Accueil > EGW 2018 : 6th European Geothermal Workshop > Abstracts > Session 2 : Reservoir Engineering > Session 2 : Poster Presentations

Session 2 : Poster Presentations

S2.1 Visualizing flow interactions between fracture and matrix using a temporo-ensemble PIV method

Mehrdad Ahkami, Thomas Roesgen, Martin O. Saar, Xiang-Zhao Kong

PDF - 420.2 ko
Ahkami et al.

In the Earth Sciences, a fractured medium is often considered to be an interacting double-continuum medium with varying transport properties. Understanding transport processes in such a double-continuum medium is crucial for understanding and optimizing mass and energy transport in various industrial and scientific applications, such as hydrogeology, geothermal energy, and geochemistry. In Engineered Geothermal Systems (EGS), fluids mainly flow through fracture networks, where the heat exchange between the matrix (i.e., heat reservoir) and fractures governs the efficiency and life time of the EGS. There, flow interactions between fracture and matrix likely govern the heat exchange rate. Several physical models (e.g., the flow transfer function (Kazemi et al., 1976 ; Lu et al., 2008 ; Abushaikha and Gosselin, 2008)) have been proposed in order to describe the fluid flow interactions between fractures and rock matrix. However, validation of these models requires laboratory experimental observations.

In the presented study, Particle Image Velocimetry (PIV) methods are used to delineate a two-dimensional (2D) fluid flow field in a well-characterized fractured porous medium that is produced using 3D printing technology. This medium consists of two matrices with two different pore sizes, each matrix containing one dead-end fracture and one flow-through fracture. This configuration allows us to quantify the effect of pore-space heterogeneity on flow behaviour at the interface between the porous rock matrices and both the dead-end and the flow-through fractures.

The utilized PIV method is capable of reducing the size of the interrogation window (and velocity vector resolution) down to a single pixel. Such small interrogation windows enable the characterization of pore-scale flow features in a large Field of View (FOV) which involves various hydraulic property heterogeneities. The results of this study illustrate the effect the fracture geometry and the permeability of the surrounding matrices have on fluid flow interactions between different regions and can be used to calibrate future numerical simulations in fractured porous media.

S2.2 Thermo-hydraulic modeling of a deep fault-related fracture system in the Upper Rhine Graben using MOOSE/TIGER

Robert Egert, Maziar Gholami Korzani, Sebastian Held, Thomas Kohl, Ingrid Stober

PDF - 456.2 ko
Egert et al.

The scope of the research project GeoFaces (BMWi : 0324025C) is the analysis of thermo-hydro-mechanic properties of possible geothermal reservoirs in the Upper Rhine Graben (URG) in SW-Germany / E-France. The aim is to quantify the amount of fluid circulation along or through joint faces (fractures and faults) under recent geological conditions.

Location and extension of geothermal reservoirs in the URG are well known through geophysical exploration (e.g. 3D seismic survey). A lack of understanding of the thermic, hydraulic, (geo-) mechanic, and chemical (THMC) processes in the reservoir and their mutual influence, might lead to problems concerning geothermal exploration and evaluation of the geothermal potential.

This presentation highlights the combination of different datasets to forecast the physical behavior of an Enhanced Geothermal Reservoir (EGS) in the URG. The former international geothermal research project in the URG, Soultz-sous-Forêts, targets a fault-related granitic reservoir with elevated temperatures at 5.000 m depth. During the long-term research activities, a large scientific and experimental database has been created, which offers the opportunity to characterize the geothermal reservoir in a detailed way.

These data, e.g. geophysical, hydraulic, temperature and seismic measurements, were evaluated and used as input parameters and for the definition of the boundary conditions in a FE-modeling of the geothermal reservoir at the Soultz site. Sausse et al. (2010) developed a 3D geological model of the fracture network at the location of Soultz-sous-Forêts. Held et al. (2014) extended this structural model to an input for a thermo-hydraulic evaluation of the long-term reservoir performance. The new simulation extends the model of Held et al. (2014) and uses hydraulic and thermal datasets to improve the understanding of the fracture network of the Enhanced Geothermal System. A tracer test, made in 2005 (Sanjuan et al., 2006), allows the evaluation of the interconnection of the different wells and the quantification of the flow field in the influenced fractures between the principle wells. We simulated a long-term production scenario with the MOOSE-based application TIGER. The application gives us the opportunity to quantify the geothermal potential by solving hydro, thermal and transport processes in a fully-coupled manner.

S2.3 Feasibility of combining natural gas recovery and CO2-based geothermal in deep natural gas reservoirs

Justin Ezekiel, Anozie Ebigbo, Benjamin Adams, Martin O. Saar

PDF - 265.2 ko
Ezekiel et al.

There is potential for utilizing supercritical CO2 (scCO2) for improving and maintaining the reservoir pressure during the production of natural gas, so as to increase natural gas recovery. This is conventionally referred to CO2-enhanced gas recovery (EGR). It has also been proposed that scCO2, due to its high expansivity and low kinematic viscosity, can be utilized as a working fluid for heat recovery from sedimentary reservoirs, referred to as CO2-plume geothermal (CPG) systems. In both systems, the storage of CO2 in the reservoirs is a favourable by-product. In deep, and hot, natural gas reservoirs, there are clear synergy effects in combining these two systems (CO2-EGR and CPG) – including shared infrastructure and working fluid – which can be exploited. In this study, we investigate the feasibility, in terms of energy co-production and associated CO2 geological storage, of integrating these two systems in deep, porous and permeable natural gas reservoirs. A summary of some existing hot natural gas reservoirs worldwide and their respective reservoir properties has been investigated and are presented in this study. Using key information obtained from the different examples of these natural gas reservoirs, an anticlinal natural gas reservoir model is set up. Using this model, a reservoir simulation study is carried out, using TOUGH2, to evaluate the natural gas recovery and heat-mining performance of the system. Two stages of development are considered : when there is only natural gas production (Stage 1), and during simultaneous scCO2 injection and production stage (Stage 2). Stage 2 includes the processes of CO2-EGR, pressure recovery and CPG. Results show a high heat-mining potential for both stages, with Stage 1 having a better heat-mining rate than Stage 2. The results also show that a significant part of the natural gas reservoir could be used for CO2 storage. Coupling the reservoir model with a wellbore heat-transfer model is also important in order to find the temperature and pressure of the produced fluid at the wellhead. Hence, at any given time, the temperature and pressure of the produced fluid are numerically calculated as it rises through the production well. The obtained results confirm that the combined system can improve energy production and sustain the useful life of the gas field, for a longer period of time, as compared to the conventional CO2-EGR or CPG systems when operated in isolation. In general, the additional energy produced and CO2 sequestered in the gas reservoir can increase the gas field’s overall system efficiency and have a positive effect on the final costs of electricity generation.

S2.4 MOOSE/TIGER : New High-Performance Simulator for Nonlinear Coupled THMC Processes

Maziar Gholami Korzani, Thomas Kohl

PDF - 466.6 ko
Gholami Korzani and Kohl

The main key to the utilization of thermal energy is to understand the controlling factors of the thermal fields in reservoirs. It has been shown during the last decades that temperature can be highly affected by the groundwater circulation in fractures and faults which causes thermo-convective heat and mass transfers.

This paper introduces a new numerical tool, called TIGER, for modelling the heat and mass transfers in 3D. TIGER is a MOOSE-based (Gaston et al. 2009) application developed by Institute of Applied Geosciences, KIT. It is capable of modelling faults and well paths as (lower dimensional) discrete features in order to simulate thermal-hydro-solute transports in a fully-coupled and fully-implicit manner for anisotropic porous media. The main purpose of this study is to show practical applications of this code to better understand temperature anomalies and to avoid model simplifications for surrounding boundaries in geothermal reservoirs. There are several problems provided for showing the capabilities of this code in the simulation of diverse geothermal applications.

S2.5 Impact of effective normal stress on two-phase ow through a single fracture with rough surfaces

M. M. Grimm Lima, D. Vogler, P. Schädle, M. O. Saar, X.-Z. Kong

PDF - 443.7 ko
Grimm Lima et al.

Supercritical CO2 has been proposed to serve instead of water as the underground heat transmission fluid for Enhanced Geothermal Systems (EGS) projects, due to its superior thermophysical properties. The economical feasibility of fractured deep geothermal reservoirs, which are highly abundant, is tightly coupled to factors that determine the reservoir productivity and injectivity, such as the rock transmissivity and flow rates in fractures. Even though fracture apertures and corresponding transport properties are associated to the effective normal stress acting on the fracture, techniques under zero-stress conditions, such as laser scanning or photogrammetry, are commonly used to determine fracture aperture fields. In this study, fracture geometries of naturally-fractured granite cores from the Grimsel Test Site (GTS) in Switzerland are used to numerically model CO2 injection into these fractures under different effective normal stress conditions. Photogrammetric scanning is used to map the fracture surfaces of 2.5  3.0 cm, which are later matched to obtain the fracture aperture fields at the zero stress condition. The aperture distributions under different effective normal stresses (0.25 to 10 MPa) are then obtained by means of a Fast Fourier Transform (FFT)-based convolution numerical method. Finally, we perform two-phase flow simulations of brine displacement with CO2 injection within the aforementioned aperture fields, using an in-house application based on the MOOSE framework. Analyses on the resulting CO2 saturation patterns enable investigation of the relationships coupling effective normal stress, multiphase flow channeling and fracture transmissivity. The obtained results will assist the evaluation of applications such as enhanced geothermal systems using CO2 and geological CO2 sequestration in fractured reservoirs.

S2.6 Anisotropy and flow channeling with shearing in rough single fractures

Sophie Marchand, Michael Selzer, Olivier Mersch, Martin Schoenball, Fabian Nitschke, Jean Schmittbuhl, Britta Nestler, Thomas Kohl

PDF - 1.4 Mo
Marchand et al.

Developing and operating deep geothermal reservoirs require a comprehensive understanding of the reservoir permeability. Rock features such as fractures have a crucial impact on permeability and then on the geothermal reservoir sustainability. Thus, the detailed understanding and appropriate quantification of single fracture flow is a major step forward for modelling flow in fractured reservoirs. In these media, fluid flow is affected by the roughness which is the high frequency components of the measured surface as well as by mechanical deformations such as shearing. The associated spatial heterogeneities lead to flow behaviours such as channeling and anisotropy [1]. Both phenomena are crucial processes for the prediction of the hydraulic conductivity in fractures. Channeling refers to the flow concentration along preferential pathways. Anisotropy quantifies the local fluid velocity orientation deviation from the hydraulic pressure gradient direction. The description and quantification of these flow behaviours depend on the flow law applied and its consideration of surface roughness through notably the fracture aperture definition. If the flow is laminar and behaves locally as a Darcy flow, the widely used equation to evaluate the impact of aperture variability and roughness on fluid flow is the local cubic law - LCL [2]. Indeed, the application of LCL is widely used due to its simplicity and its low computational costs. However, a critical issue related to LCL is the definition of the local aperture which is used in this equation [3]. The common practice is to measure the aperture vertically, i.e. an identical direction is considered everywhere. Nevertheless, this practice leads to overestimated fluid flow [1]. To counter this deviation from the experimental results, Ge [4] suggested to improve LCL representativity by considering the effective aperture which is defined as the normal to the local orientation of the centerline between both fracture surfaces. Therefore, through this second definition, the area perpendicular to the local flow is taken into account.

In this study, vertical (a_ver) and effective (a_eff) apertures are compared. The aim is to evaluate the impact of choosing the aperture definition used most often over one taking into account the tortuosity of the flow path on the results. On the basis of these two aperture definitions, we aim at quantitatively study the evolution of anisotropy and channeling processes in slightly sheared factures as well as comparing it through the use of a_ver or a_eff. We perform this study through numerical steady state simulation of laminar flow in self-affine fractures using LCL. We consider 8 offsets of shearing displacement from 0.31 mm to 2.5 mm and for each of them we generated 70 distinct fractures. These 560 realizations are 2D and based on the fracture apertures which are 3D characteristics of it. Moreover, they are scaled on a real sample of granite with a roughness exponent of 0.8 and a total length of 640 mm.

For each of the previous simulations, we extract the anisotropy values with shearing for both aperture definitions. To define the anisotropy, we use the shearing direction as reference for the pressure gradient direction which is perpendicular (⊥) or parallel (∥) to it. Thus, anisotropy refers to the ratio of the sum of the positive flow velocities in the fracture outlet layer for the case ⊥ (〖OV〗_⊥) over the one obtained in the case ∥ (〖OV〗_∥) as defined in figure 1. Here we find that anisotropy distributions shift faster toward higher values with a shearing displacement below 1.88 mm for a_ver and 0.94 mm for a_eff. Above these thresholds, the anisotropy distributions are quasi-identical with shearing and show larger ranges. We conclude that optimal shearing offsets to stimulate perpendicular flows over parallel ones are founded below the previous thresholds. High shearing leads to monotonous values of the anisotropy which indicates identical variations of 〖OV〗_⊥ relative to 〖OV〗_∥. Thus, gains in hydraulic conductivity using a perpendicular pressure gradient are increasing below the thresholds and stay constant thereafter. In average, 〖OV〗_⊥ is 10% higher than 〖OV〗_∥ with extremal values ranging for a_ver and a_eff respectively from 8% and 0.95% to 30% and 40%. To interpret these differences in the anisotropy value ranges, we study the evolution of channeling with shearing for both aperture definitions as this phenomenon has been shown as one of the main drivers of anisotropy [5].

For each simulated fracture, channels are identified as the area having a flow rate value above the third quartile (Q75). From these channels, we extract two indicators to follow channeling evolution. The first one (I_1) quantifies the area of flow rates above Q75 belonging to the channels and the second one (I_2) quantifies the continuity of the flow path by measuring the maximal channel length in a given fracture. Using these indicators, we notice that in the a_eff case regardless of the shearing displacement I_1 is more spread than in the a_ver case and this for both gradient pressure directions. Whereas I_2 variations are similar except in the case a_ver for ∥ where I_2 present the largest range than any other cases associated with the smallest range of I_1 for a given shearing. We identify that the larger variations of I_1 for a_eff can explain the anisotropy ranges for this aperture definition. Nevertheless, observed significative difference in the variations of I_1 and I_2 regarding aperture definition may lead to different interpretation of the channeling evolution. Using a_ver for instance with ∥ the main factor of channeling development is I_1 as values are spread over I_2 in a given offset. However, based on a_eff, channeling evolve with the effects of both I_1 and I_2.

We have shown that the selection of the aperture definition through LCL application is crucial to interpret processes such as anisotropy and channeling. Even though a_eff has been established as a more representative aperture definition regarding real fluid flow [4], it does not imply directly that it is the most suitable to describe the physical processes under study. Indeed, the adequate aperture definition must be selected by comparison of experiments studying identical physical phenomena. Thus, further development of this work will be focused on comparing the obtained numerical results with related experiments from literature and from direct laboratory work.

S2.7 Fluid pressure drops during stimulation of segmented faults in deep geothermal reservoirs

Guillem Piris, Albert Griera, Enrique Gomez-Rivas, Ignasi Herms, Mark W. McClure, Jack H. Norbeck

PDF - 455.4 ko
Piris et al.

Hydraulic stimulation treatments required to produce deep geothermal reservoirs present the risk of generating induced seismicity. Understanding the processes that operate during the stimulation phase is critical for minimizing and preventing the uncertainties associated with the exploitation of these reservoirs. It is especially important to understand how the phenomena of induced seismicity is related to the pressurisation of networks of discrete fractures. In this study we use the numerical simulator CFRAC to analyse pressure drops commonly observed during stimulation of deep geothermal wells. We develop a conceptual model of a fractured geothermal reservoir to analyse the conditions required to produce pressure drops and their consequences on the evolution of seismicity, fluid pressure, and fracture permeability throughout the system. For this, we combine two fracture sets, one able to be stimulated by shear mode fracturing and another one able to be stimulated by opening mode fracturing. With this combination, the pressure drop can be triggered by a seismic event in the shear-stimulated fracture that is hydraulically connected with an opening-mode fracture. Our results indicate that pressure drops are not produced by the new volume created by shear-dilatancy, but rather by the opening of the conjugated tensile fractures. Finally, our results show that natural fracture/splay fracture interaction can potentially explain the observed pressure drops at the Rittershoffen geothermal site.

S2.8 Probabilistic analysis of fault stability at the Bavarian Molasse basin

Robin Seithel, Thomas Kohl

PDF - 378.2 ko
Seithel and Kohl

The geothermal development in the south German Molasse Basin is successful for district heating as well as power generation. This requires optimized reservoir management, not only for hydraulic and thermal issues but also to reduce the hazard of induced seismicity.

We analyze all available borehole data to better characterize the regional stress field. Technical pressure test data (cementations pressure, formation integrity tests, leak-off tests) exists for the Tertiary overburden but not for the upper Jurassic reservoir. The analysis highlights a stress drop from a Sh-gradient of 16-18 MPa/km at the Tertiary to a Sh-gradient of 15 MPa/km at the upper Jurassic. Stress limitation concept of the hydrostatic Tertiary at a typical Sh-gradient of 18 MPa/km indicates a SH-gradient of 1.09 Sv (μ=0.3) to 1.22 Sv (μ=0.4) near to the transition from normal to strike-slip faulting regime. Under-hydrostatic conditions and a typical Sh-gradient of 15 MPa/km at the upper Jurassic, limits the SH-range to 1.2 Sv (μ= 0.6) and 1.43 Sv (μ= 0.8).

In order to provide further knowledge of the tendency of a fault structure to slip and the interplay with micro-seismic observation we use a Coulomb-Failure Model. A Monte-Carlo-Simulation define a critical probability for reactivation to address the geological uncertainty for a fault pattern and under the contemporary tectonic stress. This method is applied to the fault dataset in the greater Munich area and the project locations (Unterhaching, Poing and Sauerlach / Dürrnhaar) where micro-seismicity has been observed in recent years. We compare the observation of micro-seismic events to the critical probability of the nearby fault structures. It highlights a potentially critically stressed fault plane at the Unterhaching site. Fault structures at the Poing site are not critically stressed in the regional stress field. But the time delay of the onset of the seismicity in comparison to the begin of geothermal circulation indicates a slow triggering process within the reservoir. We interpret this situation by a stress heterogeneity caused by circulation (thermo-elastic or poro-elastic) leading to a localized stress rotation. A clockwise rotation of SH by 20° results in a critically stressed fault segment of very limited length, which might be responsible for the perceived seismicity of Magnitude 2.1. At the Sauerlach and Dürrnhaar site this study indicates a low reactivation potential of the nearby fault structures and minor reactivation potential for local operation induced stress rearrangement. Our study shows that with a high quality data management (Fault zone, Stress field) an optimized development strategy can be defined which minimize the triggering of micro seismicity.

3 octobre 2018