Indirect Monitoring of Hydrocarbon Reservoir Deformations by Measuring Ground Surface Movements

Kamelia Atefi Monfared, Leo Rothenburg

In the proceedings of: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Mine Waste Disposal

ABSTRACT: Subsurface pressure changes generated during hydrocarbon production, waste/water reinjection will induce surface movements. It is important to predict these induced surface deformations. On the other hand, surface deformation monitoring can be applied to indirectly monitor subsurface deformations. Two numerical models were developed in this study based on the unidirectional deformation technique: 1) direct model: predicts induced surface deformations due to reservoir volumetric changes; 2) inverse model: simulates subsurface volume changes using surface deformation measurements. In order to solve the inverse ill-posed problem, a regularization technique was developed in this study. A detailed sensitivity analysis was also performed to determine the most suitable surface deformation measurement resulting in the best inverse simulation. Tilt measurements we found to be better input than vertical displacements for modeling subsurface deformations.

RÉSUMÉ: Les variations de pression du sous-sol générés pendant la production d'hydrocarbures, déchets/réinjection d'eau va entraîner les mouvements de surface. Il est important de pouvoir prédire les déformations induites de surface. D'autre part, la surveillance de la déformation de surface peut être appliquée de manière indirecte pour surveiller les déformations du sous-sol. Dans cette étude, deux modèles numériques ont été développés basée sur la technique de déformation unidirectionnelle: 1) modèle direct: prédit les déformations de surface induites par les changements volumétrique de réservoir, 2) modèle inverse: simule les variations de volume du sous-sol en utilisant des mesures de déformation de surface. Afin de résoudre le problème inverse mal posé, une technique de régularisation a été développée dans cette étude. Une analyse de sensibilité bien détaillée a été aussi effectuée pour déterminer la plus convenable façon de mesure de déformation de surface, ce qui produit une meilleure simulation inverse. Nous avons trouvé que les mesures de pente est meilleur choix de paramètre mesuré que les déplacements verticaux pour la modélisation les déformations du sous-sol. 1 INTRODUCTION It has long been recognized that the withdrawal or injection of any kind of fluid or material from or into the subsurface will generate underground deformations. Such subsurface deformations induce ground level movements. Induced surface movements have significant environmental impacts and have been an issue of concern in hydrocarbon related projects. Failure of underground utility lines, well casings, and pipelines; structural damage and foundation settlements (Hu et al., 2004); and wetland loss are some common impacts of induced surface deformation occurring in hydrocarbon related projects. It is thus significantly important to be able to predict induced deformations. Calculating surface deformations using subsurface volumetric changes in the reservoir rock is referred to as solving for the direct case. Two approaches have been commonly applied in literature for modeling subsurface deformations: a) nucleus of strain approach (Geertsma, 1973); through which subsurface deformations are modeled as single point sources that expand or compact in all directions, representing expansion or compaction; b) unidirectional deformation technique (Okada, 1985). Induced ground surface deformations are measurable quantities and are typically measured as: vertical displacements; horizontal displacements; and tilts, which are the gradient of surface deformations. Being easy to monitor and sensitive to subsurface pressure changes (Geertsma, 1957; Segall, 1985; Vasco, 2004), surface deformation data can be used to indirectly monitor subsurface deformations. Inverse monitoring has considerable potential use in fast-paced projects (steam injection/steam-assisted gravity drainage; waste injection projects) in which continuous monitoring of subsurface deformation is vital. Applying surface deformation measurements to solve for the subsurface deformation sources is referred to as solving for the inverse case. Forward and inverse models have been previously studied and reported on in the literature (Bruno et al., 1992; Dusseault et al., 2002; Vasco et al., 2002; Rothenburg et al., 1994; Vasco et al., 2008; Kroon et al., 2008; Carnec et al., 1999; Klemm et al., 2010); Most of these models are based on the nucleus of strain approach (Dusseault et al., 1993; Kroon et al., 2008; Vasco et al., 2002). Previous studies on direct and inverse simulations in the hydrocarbon related projects were mostly focused on measurements of vertical displacements (Bilak, 1989). Fewer studies have focused on horizontal deformations or tilt measurements: The reason is that displacements induced as a result of reservoir compaction/dilation majorly occur in a vertical direction, because of the

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