Joshua A. White

Geomechanical behavior of the reservoir and caprock system at the In Salah CO2 storage project

Significance In Salah is one of the largest carbon capture and storage projects to date and has played a central role in demonstrating the feasibility of onshore sequestration of CO2 in deep saline aquifers. The unique field experience at In Salah provides a valuable case study in managing commercial-scale CO2 injections. In particular, the current work highlights the importance of geomechanics and integrated monitoring in understanding field behavior and managing storage risk. Almost 4 million metric tons of CO2 were injected at the In Salah CO2 storage site between 2004 and 2011. Storage integrity at the site is provided by a 950-m-thick caprock that sits above the injection interval. This caprock consists of a number of low-permeability units that work together to limit vertical fluid migration. These are grouped into main caprock units, providing the primary seal, and lower caprock units, providing an additional buffer and some secondary storage capacity. Monitoring observations at the site indirectly suggest that pressure, and probably CO2, have migrated upward into the lower portion of the caprock. Although there are no indications that the overall storage integrity has been compromised, these observations raise interesting questions about the geomechanical behavior of the system. Several hypotheses have been put forward to explain the measured pressure, seismic, and surface deformation behavior. These include fault leakage, flow through preexisting fractures, and the possibility that injection pressures induced hydraulic fractures. This work evaluates these hypotheses in light of the available data. We suggest that the simplest and most likely explanation for the observations is that a portion of the lower caprock was hydrofractured, although interaction with preexisting fractures may have played a significant role. There are no indications, however, that the overall storage complex has been compromised, and several independent data sets demonstrate that CO2 is contained in the confinement zone.

Hydromechanical Modeling of Unsaturated Flow in Double Porosity Media

AbstractGeomaterials with aggregated structure or containing fissures often exhibit a bimodal pore size distribution that can be viewed as two coexisting pore regions of different scales. The double-porosity concept enables continuum modeling of such materials by considering two interacting pore scales satisfying relevant conservation laws. This paper develops a thermodynamically consistent framework for hydromechanical modeling of unsaturated flow in double-porosity media. With an explicit treatment of the two pore scales, conservation laws are formulated incorporating an effective stress tensor that is energy-conjugate to the rate of deformation tensor of the solid matrix. A constitutive framework is developed on the basis of energy-conjugate pairs identified in the first law of thermodynamics, which is then incorporated into a three-field mixed finite-element formulation for double-porosity media. Numerical simulations of laboratory- and field-scale problems are presented to demonstrate the impact of d...

Scalable algorithms for three-field mixed finite element coupled poromechanics

Abstract We introduce a class of block preconditioners for accelerating the iterative solution of coupled poromechanics equations based on a three-field formulation. The use of a displacement/velocity/pressure mixed finite-element method combined with a first order backward difference formula for the approximation of time derivatives produces a sequence of linear systems with a 3 × 3 unsymmetric and indefinite block matrix. The preconditioners are obtained by approximating the two-level Schur complement with the aid of physically-based arguments that can be also generalized in a purely algebraic approach. A theoretical and experimental analysis is presented that provides evidence of the robustness, efficiency and scalability of the proposed algorithm. The performance is also assessed for a real-world challenging consolidation experiment of a shallow formation.

Probabilistic geomechanical analysis of compartmentalization at the Snøhvit CO2 sequestration project

Pressure buildup caused by large-scale CO2 injection is a key concern during a carbon sequestration project. Large overpressures can compromise seal integrity, reactivate faults, and induce seismicity. Furthermore, pressure buildup is directly related with storage capacity. In this work we study the geomechanical response to CO2 injection at Snohvit, to understand the potential for fault reactivation, leakage, and contamination of the producing interval through bounding faults. Furthermore, we evaluate the potential contribution of a structural component to reservoir compartmentalization. We combine simplified analytical models, based on critically stressed fracture theory and a Mohr-Coulomb failure criterion, with a rigorous sensitivity analysis. Large stress uncertainties are present and reflected in the modeling results. It was found that under the most likely stress state the faults are fairly stable and caprock hydrofracturing would be expected before fault reactivation. In most of the analyzed cases, the critical pressure perturbation needed for reactivation is above 13 MPa, which was the limiting pressure increase before reaching the fracture pressure. Faults were found to be ~ 20% less stable when considering variations in SHmax orientation. In those cases, fault reactivation could be expected before caprock failure if injection continued. However, if the pressure increase did reach the critical values for seal failure estimated under the worst case (and least likely) stress state, no indication of such failure can be observed in the measured pressure response. Finally, the potential role of a structural component in the compartmentalization and fluid migration is difficult to assess due to the stress state uncertainty.

Managing geologic CO2 storage with pre-injection brine production: a strategy evaluated with a model of CO2 injection at Snøhvit

CO2 capture and storage (CCS) in saline reservoirs can play a key role in curbing CO2 emissions. Buildup of pressure due to CO2 injection, however, can create hazards (wellbore leakage, caprock fracturing, and induced seismicity) to safe storage that must be carefully addressed. Reservoir pressure management by producing brine to minimize pressure buildup is a potential tool to manage these risks. To date, research studies on the effectiveness of brine production have largely focused on generic, hypothetical systems. In this paper, we use data from the Snohvit CCS project to perform a data-constrained analysis of its effectiveness under realistic geologic conditions. During the first phase of the Snohvit project, CO2 was injected into the compartmentalized Tubaen Fm. with lower-than-expected injectivity and capacity, which resulted in pressure buildup sooner than was expected. Using a reservoir model calibrated to this observed behavior, we analyze an alternative scenario in which brine is produced from the storage unit prior to injection. The results suggest that pre-injection brine production in this particular formation would be 94% efficient on a volume-per-volume basis – i.e. for each cubic meter of brine removed, an additional 0.94 cubic meters of CO2 could have been injected while maintaining the same peak reservoir pressure. Further, pressure drawdown observed during brine production is a mirror image of pressure buildup during CO2 injection, providing necessary data to estimate reservoir capacity before CO2 is injected. These observations suggest that this approach can be valuable for site selection and characterization, risk management, and increasing public acceptance.

Slope stability assessment using stochastic rainfall simulation

Abstract Many regions around the world are vulnerable to rainfall-induced landslides and debris flows. A variety of methods, from simple analytical approximations to sophisticated numerical methods, have been proposed over the years for capturing the relevant physics leading to landslide initiation. A key shortcoming of current hazard analysis techniques, however, is that they typically rely on a single historical rainfall record as input to the hydromechanical analysis. Unfortunately, the use of a single record ignores the inherently stochastic nature of the rainfall process. In this work, we employ a Markov chain model to generate many realizations of rainfall time series given a measured historical record. We then use these simulated realizations to drive several hundred finite element simulations of subsurface infiltration and collapse. The resulting slope-stability analysis provides an opportunity to assess the inherent distribution of failure statistics, and provides a much more complete picture of slope behavior.

A Thermoplasticity Model for Oil Shale

Several regions of the world have abundant oil shale resources, but accessing this energy supply poses a number of challenges. One particular difficulty is the thermomechanical behavior of the material. When heated to sufficient temperatures, thermal conversion of kerogen to oil, gas, and other products takes place. This alteration of microstructure leads to a complex geomechanical response. In this work, we develop a thermoplasticity model for oil shale. The model is based on critical state plasticity, a framework often used for modeling clays and soft rocks. The model described here allows for both hardening due to mechanical deformation and softening due to thermal processes. In particular, the preconsolidation pressure—defining the onset of plastic volumetric compaction—is controlled by a state variable representing the kerogen content of the material. As kerogen is converted to other phases, the material weakens and plastic compaction begins. We calibrate and compare the proposed model to a suite of high-temperature uniaxial and triaxial experiments on core samples from a pilot in situ processing operation in the Green River Formation. We also describe avenues for future work to improve understanding and prediction of the geomechanical behavior of oil shale operations.

Rock Moisture Dynamics, Preferential Flow, and the Stability of Hillside Slopes

This chapter investigates the relevant hydrologic and geotechnical processes triggering failure of steep hillside slopes under rainfall infiltration. Despite decades of extensive study, the fundamental controls responsible for this commonly observed slope failure mechanism are yet to be quantified. The work focuses on the triggering mechanisms of slope failure induced by rainfall events and highlights the multiphysical nature of the problem. In hillside slopes, fluid supply from the rain and fluid input from the fractures of an underlying bedrock create moisture dynamics that could undermine the stability of slopes. The impact of such dynamics is difficult to predict, let alone quantify. In this chapter, the influence of rainfall input into the slope surface and the accompanying rock moisture dynamics are investigated using a hydromechanical model that couples the interaction between fluid flow and solid deformation. Both single-porosity and double-porosity formulations are employed, the latter formulation pertaining to the case where the solid matrix exhibits two dominant porosity scales. Nonlinear finite element simulations of the failure of hypothetical hillside slopes similar in configuration to the two well-documented test slopes, the CB1 and Ruedlingen test slopes, reveal the impacts of slope/bedrock topography, rainfall history, rock moisture dynamics, and preferential flow pattern on the failure of hillside slopes.

Analysis of fault leakage from Leroy underground natural gas storage facility, Wyoming, USA

Leroy natural-gas storage site is an anticlinal, fault-bounded, aquifer-storage system located in Wyoming, USA. Based on its abundant data, uncontrolled leakage history and subsequent control by the facility operators, a modeling framework was developed for studying reservoir behavior, examining pressure and gas-inventory histories, as well as gas and brine leakage, and evaluating the sensitivity of that behavior to uncertainty about reservoir properties. A three-dimensional model capturing the bounding fault, layered geologic stratigraphy, and surface topography was calibrated by history data of reservoir pressure and gas inventory. The calibrated model predicted gas arrival at the ground surface that was consistent with the timing of observed gas bubbling into a creek. A global sensitivity analysis was performed to examine the parameters influencing fault leakage, and a geomechanical stability analysis was conducted to investigate the likelihood of fault reactivation. In general, it is shown that a discrete leakage pathway is required to explain the observed gas leakage and its subsequent operational control by reducing reservoir pressures. Specifically, the results indicate that fault leakage is a plausible explanation for the observed gas leakage. The results are relevant to other natural-gas storage sites, as well as other subsurface storage applications of buoyant fluids, such as CO2.RésuméLe site de stockage de gaz naturel de Leroy est un système aquifère dans un anticlinal bordé par faille, localisé dans le Wyoming, USA. A partir de données historiques abondantes sur les fuites non contrôlées et des contrôles postérieurs par les opérateurs de l’installation, un modèle conceptuel cadre a été développé pour étudier le comportement du réservoir, en examinant les séries chronologiques de pression et de réserves de gaz, ainsi que les fuites de gaz et de saumure et en évaluant la sensibilité des réponses aux incertitudes sur les propriétés du réservoir. Un modèle tridimensionnel incluant la faille bordière, la stratification géologique et la surface topographique a été calibré avec les données historiques des pressions et réserves de gaz du réservoir. Le modèle calibré prédisant l’arrivée du gaz à la surface du sol est conforme au temps de dégagement gazeux observé dans un ruisseau. Une analyse globale de sensibilité a été effectuée pour examiner les paramètres influençant la fuite par la faille, et une analyse de stabilité géomécanique a été réalisée pour étudier la probabilité de la réactivation de la faille. De manière générale, on a montré qu’un passage distinct est nécessaire pour expliquer la fuite de gaz observée et son contrôle opérationnel ultérieur par la diminution des pressions dans le réservoir. En particulier, les résultats indiquent que la fuite par la faille est une explication plausible de la perte de gaz observée. Les résultats sont applicables à d’autres sites de stockage de gaz naturel, ainsi qu’à des applications de stockage en sub-surface de fluides volatils, tels que le CO2.ResumenEl sitio de almacenamiento de gas natural de Leroy es un sistema de almacenamiento acuífero anticlinal limitado por falla, localizado en Wyoming, EEUU. Basados en abundantes datos de su historia de filtraciones no controladas y el subsecuente control por los operarios de la instalación se desarrolló un esquema de modelado para estudiar el comportamiento del reservorio, examinando la historia e inventario de las presiones de gas, así como de las filtraciones de gas y de salmuera, y para evaluar la sensibilidad de aquel comportamiento con respecto de las incertidumbres de las propiedades del reservorio. Se calibró un modelo tridimensional que abarca el contorno de la falla, la estratigrafía geológica en capas y la superficie topográfica mediante los datos históricos de la presión y el inventario de gas del reservorio. El modelo calibrado predijo el arribo del gas a la superficie del terreno que fue consistente con el tiempo del burbujeo de gas observado en un arroyo. Se realizó un análisis de sensibilidad global para examinar los parámetros que influyen en la filtración en la falla, y se llevó a cabo un análisis de estabilidad geomecánica para investigar la probabilidad de la reactivación de la falla. En general, se muestra que se requiere una trayectoria discontinua para explicar la filtración de gas observada y su subsecuente control operacional mediante la reducción de las presiones del reservorio. Específicamente, los resultados indican que la filtración es una explicación plausible para la pérdida de gas observada. Los resultados son relevantes respecto de otros sitios de almacenamiento de gas natural, así como para otras aplicaciones de almacenamientos subsuperficiales de fluidos flotantes, tal como el CO2.摘要Leroy地下天然气储藏库是位于美国怀俄明州的一个背斜式的以断层为西边界的含水层储气系统。基于其详细的操作,泄漏及随后的控制历史数据,我们创建了一个模型体系来研究气库状态,模拟压力和库存气量的历史以及气和水的泄漏,并且评估气库状态对地质水文参数的敏感性。其中三维气体运移模型囊括了储气库的边界断层,地层结构和表面地形。储气库的压力和储存量历史数据用来校正这个三维模型。运用校正后的模型预测得到的通过断层泄漏到地表的气量和时间符合观测数据。全局敏感性分析评估了各种气库参数对断层气体泄漏的影响。地质力学稳定性分析对断层重新激活进行了风险评价。我们的研究表明通过断层泄漏的机制解释了Leroy储气库的泄漏以及随后的减压控制历史。本文的研究结果对其他相关地下储气库,比如CO2地质储藏研究有重要的借鉴作用.ResumoO local de armazenamento de gás natural de Leroy é um sistema de aquífero-armazém num anticlinal, limitado por falhas, localizado em Wyoming, EUA. Com base em dados abundantes, no historial da percolação não controlada e no subsequente controlo pelos operadores da estrutura, foi desenvolvida uma rede de modelos para estudar o comportamento do reservatório, examinando o inventário dos dados históricos da pressão e do gás, bem como a percolação de gás e salmouras, e avaliando a sensibilidade deste comportamento com a incerteza sobre as propriedades do reservatório. Um modelo tridimensional englobando as falhas de fronteira, a estratigrafia das camadas geológicas e a superfície topográfica foi calibrado com base nos dados históricos da pressão no reservatório e do inventário de gás. O modelo calibrado previu uma chegada de gás à superfície do solo consistente com os tempos observados da presença de bolhas de gás num pequeno ribeiro. Foi realizada uma análise de sensibilidade global para examinar os parâmetros que influenciam a percolação nas falhas e foi efetuada uma análise da estabilidade geomecânica para investigar a probabilidade de reativação das falhas. Em geral, é demostrado que é necessária uma via de percolação discreta para explicar a fuga de gás e o seu subsequente controlo operacional por redução de pressão no reservatório. Especificamente, os resultados indicam que a percolação através da falha é uma explicação plausível para a fuga de gás observada. Os resultados são relevantes para outros locais de armazenamento de gás natural, bem como para outras aplicações de armazenamento subterrâneo de fluidos flutuantes, tais como o CO2.

High-Dimensional Intrinsic Interpolation Using Gaussian Process Regression and Diffusion Maps

This article considers the challenging task of estimating geologic properties of interest using a suite of proxy measurements. The current work recast this task as a manifold learning problem. In this process, this article introduces a novel regression procedure for intrinsic variables constrained onto a manifold embedded in an ambient space. The procedure is meant to sharpen high-dimensional interpolation by inferring non-linear correlations from the data being interpolated. The proposed approach augments manifold learning procedures with a Gaussian process regression. It first identifies, using diffusion maps, a low-dimensional manifold embedded in an ambient high-dimensional space associated with the data. It relies on the diffusion distance associated with this construction to define a distance function with which the data model is equipped. This distance metric function is then used to compute the correlation structure of a Gaussian process that describes the statistical dependence of quantities of interest in the high-dimensional ambient space. The proposed method is applicable to arbitrarily high-dimensional data sets. Here, it is applied to subsurface characterization using a suite of well log measurements. The predictions obtained in original, principal component, and diffusion space are compared using both qualitative and quantitative metrics. Considerable improvement in the prediction of the geological structural properties is observed with the proposed method.