• Energy Research

Sequestration of carbon dioxide (CO2) in deep saline aquifers has emerged as a mitigation strategy for reducing greenhouse gas emissions to the atmosphere. The large amounts of supercritical CO2 that need to be injected into deep saline aquifers may cause large fluid pressure buildup. The resulting overpressure will produce changes in the effective stress field. This will deform the rock and may promote reactivation of sealed fractures or the creation of new ones in the caprock seal, which could lead to escape paths for CO2. To understand these coupled hydromechanical phenomena, we model an axisymmetric horizontal aquifer-caprock system. We study plastic strain propagation patterns using a viscoplastic approach. Simulations illustrate that plastic strain may propagate through the whole thickness of the caprock if horizontal stress is lower than vertical stress. In contrast, plastic strain concentrates in the contact between the aquifer and the caprock if horizontal stress is larger than vertical stress. Aquifers that present a low-permeability boundary experience an additional fluid pressure increase once the pressure buildup cone reaches the outer boundary. However, fluid pressure does not evolve uniformly in the aquifer. While it increases in the low-permeability boundary, it drops in the vicinity of the injection well because of the lower viscosity of CO2. Thus, caprock stability does not get worse in semi-closed aquifers compared to open aquifers. Overall, the caprock acts as a plate that bends because of pressure buildup, producing a horizontal extension of the upper part of the caprock. This implies a vertical compression of this zone, which may produce settlements instead of uplift in low-permeability (k≤10-18 m2) caprocks at early times of injection. Peer Reviewed

Clear understanding of coupled hydromechanical effects, such as ground deformation, induced microseismicity and fault reactivation, will be crucial to convince the public that geologic carbon storage is secure. These effects depend on hydromechanical properties, which are usually determined at metric scale. However, their value at the field scale may differ in orders of magnitude. To address this shortcoming, we propose a hydromechanical characterization test to estimate the hydromechanical properties of the aquifer and caprock at the field scale. We propose injecting water at high pressure and, possibly, low temperature while monitoring fluid pressure and rock deformation. Here, we analyze the problem and perform numerical simulations and a dimensional analysis of the hydromechanical equations to obtain curves for overpressure and vertical displacement as a function of the volumetric strain term. We find that these curves do not depend much on the Poisson ratio, except for the dimensionless vertical displacement at the top of the caprock, which does. We can then estimate the values of the Young's modulus and the Poisson ratio of the aquifer and the caprock by introducing field measurements in these plots. Hydraulic parameters can be determined from the interpretation of fluid pressure evolution in the aquifer. Reverse-water level fluctuations are observed, i.e. fluid pressure drops in the caprock as a result of the induced deformation that undergoes the aquifer-caprock system when injecting in the aquifer. We find that induced microseismicity is more likely to occur in the aquifer than in the caprock and depends little on their stiffness. Monitoring microseismicity is a useful tool to track the opening of fractures. The propagation pattern depends on the stress regime, i.e. normal, strike slip or reverse faulting. The onset of microseismicity in the caprock can be used to define the maximum sustainable injection pressure to ensure a permanent CO2 storage. This work has been funded by Fundación Ciudad de la Energía (Spanish Government) (www.ciuden.es) through the project ALM/09/018 and by the European Union through the “European Energy Programme for Recovery” and the Compostilla OXYCFB300 project. We also want to acknowledge the financial support received from the ‘MUSTANG’ (www.co2mustang.eu) and ‘PANACEA’ (www.panacea-co2.org) projects (from the European Community's Seventh Framework Programme FP7/2007-2013 under grant agreements n° 227286 and n° 282900, respectively). Peer reviewed

Finding a numerical method to model solute transport in porous media with high heterogeneity is crucial, especially when chemical reactions are involved. The phase space formulation termed the multi-advective water mixing approach (MAWMA) was proposed to address this issue. The water parcel method (WP) may be obtained by discretizing MAWMA in space, time, and velocity. WP needs two transition matrices of velocity to reproduce advection (Markovian in space) and mixing (Markovian in time), separately. The matrices express the transition probability of water instead of individual solute concentration. This entails a change in concept, since the entire transport phenomenon is defined by the water phase. Concentration is reduced to a chemical attribute. The water transition matrix is obtained and is demonstrated to be constant in time. Moreover, the WP method is compared with the classic random walk method (RW) in a high heterogeneous domain. Results show that the WP adequately reproduces advection and dispersion, but overestimates mixing because mixing is a sub-velocity phase process. The WP method must, therefore, be extended to take into account incomplete mixing within velocity classes.

SignificanceGeologic carbon storage remains a safe option to mitigate anthropogenic climate change. Properly sited and managed storage sites are unlikely to induce felt seismicity because (i) sedimentary formations, which are softer than the crystalline basement, are rarely critically stressed; (ii) the least stable situation occurs at the beginning of injection, which makes it easy to control; (iii) CO2will dissolve into brine at a significant rate, reducing overpressure; and (iv) CO2will not flow across the caprock because of capillarity, but brine will, which will reduce overpressure further. Furthermore, CO2leakage through fault reactivation is unlikely because the high clay content of caprocks ensures a reduced permeability and increased entry pressure along localized deformation zones.

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%. This work has been funded by EU project TRUST, grant agreement number 309067, and by and the Spanish project MEDISTRAES (CGL2013-48869 and CGL2016-77122) . Peer reviewed

Reactive transport (RT) couples bio-geo-chemical reactions and transport. RT is important to understand numerous scientific questions and solve some engineering problems. RT is highly multidisciplinary, which hinders the development of a body of knowledge shared by RT modelers and developers. The goal of this paper is to review the basic conceptual issues shared by all RT problems, so as to facilitate advance along the current frontier: biochemical reactions. To this end, we review the basic equations to point that chemical systems are controlled by the set of equilibrium reactions, which are easy to model, but whose rate is controlled by mixing. Since mixing is not properly represented by the standard advection-dispersion equation (ADE), we conclude that this equation is poor for RT. This leads us to review alternative transport formulations, and the methods to solve RT problems using both the ADE and alternative equations. Since equilibrium is easy, difficulties arise for kinetic reactions, which is especially true for biochemistry, where numerous frontiers are open (how to represent microbial communities, impact of genomics, effect of biofilms on flow and transport, etc.). We conclude with the basic 10 issues that we consider fundamental for any conceptually sound RT effort.