Fracture Propagation Driven by Fluid Outflow from a Low-permeability Aquifer

Deep saline aquifers are promising geological reservoirs for CO2 sequestration if they do not leak. The absence of leakage is provided by the caprock integrity. However, CO2 injection operations may change the geomechanical stresses and cause fracturing of the caprock. We present a model for the propagation of a fracture in the caprock driven by the outflow of fluid from a low-permeability aquifer. We show that to describe the fracture propagation, it is necessary to solve the pressure diffusion problem in the aquifer. We solve the problem numerically for the two-dimensional domain and show that, after a relatively short time, the solution is close to that of one-dimensional problem, which can be solved analytically. We use the relations derived in the hydraulic fracture literature to relate the the width of the fracture to its length and the flux into it, which allows us to obtain an analytical expression for the fracture length as a function of time. Using these results we predict the propagation of a hypothetical fracture at the In Salah CO2 injection site to be as fast as a typical hydraulic fracture. We also show that the hydrostatic and geostatic effects cause the increase of the driving force for the fracture propagation and, therefore, our solution serves as an estimate from below. Numerical estimates show that if a fracture appears, it is likely that it will become a pathway for CO2 leakage.Comment: 21 page

Simulation of rock salt dissolution and its impact on land subsidence

Extensive land subsidence can occur due to subsurface dissolution of evaporites such as halite and gypsum. This paper explores techniques to simulate the salt dissolution forming an intrastratal karst, which is embedded in a sequence of carbonates, marls, anhydrite and gypsum. A numerical model is developed to simulate laminar flow in a subhorizontal void, which corresponds to an opening intrastratal karst. The numerical model is based on the laminar steady-state Stokes flow equation, and the advection dispersion transport equation coupled with the dissolution equation. The flow equation is solved using the nonconforming Crouzeix-Raviart (CR) finite element approximation for the Stokes equation. For the transport equation, a combination between discontinuous Galerkin method and multipoint flux approximation method is proposed. The numerical effect of the dissolution is considered by using a dynamic mesh variation that increases the size of the mesh based on the amount of dissolved salt. The numerical method is applied to a 2D geological cross section representing a Horst and Graben structure in the Tabular Jura of northwestern Switzerland. The model simulates salt dissolution within the geological section and predicts the amount of vertical dissolution as an indicator of potential subsidence that could occur. Simulation results showed that the highest dissolution amount is observed near the normal fault zones, and, therefore, the highest subsidence rates are expected above normal fault zones

Application of upscaling methods for fluid flow and mass transport in multi-scale heterogeneous media : A critical review

Physical and biogeochemical heterogeneity dramatically impacts fluid flow and reactive solute transport behaviors in geological formations across scales. From micro pores to regional reservoirs, upscaling has been proven to be a valid approach to estimate large-scale parameters by using data measured at small scales. Upscaling has considerable practical importance in oil and gas production, energy storage, carbon geologic sequestration, contamination remediation, and nuclear waste disposal. This review covers, in a comprehensive manner, the upscaling approaches available in the literature and their applications on various processes, such as advection, dispersion, matrix diffusion, sorption, and chemical reactions. We enclose newly developed approaches and distinguish two main categories of upscaling methodologies, deterministic and stochastic. Volume averaging, one of the deterministic methods, has the advantage of upscaling different kinds of parameters and wide applications by requiring only a few assumptions with improved formulations. Stochastic analytical methods have been extensively developed but have limited impacts in practice due to their requirement for global statistical assumptions. With rapid improvements in computing power, numerical solutions have become more popular for upscaling. In order to tackle complex fluid flow and transport problems, the working principles and limitations of these methods are emphasized. Still, a large gap exists between the approach algorithms and real-world applications. To bridge the gap, an integrated upscaling framework is needed to incorporate in the current upscaling algorithms, uncertainty quantification techniques, data sciences, and artificial intelligence to acquire laboratory and field-scale measurements and validate the upscaled models and parameters with multi-scale observations in future geo-energy research.© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)This work was jointly supported by the National Key Research and Development Program of China (No. 2018YFC1800900 ), National Natural Science Foundation of China (No: 41972249 , 41772253 , 51774136 ), the Program for Jilin University (JLU) Science and Technology Innovative Research Team (No. 2019TD-35 ), Graduate Innovation Fund of Jilin University (No: 101832020CX240 ), Natural Science Foundation of Hebei Province of China ( D2017508099 ), and the Program of Education Department of Hebei Province ( QN219320 ). Additional funding was provided by the Engineering Research Center of Geothermal Resources Development Technology and Equipment , Ministry of Education, China.fi=vertaisarvioitu|en=peerReviewed

Physical Pictures of Transport in Heterogeneous Media: Advection-Dispersion, Random Walk and Fractional Derivative Formulations

The basic conceptual picture and theoretical basis for development of transport equations in porous media are examined. The general form of the governing equations is derived for conservative chemical transport in heterogeneous geological formations, for single realizations and for ensemble averages of the domain. The application of these transport equations is focused on accounting for the appearance of non-Fickian (anomalous) transport behavior. The general ensemble-averaged transport equation is shown to be equivalent to a continuous time random walk (CTRW) and reduces to the conventional forms of the advection-dispersion equation (ADE) under highly restrictive conditions. Fractional derivative formulations of the transport equations, both temporal and spatial, emerge as special cases of the CTRW. In particular, the use in this context of L{\'e}vy flights is critically examined. In order to determine chemical transport in field-scale situations, the CTRW approach is generalized to non-stationary systems. We outline a practical numerical scheme, similar to those used with extended geological models, to account for the often important effects of unresolved heterogeneities.Comment: 14 pages, REVTeX4, accepted to Wat. Res. Res; reference adde

Application and Parameterization of a Semi-Analytical/Numerical Method for Modeling Matrix Diffusion Effects in Groundwater Chemical Transport

The back diffusion of dissolved chemicals from low permeability zones to aquifers can cause contaminant plumes to persist long after remediation (Chapman and Parker, 2005). Because of the complicated nature of some field sites, the effect of back diffusion on plume persistence can sometimes be ambiguous. A novel approach for simulating matrix diffusion effects was previously adapted from geothermal reservoir modeling, which combines numerical and analytical methods by discretizing only the high permeability parts of the aquifer and treating the matrix diffusion flux into the high permeability gridblocks as a concentration dependent source/sink term (Falta and Wang, 2017; Muskus and Falta, 2018). This semi-analytical/numerical method, as a result, is not as computationally intensive as conventional matrix diffusion modeling methods.The objective of this research is to better understand the parameterizations that affect the back diffusion signal in a chemical transport model, which is accomplished by applying the semi-analytical/numerical modeling method to theoretical scenarios that are representative of field conditions. This research aims to develop a better intuition for back diffusion effects that can be applied to future field studies. From this study, it was concluded that the observation of the most significant back diffusion effects in any aquifer system is dependent on monitoring well location relative to the highest concentrations within the aquifer and the low permeability/high permeability interfaces. The initial source concentration is critical for determining the magnitude at which back diffusion affects aquifer concentrations, which in some cases can be below the MCL. The low k zone degradation rate was found to be a key parameter for determining the magnitude of plume persistence caused by back diffusion. Diffusive mass flow was shown to be governed by porosity and the geometric parameterization for embedded low k material or fractures. Lastly, partial source zone remediation that results in a residual source mass can cause plume persistence that looks similar to the effect of back diffusion, and the relative contributions of a residual source mass and back diffusion to overall plume persistence are determined by the amount of source mass removed, the amount of low k material or fractures in the aquifer system, the location in the aquifer relative to the source zone, and the low k zone parameterization. Finally, a field site was assessed where the gained insights from this study were used to determine the potential risk for back diffusion at the site and to develop a model to evaluate any observed plume persistence for back diffusion

A macroscale hydrogeological numerical model of the Suio hydrothermal system (Central Italy)

The complex behaviour of the Suio hydrothermal system (central Italy) and its potential exploitation as a renewable energy source are still unclear. To quantitatively evaluate the geothermal resource, the Suio hydrothermal system has been investigated with a hydrogeological numerical model that couples fluid flow, thermal convection, and transport of diluted species inside a hybrid continuum-discrete medium. The numerical model, calibrated and validated with available and new experimental data, unveiled the complex behaviour of the hydrothermal system. The normal tectonic displacements, the fracturing of the karst hydrostructure, and the aquitard distribution strongly influence the hydrothermal basin. In particular, a dual fluid circulation, sustained by steady-state thermal and pressure gradients, modulates the hydrothermalism at the several springs and wells. The presence of a medium to a low-temperature reservoir allows for potential exploitation of the geothermal resource

Modelling Fracture Propagation in Shale Cap Rocks Cooled by CO2 Injection

Imperial Users onl

Enhanced NAPL removal and mixing with engineered injection and extraction

Aquifer remediation with in-situ soil washing techniques and enhanced oil removal typically involves the injection of liquid solutions into the geological formation to displace and mobilize non-aqueous phase liquids (NAPLs). The efficiency of these systems is oftentimes low because the displacing fluid bypasses large quantities of NAPL due to the inherent complexity of a heterogeneous natural system. Here, engineered injection and extraction (EIE) generated by rotating periodic injection is proposed as a method to enhance NAPL removal and mixing. To evaluate the method, we perform two-phase flow simulations in multiple realizations of random permeability fields with different correlation structures and connectivity between injection and extraction wells embedded in a five-spot pattern. Results show that EIE can significantly improve removal efficiency and mixing depending on several controlling factors. The effects of EIE are more significant under unfavorable conditions, that is, when injection and extraction wells are well-connected through preferential channels, permeabilities are highly heterogeneous, and/or the mobility ratio between the wetting and the non-wetting fluids is larger than one. Removal efficiency reaches its maximum value when the Kubo number is close to one, that is, when the saturation front travels one range of the permeability field in an injection pulse. These effects can develop in just a few cycles. However, removal efficiency should undergo first an early stage with detrimental effects in order to maximize removal in the long term. EIE not only enhances NAPL removal and mixing but also reduces the uncertainty, making the system more reliable and less dependent on heterogeneity.This work was partially supported by the European Commission, through project MARSOLUT (grant H2020-MSCAITN-2018); by the Spanish Ministry of Economy and Competitiveness, through project MONOPOLIOS (RTI 2018-101990-B-100, MINECO/FEDER); and by the Catalan Agency for Management of University and Research Grants through FI 2017 (EMC/2199/2017).Peer ReviewedPostprint (published version