Conceptual model development using a generic Features, Events, and Processes (FEP) database for assessing the potential impact of hydraulic fracturing on groundwater aquifers

<p>Hydraulic fracturing for natural gas extraction from unconventional reservoirs has not only impacted the global energy landscape but has also raised concerns over its potential environmental impacts. The concept of <q>features, events and processes</q> (FEP) refers to identifying and selecting the most relevant factors for safety assessment studies. In the context of hydraulic fracturing we constructed a comprehensive FEP database and applied it to six key focused scenarios defined under the scope of FracRisk project (<a href="http://www.fracrisk.eu" target="_blank">http://www.fracrisk.eu</a>, last access: 17 August 2018). The FEP database is ranked to show the relevance of each item in the FEP list per scenario. The main goal of the work is to illustrate the FEP database applicability to develop a conceptual model for regional-scale stray gas migration.</p

Heletz experimental site overview, characterization and data analysis for CO2 injection and geological storage

International audienceThis paper provides an overview of the site characterization work at the Heletz site, in preparation to scientifically motivated CO2 injection experiments. The outcomes are geological and hydrogeological models with associated medium properties and baseline conditions. The work has consisted on first re-analyzing the existing data base from ∼40 wells from the previous oil exploration studies, based on which a 3-dimensional structural model was constructed along with first estimates of the properties. The CO2 injection site is located on the saline edges of the Heletz depleted oil field. Two new deep (>1600 m) wells were drilled within the injection site and from these wells a detailed characterization program was carried out, including coring, core analyses, fluid sampling, geophysical logging, seismic survey, in situ hydraulic testing and measurement of the baseline pressure and temperature. The results are presented and discussed in terms of characteristics of the reservoir and cap-rock, the mineralogy, water composition and other baseline conditions, porosity, permeability, capillary pressure and relative permeability. Special emphasis is given to petrophysical properties of the reservoir and the seal, such as comparing the estimates determined by different methods, looking at their geostatistical distributions as well as changes in them when exposed to CO2

Saltwater Upconing and Decay Beneath a Well Pumping Above an Interface Zone

Saltwater, or brine, underlies fresh water in many aquifers, with a transition zone separating them. Pumping fresh water by wells located above the transition zone produces upconing of the latter, eventually salinizing the pumped water, forcing shut-off. The salinity of the pumped water depends on the pumping rate, on the location of the well's screen, on the fresh water flow regime, and on the difference in density between fresh and salt water, expressed as a dimensionless factor called density difference factor (DDF). Following the well's shut-off, the upconed saltwater mound undergoes decay, tending to return to the pre-pumping regime. In this paper, the upconing-decay processes in an axially symmetrical system are investigated to discover how they are affected by the DDF and by the dispersivities. The code FEAS-Brine, developed for the simulation of coupled density-dependent flow and salt transport, is used. In this code, the flow equation is solved by the Galer:wqkin finite element method (FEM), while the advective-dispersive salt transport equation is solved in the Eulerian-Lagrangian framework. This code does not suffer from the instability constraint on the Peclet number in the vicinity of the pumping well, where advection dominates the salt transport. Simulation results show that upconing is very sensitive to the DDF, which, in our work, is in the range from 0 (for ideal tracer) to 0.2 (for brine). It is shown that for the DDF of 0.025 (for seawater), local upconing occurs only for low iso-salinity surfaces, while those of high salt concentration, practically, do not shift toward the pumping well. For an ideal tracer, all iso-salinity surfaces rise toward the pumping well. For brine, however, only iso-salinity surfaces of very low salinity upcone towards the pumping well. The decay process is lengthy; it takes a long time for the upconed saltwater to migrate back to the original horizontal transition zone prior to pumping. However, the wider transition zone caused by hydrodynamic dispersion can never return to the initial one. This indicates that once a pumping well is abandoned because of high salinity, it can be reused for groundwater utilization only after a long time

Characterization of formation properties for geological storage of CO2 - Experiences from the Heletz CO2 injection site and other example sites from the EU FP7 project MUSTANG Preface

International audienceSaline aquifers in deep sedimentary formations are considered the primary candidates for geological storage of CO2, due to their large volumetric capacity that would be sufficient to meet the needs for CCS storage space in a global scale. In comparison to depleted oil and gas reservoirs the deep saline aquifers are, however, less investigated as there has previously been little economic interest in them. Effective methods for characterizing the storage aquifers are crucial for a successful implementation of any CCS project and therefore being developed and transferred from other applications in a various CCS projects.The objective of EU FP7 funded project MUSTANG project (A multiple space and time scale approach for the quantification of deep saline formations for CO2 storage, www.co2mustang.eu) has been to develop methods for characterizing saline aquifers and for understanding their properties. An essential part of this was establishment of a CO2 injection site at Heletz, Israel, where small-scale scientifically motivated injection experiments are planned to be carried out in presently ongoing continuation projects TRUST (http://trust-co2.org/) and CO2QUEST (www.co2quest.eu).While a large number of publications has been already produced based on the work in the MUSTANG project and its successors, and published in different journals, including this one, the objective of this Special Edition has been to gather some of the concluding work in the same journal issue. The focus has in particular been in summarizing the site characterization work carried out at the Heletz site, thereby providing readers an easy overview of the findings and characteristics of the site. In addition, some related work from some of the other sites investigated in MUSTANG project is also included, namely work from the sites Hontomin, Maguelone and a natural analog site in the North Sea. The results are intended to add to our understanding on relevant properties of geological formations as candidates for CO2 geological storage

Hydrogeological characterization of the Heletz Sands reservoir, Heletz (Israel) as a preliminary step towards CO2 injection experiments

Bensabat, Jacob et al.Ponencia presentada en la European Geosciences Union General Assembly, celebrada en Viena del 7 al 12 de abril de 2013.One the major components of the EU-FP7 funded MUSTANG project is to conduct a highly controlled se- ries of CO 2 injection experiments, aimed at determining field values of key CO 2 trapping mechanisms such as dissolution and residual trapping and to establish a comprehensive and consistent dataset for model validation. Prior to injecting CO 2 there is a need to achieve a sufficient degree of hydrogeological characterization of the reservoir. In what follows we present a sequence of hydrologic tests to be conducted at Heletz and their expected contribution to the understanding relevant hydrogeology. These include: 1) Chemical characterization of the formation fluid; 2) Flowing Fluid Electrical Conductivity log, aimed at determining the vertical variability of the reservoir permeability in the near well vicinity; 3) Water pulse and pumping tests, aimed at determining the reservoir scale hydraulic properties; 4) Thermal test, aimed at determining the value of the heat transfer coefficient from the reservoir to the borehole fluid, which is responsible for the heating of injected fluid in the borehole; 5) two-well injection and pumping of water and tracers test, in order to determine the impact of heterogeneity on the hydraulic parameters and to identify preferential flow paths in the reservoir. This paper presents the design and planning of the experiments, the results obtained in field and a preliminary interpretation.Peer reviewe

Model analysis of CO2 residual trapping from single-well push pull test based on hydraulic withdrawal tests : Heletz, residual trapping experiment I

Residual or capillary trapping is one of the key trapping mechanisms for geological storage of CO2. Yet, very few studies so far have attempted to estimate the residual trapping and the related characteristic parameter, residual saturation, in situ. At Heletz, a pilot CO2 injection site in Israel a single-well push-pull experiment to estimate residual gas saturation in situ was carried out during autumn 2016. The main characterization method was hydraulic withdrawal tests. The residually trapped zone was also created by means of fluid withdrawal, by first injecting CO2 and then withdrawing fluids leaving behind the immobile residual CO2. This paper presents the first model interpretation of the experimental results. Numerical modeling with TOUGH2/ECO2N was carried out to model the entire test sequence, the focus being in matching the collected pressure, temperature and flow data as well as observations of gas content in the borehole. The experimental results could be well fitted with the model that also is in agreement with previously collected petro-physical data. The results indicate a somewhat lower residual gas saturation than that measured in the laboratory, the estimated maximum residual saturation from the field experiment being 10% and the corresponding value from the core 20%. The results also indicate that most of the CO2 entered the upper reservoir layer, thus actually giving an estimate of the effective residual trapping in that layer. Overall, pressure response gave a clear signal and was an effective method in getting an estimate of the effective residual trapping in the interval tested

Interwell field test to determine in-situ CO2 trapping in a deep saline aquifer : Modelling study of the effects of test design and geological parameters

An interwell field test to determine residual phase and dissolution trapping of CO2 is being designed at Heletz, Israel. Effects of test-design options and geological parameters were investigated using numerical modelling. It was found that the interwell distance has large influence on the feasibility of the test both in terms of creation of a zone of residually trapped CO2 and detection of the time when such zone has been created. The optimal distance is site-specific and depends on formation properties. Alternating CO2 and brine injections slightly increased residual trapping, but did not facilitate creation of a well-defined zone of trapping

Role of critical gas saturation in the interpretation of a field scale CO2 injection experiment

Residual trapping of CO2, typically quantified by residual gas saturation (Sgr), is one of the main trapping mechanisms in geological CO2 storage (GCS). An important additional characteristic parameter is critical gas saturation (Sgc). Sgc determines at what saturation the trapped gas remobilizes again if gas saturation increases due to exsolution from the aqueous phase, rather than from further gas injection. In the present study, a pilot-scale CO2 injection experiment carried out at Heletz, Israel, in 2017, is interpreted by taking critical saturation into account. With regards to this experiment, the delayed second arrival peak of the partitioning tracer could not be captured by means of physical models. In this work, the hysteretic relative permeability functions were modified to account for Sgc. The results showed that accounting for the effect of Sgc during the secondary drainage indeed captured the observed delayed peak. The difference between the values of Sgr and Sgc, influenced both the time and peak height of the tracer arrival. To our knowledge this is first time that critical gas saturation has been considered in field scale analyses related to GCS. Accounting for Sgc is relevant where gas saturation during secondary drainage increases due to gas phase expansion or exsolution from the aqueous phase. This will happen in situations where pressure depletion occurs, e.g. due to gas leakage from fracture zones or wells or possibly because of pressure management activities. The findings also have implications for other applications such as underground gas storage as well as for geothermal reservoir management

A methodology for the interpretation of aquifer tests : Application to CO2 residual trapping experiments at the Heletz site

Estimation of trapped CO2 is essential for assessing the potential of a site for geological carbon storage. In situ residual trapping can be obtained through Residual Trapping Experiments (RTE). RTE experiments consist in performing characterization tests e.g. hydraulic, thermal and tracer tests before and after creating the residually trapped zone of CO2 and estimating residual saturation from the differences between the two tests. We introduce a methodology for interpreting residual drawdowns from hydraulic tests, and specifically those performed before and after the creation of the residually trapped zone. Martinez-Landa et al. (2013) demonstrated that the reduction of hydraulic conductivity and the increase in storativity within the trapped CO2 zone can produce early time differences that are significant. However, our interpretation is hindered by the fact that accurate measurement of early time (a few minutes) response is difficult because the large inertia of the system prevents us from rapidly establishing a controlled constant flow-rate. This is particularly true for the RTE test at Heletz, where water withdrawal during the hydraulic tests had to be performed by air-lift. To resolve this difficulty, we use the proposed methodology which avoids instabilities derived from changes in flow rates. Our approach consists of four steps: (1) filtering of natural trends in heads to ensure good definition of drawdowns; (2) transformation of residual drawdowns into constant pumping test drawdowns, by using the Agarwal or other methods, while accounting for flow rate variations during the pumping phase; (3) computation of smooth log-derivatives to prepare diagnostic plots to aid in conceptual model identification; and (4) quantitative interpretation. The application of our approach to the Heletz RTE experiment gave rise to diagnostic plots consistent with theoretical expectations and a residual CO2 saturation of about 10%

CO2-rich brine percolation experiments through Heletz reservoir rock samples (Israel): Role of the flow rate and brine composition

International audienceHeletz sandstone (Israel) reactivity is studied by means of CO2-rich brine percolation experiments and modeling. Experiments are performed at in situ temperature and pressure conditions with two distinctly different injection flow rates and for two different brine compositions: the genuine Heletz brine and a similar brine but in equilibrium with gypsum. The Heletz sandstone is composed of quartz, dolomite, ankerite, K-feldspar, clay minerals and minor amounts of gypsum and pyrite. The main reaction is the dissolution of the carbonates which is fastest for the highest flow rate injection and for the genuine Heletz brine. Permeability increases during the five percolation experiments, regardless of flow rate and brine type. This increase in permeability is associated to a decrease in porosity during the gypsum-equilibrated brine experiments due to gypsum precipitation. The precipitation of secondary minerals (kaolinite, muscovite and smectite) is mainly controlled by the dissolution of K-feldspar and largest at low flow rate injection, indicating that these precipitation processes are diffusion-controlled and localized in immobile zones where favorable chemical micro-environments exist. These processes are quantified by means of a reactive transport model. These results indicated that the CO2 injection at the Heletz site should trigger both permeability increase in interface between brine and CO2, which may extend over a large extension and where mixing processes are very active, especially near the well under intermittent injection conditions