutions (in terms of reservoir characterization) within an acceptable error, by allowing the interpretation of surface and downhole measurements in conjunction, instead of separately. From this point of view T2Well-EWASG can effectively be used as a tool which allows an improvement of reservoir engineering activities. Finally, the huge amount of data managed during these activities has permitted to test and project the improvement of pre- and post- processing tools specific for TOUGH2 created by the geothermal research group of DICAM. In particular, the pre- and post-processing tools have been validated with a case study dealing with the migration of non-condensable gases in deep sedimentary formation

TOUGH2Viewer tutorial

TOUGH2Viewer is a Java program capable of displaying unstructured (Voronoi complying) grids, locally refined and structured grids (complying with the MESHMAKER iTOUGH2 format). In particular, TOUGH2Viewer allows to navigate through a 3D grid compatible with the iTOUGH2 data file format and see: (1) the thermodynamic variables and the material (namely, petrophysical properties) of each block; (2) maps of isovalues (2D) of all thermodynamic variables; (3) isosurfaces (3D) of all thermodynamic variables; (4) flows of mass and heat between blocks; (5) spatial profiles of thermodynamic variables, of a selected set of blocks, long a Cartesian directions; (6) time plot of thermodynamic variables of a selected block. All commands to manage a 3D visualization (zooming, pan, rotations) are CAD complying

3D Voronoi Pre- and Post- Processing Tools for Using the Tough2 Family of Numerical Simulator for Hydrocarbon Gas Migration

The TOUGH family of multi-component, multiphase numerical reservoir simulators have a well know and a long history of applications in different fields of mass and heat transport in porous media. The use a full three-dimensional (3D) unstructured grid permits a great degree of flexibility to reproduce the geometry of complex geological formations and performs accurate reservoir numerical simulations. The full 3D Voronoi tessellation approach also allows reproducing the geometry of geological formations (useful, for example, in directional drilling). In this work, we present some applications of the 3D Voronoi pre- and post-processing software tools dedicated to the TOUGH family of codes (developed at the DICAM Department of the University of Bologna by the Geothermal research group), to study problems of gas migration in hydrocarbon reservoirs. In particular, a small set of 3D grids of a deep sedimentary formation has been created with VORO2MESH, and the simulation results analysed with TOUGH2Viewer. VORO2MESH is a software coded in C++ able to rapidly compute the 3D Voronoi tessellation for a given domain and to create a ready-to-use TOUGH2 MESH file, up to million blocks. It is based on the well-known and powerful open source voro++ library. The new extended version of the TOUGH2Viewer post-processor was used to easily inspect the 3D Voronoi discretization and to better manage the numerical simulation results. The software, written in Java, handles the visualization of both 3D grids (structured and unstructured) and simulation results. This study shows the effectiveness of these tools, and that the use of unstructured grids, instead of structured grids, substantially improve both the reproduction of the geological model and the TOUGH simulation results

INTERPRETATION OF PRODUCTION TESTS IN GEOTHERMAL WELLS WITH T2WELL-EWASG

A crucial aspect of geothermal reservoir engineering is the performance and interpretation of field tests, such as production tests, flowing temperature and pressure logs - both during injection and production, and down-hole pressure transient recording. Such tests are of great importance in order to evaluate the reservoir properties and to predict wells production capacity. However, production tests duration is often limited by environmental constraints related to the disposal of brines, and the running of downhole measurements is constrained by safety and cost considerations related to the use of expensive tools in high temperature high productive wells. From this point of view, the capability to simulate the whole production tests can be extremely useful to improve the data interpretation and reduce the impact of lacking field data. In particular, the coupled simulation of transient fluid flow within the wellbore and the reservoir with the matching of output production curve, flowing logs, and downhole pressure transients, should result in a more reliable evaluation of reservoir properties. This can be done using T2Well, a coupled wellbore-reservoir numerical simulator, with proper equation of state (EOS) modules. EWASG is an EOS module for high enthalpy geothermal fluids consisting of mixtures of water, salts and a non-condensable gas (NCG). We have recently plugged it into T2Well and also improved the analytical computation of heat exchange between the well and the surrounding formations at early time, which may be important for reproducing the transient results of production tests. The resulted wellbore-reservoir simulator, T2Well-EWASG, is capable to simulate high enthalpy geothermal wells. We validated T2Well-EWASG against flowing pressure and temperature logs taken from published literature

Interpretation of production tests in geothermal wells with T2Well-EWASG

In the geothermal sector, being able to simulate production tests by combining surface and downhole measurements can be extremely useful, improving data interpretation and reducing the impact of unavailable field data. This is possible with T2Well, a coupled wellbore-reservoir simulator. We plugged the EWASG equation of state for high enthalpy geothermal reservoirs into T2Well and extended the function to analytically compute the heat exchange between wellbore and formation at the short times. Changes to the analytical heat exchange function were verified by comparison with wellbore-formation heat exchange numerically simulated. T2Well-EWASG was validated by reproducing the flowing pressure and temperature logs taken from literature, and by using the software for the interpretation of a short production test. Simulation results indicate that T2Well-EWASG can be effectively used to improve the interpretation of production tests performed in geothermal well

INTERPRETATION OF PRODUCTION TESTS IN GEOTHERMAL WELLS WITH T2WELL-EWASG

A crucial aspect of geothermal reservoir engineering is the performance and interpretation of field tests, such as production tests, flowing temperature and pressure logs - both during injection and production, and down-hole pressure transient recording. Such tests are of great importance in order to evaluate the reservoir properties and to predict wells production capacity. However, production tests duration is often limited by environmental constraints related to the disposal of brines, and the running of downhole measurements is constrained by safety and cost considerations related to the use of expensive tools in high temperature high productive wells. From this point of view, the capability to simulate the whole production tests can be extremely useful to improve the data interpretation and reduce the impact of lacking field data. In particular, the coupled simulation of transient fluid flow within the wellbore and the reservoir with the matching of output production curve, flowing logs, and downhole pressure transients, should result in a more reliable evaluation of reservoir properties. This can be done using T2Well, a coupled wellbore-reservoir numerical simulator, with proper equation of state (EOS) modules. EWASG is an EOS module for high enthalpy geothermal fluids consisting of mixtures of water, salts and a non-condensable gas (NCG). We have recently plugged it into T2Well and also improved the analytical computation of heat exchange between the well and the surrounding formations at early time, which may be important for reproducing the transient results of production tests. The resulted wellbore-reservoir simulator, T2Well-EWASG, is capable to simulate high enthalpy geothermal wells. We validated T2Well-EWASG against flowing pressure and temperature logs taken from published literature

Perfidi Filters validation: from Nuclear Magnetic Resonance Relaxometry to Magnetic Resonance Imaging

In order to well distinguish di\ufb00erent tissues of the human body by magnetic resonance imaging (MRI), it is of great importance to \ufb01nd procedures to improve the image contrast. In particular, a valuable feature is to image only speci\ufb01c parts of organs and/or tissues while ignoring all the others. Dedicated MRI sequences able to \ufb01lter the 1H nuclei signals based on the di\ufb00erent longitudinal relaxation times (T1) of the tissues have been developed. Standard signal selection/attenuation sequences, such as the Short Time Inversion Recovery and Multiple Inversion Recovery, have the e\ufb00ect to zero the signal for a discrete number of T1 values. Parametrically Enabled Relaxation Filters with Double and multiple Inversion (PERFIDI) sequences act on a range of T1 values and behave as an electronic band-pass or high-pass or low-pass \ufb01lters. PERFIDI \ufb01lters are therefore primarily focused on the components that pass through, rather than on those that are blocked. These \ufb01lters have been developed and tested by nuclear magnetic resonance relaxometry. Here, these sequences have been validated for MRI on phantom samples to mimic T1 distributions present in tissues. Preliminary applications show that PERFIDI \ufb01lters can e\ufb00ectively work on a range of T1 values to give well contrasted images

3D VORONOI PRE- AND POST- PROCESSING TOOLS FOR THE MODELING OF DEEP SEDIMENTARY FORMATIONS WITH THE TOUGH2 FAMILY OF CODES

Full three-dimensional (3D) unstructured grids offer a great degree of flexibility to perform accurate reservoir numerical simulations. However, when the space discretization is done using the Integral Finite Difference Method (IFDM), the requested orthogonality between the segment connecting the nodes and the blocks interface area complicates the construction of grids with irregular shape blocks. Nevertheless, the full 3D Voronoi approach guarantees the IFDM constraints and allows reproducing geological formations geometry, to follow the shapes of objects such as faults and directional wells, and allows increasing grid resolution in volumes of interest (local grid refinement). Here we present applications of the 3D Voronoi preand post-processing software tools dedicated to the TOUGH family of codes, (developed by the Geothermal research group of the DICAM Department, University of Bologna), to study the migration of non- condensable gases (NCG) in deep sedimentary formations at basin scale. Several algorithms, mainly developed by the scientific community, are already available to calculate the Voronoi tessellation from a given set of seed points. In particular, the voro++ library is a well-known and powerful open source code to carry out 3D computations of the Voronoi tessellation. Based on voro++, VORO2MESH is a new software coded in C++ able to rapidly compute the 3D Voronoi tessellation for a given domain and to create a ready to use TOUGH2 MESH file, up to million blocks. The program can also directly generate the set of Voronoi seed points, using a set of geological surfaces as input. The resulting grid is a mixed grid with regular blocks (orthogonal prisms) and irregular blocks (polyhedron Voronoi blocks) at the contact between different geological formations. The use of regular blocks in regions sufficiently far from the contact surface allows limiting the number of connections. In order to easily inspect the 3D Voronoi discretization and to better visualize the subsequent numerical simulation results, the functionality of the TOUGH2Viewer post-processor has been extended. The software handles the visualization of 3D grids (structured and unstructured), the 3D vector representation of heat and mass fluxes, the iso-surfaces of the simulated variables and 2D contour maps. Different 3D grids of a deep sedimentary formation have been created with VORO2MESH, and the simulation results analysed with TOUGH2Viewer. In particular, the migration of NCG in a large sedimentary formation occupying an area of about 25,000 km2 and with an average thickness of 800 m, extending from -2000 to -7000 m asl, has been simulated using TOUGH2-TMGAS. The use of unstructured grids as compared to the use of structured regular grids has substantially improved both the reproduction of the geological model and the simulation results of the NCG migration

3D Voronoi grid dedicated software for modeling gas migration in deep layered sedimentary formations with TOUGH2-TMGAS

As is known, a full three-dimensional (3D) unstructured grid permits a great degree of flexibility when performing accurate numerical reservoir simulations. However, when the Integral Finite Difference Method (IFDM) is used for spatial discretization, constraints (arising from the required orthogonality between the segment connecting the blocks nodes and the interface area between blocks) pose difficulties in the creation of grids with irregular shaped blocks. The full 3D Voronoi approach guarantees the respect of IFDM constraints and allows generation of grids conforming to geological formations and structural objects and at the same time higher grid resolution in volumes of interest. In this work, we present dedicated pre- and post-processing gridding software tools for the TOUGH family of numerical reservoir simulators, developed by the Geothermal Research Group of the DICAM Department, University of Bologna. VORO2MESH is a new software coded in C++, based on the voro++ library, allowing computation of the 3D Voronoi tessellation for a given domain and the creation of a ready to use TOUGH2 MESH file. If a set of geological surfaces is available, the software can directly generate the set of Voronoi seed points used for tessellation. In order to reduce the number of connections and so to decrease computation time, VORO2MESH can produce a mixed grid with regular blocks (orthogonal prisms) and irregular blocks (polyhedron Voronoi blocks) at the point of contact between different geological formations. In order to visualize 3D Voronoi grids together with the results of numerical simulations, the functionality of the TOUGH2Viewer post-processor has been extended. We describe an application of VORO2MESH and TOUGH2Viewer to validate the two tools. The case study deals with the simulation of the migration of gases in deep layered sedimentary formations at basin scale using TOUGH2-TMGAS. A comparison between the simulation performances of unstructured and structured grids is presented